Airplane on a Treadmill?

Oh, no. Not again? :-)

Okay, actually the tread will make no difference. Imagine the aircraft floating like a hovercraft and you ignite the jet engines. That sucker still going forward!

The wheels of the aircraft only support the plane and reduce friction as it takes off. You can put skis on the plane and it will take off, too.

In the end, the wheels will spin faster (about two times faster) and the takeoff distance might be a little longer due to the increase in friction, but the aircraft will still move forward relative to the surrounding air and eventually fly when the airspeed is high enough to support takeoff.

The critical factor is the airspeed under/over the wings.

Aircraft carriers head into the wind for launch and retrieval.

Early in the second world war when the Royal Navy was flying torpedo bi plains there were recorded cases of air crew having to secure the plain on landing as the air speed over the deck was incline to provide lift off for the now much lighter aircraft.

But isn't that the opposite of what is described in this scenario?

The air going over the wings is the actual speed of the wind, whereas in the treadmill scenario there is no air going over the wings, thus no lift.

Dominating factor is the RELATIVE WIND at the airfoil (wing) board of attack, and NOTHING else. Having relative wind means laminar flow over the wings, resulting in differential pressure between the upper part and the lower part of the airfoil. to finally produce lift. hence, flight. It doesn't make any difference whatsoever if the airplane has ground speed or not. (your treadmill in this case). Watch birds hovering, wings spread, head pointing into the wind, but zero ground speed. Better than that, you can move the earth below at ANY direction. it will not make any difference at all.

WANGITO

Let us discuss in a different way.

Should the craft to stay stationary on a moving runway, its propeller or jet have to do work? This will force air around the wing. Grater the speed of runway, greater will be the force by propeller or jet which means great air movement around the wings. At one stage this has to lift the craft. And craft should start to fly.

Let us imagine it enters in a big wind tunnel wherein the wind velocity is the same of this craft. It is again going to be stationary.

Are not this imaginary conveyer and the wind tunnel comparable?

Depends on the wind. The ground makes no difference.

Used to watch a flying club 'play' in high winds with Cessna's - taking off by standing still - at least as far as the ground was concerned. Once even saw two of them go up at about ten mph backwards. It was pretty funny looking.

If the wheels made aircraft fly, then our cars would float upwards off the highway even when we were dirving straight.

Basically, you are trying to hold the plane stationary using the friction of the rotating landing wheels, which in effect would be the same as clamping them down. So I imagine that you would reach a point where the landing gear fails, and the plane drops onto the conveyor, with a result much like being dropped onto a giant, super high-speed belt sander. If the plane was unmanned, I would pay a buck or two to see that!

You are making the fundamental mistake that the wheels are what makes the aircraft move. The forward motion of the aircraft is due to the backwards thrust produced by the engines. Since your conveyor is not changing the speed of the air then it would have little effect on the aircrafts forward motion with the exception of the increased friction on the wheel bearings and tires.

An aside is that most modern jet aircraft produce so much thrust that they move and given enough runway can even take off with the breaks on.

Conversely, would it be possible to use the conveyer belt concept to land an aircraft to achieve shorter runways ?

No. On landing, the tires are already close to their limiting speed. Landing on a counter-moving conveyor would increase wheel speed too much, causing blowouts. Also, remember that the deceleration rate will depend upon the brakes' and tires' ability to absorb energy. If the runway were moving in reverse, the brakes would have to deal with double the wheel speed -- something likely to cook them quickly.

Simple answer: The question is illogical.

I think the question is logical, although a little ambiguous. Perhaps the "speed" should be specified as ground or true airspeed (or the wind could be specified as calm, in which case those two speeds are the same.) But it doesn't matter whether the conveyor system tracks the ground speed or airspeed; it doesn't matter if there is a headwind or tailwind; the result is the same either way: the airplane takes off as is normally would: the drag from spinning the wheels faster would be far, far less than the drag of (for instance) floats on a float plane.

The illogic comes in only when some of the people reading the question assume the conveyor is keeping the plane's speed at zero -- nowhere in the premise does it say that. Further, assuming that a conveyor could act through freely spinning wheels to significantly retard a plane against hundreds or thousands of pounds of thrust seems very naïve.

The precise speed of the conveyor matters little. The question is analogous to asking: if you place a powered model airplane on a very long grocery checkout conveyor so that the belt would push the plane backwards if it had brakes, then, (given no brakes and wheels with virtually no friction) can the plane takeoff when you release the prop (or fire up the motor)? Of course.

Suppose you roller skate onto a moving walkway going against you. Do you suddenly stop or get pushed get pushed backwards? Of course not. Suppose you pull your wheel-around luggage onto a moving walkway going the wrong way. Does your luggage suddenly get yanked from your hand? Of course not.

The question is not perfectly unambiguous, but it is logical. What is illogical is the answers given by so many posters. Hero's first answer is right. This is the sort of basic stuff one would hope all engineers, not just aeronautical engineers, would know (even EE's have to take physics, don't they?)

One point on the initial question:

"This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction) instantly."

The conveyor wouldn't move an inch until the plane started moving forwards!

"airspeed will be 0 while ground speed will be whatever speed the treadmill is."

If the airspeed was zero the conveyor wouldn't move!

I agree that the question is illogical, I could go on but the sheer ignorance on display is winding me up!!

The plane will never take off to fly 'cos the speed of the plane relative to the airspeed will always be zero. It is not the wheels that make an aircraft to lift up and fly but the action of the air over the wings generate lift and since the relative speed of the wings to that of the sorrounding air is zero then no lift and the plane will remain at a fixed position relative to a stationary observer.

Esan, Korede O. (Engr.), Nigeria.

Esankoredu you are correct when you say that it is the flow of air over the wings that lifts the aircraft up and this is called the air speed and is indicated by an instrument called the Air Speed Indicator (ASI). The ASI is a pressure gauge that displays the difference between the static air pressure and the dynamic pressure. The dynamic pressure is the pressure that is taken from a tube (Pito Tube) that points directly into the flow of air. The difference between these two pressures is proportional to the speed of the aircraft through the air and is usually measured in knots.

If you had a car instead of an aircraft the car would be stationary because the car uses the ground the produce the force that moves it forward.

An aircraft on the other hand uses the air itself to produce the forward thrust and since the air is not effected by the conveyor belts motion it would have no effect on the thrust and forward motion of the aircraft. The only thing that would change is the wheels would spin twice as fast.

Everything to do with an aircraft is related to it airspeed not ground speed. You can't directly measure the ground speed of an aircraft. Ground speed needs to be calculated from the airspeed, wind speed and direction, humidity, barometric pressure at sea level and altitude.

This was almost the most ridiculous thread I've seen to date..

To prove to the original questioner that ground speed is irrelevant, he/she can try this little experiment. You might want to get a friend to video-tape this so you can post it for all of us to see.

Find a place with moving sidewalks - often large airports have them. Make yourself a paper plane (or better yet buy one of the styrofoam ones).

Stand on a non-moving part of the floor, point your model forward in front of you and drop it. Observe its fall.

Get on the moving sidewalk and walk against it so you are standing still relative to the walls. (People may get annoyed, so do it when there are few around.) Point your model forward in front of you and drop it. Observe its fall. Be careful not to step on it when it hits the moving walkway and comes toward you.

Let us know the result.

Let's specify that you are doing this on a calm day, windspeed is zero relative to the ground. If you set up your system as described, the speed of the plane is zero with respect to the ground, we have already specified that the speed difference between the air and ground is zero. Well, there is then zero difference between the speed of the plane and the speed of the air, therefore no lift.

Smeging hell.

Ok look at it this way. Everything starts off stationary with the plane pointing down the runway. Now the plane sucks air into its engines and blows it out the back pushing the aircraft forward. As this happens the conveyor starts going in the other direction. But the aircraft is on wheels that provide very little resistance so the fact that the ground is now moving backwards has little effect on the forward motion of the aircraft. All that happens is that the wheels will spin twice as fast as they would if the runway didn't move.

The aircraft is moving through the air, all it is doing is rolling along the ground. The movement of the conveyor will not cause the air to move so it will not effect the aircraft. Remember this is not a car that propels itself by pushing on the ground it is an aircraft that propels itself by pushing air through it engines.

Another way to look at it is this. Imagine that the wheels are perfect and produce no friction. If the aircraft were stationary on a stationary conveyor belt nothing would be moving. Now lets start the conveyor belt. Because there is no friction between the conveyor belt and the aircraft the would be no force acting on the aircraft so it wouldn't move. OK given that there is a certain amount of friction between the aircraft and conveyor the aircraft would eventually start to move backwards but this would take time. In our situation we have the engines running and pushing the aircraft forward overcoming the friction. The result would be an increased take off run and could possibly damage the wheels because they would be spinning twice as fast as normal but the aircraft would still move through the air and take off.

If you read my first response Masu (#5), you would realize that's pretty much what I said. Since there is no movement through the air, forget about the wings. Think of it as a jet car, and the treadmill is using rolling resistance to counteract forward movement. However, one possible way it might take off, is if the rear landing gear dropped off first, and the resulting upward vectored thrust caused the plane to launch like a Harrier (jet) or an Osprey (prop).

