How Do Helicopters Get Forward Thrust?

Hi Mike,

Check out this link. It might help.

http://www.aerospaceweb.org/question/helicopters/q0084.shtml

Thanks Jibril. I understand how it works.

Jibril, one more question, how stiff the blades are? I have seen the blades of some 4-6 meters long aluminum sandwiched in between steel plates. I just wonder how these things work?

Also the sailplanes have a thin long (7-15meters) aerofoils, how they construct?

Thanks!

hi friend i am HIMANSHU SHARMA

the thurst should be produced by bypass duct from the engine & by using a flaps & slats in the wings.

in the engine takes a air from atmosphere a 100% air & uses a only 20% & 80% going out through the byepass duct which helps in planes going to the forward direction

Himanshu sharma,

Learning aerodynamics will do you a lot of good, Your answer is a total nonsense. What in heavens sakes has a bypass duct to do with helicopter main or tail rotor blades?

Wangito.

Flaps and slats are control surfaces and have nothing whatsoever to do with thrust. They actually are used to alter the surface area of the wings on fixed wing A/C allowing for more controllable flight at slower airspeeds.

Hello HIMANSHU SHARMA

me thinks you are a little confused............? What you sort of describe sounds more like a V/stol.......Very short take off and landing. Or Vertical takeoff and landing (VTOL). As in the Harrier Jet fighter. It aint how a helicopter moves forward, not as I understand it anyway. No offence intended

babybear

By flapping the horizontal blade. When the blade is in rear semicircle it is turned to get more trust, while the opposite blade in forward circle is turned opposite to get less trust. (The same happens when the helicopter fly sideways or turns right and left).

I'm not an aeronautic engineer, but I have worked on both helicopter and fixed wing aircraft as an avionics tech. Fixed wing aircraft get lift due to the upper wing surface being longer than the bottom which increases the speed of the airflow creating lower pressure.Thrust is generated by props or jet exhaust. The same principle applies to rotary wing (Helicopters). The major difference with helicopters is that as the blades rotate the opposite ones are 180 deg in angle of attack (pitch). The forward, reverse, and sideways motion is controlled by the collective control stick which,in turn, controls the swash plate. This swash plate pivots to direct the rotor wash in the opposite direction of intended direction. Speed is controlled by the cyclic stick which either increases, or, decreases the angle of attack (pitch) deternmining how much thrust is generated. The yaw is controlled by the rudder pedals that operate: the tail rotor,tail ducted fan(Dauphine type), or the rotatateable nozzles (NOTAR [No Tail Rotor]).

There are a few differences with dual main rotor birds ie. Ch-46 Sea Knight, Ch-47 Chinook, etc.

When I worked on helicopters we had a saying: "Helicopters don't fly. They beat the air into submission!"

This is as simple as I can make it. Hope it helps

Excelent explanation JMR !!!

All helicopter motions are controlled by the cyclic, fwd, left, right, aft and hovering through the swash plate.The pitch (angle of attack of the main rotor) is contrlled by the collective.

Want to know what the Jesus nut does in a helicopter ?

I holds the rotor on. If it fails, get ready to meet Jesus.

Then, that is an appropiated name for that nut.

re: Jesus nut

When it comes loose,; Tuck your head between your legs and kiss your butt goodbye!

Well my freind, at that moment is better be in peace with the creator and have your sings confeseeesd...........

Re: Jesus nut---

Is it the one that connects the linkage to the tail rotor?

No it is not. It holds the plate and linkages of main rotor assambly.

Re: Jesus Nut (& bolt), that is EXACTLY what it is called. Thats what my instructor use to tell me. If it comes off....... "JESUS" here I come!

It is unbelievable how much bogus there is in so called popular science. This reason "why an aeroplane flies" is the one I hate the most. No offence noshorts, you are just a victim, but that lenght of surfaces is not the reason. If you wan't to have real reason, write aerofoil to wikipedia, but putting it very simple: If you put an object to a flow it turns to flow. This turning causes according to Newton's law an opposite force effecting to an object. If object is positioned so that the flow meet the object on it's lower surface, this force has 2 components: backwards (sometimes called drag) and upwards (sometimes called lift). If the flow meets upper surface, it causes downward component (sometimes called downforce).

There is a reason why aerofoil is usuallu made as it is, but it is not a requirement. If you still dont believe me, fold an aeroplane from a sheet of paper. It doesn't have differences in the length of surfaces and still it flies.

You wrote:"No offence noshorts, you are just a victim,"

The only victim here is you sir, a victim of your total ignorance.

