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Is DownWard Air Flow Necessary For Lift?

09/12/2014 4:00 AM

Dear gurus- This question is for physicists and aerodynamic experts.Several times I introduced here myself, and emphasized that I have no formal high education, but I'm very interested in science and engineering, and I'm " hyper creative".For this reason I've done my best to understand aerodynamic.I read many articles, and built many airfoils, which I used to attach to some device that hold them on one side, the other side I hold in my hand.,all this while I'm biking.So I could watch the wing's lift at different angles.And I had discussions with some engineers, who had done their best to teach me aerodynamic.A very interesting point was that as time passed- the explanations changed! I'm talking about a period of 60 years!
As a youngster I was taught that lift is resulted through air flow above wing's curvature, which causes low air pressure .Then- they told me about magnus effect, which isn't in contradiction for the first explanation.But latter I was told about lift created by "downwash' of the airstream, and about the vortices that create lift.Some of the experts claimed that lift can be gained only as result of a down force of the air stream.I have done many experiments, and I'm not going to tell you what were the results.Since I intend to open a discussion with Israeli experts, I'm eager to know what you do think about it:1]Lift will be gained only as result of some downward air flow, resulted by different configurations and devices.2]Lift can be gained in any fluid [air , liquid] , downward flow will be only the side effect of the upward motion.I'm eager to pick as many opinions about this topic as possible.Or- I'm wrong- and the topic isn't disputable.

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#1

Re: Isdown ward air flow necessary for lift

09/12/2014 4:10 AM

Newton's Laws of Motion are applicable: in order for a massive body to be driven upwards by a fluid, that fluid has to experience a drive downwards as a result.

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#2
In reply to #1

Re: Isdown ward air flow necessary for lift

09/12/2014 4:22 AM

Thank you, but my question was if the down wash is side effect or the cause for the up motion?

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#3
In reply to #2

Re: Isdown ward air flow necessary for lift

09/12/2014 6:07 AM

"Inevitable consequence" would be an accurate description.

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#4
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Re: Isdown ward air flow necessary for lift

09/12/2014 6:19 AM

OKAY!

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#8
In reply to #2

Re: Isdown ward air flow necessary for lift

09/12/2014 5:06 PM

It doesn't necessarily have to cause an 'up motion'. The wing of an airplane in level flight doesn't have upward motion, but it still has an upward force (lift) equal to the weight of the airplane. PWS has done a good job of explaining this.

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#7
In reply to #1

Re: Isdown ward air flow necessary for lift

09/12/2014 4:17 PM

this is a constant, you can't change this fact

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#5

Re: Isdown ward air flow necessary for lift

09/12/2014 8:54 AM

Force is rate of change in momentum (momentum = mass x velocity). By deflecting the airstream downward, you are changing the momentum of the air, and this creates the lift force.

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#6

Re: Is DownWard Air Flow Necessary For Lift?

09/12/2014 2:43 PM

Look up Bernoulli Effect.

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#10
In reply to #6

Re: Is DownWard Air Flow Necessary For Lift?

09/12/2014 9:06 PM

I think it has been shown that the Bernoulli principle does not explain what takes place with wings. It is useful for carburetors, eductors, etc., but not wings.

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#12
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Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 8:10 AM

Look up the flow equation.

If the velocity of air increases over the top of the wing because of the longer path it takes, there must be a decrease in pressure, or change in temperature, or both. Bernoulli, reduced pressure = lift.

The wing has an angle of attack, which is normally such that the air flowing under the wing is deflected downwards. Newton, air down = wing up = lift.

The flow equation has five components;

Internal energy of the fluid (air), not important here

Kinetic Energy of the fluid, 0.5*m*v*v, as air speed varies so does KE

Potential Energy of the fluid, m*g*h, not important here

Energy due to pressure and volume, p*V, varies as the air speed v varies

Energy added or subtracted, eg heat, not important here

There are two important factors, KE and pV changes. If one changes, the other changes. Between them these two provide the lift.

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#14
In reply to #12

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 10:05 AM

Google 'how do wings work'. Here's one example.