I agree that the whole idea is preposterous, but still fun to think about.

Sorry sir, but you are lacking minimum knowledge and understanding of aerodynamics. Go back to high school, or better and maybe more efficient, get yourself a good book on aerodynamics. Want some recommendation, ISBN 0-8306-2390-6 (H Smith, The Illustrated Guide To Aerodynamics) would be an excellent choice. After reading AND understanding it, you will not make yourself ridiculous by submitting threads like this one...

Wangito.

As I stated above, given zero air velocity, the velocity imparted by the engines would be canciled out by the velocity imparted by the belt, giving no net velocity in the calm air therefore no lift. It is possible that a plane might fly given no ground velocity if it has a headwind, but not in calm air.

you plain dumb. go to school, lern.

Re posts 15 and 18:

I'm afraid you're misreading the question. The runway can move (like a giant conveyor belt). The ground around the runway does not move in relationship to calm air, the earth in general, etc. Therefore your assertion that the ground speed is zero is a fallacy. In fact, the conveyor can only begin to move if the ground speed is greater than zero, just as the question states. Given that the wheels rotate with friction that is negligible (Fr = .013 or so) the fact they will spin at double their usual rate as the aircraft accelerates makes little difference. The aircraft's engines do not transmit their thrust through the wheels. As you said, the air is still, so the aircraft will accelerate as usual, attaining a speed relative to the still air just as it usually will. The additional friction from spinning the wheels faster is actually much less that the additional friction from taking off on, for example, grass (where the Fr = about .07).

If this makes no sense to you then look at it this way: Suppose the aircraft is sitting on glare ice. The brakes are locked on. The aircraft will still take off, just as an aircraft on skis and snow will.

If you are still not convinced, then put a wind-up plane on a grocery store conveyor. Ask the checkout person to start the conveyor just as you release the plane's prop. The plane will rush down the conveyor on its relatively frictionless wheels (and probably crash into the little wall at the end before lifting off).

the conveyor is tasked with keeping the planes motion with regards to the world at zero. It can only do that by moving backwards at such a speed that the small wheel resistance becomes large enough to offset the planes thrust. That will occur at a high speed, however, since we have hitherto unseen conveyors built into runways we also have unbreakable wheels and tires, so at some speed the wheel drag will balance the engine thrust and the plane will not advance.....just the conveyor doing it's job

This thread causes me worry about engineering education.

My two cents, assuming that the airplane has adequate propulsion, props or jet, and ruling out extreme wind conditions:

1. Engines WILL move air and create thrust (that's what airplane engines do).
2. Thrust WILL move the airplane forward, relative to a stationary ground point.
3. The conveyer affects speed of the free-spinning wheels only - has nothing to do with the dynamics of airplane, atmosphere and stationary reference point.
4. Continuing with #2, the airplane will gain velocity relative to stationary ground (and airspeed too), and acquire lift.

Oh, please.

If it were a car with wings, so that the engine turned an axle that propelled the car along the runway via the wheels, then you have a question, though still an elementary one. Without the conveyor runway, it would keep lifting off slightly, then contacting the ground again, and the force available at the wheels would be too slight at near-liftoff to do much accelerating. In fact, even in the conveyor situtation you will get some adhesion of boundary layer air to the quickly moving conveyor belt, and possibly reach sufficient lift in ground effect to lift the car somewhat higher than the resting position of the shocks.

The question itself has too many flaws, however. "control system that tracks the plane's speed" indeed - to what speed are you referring? In the car-with-wings scenario you would be tracking the "speed" of the wheels, not the car.

"Will the plane be able to take off?"

Of course. Now we're talking about a "plane," short for AIRplane. An airplane, as you noted, has "thurst"(thrust) from it's engines, applied to the AIR, not the ground. Conservation of momentum applies, and the reactive force of grabbing air from in front of the plane and pushing it behind the plane will result in the plane moving forward, unless the brakes are of sufficient strength to grab the conveyor, until the thrust is greater than the static friction and the tires start to skid/slide along the conveyor anyway, leaving a black mark down the conveyor until the tread is worn away sufficiently to blow the tire.

As the AIRplane begins to move forward with respect to the air mass, it will take off. Period. Even with a planar (non-airfoil) wing cross-section, any positive angle of attack will provide lift based upon the velocity of the air that pushes the lower surface, and therefore the attached body of the plane. There are even valid arguments about the true source of the lift of an airfoil (whether it simply affects the effective angle-of-attack seen by the wing, increasing upward push, rather than the traditional assumption of 'negative pressure' causing 'lift'). But whatever the theoretical understanding, it will take off completely disregarding the movement of the pavement beneath it.

e.g. Navy planes could land at nearly zero relative velocity to the deck of a carrier travelling sufficiently quickly, into the wind.

And as for the moving walkway in an airport experiment, rather than dropping your paper airplane: Get a wound rubberband balsa model and let go of it while standing still beside the walkway, versus walking at the same speed as the walkway. That is a more equivalent exercise.

Same result. The plane flies based on it's airspeed.

Extreme example? With enough thrust, some heavier-than-air craft have been known to go straight up with zero ground speed. Shuttle launch, anyone?

If you read the original conditions, it says that the runway adjusts to the Aircraft Speed, as you add more velocity from the engine thrust the runway also goes faster in the opposite direction. I read that as meaning there is no net ground speed, if this is the case there is no air speed either.

OK this is all I can stand!!

An airplane requires air molecules under (and over) the wings to give lift. It won't fly in a vacuum, it requires rapidly flowing air under and over the wings to take off! Piss on the the runway whether it's moving or not!!!It doesn't matter!!

Thank You Very Much 8-\

Guest You said in post #23

"the runway adjusts to the Aircraft Speed, as you add more velocity from the engine thrust the runway also goes faster in the opposite direction. I read that as meaning there is no net ground speed"

which is correct, you then went on further to say

"if this is the case there is no air speed either."

Which is incorrect.. You have forgotten to allow for the fact that the air has remained stationary and since the runway is moving backwards it equivalent to moving the air in the opposite direction. This means that there is airflow over the wings and therefore the aircraft would fly.

Look at the diagram. The conveyor motion (shown in red) turns the wheels of the aircraft (shown in red). The air (shown in blue) however doesn't move because it is not connected to the conveyor. It is this stationary air that is sucked into the engine that produces the thrust (also shown in blue). Since the thrust is generated from the stationary air the resultant Aircraft Motion (also shown in blue) would be in relation to the air and not the conveyor. Since the aircraft is moving through the air it would be able to take off. There are two separate sets of vectors here. The conveyor, wheels set (shown in red) and the aircraft, thrust, aircraft motion set (shown in blue). The only connection between the two sets of vectors is the bearings in the wheels and as it is designed to minimize friction this has a negligible effect on the air, thrust, aircraft motion set of vectors.

PS don't worry about negative feedback, it is important to learn and the only way you can learn is to keep asking questions until you understand. By the way I have a pilots license and have experience on single engine light aircraft as well as gliders an have been flying for 30 odd years now so I do know what I am talking about..

Hello Masu,

I have a question...

Suppose for a minute that the airplane on the treadmill is a glider, and it's "zero" forward motion, in reference to the treadmill is being obtained by another set of pulleys turning the opposite direction to the conveyor belt pulleys towing the glider forward, so that to a side spectator (or the airmass if you want,) the glider looks stationary, but in reality it's ground speed equals the treadmill speed. Correct? How than, will the glider develop relative wind (hence lift) if it is in zero speed relative to the airmass in front?

"You have sufficient relative wind over the airfoil, it will fly. If not it will not"

Please correct me if I am wrong.

Wangito.

Well, I guess the children are throwing a temper tantrum so I will excuse myself now.

While you're experimenting with ideas such as this one, why don't you put a fan in a sailboat filling the sails with wind and see how far you go.

But I digress..

Tried that... only got 30 meters from the dock. (Length of the extension cord.)

... see Challenge Question:

We are Sailing (again), Posted August 10, 2006 6:45 PM

Maybe the idea of an airplane taking off on a sheet ice runway, with wheels free to independently spin forward or backward at any speed, or just skid, can hammer home the point that the conveyer speed is irrelevant to the airplane moving forward through the atmosphere, propelled by engine thrust, and gaining lift over the airfoils.

When it comes to getting air to move over and under the wings maybe the best thing we could do is to run the conveyor belt in the direction of take off to assist the aircraft. A bit like an aircraft carrier (Big Ship) heading into the wind or using a catapult to assist aircraft launch.

What else could we do to get this thing into the air more efficiently?

If we could assist or limit that peak thrust requirement on engines it may be of some benefit. If nothing else in the amount of fuel that is required to get up into the sky. It could also reduce local air polution.

BlueAusieBoy your idea would work but you would need to have the wheel breaks on otherwise the same thing would apply and you would impart no force on the aircraft. As to saving fuel you still need to impart the same amount of energy on the aircraft plus the extra energy to move the runway so there would be no real gain. The only reason they use catapults on aircraft carriers is the lack of space.