No offence Nifalen but your post is concentrated bull. You don't think so? post it as a thread and see for yourself...

Wangito.

Hello Nifalen,

some of what you say makes a little sense. But you can't use a paper aeroplane as an example. Because ALL paper planes crash. They fly down in various ways and directions but, always crash.

The air over the top of the wing on a proper aircraft, does have further to go because the wing shaped with the wing centre-line being flexed up and it is built like this to give constant lift. Passenger planes have larger wings which give more lift I imagine for safety reasons as well as efficiency.

babybear

First of all to Wangito: I know my post was a bit brutal. I am a tolerant man, but my blood starts to boil when I see this type of pseudo science. Again noshorts is a victim on my mind for some a#*hole has told him the function of aerofoil the wrong way. It most ceartainly is not his fault. I really don't understand why this "particle moving faster causes suction" thing has got so popular. That is told here in Finland to people takingcourses for pilot's license as well as in schools.

To babybear: I'm sure my example was not too good. A paper plane is a very simple glider, but no a particularly good one. It was just so that people would understand that the standard form of aerofoil is no a requirement. Now there are many other simple ways to see why this thing about sucking the plane up is bogus. Here is one:

Stick your hand out of the car window as if it was a wing whiledriving so that your palm is downwards. Now twist your hand a bit so that your palm is facing less than 45 degrees forward and you feel lift. Then twist it so that it is facing backwards and the force you feel is downforce. I don't think you feel suction on the back of your hand pulling it upwards.

If you want to have a better explanation, please refer to wikipedia. They got the aerofoil right there. I just tried to put it as simple as I could.

Nifalen,

Even a brick will "fly" if you put wings on it!!!!!

The major point that you are missing is about the efficiency coefficient. Your "aerofoil, (hand)" may lift a Sopwith Camel into the air or a Piper cub, but, if you need higher speeds and more maneuverability then you must increase the lift and drag coefficients.

Do you realize that the propellors on prop planes have variable pitch. (which is what you describe by putting your hand out of a moving car?) This is increasing or decreasing the AOA (angle of attack) determining how much air is displaced

Even a brick will "fly" if you put wings on it!!!!!

I havn't heard that one before, only "Given enough thrust you can make anything fly"

This was in reference to the F-4 Phantom(flying brick) It was said that if you put wings and a large enough engine even a brick would fly. At least this is what was said when we were still flying F-4's in the Corps.

Hello noshorts,

firstly can I say you have given some great advice on this post!

I have not heard it called a brick, but, the same can be said of the Euro-fighter. If it had no computers it simply would not fly. I understand this is done when designing for 'maneuverability'?. I don't know about the helicopters but the fighters and modern passenger planes are 'fly by wire now. Which mean they can only be flown if the computers are working? I wonder what sort of 'back-up' they have for a total power failure?

babybear

Most fly-by-wire systems here in the states are triple redundant. In military aircraft the final fail-safe is an ejection seat.

Hello noshorts:

I see. The same is probable for most 'fly by wire' craft?

Strictly speaking I suppose you could say the final 'fly by' you mention is 'fly by cord'?

babybear

Hey! Babybear.

What do you mean, 'All paper planes crash' ? Every plane crashes. Landing is a controlled crash.

Put a motor and an infinite amount of fuel on a 'paper plane', with some controll tabs and the thing will fly forever.

Now, if you mention 'efficiency' then that's something else.

Put a motor and surface contrlolls onto a sheet of plywood and it'll also fly. Use angle of attack and airflow velocity to compensate for lack of airfoil profile.

Cheers,

Stu

Hello Stueywright:

Angle of attack is the crucial phrase here. Twist the front of the wings so they are high than the rear of them, and you have the beginnings of a proper plane. The angle of attack sort of compensates for any lack in aerofoil shape. It will be really difficult to control though.

Stay safe

babybear

No offense taken. I do, however, know what I'm talking about. If the surface area on the top of an airfoil is greater than the bottom:

• length of travel is increased
• speed of air travel is also increased
• pressure is decreased
• lift is generated

Wikipedia isn't always correct.

Slats extend from the leading edge of a wing surface, while flaps extend from the trailing edge. They both serve the same function: increase surface area, increase slow speed maneuverability, prevent stall conditions. This is why they are primarily used in take-off and landing.

There are also vortex generators on the top of some wing designs. These generate vortices to stabilize the laminar flow to increase efficiency.

Good that you haven't taken the offence and I'm sorry again.