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#20
In reply to #10

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 2:30 PM

From the wiki:

"Bernoulli's principle can be used to calculate the lift force on an airfoil if the behaviour of the fluid flow in the vicinity of the foil is known. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface, then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lifting force. Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations - established by Bernoulli over a century before the first man-made wings were used for the purpose of flight."

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#24
In reply to #20

Re: Is DownWard Air Flow Necessary For Lift?

09/14/2014 8:25 AM

I'm not an aerodynam/physic/expert, but my reading indicates that the Bernoulli explanation has fallen out of favor. It surely doesn't explain the action on a fabric sail, which is much like a wing turned vertical but has virtually no thickness; large jet wings, such as a 747, when in high lift mode such as for landing; and bird's wings, which are also cupped and have little airfoil shape.

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#9

Re: Is DownWard Air Flow Necessary For Lift?

09/12/2014 5:09 PM
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#11

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 8:10 AM

I am thoroughly confused over your use of the term "downwash". Perhaps if you can precisely define that my confusion can be alleviated.

Let me describe a scenario:

If I mount an airfoil (constant cross section, no sweep, no taper) full span across the width of the test throat of a wind tunnel, I can adjust the angle of attack such that it produces enough lift to counteract gravity's acceleration. The airfoil is then said to be in equilibrium.

An investigation of the airflow across a properly designed airfoil will show that there is laminar flow, no vortices and, I believe, if I understand your use of the term, no "downwash".

Thus, it can be proven that lift can be generated simply by accelerating the airflow across the top surface of the airfoil, producing a zone of relative low pressure versus the air pressure under the airfoil.

Hooker <-- who helped design, build and install many an airfoil in wind tunnels

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#13
In reply to #11

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 9:37 AM
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#15
In reply to #13

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 10:13 AM

Sorry, but I don't get your point. Are you suggesting that Bernoulli's Principle has something to do with "downwash", whatever it is?

I'm well aware of Bernoulli's principle. It's one of the operating principles used when designing the necked down test section of a variable flow wind tunnel.

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#16
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Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 10:56 AM

As a flight instructor I found the demonstration depicted in the video help some student pilots gain an intuitive understanding of how an airfoil can generate lift.

I did not intend to make a point but if there is one, its that neither a wind tunnel or mathematics are necessary for a practical understanding the effect.

In answer to the OP's questions:

1]Lift will be gained only as result of some downward air flow, resulted by different configurations and devices

I agree with those who've stated that "downdraft" it is not necessary to observe "lift"

2]Lift can be gained in any fluid [air , liquid] , downward flow will be only the side effect of the upward motion

IMO, this is not a true statement. eg: there may only be an "upward force" without upward motion co-incident with "downward flow."

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#17
In reply to #16

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 11:55 AM

If an airfoil deflects the airflow with a downward velocity of 1 meter/sec and it deflects 1 kg of air per second, then the lift force would be 1 kg m/sec2 or 1 newton of force.

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#18
In reply to #16

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 12:21 PM

But, but, but, the video is essentially a wind tunnel. It just didn't have the containing walls!

Also, without a controlled environment I could also make a demonstration that could be construed differently. Partially introduce your finger perpendicularly into a stream of water coming down out of a faucet. Though the end effect is the same as blowing over the paper, in itself the effect could be described otherwise, particularly if you introduce the concept of friction (drag) between the medium and the body acting on it.

Just saying, because without defining parameters and controls your students are accepting your demonstration essentially on your word alone.

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#19

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 1:14 PM

As I read your query I think you are on the right track. Lift is created by deflected air. The Bernoulli equation was always used to explain it but that is just a simplification. That equation assumes the the two airstreams, one on the top of the wing and the second on the bottom, are connected. In reality they are not. (although by coincidence they do arrive at the trailing edge simultaneously). The wing produces lift by deflecting air downwards thereby causing the equal and opposite reaction upwards. Think about it. Do you really believe that a 300,000 pound 747 is lifted without 300,000 pounds of air deflected down? Additionally a propeller is an airfoil shape rotated in a circle. I don't know anyone who thinks the plane is sucked forward. If so there would not be much P-factor to correct for. And lastly if it is the airfoil shape that creates lift how then does a plane fly upside down?

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#21
In reply to #19

Re: Is DownWard Air Flow Necessary For Lift?