No airspeed, no lift, no take off. But ...

If it was a prop plane (prop at the front), and you could tilt the wings (or use huge flaps) to divert the thrust down, you could end up with a very bad immitation of a Harrier, which (given enough thrust) would probably flip over & make life very uncomfortable for the pilot.

There's also the possiblility that the drag of the moving runway could generate enough airspeed to get a bit of lift, but as soon as the plane moved out of this 'ground effect zone', the airspeed would be lost - no lift. But maybe, if you got everthing just right, it may hover for a while (before nosing into the rolling runway, smashing the prop and again giving the pilot a bad day).

Happy Friday .

JohnDg you said

"the drag of the moving runway could generate enough airspeed to get a bit of lift"

which means you understand that the conveyor belt has little effect on the air.

Try looking at it this way. You are in a glider that is going to be launched by being pulled along the runway with a cable that is attached to a winch. The winch is at the far end of the runway and IS NOT on the conveyor belt. As the winch which is stationary starts pulling in the cable the glider would start to move forward through the air which is also stationary and eventually take off. The fact that the runway conveyor belt was also moving in the opposite direction would have little effect on the glider becoming airborne because the winch is not on the conveyor so it can still pull the glider forward through the air.

Now lets look a propeller driven aircraft. The propeller turns and bites into the stationary air forcing the air backwards and the aircraft forward. Now the conveyor belt starts moving but it doesn't have any effect on the air and since it is the air that is moving the aircraft forward it will continue to move forward picking up speed until it takes off.

Finally if it were a jet aircraft the same thing applies it's the air not the conveyor belt that the engines use to generate the thrust that moves the aircraft forward so it too would take off.

Look closely at the diagram and description in post #28 and you should understand why the aircraft will take off.

OH and by the way it's already Saturday where I am.

Good points, well made. I put my hands up to one too many at lunch time .

Bloody hell, I just realized its my birthday already. I'm off to get pissed, have a good day BYE!

Happy Birthday!

Drinks all 'round

I can't believe this ridiculous and childish thread is going o and on and on. Let's wrap it up:

Enough relative wind over the airfoil, it will fly. Not enough it will not. period.

And let's move on to at least college level physics... Don't forget this is an engineering CR.

I think you're missing a point here.

This is called Conference Room 4. The original concept (unless I've lost the plot myself) is that folks come together & chat about this 'n' that, admittedly mostly engineering-type stuff, asking questions of others in the room who may have a handle on something they're fretting about. Nevertheless, odd asides are permitted. If folks aren't interested, they'll drift away. BTW the odd joke here and there is also allowed.

I'll grant that this message is not, perhaps, made as clear in the current FAQs as it was when I joined, but I think (and hope) the idea is still there.

CR4 is a big room. If you don't like the conversation where you're standing, go and find another. There are plenty going on.

The engine makes the plane move. As it tries to move the conveyor tries to stop it. All the conveyor has in the rolling friction of the wheels, so it might have to make the conveyor run backwards at 1000 MPH as it tries to stop the planes' forward motion.

That is the only way it can influence the plane.

What we have is a huge test your wheels to destruction mechanism

At some speed, 1000 MPH ? the plane while rolling still will blow a tire/wheel and that will fall to the conveyor and the other one will be at ~1000 MPH? and the plane will have a stressful instant as it is made to spin in one spot. So I suggest we have two conveyors, one under each wheel set, so thay can operate independantly.

The engine makes the plane move. As it tries to move the conveyor tries to stop it. All the conveyor has in the rolling friction of the wheels, so it might have to make the conveyor run backwards at 1000 MPH as it tries to stop the planes' forward motion.At some speed, 1000 MPH ? the plane while rolling still will blow a tire/wheel and that will fall to the conveyor and the other one will be at ~1000 MPH? and the plane will have a stressful instant as it is made to spin in one spot.

One Thousand MPH is very fast. It is in fact faster than Mach 1, which is approximately 760 MPH at sea level. If the airplane is not moving through the air there is no airspeed. Hence the airplane is not moving forward or reverse.

However, the wheels of the airplane are revolving at a terriffic speed. But the rotation of a wheel will not cause the vehicle to which it is attached to fly. Therefore the airplane is still sitting still relative to the air surrounding the airplane. If no air is moving across the wings there will be no lift so the plane will continue to be in contact with the surface of the platform on which it is located no matter how much thrust the engines generate.

The forward thrust of the airplane engine is negated by the reverse thrust of the conveyor. There is No forward movement through the air by the airplane. If there is no movement the airplane is parked.

You will have the same results if you remove the propeller from the airplane and start the engine. You can increase the engine rpm to maximum and the plane will not begin flight because there is forward movement through the air surrounding the airplane.

No Airspeed, No Flight. Therefore the airplane will not fly.

Where on earth are you and others getting the notion that the conveyor is "trying" to stop the plane??? ("As it tries to move the conveyor tries to stop it.") Is this conveyor imagined to have some evil brain lurking within? The premise says only "This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction) instantly."

Therefore, when the plane is motionless, the conveyor is motionless. The plane will move just as it moves on a standard, motionless runway. The pilot applies full throttle, and the plane begins to move. The conveyor starts to move in the opposite direction. But there is no cable from the conveyor to the plane. The rolling friction is insignificant in comparison to the inertia of the plane. The plane's thrust is relatively enormous and has nothing to do with the wheels, or skis, or floats or anything else the plane is resting upon. So the plane accelerates to 10 knots. The conveyor adjusts its speed to ten knots in reverse, but still has no hold on the plane because the wheels are virtually frictionless (and, in any case, have about 1/7^th the friction of plane wheels on a grass runway, from which we KNOW that airplanes can take off.) So nothing is impeding the plane, and it continues to accelerate. It gets to 20 knots, and the conveyor runs at 20 knots in reverse. Again, nothing has changed; the pilot has not applied the brakes, an anchor has not dropped from plane to runway. From the pilots perspective this would be a normal takeoff, save the fact that the centerline lights would be coming at him twice as fast. He has no indicator that gives wheel speed, so everything in the cockpit looks normal: airspeed coming up as normal. So the plane continues to accelerate. Where in this process does the plane suddenly come to a screeching halt??

And where does the plane get up to 1000 mph to produce a conveyor speed of 1000 mph??? If it is going 1000 mph, it will long since have been flying. If the conveyor is moving at any speed in reverse greater than the plans liftoff speed, THE PLANE IS FLYING.

Masu flies, and says the plane will take of normally. I fly, and say the plane will take off normally. Anonymous Hero has a good grasp of physics, and says the plane will take off normally. But don't fall for an ad hominem argument. Read the premise. Use a little logic. The conveyor cannot move unless the plane is moving. Therefore the plane must move for the conveyor to move. If the plane moves, it is starting to generate airspeed, exactly as it does in a normal takeoff. The fact that the conveyor spins the wheels twice as fast has nothing to do with the physics of flight. The wheels don't make the plane fly. The wheels do not indicate the plane's speed.

If you measure the plane's speed relative to the conveyor, rather than relative to ground or air, then the conveying logic becomes nonsensical: as soon as the conveyor starts to move it would accelerate to match its own speed??? A ridiculous notion, and clearly not the intent of the question.

Guest, you didn't read post #10 did you? An aircraft's speed is measured by an instrument called and Air Speed Indicator that, strange as it may seem, measures its speed through the air. There is no instrument that measures ground speed, read post #10 for how it all works. If the aircraft has a 10Kt air speed then the conveyor has a 10Kt backward speed not the 1000Kt or something speed you suggest in an attempt to stop the 10Kt speed of the aircraft.

.

At the risk of being as pedantic as several others, there IS an instrument that measures ground speed cleverly labelled "Ground Speed". Newer ones likely use GPS signals, but in the past there were radar and other versions. It would be interesting (but moot) to try the whole experiment with an 'old' version and see what it thought.

Jdst true there are instruments like this but they don't' directly measure ground speed like a car but rather measure for example some effect like Doppler shift then calculate the ground speed. However in this case all these instruments would measure the progress of the aircraft in relation to the stationary ground not the moving runway so it still wouldn't stop it from taking off.

I agree with every word you say, What I am saying is, Let's keep a certain technical level...

Wangito.

Ok, pax vobis, I'm really beginning to agree with you here. Thing is, we either walk away, or shout. Unfortunately, shouting won't help . So we go?

well, if the speed of the conveyor is exactly the same as the plane in the other direction, then it will be unable to prevent the plane from taking off, as the rolling drag is not enough.

I had read the first post to mean the conveyor would go to any speed to prevent the plane from moving at all, which leads to the high speed scenario.