I agree that wikipedia is not always correct. This time it is not the perfect explanation there, but very simple and understandable.

I'm sure you know a lot about flight. The explication you gave however is wrong. It even has a name: Equal Transit time fallacy. It is very commun and even taught in some countries to pilots. I opened a new thread about it here:

http://cr4.globalspec.com/thread/24105?frmtrk=cr4sd#newcomments

The true mechanism of lift force is complicate. You should know an effect called circulation in order to fully understand it. However the reason itself is very simple. It is about turnin the airflow (though a wing can only cause a local disturbance). The pressure difference exist allright, but talking about it will just confuse people.

Hello Nifalen:

I look forward to viewing your new thread. I still think you may be confusing fact with theory though. And, yes, do not trust Wikipedia, especially since it is now allowing any who view it to alter or edit the contents..........BIG MISTAKE (on Wikipedia's part)!

The pressure difference exist alright, but talking about it will just confuse people.

The pressure difference is the single most important thing which has to be mentioned. Without it there would be no flight at all.

Talking about it will not and cannot confuse people as most people understand the way this works before they may train as a Pilot etc. It is certainly not going to confuse someone. What could be confusing is the different terms you may use and others have used for lift. It is called lift because that is exactly what happens to the object when taking off.

I really do look forward to reading your thread. You seem to know the basics in some posts then, in others you just make no sense. Sorry, but you do seem to try and make your ideas of flight and how and why it happens fit your thinking.

This is not an insult in any way, just a discussion

babybear

"8. in order to achieve a forward flight, this "disc is mechanically tilted forward. in doing this, the nose of the helicopter tilts down, and the center of thrust is no longer vertical,and the helicopter will "try" to correct it, causing a forward motion. (this is the answer to your question...) "

I gotta the answer.

Thank all you guys for the help.

Ruban

Mike,

I guess what the guys are trying to say is that the helicopter through it's cyclic pitch changing ability, 'paddles' it's way forward. The receeding blade assumes more pitch than the advancing blade and it essentially paddles it's way through the air with the resultant torque being negated by the tail rotor, which in itself can adjust it's pitch so as to exert more or less force resuting in controlled yaw (or, turning). The collective pitch controlls lift . It is rarely that any two of the main rotor blades are at the same pitch (on the ground, running up) due to the everchanging requirement of controlling the vehicle in an unstable environment.

8. is not actually correct. There is some of this phenomenon present, but not enough to effect forward flight.

Any clearer?

Cheers,

Stu.

I think the idea that the helicopter "paddles" it's way forward is totally wrong. It's true that the receding blade has more pitch, but that is just to compensate for less speed relative to the air.

If equal pitch were maintained through 360° rotation and the rotor was tilted forward the helicopter would still move forward, but it would also roll because the blades would have more airspeed (therefore more lift) when on the forward moving side of the rotor.

The main component of forward thrust comes from the angle of the rotor disk.

The main component of forward thrust comes from the angle of the rotor disk.

No it isn't.

Go down to your local airport and to the Helicopter hangars and ask them.

The only helicopter which achieves forward thrust by 'tilting' the rotor forward is the one invented just a few years ago by a bloke who's name currently escapes me. This machine has twin, contra-rotating lift fans, using proportional speed differential of the fans to controll yaw, and a tail rotor which is in the HORIZONTAL aspect, using the variable pitch fan so as to lift and depress the boom to adjust the attitude of the entire craft to achieve forward/reverse thrust.

If equal pitch were maintained through 360° rotation and the rotor was tilted forward the helicopter would still move forward, but it would also roll because the blades would have more airspeed (therefore more lift) when on the forward moving side of the rotor.

This is a true statement, however, the Question of the thread asks how the machine moves, and the pitch of the receding blade is increased to the point where the vertical thrust is in equilibrium with that of the advancing blade and it 'paddles' it forward, just as surely as the oars of a rower in a scull. Remember lift/drag co-efficients, when you were getting your license?

I know that 'paddles' is probably a bit low a term for us technophiles but sometimes you just gotta say it like it is.

Now think about this: Take a close look at a toystore fixed pitch rotor helicopter. Does it need any of the very complicated pitch controll mechanism on the rotor to achieve flight? No. Is it a stable aircraft? Why is it that a Sikorsky style craft is considered an 'unstable' one. (Sikorsky was the inventor remember)

Cheers,

Stu

Good on you wangito! Until I got to your post I was getting my fill of all the bs. When I finally got through the garbage I was going to post the answer you said so well,"in order to achieve a forward flight, this "disc is mechanically tilted forward. in doing this, the nose of the helicopter tilts down, and the center of thrust is no longer vertical, and the helicopter will "try" to correct it, causing a forward motion. (this is the answer to your question...)