09/13/2014 2:49 PM

Money and fuel is what keeps airplanes in the air. Neither one cares which way is up.

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#22
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Re: Is DownWard Air Flow Necessary For Lift?

09/14/2014 7:20 AM

.........bugga..........

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#23
In reply to #19

Re: Is DownWard Air Flow Necessary For Lift?

09/14/2014 8:14 AM

Actually, they don't arrive at the trailing edge simultaneously - not always, at least.

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#25
In reply to #19

Re: Is DownWard Air Flow Necessary For Lift?

09/14/2014 9:05 AM

A plane can fly upside down by ensuring the angle of attack of the wing (and the fuselage) is upwards, as if the plane were the right way up. The fuselage contributes to the lift.

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#26

Re: Is DownWard Air Flow Necessary For Lift?

09/15/2014 7:46 AM

P1V1=P2V2 as air molecules take separate paths at the start of the airfoil V2 above the wing is faster than V1 so P2 is lower than P1 so the plane is actually sucked up into the air. This is the same principle that sails use to suck sail boats across the water at faster than the ambient airspeed.

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#27
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Re: Is DownWard Air Flow Necessary For Lift?

09/15/2014 2:38 PM

Thats a good explanation for the Bernoulli equation but there is no reason for V2 to be faster than V1. Thats what I was trying to explain before. The two airstreams are not connected. Now the top airstream is deflected upwards by the curvature of the airfoil and that deflection is what causes the lower pressure over the wing. When a plane flies upside down again it is deflected air which produces the lift not pressure differential. Remember ,it is the wing which is moving thru static air. The air molecules have inertia and when they are displaced they have momentum which slows their return to their static position. That is what causes the void above the wing and the lower pressure there. And by canting the wing with a suitable angle of attack the same effect is produced when the airfoil is upside down thereby also creating lift.

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#28
In reply to #27

Re: Is DownWard Air Flow Necessary For Lift?

09/15/2014 4:01 PM

Planes can fly on horsepower alone, without any "lift", as demonstrated by high powered air show demonstrations. Does not explain lift, tho'. Also, how do trailing flaps, that are adjustable, fit in with angle of attack? Do they deflect "more" air, or change the shape of the foil to influence the angle of attack?

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#29
In reply to #28

Re: Is DownWard Air Flow Necessary For Lift?

09/15/2014 4:27 PM

"Planes can fly on horsepower alone, without any "lift", as demonstrated by high powered air show demonstrations"

I think you may be confused with your forces here. Allow me to explain the basics.

Any object moving through the air has four forces acting on it at once, Lift, Thrust, Drag, and Gravity. Lift and Gravity are in direct opposition, as is Thrust and Drag.

For a simple example, let us take a baseball hit in a fly ball to the outfield. It's initial direction and speed after the brief kiss with the bat can be shown as one vector, or divided into two, showing the initial Lift and Thrust. As long as aerodynamics do not change midflight, Drag is a simple equation, speed*the drag coeficient, the faster a thing moves, the more Drag it generates. Gravity, as we know, is a constant value. As the baseball is not accelerating, it is not generating any Lift or Thrust, so those vectors are zero, Drag is a decreasing vector, and Gravity is a constant. As you integrate the four forces (two of which are zero) with the ball's velocity vector, you will see the velocity change over time so the ball follows a roughly parabolic path until it hits the ground (or is caught). If the ball were batted so it would drop into an endless, bottomless chasm (just to show how the math works) its Drag would decrease to zero along with its forward speed, eventually falling straight down as Gravity becomes the only force acting on it.

A propeller-driven plane normally generates Thrust with its propeller, and the airflow over the wings generates Lift. Drag and Gravity have already been discussed. When these forces are in the proper balance, Thrust overcomes Drag by enough that the plane moves forward so the Lift generated by the wings will match Gravity. During the 'steep climb' stunt you are alluding to, the plane noses up to the point where the propeller is generating Lift directly from Thrust, and the wings are now mere decoration, worse than useless in fact, as they still generate Drag, which is now in line with Gravity.