All that will happen is the plane will acceerate and takeoff at 160 knots(say) and the conveyor will frantically wheen in the other direction, = wheels rotate at 320 knots

Nobody's shouting...Everyone's technical:) A plane that's travelling 0 mph ground speed (Xforward speed cancelled out by the -Y reverse speed )= 0 relative ground speed. Lets say that there is no ambient wind. The air speed indicator should read 0. Unless I'm mistaken, a plane cannot take off with 0 air speed. The following is a paste from The Physical Description of Flight:

Let's compare two figures used to show streamlines over a wing. In figure 2 the air comes straight at the wing, bends around it, and then leaves straight behind the wing. We have all seen similar pictures, even in flight manuals. But, the air leaves the wing exactly as it appeared ahead of the wing. There is no net action on the air so there can be no lift! Figure 3 shows the streamlines, as they should be drawn. The air passes over the wing and is bent down. Newton's first law says that them must be a force on the air to bend it down (the action). Newton's third law says that there must be an equal and opposite force (up) on the wing (the reaction). To generate lift a wing must divert lots of air down.

Fig 2 Common depiction of airflow over a wing. This wing has no lift.

Fig 3 True airflow

over a wing with lift, showing upwash and downwash

Could we create an experiment using a RC fixed-wing aircraft and a human treadmill to finally put this thread to rest?

Yea but as I keep saying aircraft measure their speed through the air and there is no way for them to measure the ground speed. When you say the planes speed it always means air speed not ground speed. Read my earlier posts on how it all works.

Yes I understand. There is , however, ground speed present whether you can measure it or not , along with air speed. What do you think of a miniature experiment of a small model aircraft (powered - gas or electric) sitting on an excersize treadmill. You start the plane and the treadmill. The plane begins moving forward regardless of wheel/ treadmill speed until it takes off. I believe we are all arguing the same point! The plane is not going to be "hovering" over the treadmill though. I guess this was the misconception? Or am I missing the point?

the only way the treadmill can stop the plane is by drag through the wheels(whatever speed they roll at). If the engine thrust can overcome this drag the plane will move. Free rolling wheel will have little drag until they are going many hundreds of miles per hour, and may well fail catastrophically, as drag in this case is fairly linear function, made up of several components that might have a combined exponent of 1.2-1.4. If the wheel are allowed to use brakes they can stop the plane or slow it to a speed the conveyor can match

OK. I've re-read this entire thread and I believe everyone is in agreement that the plane will take off (with the wheels/ tires moving at 2xV.) probably damaging them.

The only one who disagreed is the Guest who originally started this thread and I don't know if he's still here or not. Probably got board and left..

You wrote: "However, ground speed present whether you can measure..." Where did you take this conclusion from? What if the airplane has an airspeed of 100mph and is flying into 100mph headwind, what would be the ground speed than? (whether you can measure it or not...)

Wangito

I think your frustration is starting to get you rattled. What you've said all along is, of course, correct: the plane will take off just fine. That some have presented detailed arguments for an alternate reality, based on reading into the premise something not stated at all (namely that the conveyor will somehow stop the plane, something it could only do if the planes brakes were on -- or severely dragging at precisely the right value to cancel thrust -- or if there was a cable from conveyor to plane) boggles my mind too.

There are, however, ways to measure ground speed: directly via GPS or radar, semi-directly through inertial and other electronic nav systems that do the math for you, and my favorite: indirectly by calculating with an antique E6B calculator. And whether you measure ground speed or not, it still exists. But in this example it makes no difference. Unless the headwind or tailwind component is so high that the aircraft cannot take off because it legally or prudently cannot do so, the effect of wheels rolling at twice normal speed will have no appreciable effect. (Granted, the really, really pedantic among us might say "On a jet with tires rated for only 50% higher than typical liftoff speed, then you might very well blow the tires before you attain liftoff." But answers of that sort are clearly not the intent of the question. If that were the intent, then we'd need to know the takeoff weight, temperature, winds, max permissible ground speed, and other flight manual data…)

At the first it stated that the conveyor was able to counter all motion of the plane. By definition the plane cannot take off. This then means we must explore the limiting conditions with the conveyor having only rolling drag to stop the plane it needs to be able to go fast enough that the rolling drag will counter the thrust, and that will only happen at extreme speeds that would destroy any type of wheel.

Let us pose a similar problem. We have a sphere that is 100 feet in diameter that is buried in a surface with a small portion 20 feet in diameter showing. Like the ball in a giant ball point pen. It also has a tracking mechanism and rotates to place a prisoner that we have placed in the exact center at all times. A couple each of whom weighed the same could easily escape. With unequals, one could escape as one could reach the edge while one was still higher on the ball. If that one stepped off, the other would then be rotated back into the central trapped zone.

I don't know about every aircraft but Concorde is (sorry, sadly it should be was) rated to take off with all the tires blown. Also with most modern aircraft the engine thrust is greater than the maximum breaking force that can be applied. Even with the breaks on they will move and with enough runway can even take off.

I can't believe I am doing this.... Let me answer you again, and let's try to put an end to this.

Will the airplane take off? under the same conditions as you describe them- NO.

And here you got everybody confused: "ground speed will be whatever speed the treadmill speed is". Bull. (excuse my French)

If the "treadmill ground speed is whatever...." that means that the treadmill is NOT STAIONARY. and that it is advancing over the earth. and this is, I believe, NOT what you wanted to say.

While ON your treadmill your airplane is stationary in reference to the earth, this means NO relative wind, NO lift, NO fly.

Let this thread rest in peace.

(_{TEE HEE HEE!} )

Well, This was my first attempt at a treadmill, and we did not go anywhere...

We're kidding here right? I've allready been caught once today (think I'll stop shaving, or abrading at least).

Yes, you are on it, just take another few steps and you will be unable to leave

Treadmill won't do anything but make the wheels spin faster. If the wheels were dirven directly by the engines, it would make a differenct. If the plane was put into a windtunnel, or a suffecient amount of wind were blown against the front of it, it would prevent it from moving forward, but not from flying. Instead of flying forward, it would hover in place until the wind would cease (models of the plane do this during the design stage at the factory).

In the scenario described in this question, the plane would not take off. There is no wind over the wings.

Upon further review and from discussions with some people, I don't know what the plane would do without more information. The problem with my answer above was my assumption that the plane couples perfectly with the treadmill runway. This is not correct. If the plane has wheels and the wheel's bearings are frictionless, the treadmill would only effect the wheels of the plane by making them spin faster, not the plane itself. It would be the same as if the plane was hovering, it simply doesn't see the treadmill and therfore doesn't care what it's speed is.

HOWEVER-

Wheel bearings do have friction. Furthermore, the friction of a bearing is dependent on both weight of the load on the wheel and speed of rotation. So a big heavy plane on small wheels would experience some serious friction and would couple, at least partially, with the treadmill beneath it.

So basically, the plane might be able to take off, but it would take a whole lot of thrust, much more than normal, to get up to speed. The friction in the bearings will couple the plane at least partially to the treadmill runway reference frame, which than cancels out some of the forward thrust.

Thanks to Dan for bringing the wheel issue to my attention.

The initial assumption I made was that we had a 'rolling road' type configuration, where the vehicle (car, truck, or in this case plane) was constrained to have no horizontal motion.

Removing that restriction, and having done a bit of re-reading and reflecting, it's still all down to interpretation of the original post.

I now feel (given an ideal servo system running the belt) that unless we have frictionless wheel-bearings (or wheels!) the situation in my post #32 still obtains.

BTW - you're very late this party, Roger; are you trying to resurrect it?

You Wrote "BTW - you're very late this party, Roger; are you trying to resurrect it?"

Actually I got into the discussion with this guy I know about it and I did a google search for information which pointed me to this thread! I didn't even know this thread existed. Crazy isn't it? CR4 is starting to grow up.

I reckon it's already grown up (or it's certainly well on its way) - I've done several googles which have led me back to CR4.

(Not that being discoverable by a google search is a measure of maturity per se, but it indicates a certain presence in the 'Big Grown-Up' world).

The thrust that modern engines produce is phenomenal and can nearly always overcome the breaks on an aircraft. There are even cases on wet and icy runways of aircraft taking off with the brakes on and in one case back in the late 60s a DC-8 got airborne on a partially icy runway with the wheels locked. Trying to overcome engines that can produce ½ MN of thrust with a handful of bearings that are specifically designed to support a massive weight at high speed is going to be difficult. As for the tires bursting Concorde at least is capable of continuing a take off run with all the tires blown out.

Keep in mind that the rollers that support a conveyor would be spinning at a phenomenal speed when compared to the aircraft wheels. It is therefore safe to assume that the conveyor is constructed from a massless belt and frictionless rollers so why wouldn't the wheel bearings in the aircraft be frictionless. If we look at if from this hypothetical point then there would be no retarding force on the aircraft form the wheel bearings. You cant have it both ways, its all hypothetical or all real world technology.

Weather you look at it from the real or hypothetical worlds the treadmill would never be able to overcome the thrust and stop the aircraft from becoming airborne.

Thrust is not what makes an airplane fly; it's air-speed and angle of attack of air on and around the wings. On the other hand, how about the experiment done under-water (the water being a dense version of air, so to speak.) A person is lying on his stomach on the treadmill with wheels attached--like a under car "crawler". He has a powered propeller attached to his feet. The treadmill starts, his wheels spin and he fires up the rear prop. With his hands at the proper upward angle he would "take off", I believe. Same thing in air; forward motion and therefore lift would be inevitable regardless of treadmill/ wheel speed.