I can't understand why persons who have no idea what the question was, much less the answer, ever bother to post.

It's better to be silent and be thought a fool than to open your mouth and prove it.

Simply, horizontal flight attains from the main rotor, after the ship is pitched so that the tail is high and the main rotor lifts the ship up and forward instead of just up.

...but this does not apply to any except top (and very expensive) , hobby class RC miniature copters; these are not capable of direct forward thrust.

The petrol powered hobby RC helicopters are exactly the same construction as for full scale helicopters, and possibly just as hard to fly, or even harder, as in a real one, your controlling the attitude with both your feet, 1 hand, and throttle with the other hand, in a RC one, your controlling everything with 2 fingers.

The tail rotor or anti torque acts like a rudder of a fixed wing, also is to prevent the aircraft to rotate in its own axe, if you remove the tail rudder the choper will rotate uncontrollable opposite of the motion of the main rotor.

The forward motion come from the tilt of the main rotor through the swash plate that is controlled by the cyclic. Remember the the rotor does the same function as the wing of an airplane.

Imagine the helicopter engine running blowing air downward, imagine the circle described by the rotation of the rotor independently of the fuselage a see it tilt forward, tha is where the fwd motion comes, done through the cyclic and the collective.

By pushing the cyclic column forward, the front blade gets lowered while the rear blade gets raised. This creates the effect of tilting the rotor disc (the imaginary disc formed by the rotation of the blades) forward. The same thrust that lifts the helicopter up is now also driving it forward. By the same principle, the helicopter is able to fly backwards or sideways by tilting the rotor disc in the appropriate direction.

It leans forward.

I know your question has been answered now but, a really simple thing to do is try throwing a 'Frisbee'. You can throw it at different 'attack angles' and, you will see it goes hardly any distance if thrown at about 70 degrees or so but, when thrown almost horizontally, it acts a little like a helicopter tacking off slowly.

I realise this is perhaps a little childish but is worth giving it a go to see the best angle of attack to go forward and gain speed?

babybear

So long as the thrust is 100% in opposition to gravity, the helicopter will hover in one spot. This is assuming still air.

A complex coupling at the hub enables the blade to tilt down or up (among other things). Depending on where they blade is when this happens, this changes the thrust vectors. Some of the downward thrust is diverted and, depending where the blade is determines how much and in what direction.

Unfortunately, this propulsion method is not as efficient as Sikorsky and others would like. It is one of the limiting factors in the speeds that helicopters can attain. The Boeing "Tilt Rotor" aircraft is one attempt to harness the vertical takeoff capability of a helicopter and still attain the speeds capable only with fixed wing aircraft.

Frankly, I think that aircraft is a needlessly expensive and dangerous mistake.

Helicopters are dangerous enough with virtually no systems redundancy to speak of. The complexity of the tilt rotor makes matters demonstrably worse and has resulted it a horrible safety record during it's development.

Unfortunately, this propulsion method is not as efficient as Sikorsky and others would like. It is one of the limiting factors in the speeds that helicopters can attain.

As far as I understand, the main limiting factor of the helicopter top airspeed is not so much the lack of forward thrust, but dissymmetry of lift. All helicopters have "no exceed" speed when their retreating blade would simply stall and they would just spin out of control if they try to go faster. Some helicopters (e.g. Ka-50 "Hokum") are actually capable of producing enough thrust to exceed this speed, but they still wouldn't do that. So, I suspect this is the main reason why no one is installing additional turbofans or something like that on helicopters for extra thrust - they simply do not need any more thrust than their main rotor can provide since it won't do them much good.

Now a couple of question for those who have actually flown a helicopter:

1. As far as I understand, fore-aft cyclic controls is how you manage both thrust and pitch. Is there a way of getting it to climb without generating reverse thrust or getting it to dive without increasing forward thrust?

2. Since pitch of advancing and retreating blades gets adjusted to compensate for dissymmetryof lift, does it mean that sideway cyclics would develop a bias at high airspeeds? I would imagine that rolling it to the side of the advancing blade would become nearly impossible at airspeeds just below "no exceed", since there would be little margin left for increasing AOA of the retreating or decreasing AOA of the advancing, but rolling to the opposite direction should be no problem. It it actually the way it is, or am I missing something?