Very few planes are designed to do this safely, and most that try will experience what is known as 'stall,' where the Thrust+Lift cannot match Drag+Gravity, forward and upward motion cease, and the plane tumbles helplessly towards the Earth. At this point, since the plane is moving in the direction of Gravity, Drag is now opposing gravity, there is a point where Drag and gravity reach equilibrium, it's called Terminal Velocity, it's the maximum speed an item can reach in free-fall.

If a plane has stalled out, the pilot may have a chance to avoid flying face-first into his own grave. As the plane falls, the wings are still generating lift perpendicular to the direction of travel, if he's not in a talespin, he might be able to pull up enough so he can use the velocity generated by Gravity to pull the plane the right way up again and bring the four forces into their normal positions again.

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#33
In reply to #29

Re: Is Downward Air Flow Necessary For Lift?

09/16/2014 7:51 AM

I do not mean this comment to contradict what you're written, however, you state "As the baseball is not accelerating, it is not generating any Lift or Thrust ...".

To be accurate, the ball is accelerating which you indirectly state "you will see the velocity change over time ". If you recall "a = dv/dt".

As a matter of fact, even though gravity is a constant, it causes acceleration in free fall. To be more precise, it's the acceleration of gravity that's usually considered a constant on Earth (but that depends on your location as it varies from lat/long location to lat/long location). The mass of the ball times the gravitational constant (32.174 ft/s/s or 9.81 m/s/s) results in the gravitational force causing the acceleration which slows the balls vertical movement upward until it stops and speeds it's vertical movement downward.

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#35
In reply to #33

Re: Is Downward Air Flow Necessary For Lift?

09/16/2014 9:21 AM

Perhaps I was a bit unclear, I should have said 'As the baseball is not producing its own acceleration..'

I was trying to point out the difference between a baseball and a powered vehicle. a plane can 'accelerate' (add to its velocity) using its motor, a baseball has only its own mass and inertia to maintain its speed and course.

Maybe I was oversimplifying, but I was trying to show the balance between the four forces on flying objects, to point out the misstatement the earlier poster said when they claimed a stunt plane could fly without Lift.

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#36
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Re: Is Downward Air Flow Necessary For Lift?

09/16/2014 9:26 AM

I believe you need to distinguish between ballistic flying objects and non-ballistic (lift producing) objects.

The baseball is a ballistic object, though the spin, if any, may slightly influence its trajectory.

Hooker

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#30
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Re: Is Downward Air Flow Necessary For Lift?

09/15/2014 4:38 PM

So to make a long post short (too late):

Lift is any force that moves in opposition to Gravity, regardless of the source.

Thrust is the force that accelerates a thing 'forward' (whichever direction that may be in relation to Gravity).

Drag, is a 'phantom force' that opposes Thrust.

A rocket generates Thrust straight up, so its Thrust is its Lift, and therefore Drag and Gravity are also in parallel.

A free-falling object is accelerating die to Gravity alone, to Gravity and Thrust are the same, and Drag parallels Lift.

A hot-air balloon floats in the air because the Lift generated by the balloon envelope's buoyancy matches Gravity at that altitude, if the balloon were higher, it would sink, if it were lower it would rise. (You could thing of the balloon as 'bobbing like a cork' on the sea of air that is just thin enough to support it). Any Thrust generated is from the actions of the winds at that height. If we look at the math from the inertial reference of the balloon, it is 'Drag' that seeks to change the balloon's speed to match the surrounding wind.

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#31
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Re: Is Downward Air Flow Necessary For Lift?

09/15/2014 6:35 PM

adreasler has an interesting sense of humor

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#34
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Re: Is Downward Air Flow Necessary For Lift?

09/16/2014 9:15 AM

(drops into bad Joe Pesci 'Goodfellas' inpersonation)

What do you mean, I'm interesting?

'Interesting,' how? Do I produce interest? Do I take money you've deposited with me, loan it out to someone else, collect an additional fee from them when they pay back the money, and share some of that fee with you by adding it to your account?

(Okay, I'm more ripping of the 'Goodfeathers' parody of Goodfellas, but if you're going to steal comedy bits, steal the good ones.)

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#32
In reply to #30

Re: Is Downward Air Flow Necessary For Lift?

09/15/2014 11:45 PM

Many thanks for your added data--I have not thought of some of the data in the terms you have used, Especially, LIft… I will chew a bit more on the mail--Many thanks, macs

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