Yes steve-o it's the airspeed that makes it fly but it's the thrust that gives it the airspeed and the thrust is what the treadmill needs to oppose in order to stop the aircraft from becoming airborne.

Yes. Well put.

I'm not an engineer, but was intrigued by this topic. As a former pilot, I felt the original poster was correct that the plane would never take off because its "zero" ground speed would create no airspeed over the wings. Then someone mentioned that the wheels would just turn twice as fast which would negate the zero ground speed.

This led me to two examples. In the case of an airport moving walkway, we all know that you can walk in the opposite direction and make progress. Similarly, you could push a wagon on the walkway and make progress. Guest's point was that the conveyor belt could speed up as you pushed the wagon faster so that there was zero ground speed. Theoretically, no matter how much thrust you applied to the wagon, the conveyor could speed up in the opposite direction to counteract it.

But then I thought of the example of a car on a conveyor that it left in neutral. Now tilt the conveyor downhill so that the force of gravity is giving the car forward movement. But the conveyor is matching that forward movement in the opposite direction....or is it? Will the tires simply move faster than normal as you elevate the conveyor more and more...or will the speed of the conveyor in the opposite direction adjust for that.

Is the friction of the tire against the moving conveyor going to be sufficient to carry it in the opposite direction to counteract gravity? Or is it going to spin its tires as one force tries to carry the car backwards while another thrusts it forward?

Now tilt the conveyor in the opposite direction so that it is an uphill ramp. Even stationary, it would take more jet thrust to move a car/plane "in neutral" up the ramp. Now if the ramp is suddenly moving in the opposite direction, isn't the thrust required even greater? Does it come down to the friction of the tires against the moving conveyor belt? Clearly a jet with brakes on does have sufficient tire friction with the ground to counteract a high degree of thrust

Your tilted conveyor and car scenario is rather similar to the aircraft scenario. I know, from my Bosch handbook, that car tire rolling friction is about .012, and the curve is quite flat, rising to about .0125 at 50 km/h. Therefore, if the car starts to move downhill, reaching a speed of let's say 3 kph, the conveyor will run at 3 km/h in reverse. The difference in friction at 3 km/h wheel speed vs 6 km/h wheel speed is scarcely measurable, so the car will continue to accelerate at a rate determined by the slope. If the slope is 1 in 6, then the accelerating force will be 1/6 the weight of the vehicle, while friction will be just over 1/100. So, the car will accelerate as normal. Just as the jet will, assuming the tires don't blow.

OK, guess the gravity examples were pretty lousy. And the friction is not really the issue so much as the ability of the conveyor to duplicate thrust in the opposite direction. Even if the jet slowly builds thrust to as much as 1000 mph, if the belt is moving 1000 mph backwards, the jet is standing still. Wheel circumference is 2 x pi x r= say 15'. If the conveyor moves backwards 15' during one wheel rotation, the plane remains stationary. No matter how many rotations of the wheel, if the equivalent circumference distance is moving backwards...there is no forward motion.

What about a kayaker trying to paddle upstream. The friction between the boat and water is minimal, but because the frame of reference is moving backwards, it is hard to make progress. If you stuck a propeller on the kayakers back and increased the rivers flow directly in proportion to the amount of prop thrust, he would remain stationary in the water.

Isn't that the same principle as the airplane?

Guest2:

Thrust is a force, not a speed. In the US, it's measured in pounds, but in the civilized world it is measured in "kilograms force" or somtimes newtons.

You say "if the belt is moving 1000 mph backwards, the jet is standing still." This makes no sense in light of the question's premise: "This conveyor has a control system that tracks the plane's speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction) instantly". If the jet is standing still, the conveyor speed has to be stopped. Otherwise, the conveyor is not working as described.

I wonder if you are reading the original poster's proposed, incorrect, answer as being part of the question or premise. The question ends with "Will the plane be able to take off? The poster goes on to suggest that the conveyor compensates for thrust, not speed, but that is his own misunderstanding of the question's premise.

The only way the question can make any sense is if the speed being measured is the aircraft's speed relative to the ground (which, in still air, will also be its airspeed). Then, for the conveyor to move, the plane must move relative to the ground, which it will do with ease. The conveyor will then move in reverse at equal speed, and the plane will continue to accelerate, because the conveyor has no grip on the plane (because the wheels are all but frictionless in comparison to the force pushing the plane forward : the engine thrust.) The takeoff proceeds as normal. Through what magic could the conveyor drag the plane back?

Here are some figures: A fighter's thrust can equal its weight, but for most planes, thrust is much less than the weight of the plane. The thrust of my old Beachcraft at takeoff was about 1/6 its weight, so it could accelerate at about 1/6 G (pretty leisurely). It weighed 2000#, and had about 330# of thrust at takeoff. The wheels roll with about the same resistance as car tires: .012 x the weight: 24 lbs. Clearly, that 24 lbs of drag will not stop the plane from taking off. And in fact, the incremental drag (from doubling wheel speed) is a very small fraction of that 24#. Per the Bosch Handbook, rolling friction at double takeoff speed for my small plane is about .016 x weight, an increase of just .004 or 8#. Not enough to even notice.

The kayak situation is different. There, an average paddler cannot get the kayak moving at more than about 6mph in still water: at that speed the drag of the kayak (in pounds) equals the maximum thrust (in pounds) that the paddler can exert. Therefore, if the river is flowing against the paddler at 6 mph the paddler will not move across the ground. A kayak's resistance at 6 mph is very high, as compared to rolling resistance: on the order of ten percent of displacement. At 7 mph that figure can go to 15%, and at 8 mph, to 25%. Displacement mode drag effectively hits a wall at the boat's hull speed (about 6mph for a typical kayak) increasing (around the hull speed range) with roughly the third power of speed.

Another difference, kayak vs plane, is the fact that the paddler acts against the water to move the kayak. The plane is not acting against the conveyor for its movement. It is acting against the inertia of air accelerated by its engines.

On the other hand, at very low paddling speeds, the situation is like that with the plane: one can paddle just fine. I have paddled routinely upstream in one mph flow. My speed over the ground (the reference speed for the conveyor, and in this case for the stream flow) is one mph, and my speed though the water is 2 mph. My leisurely paddle upstream is not stopped by the stream flow matching my speed over the ground, nor is the plane stopped by the conveyor.

ken fey:

Yes, the conveyor tracks the plane's speed, or in this case, because it is still on the ground, it tracks the number of wheel rotations. For every wheel rotation of 14.67' for instance, the treadmill would move backwards 14.67' exactly. So although the 50,000 fighter could slowly accelerate up to 1000 mph or 1467' per second, or 100 tire rotations per second (yikes) the treadmill would be smoothly matching that acceleration in the opposite direction, negating any forward groundspeed, or air over the wings to produce lift. That was my understanding of the design of the treadmill. Obviously, I concede that no such treadmill could ever be built.

The plane is still accelerating, and eventually moving at a constant speed...only against the moving treadmill. The wheels have considerable pressure on them with a 50,000 lb aircraft on three wheels and no lift to counter that drag. Even your 2,000 lb Cessna still has 2,000 lbs of gravity acting on it against the treadmill until you develop some airspeed over the wings to counteract that weight. But that airspeed never comes because the plane maintains position relative to the still ground (off treadmill) and still air.

I still don't see how you can paddle upstream if the river flow exactly matches your paddle speed to attain any forward progress. Even with a powered prop on the kayakers back, if the the prop makes sufficient thrust to accelerate upstream to 20 knots and the river flows downstream at 20 kts, you aren't going to make any progress.

I can see it's a matter of semantics, understanding of aircraft instrumentation, and acceptance or not of an implied logic of the question premise.

For a question like this one, presented to flyers and non-flyers alike, you can make the assumption that airspeed and ground speed are the same... in other words that the wind is calm. For the question , it makes little difference. If the wind were ten knots down the runway, the plane would simply take off a little earlier. So for this question, to this audience, you can consider ground speed and airspeed to be the same.

In planes, there is nothing to measure wheel rotational speed directly. When pilots say "ground speed" they mean speed in reference to the ground, not wheel speed. (Prior to takeoff, a pilot calculates takeoff distances, based on wind speed, temperature, altitude, etc. From these these calculations, he can determine if he can safely make the takeoff, abort/commit speed, etc.) The situation is somewhat complicated for commercial airplanes, but in essence, when you takeoff you look at only one speed: airspeed -- that's what you need to fly.

The plane, in this case, is setting on a conveyor, not directly on the ground, but in pilots' terms, it would have a "ground speed" as soon as it begins to move. If the conveyor is stationary, that speed would be the same as its speed across the conveyor. Once the plane and conveyor begin to move, then "ground speed" and wheel speed (as indicated by the non-existent speedometer) are two entirely different numbers.

If "speed" in the question's premise were the speed as measured by a speedometer (i.e wheel rotation rate) then the question is immediately and completely nonsensical: the plane moves at one mph over ground, which the speedometer senses as 2 mph (because the conveyor moves instantly rearward) which means that the conveyor now must instantly move at 4 mph, and instantly move at 8, 16, 32, etc. All this occurs while the plane's actual "ground speed" is just one mph. So in the nonsensical case, the wheel speed will instantly accelerate to a surface speed equal to the speed of light, and a rotational speed that will weld the bearings in both the conveyor and the planes wheels.

Clearly, the question was not intended to be nonsensical, so you have to assume the measured speed is the plane's speed in reference to the ground, and measured by GPS, by radar, by laser triangulation, by timing it going past ground reference points, by airspeed minus headwind component, etc, etc. (On a calm day, you'd simply use the airspeed indication.) In this case, the plane takes off normally, but with double the usual wheel rotation rate.

Google for the vector directions for thrust, lift, drag, and gravity in flight and takeoff. Lift does not counter drag.

Yes, my Cessna (actually a Beachcraft) would have 2000# acting against the conveyor. But the drag caused by the rolling friction associated with that normal force causes very slight frictional force (24#) in the direction opposing the plane's motion (perpendicular to the normal force). You write "the plane maintains position". How? Calculate the acceleration of a 2000# plane given 330# thrust and 24# of initial drag.

Re the kayaker, it is again a matter of semantics and logic. What do you mean by paddle speed? If you think that the paddle couples to the water as if to ground, and the water is moving backward at a speed equal to the paddles rearward motion, then of course, the kayak cannot move forward with reference to the river bank.

There are, again, two possible speeds to sense: over the ground, and through the water. If the speed in question is through the water (as measured by a knotmeter) then the same instant doublings of speed would occur, assuming the kayak has significant inertia. Because of the sticky and massive nature of water, however, eventually the kayak would be pushed back, the paddler would presumable give up paddling, and stream speed, the boat's speed through the water, and it's speed over the ground would trend toward zero.

The kayak, unlike the airplane, is partly submerged in a heavy fluid, and (again unlike the airplane) that fluid is capable of pushing it backwards forcefully.

The kayak problem makes more sense if the ground speed is what is sensed. In that case, if you paddle at two knots through water moving against you at one knot, then you will make one knot "velocity made good", with reference to the ground. (That one knot ground speed is what is driving the stream flow of one knot.) If you paddle faster, to make three knots over the ground, then the stream flow will adjust to three knots, and you will be paddling at your full speed, 6 knots through the water. Add a jet pack, and you could get up to a ground speed of 10 knots, giving a reverse stream flow of 10 knots, and a speed through the water of 20 knots.

Blink:

Airspeed and ground speed are NOT the same, as you know. If a Coast Guard C-130 flies into a hurricane and starts flying 200 knots (airspeed) into a 200 knot headwind, the C-130 has zero ground speed. It is not moving at all reference the earth. If it does not alter its path it will run out of gas in the same spot (assuming a stationary eye of the hurricane).

Similarly, flights from east to west nearly always take longer than flights the opposite direction due to the jet stream. The plane has the same airspeed in both directions but the ground speed is radically different due to high jetstream winds at altitude. The distance from Los Angeles to New York is exactly the same both directions yet assuming identical airspeeds, the west to east flight will take much less time with its tail wind due to faster ground speeds.

I'm assuming you know this and read the aircraft carrier scenario with an imaginary very long and very fast aircraft carrier. When it is sailing away from the direction of takeoff, it is exactly like the treadmill. As the aircraft carrier increases speed on a path of 270 degrees, the jet increases speed on the deck at the same rate on a 90 degree path. The jet is moving on the very long carrier and would eventually run off the end as the many-mile-long carrier was overcome by the ground speed of the jet on the carrier. But the jets actual ground speed versus a GPS point on the ocean would remain exactly the same.

The aircraft wheels cannot turn faster than the speed of the plane. True the plane is measuring speed through a pitot tube....which is registering zero because there is no air being forced into it since the plane has moved vis-a-vis the carrier deck, but not one inch vis-a-vis the air over that point in the ocean. You could run a car right next to the jet and it could match the jet's speed and acceleration and you could look at the speedometer and see that 120 knot-equivalent in mph being covered over the deck, yet the car would also be standing still over the same point in the ocean as the carrier's rearward movement at 120 knots countered the car and jet's forward movement.

A car's wheels cannot suddenly move faster than the speed of the car at high speed, short of locking up the brakes or having enough power to spin the tires in 4th gear. The speedometer is essentially measuring wheel rotations via a speedometer gear. Just because the plane uses a different means of thrust and different means to measure speed does not mean it is not going the same exact speed with the same number of wheel rotations as the car....if its tire diameters were identical. the distance in feet being traversed over the deck is being countered by the opposite speed and distance in feet covered by the fast carrier.....until the 10+ mile long carrier runs out of deck length...which never happens with a treadmill.

Lift does not counter drag but it normally counters the gravity of a 50,000 lb plane. The horizontal vector of the plane on the carrier deck is producing no vertical vector whatsoever, except that of gravity, because no lift is being produced since no air is moving over the wing no matter how fast it goes vis-a-vis the identically fast carrier or treadmill moving the opposite direction.

Your thrust versus drag argument proves that the plane can move forward, not up. But 330 lbs of horizontal jet or prop thrust is being countered by the identical mechanical thrust of the ground reference point in an equal and opposite direction by the treadmill or carrier.

Think of the plane moving 100 knots over the carrier deck while the carrier moves 120 knots in the opposite direction. You now have 20 knot negative ground speed and a 20 knot tailwind. The only difference is that the plane will still go off the end of the carrier eventually...only many miles "downsail" of where it started. In the case of the treadmill, if the plane traveled forward 100 knots and the conveyor moved backwards at 120 knots, the plane would soon fall off the back of the conveyor belt depending on its length. All that thrust was insufficient. The plane moved backwards, just as you would fall off the back of a conveyor if you don't keep up with it walking.

To quote Guest_2;

"You could run a car right next to the jet and it could match the jet's speed and acceleration and you could look at the speedometer and see that 120 knot-equivalent in mph being covered over the deck, yet the car would also be standing still over the same point in the ocean as the carrier's rearward movement at 120 knots countered the car and jet's forward movement."

Lets for the time being go back to the treadmill but this time we have a Ferrari formula 1 car sitting on the runway next to an FA-18. Now the starter drops the flag and Michael Schumacher plants his foot and drops the clutch in his F1 car while Ken in his FA-18 rams the throttles forward and lights up the afterburner.

As the car starts to accelerate the runway/treadmill responds by moving in the opposite direction. Now since Michael's F1 is using the wheels to push against the now moving runway/conveyor and the, now moving, runway/conveyor is reacting by moving in the opposite direction he manages to go absolutely nowhere.

However Ken in his FA-18 is sucking in vast quantities of stationary atmosphere, dumping a few tankers worth of kerosene into it, setting fire to the mixture and letting it rush out the back as a somewhat increased velocity into the, still stationary, atmosphere.. The problem is that the thrust is relative to the, still stationary, atmosphere and not the moving runway/conveyor. So while Michael in his mind blowingly fast F1 manages to go nowhere by pushing against the moving runway/conveyor Ken in his FA-18 that is sucking air in from the stationary atmosphere, and blowing it out the back at an increased velocity into the, still stationary, atmosphere will move forward with ever increasing speed through the, still stationary, atmosphere until he reaches take of speed.

The reason Michael goes nowhere while Ken manages to get airborne is the F-1 car is using the runway/conveyor to produce the forward speed. Ken in his FA-18, on the other hand, is using the, always stationary, atmosphere to move forward and since the runway/conveyor has little to no effect on the atmosphere it can't stop the plane from taking off.

Now look what you've done? I have been procrastinating on this subject for so long now that my avatar has gone blue in the face so can we please put this to bed?

You guys appear hung up on the jet thrusting against air instead of car wheels "thrusting" against the treadmill. It doesn't really matter because both plane and vehicle require a constant amount of power to travel, for instance, 120 mph or 176 feet per second. As long as the premise remains that the treadmill moves an equal 176' in the opposite direction in that same second, such thrust against stationary air is irrelevant.

You understand the real world example that forward movement of a carrier vis-à-vis "still" ocean adds upwards of 30 knots to the plane's takeoff airspeed. You also recognize that the catapult adds most of the initial forward speed to the aircraft and moves the aircraft wheels a specified number of feet per second, like the jet, despite neither directly powering the wheels. Why don't you understand that a carrier hypothetically moving the opposite direction of takeoff, is subtracting aircraft speed despite catapult speed remaining the same.

If the catapult accelerates the plane to 120 knots but the carrier is sailing 30 knots the opposite direction, the net takeoff airspeed and ground speed is only 90 knots and the plane may stall. Increase the speed of the imagined carrier to 60 knots in the opposite direction. Now the 120 knot catapult is only getting the plane up to 60 knots. Finally make the hypothetical carrier go 120 knots the wrong way. The catapult manages to get the jet to 120 knots on the carrier deck. But once over the edge it drops into the water like a stone, as the previous 120 knot tailwind of the stationary jet turned into no wind at all over the wings once released by the catapult.

OK, how about a rocket with wheels and wings. (no catapult--back to the original question) Would it take off?

Maybe I'm all screwed up but I can't get this idea out of my mind since it makes so much sense in unscientific theory.

Don't really like your rocket idea due to the implication of less thrust control. But picture your rocket as a jet and tell me what happens if the conveyor belt start turning backwards very slowly while the jet is offering no thrust. It rolls off the back, correct? (no quick movement of conveyor or you get the tablecloth effect) So will you at least concede that some jet power is required to keep it from rolling off the back as the treadmill moves?

Now start to accelerate the treadmill. Unless the jet increases thrust so that it matches treadmill reverse speed with equal forward speed, the jet rolls off the back again. Can you envision a smooth simultaneous increase in both conveyor speed and jet thrust so that the two speeds remain identical?

You guys keep asking what is holding the jet back. Nothing. It is moving, accelerating to whatever steady state speed it can reach with a given amount of thrust. Only problem is the conveyor accelerates at a matching speed preventing progress over the real ground. At 120 mph the jet is still covering 60 miles in 30 minutes....but it is all on the treadmill, covering 316,800 ft in 30 minutes on the treadmill at 176'/second.

Think of it another way. If we imagine wheels with a circumference of 17.6', the jets wheels will need to rotate 10 times a second to maintain 120 mph on a long treadmill. That equals 17.6' (one rotation) every tenth of a second, or 4.4' every .025 second. So over that .025 second what happens to the jet wheel. It moves forward 1/4 of a turn covering 4.4'. Only problem is that over the same .025 second, ground reality changed. A point on the treadmill that was 4.4' to the front previously, is now directly under the wheel not 4.4' forward, but in the exact same place, thanks to the equal speed of the treadmill. So attempted forward movement of 4.4' was countered by equal aft movement of the treadmill by 4.4'.

Look at the picture and imagine that the point on the conveyor belt directly over the front roller is 59.7' from the point on the belt directly over the rear roller. An equal amount of conveyor is underneath the rollers, so that means total conveyor belt length is 59.7' + 59.7' plus half the circumference of the rollers on each end. For simplicity, assume the rollers are the same size as the jet wheels which means adding one wheel circumference of 17.6' to the total. So 59.7+59.7+17.6= 176' of total conveyor belt length. We know that the jet at 120 mph must cover 176' per second. So all we need to do is make sure that the conveyor moves one complete rotation in one second. The plane appears to remain stationary on the conveyor belt to an observer adjacent to the conveyor, but the plane is actually traveling 120 mph. Don't ask me to do any calculus etc as it has been way too many years.

What am I screwing up? I'm not disputing that there is forward movement. But it appears to be all on the treadmill instead of over the real ground or through any still air.

Correction of my 2nd grade math.

79.2' + 79.2' + 17.6' = 176' of conveyor belt length in the above example.

"But picture your rocket as a jet and tell me what happens if the conveyor belt start turning backwards very slowly while the jet is offering no thrust. It rolls off the back, correct?"

I would respond with this question

What would happen in the above situation if the wheel bearings were frictionless?

The answer is that the aircraft would not move as there would be no force acting against it's mass, or in other words its inertia.

Now you must agree that for a conveyor the be able to run at the speeds required to stop the aircraft moving forward it would need to be constructed using frictionless rollers and bearings. Since we are using them in the conveyor we would also use them in the aircraft wouldn't we? If you use frictionless bearings in the conveyor and not the aircraft you are just cooking the books to get the result you want and any arguments you are using are invalid.

OK we havn't got frictionless bearings so the conveyor and aircraft have real world bearings. That means the conveyors multitude of bearings must produce less friction that the aircrafts small number of wheel bearings otherwise the friction on the conveyor would overcome any sort of motor using to drive the belt and you wouldn't be able to counter the thrust. So it would never work in reality either.

There you have it. It won't be able to stop the aircraft from moving in either a purely hypothetical world or in a realistic and practical world. That means no matter which way you consider it the aircraft is going to accelerate in a direction and rate commensurate with the thrust it is producing and will ultimately gain enough forward speed relative to the stationary air to become airborne.

Enough on this subject lets move on to solving the real problems the world has like where we are going to get all the energy we need without cooking the planet.

Think...

I know this is a tough question, but here it is:

Does a carrier catapult attach to a freely spinning spool of cable mounted on the side of the jet, to the tread of the jet's tires, or to the hook? Any idea why? Does the original question have anything whatsoever to do with a catapult? Did the original question stipulate that the plane is cabled to the conveyor? Did it stipulate the the planes brakes were locked on?

Another thing to ponder: according to your logic, if the plane in question was a Harrier, initially held just off the original runway/conveyor, and then accelerated just above the runway in an attempt to takeoff (mimicking a standard takeoff profile, but elevated at every point by a couple feet), it would still be driven back by the conveyor, even though it doesn't touch the conveyor. It can be demonstrated that there is a very slight amount of friction between the Harrier and any runway beneath it. (This friction comes about from laminar and turbulent shear effects.) That slight friction will hold the Harrier in place, against its full forward and vectored thrust, according to your logic. It can also be demonstrated that there is a very slight amount of friction in a planes wheel bearings through which the conveyor could apply drag to the plane. Therfore, if you can convince us that the Harrier cannot takeoff, then you will have convinced us that the plane on the conveyor cannot takeoff.

Diagram the forces at work that prevent the Harrier from taking off. Show the forces and drags involved and their magnitudes. Don't worry about getting the magnitudes exact: =/- 20% would be fine.

Having done that, diagram the forces at work in preventing the original plane from taking off. You postulate that the is some force holding the plane back. what is that force? What would it be called and what is its magnitude?

Do the same diagram for a plane resting on a hovercraft hovering above the moving runway/conveyor.

If still unconvinced, try this:

Go to an ice skating rink. Watch a brand new skater. They will try to move forward by pushing back on one skate. They go nowhere, until someone tells them that they have to turn the push-off skate partly sideways. A skate, aligned with the force against it can not transfer significant force. A wheel (as on a roller skate) aligned with the applied force can also not transfer a significant force. The rolling resistance of airplane wheels is even lower that skates on ice: ice is 2-3%percent of weight, aircraft wheels are less than 2 %. Airplane wheels cannot transfer significant force (in the direction under discussion) UNLESS THE BRAKES ARE ON.

In a level, dry parking lot, try pushing your car (with the tires correctly inflated). I can push my 3000# Accord easily. I'd say the force required is about 40#, which corresponds perfectly with the calculated value using the rolling resistance coefficient from my Bosch Automotive Handbook. Gosh, so they don't make that crap up, after all!

By applying simple calculations, and the Bosch empirical values, I can predict how much force it will take to push my car. I can predict how much force it will take to prevent my plane from moving when I'm trying to takeoff. (Amazingly, that force will just equal its thrust. So if I have 330# of thrust, then ten people, each pulling back against the plane with 33# each, could stop it from moving.) I can calculate, given the rolling resistance of my plane, its thrust, and its mass, how quickly it will accelerate down the runway. I can also calculate how much runway is used. And in each case, the calculations and real world observations match to within 10%. The incredible power of math!!! There should be a name for this kinda shit. How about engineering.

Do the math.

"Does a carrier catapult attach to a freely spinning spool of cable mounted on the side of the jet, to the tread of the jet's tires, or to the hook? Any idea why? Does the original question have anything whatsoever to do with a catapult? Did the original question stipulate that the plane is cabled to the conveyor? Did it stipulate the the planes brakes were locked on?"

I don't know. If two steam catapult cables were pulling the wheels under both wings it would probably stress the heck out of the point where the wings connect to the fuselage. It can't push the wheel tread since the wheel must turn so I think it pulls on a strong crossmember somewhere between the two front wheels.

But that is irrelevant. If you imagine a jet engine with unreal thrust so that it could take off the brakes and accelerate at an identical rate as the catapult, the principle remains the same. Better yet, put the carrier dead in the water and picture it being able to accelerate from zero to 120 mph at a rate matching the catapult or super jet, only in the opposite direction. Result? Plane remains over same GPS point as it reaches the end of the catapult or super jet's powered acceleration, and drops into the water with no airspeed.

Wow. I'll give this one more shot, and if I fail, I'll assume we simply speak much different languages.

I'll italicize your text, and put my comments in bold:

Airspeed and ground speed are NOT the same, as you know. Of course. I suggested treating them equally for the sake of this simple problem: no wind is specified, so to keep things simple, let's say it is calm (zero windspeed) . If that is the case, the values of airspeed and ground speed are the same. No, the concepts are not the same; their values are: if the plane takes off at an airspeed of 120 knots, then ground speed will be 120 knots at liftoff, given calm winds. If a Coast Guard C-130 flies into a hurricane and starts flying 200 knots (airspeed) into a 200 knot headwind, the C-130 has zero ground speed. It is not moving at all reference the earth. If it does not alter its path it will run out of gas in the same spot (assuming a stationary eye of the hurricane). I would hope everyone here is aware of this.

Similarly, flights from east to west nearly always take longer than flights the opposite direction due to the jet stream. The plane has the same airspeed in both directions but the ground speed is radically different due to high jetstream winds at altitude. The distance from Los Angeles to New York is exactly the same both directions yet assuming identical airspeeds, the west to east flight will take much less time with its tail wind due to faster ground speeds. Everyone here knows this.

I'm assuming you know this and read the aircraft carrier scenario with an imaginary very long and very fast aircraft carrier. Actually, I didn't read it until after my last post. In your description, you first write about the plane being flung forward into the air by the carrier. There is a glimmer of understanding in your post: You realize that for the carrier to impart a significant longitudinal force to the plane the brakes must be on – a condition entirely different than the takeoff condition from a runway or the imaginary conveyor. When it is sailing away from the direction of takeoff, it is exactly like the treadmill. Almost, although you seem to be saying that the carrier speed is what controls the jet's speed, rather than vice versa, as described in the question. "Slowly accelerates" is a relative term, but the jet will accelerate as fast as it can, and the carrier would have to match its acceleration. As the aircraft carrier increases speed on a path of 270 degrees, the jet increases speed on the deck (correction: over the ground way below: if we use speed over the carrier's surface as the control reference the question becomes, again, completely illogical – an instantaneous chain reaction or feedback gone awry – as described in my earlier post. Using ground speed as the reference, the speed over the deck will be increasing at twice the rate that the plane's speed is increasing over the ground way below.) at the same rate on a 90 degree path. The jet is moving on the very long carrier and would eventually run off the end as the many-mile-long carrier was overcome by the ground speed of the jet on the carrier the groundspeed of the jet on the carrier (??? ground speed is relative to the ground down at the bottom of the water, not the carriers surface.) But the jets actual ground speed versus a GPS point on the ocean would remain exactly the same. That's nonsense. For that to be the case, the jet's brakes would have to be dragging severely. From the perspective of someone on shore, the plane will accelerate normally eastward. The carrier will appear to have been pulled out from under the plane, achieving a speed of 120 knots in the opposite direction. The relative speed difference will be 240 knots. At the instant of liftoff the planes wheels will be spinning at a rate that corresponds to a tread surface speed of 240 knots.

So, yes, the carrier accelerating westward to match the plane's acceleration to the east is like the conveyor situation. In both cases, the wheel rotation rate will be twice normal, and the takeoff run will be perhaps 2% longer because of the very small incremental rolling friction drag increase at that higher wheel rotation rate. In both cases, your analysis bears no relationship to what actual occurs.

You seem to have a gross misconception about what determines a plane's rate of acceleration. The wheels do not make the plane go. Imagine yourself standing on roller skates. You throw a bushel basket of apples to a friend. You get pushed backwards as a result. That is how thrust works: the propeller (or jet) forces a mass of air rearward. The plane reacts by moving forward at an acceleration that is directly related to the force exerted upon the mass of air. If that force (thrust) is equal to the plane's weight, the plane will accelerate at 1 G. The wheels cannot significantly impede the motion of the plane relative to the earth unless the brakes are on. Thrust is a large fraction of the plane's mass. Rolling friction is a tiny fraction.

Try this: take a toy car with relatively frictionless wheels and put it on the dining room table. Yank the tablecloth out from under the car. Unless there are wrinkles in the tablecloth, the car remains relatively motionless, because the wheels cannot impart much force to the car – the rolling friction is too low.

Prove it to yourself: The figures I gave you from my plane are quite close to actual: wheel rolling friction at either normal takeoff speed or twice that is .012 - .013. You can even use .014, if you want. Say 28#. Thrust is 330# Weight is 2000#. How long will it take the plane to accelerate to its 60 knot (101 fps) takeoff speed? Ignore the increasing effects of wind resistance as the plane accelerates.

What force do you imagine can prevent the plane from accelerating normally (and in fact, more quickly than it would over grass, where the drag coefficient can be as high as .07.)

The aircraft wheels cannot turn faster than the speed of the plane. True the plane is measuring speed through a pitot tube....which is registering zero because there is no air being forced into it since the plane has moved vis-a-vis the carrier deck (It will reach 240 knots relative to the deck, and the airspeed indicator will read 120 knots) but not one inch vis-a-vis the air over that point in the ocean. Wrong. There is no force that the carrier can impart to the plane to overcome its thrust, unless the plane is tied down or its brakes are on. The plane will be moving at 120 knots over the ground and through still air at that point in the ocean. You could run a car right next to the jet and it could match the jet's speed and acceleration and you could look at the speedometer and see that 120 knot-equivalent in mph being covered over the deck, yet the car would also be standing still over the same point in the ocean as the carrier's rearward movement at 120 knots countered the car and jet's forward movement. Wrong. If you ran a car parallel to the jet, but on a land road, then their speeds would match, relative to the earth's surface. To drive alongside the plan, on the carrier deck you'd need to go 240 mph.

A car's wheels cannot suddenly move faster than the speed of the car at high speed, short of locking up the brakes or having enough power to spin the tires in 4th gear. The speedometer is essentially measuring wheel rotations via a speedometer gear. Just because the plane uses a different means of thrust and different means to measure speed does not mean it is not going the same exact speed with the same number of wheel rotations as the car....if its tire diameters were identical. the distance in feet being traversed over the deck is being countered by the opposite speed and distance in feet covered by the fast carrier.....until the 10+ mile long carrier runs out of deck length...which never happens with a treadmill. This is all gibberish: wheel rotation speed has virtually nothing to do with the problem, because with the brakes off, the wheels cannot slow the plane (other than by virtue of their tiny rolling friction, which is no more than 10% of thrust).

Lift does not counter drag but it normally counters the gravity of a 50,000 lb plane. The horizontal vector of the plane on the carrier deck is producing no vertical vector whatsoever, except that of gravity, because no lift is being produced since no air is moving over the wing no matter how fast it goes vis-a-vis the identically fast carrier or treadmill moving the opposite direction.

Your thrust versus drag argument proves that the plane can move forward, not up. Read about flight. If the plane moves forward at takeoff speed, it can fly, you simply need to manipulate the controls to make it do so. If I have proved that the plane will accelerate to takeoff speed, I've proved that it can fly. But 330 lbs of horizontal jet or prop thrust is being countered by the identical mechanical thrust of the ground reference point (nonsense: ground reference points have no thrust) in an equal and opposite direction by the treadmill or carrier. By what imaginary force??? The only retarding force the carrier or conveyor can exert is through the tires which can only resist with their rolling friction, which is a small fraction of thrust.

Perhaps, when you said equal and opposite, you were thinking about thrust, and reaction. If that is the case, then there may be hope. The problem is that thrust does not bear against the carrier or the conveyor. The thrust of my plane is the effect of its continuously taking a masses of air and throwing it violently backwards. In your way of thinking, the plane would stop accelerating as soon as it lifts off.

Think of the plane moving 100 knots over the carrier deck while the carrier moves 120 knots in the opposite direction. (But why?? That situation will never occur: If the plane is moving at 100 knots over the ground, the carrier is moving 100 knots in reverse, and the plane's speed relative to the carrier is 200 knots.) You now have 20 knot negative ground speed and a 20 knot tailwind. The only difference is that the plane will still go off the end of the carrier eventually...only many miles "downsail" of where it started. In the case of the treadmill, if the plane traveled forward 100 knots and the conveyor moved backwards at 120 knots, the plane would soon fall off the back of the conveyor belt depending on its length. All that thrust was insufficient. The plane moved backwards, just as you would fall off the back of a conveyor if you don't keep up with it walking.

I doubt this will change your thinking. In any event, I must move on.

Murphy law at it's best. (US congress)

1. NASA Budget decision-1hour 32 minutes.
2. Pentagon lunch menu - 24 days 3 hours 2 minutes.
3. If You didn't get the massage, just keep on investing your time in the stupidest thread ever to be seen on the CR-4.

Wangito

Well put!

I give up.

Amen and Hallelujah. Enough is Enough!!!

The only thrust from the engines of the aircraft would be that required to keep it from rolling backward with the treadmill. Unless the wheels were really gummed up, this would be minimal.

If the treadmill attempted to stop the aircraft's forward motion by speeding up, when the pilot added a bit more thrust, it would very soon be going incredibly fast - in the case of a commercial jetliner - the treadmill would have to be spinning the wheels in the tens of thousands of knots.

Think of it - 28,000 kg of thrust (767) being countered by the friction inherent in a bunch of well-lubricated roller bearings.

In reality, the system would fall apart - likely treadmill belt first. The strength required for the treadmill to turn the 'corner' at the end, when it's going, say 3,000 feet per second (~ 2000 mph) would be considerable.

And the second half...

If the treadmill were long enough, all that would happen would be the wheels of the aircraft would be going twice as fast as normal when it took off.

In other words, because the wheels present minimal drag, the aircraft would, if the pilot chose, be able to take off normally. So, in still air, our 767 would rise off the ground at about 120 mph (air speed) with it's happy little wheels turning at 240. Some might pop, most might not, but I think lots of tires could take that speed. If we were flying into a 20 mph headwind, the aircraft ground speed need only be 100, the treadmill matching that would be going 100 the other way, making the tires think everything was happening at 200.

If you've ever taken off on a patchy icy runway, you can see how the effect of the 'road' lessens with airspeed, and what a relief it is when the wheels lift and the aircraft is back in control of itself.