Inverted Flight

How I love the lift discussion!

I'm a professional pilot with 9000 hours of flying time. Only within the last couple of years have I felt that I've begun to learn the true story about the production of lift (and even still, I can't claim to understand most of the physical actions at work in the production of this elusive force). Most aerodynamicists will tell you the story that Seagull points out in his posts...and that is to elimate Newton from the story.

Let me explain how my convulated and confused understanding of lift developed...and I believe it can be transferred to thousands of pilots just like me...who do not have an aero-engineering background. When I learned to fly in the 1980s the FAA published AC61-21A, the Flight Training Handbook. This was the authoritative source regarding all things surrounding primary flight training. (The AC is still around although has been superceeded...I don't have the latest copy.) My CFI taught out of this book religiously, I nearly memorized it, and by the time I became a CFI my copy was so beat up....well you get the point. This is the knowledge you had to bring before the FAA to pass any checkride.

The last chapter of AC61-21A pertained to the principles of flight and aircraft performance. The FAA described in seven paragraphs how Newtons 3rd law of motion was a major contributor in the production of lift. The FAA described that when the flat lower surface of the wing was inclined to the direction of flow, the air would deflect off of the surface and rebound downward causing a certain amount of lift by Newton's account. This AC, in my opinion, also contained verbage that led to the misunderstanding of the "equal time theory"...the one that theorizes that a parcel of air on the upper leading edge and one of the bottom leading edge of the wing must meet at the trailing edge at the same time.

This is all wrong...according to modern aerodynamicists...however, generations of pilots were trained in this manner...and probably still are today.


FWIW, here is what my reading and research of many engineering sources has led me to believe is a more accurate account of the production of lift.

1) Bernoulli is mostly responsible for the production of lift. However, the acceleration of air over the top of the wing is not due to the distance that air must travel over the top of the wing...however it is accelerated due to the ROTATION of air imparted by an increasing angle of attack made more efficient by the use of camber. (Hence an inverted aircraft can produce lift opposite weight by increasing the AOA...only in reverse of normal.)

2) Newton comes into play during WING DOWNWASH. As the air flowing over the top of the wing reaches the trailing edge, it departs the wing in a backward and downward manner. Newton's 3rd law responds with a forward and upward force. I believe a discussion of induced drag could begin here...but I'm not about to go there.

3) There is no truth to the "skipping stone" theory of lift. This is the one that states lift is produced by air deflected off the lower surface of the wing and produces an upward force. (Personally, production of lift here makes sense to me...but the aerodynamicists can prove and categorically discount this has any affect on lift.)

Seagull, you must be patient. Thousands of pilots have been trained improperly regarding the production of lift. While it can be frustrating for you...it's not really their fault. This is how most pilots have been trained their entire lives. I had been flying for 15 years before I really started to learn "the truth about lift". You would probably be a good candidate to write "The Rest of the Story" or some type of training piece about this topic.
 
I have read that link. I would say that it does not explain lift at all. First, they completely miss the boat on what is meant when we say "Bernoulli". Second, the fact that air is diverting downward is, again, a result of lift. Heck, by that notion, wing tip vorticy increase the downwash, so they should make for MORE lift, but they don't!

In reference to other reading material, I would recommend the books I referenced previously.

B767, good post, your item 2 I would take issue, as it is a classic misapplication of Newton's 3rd law. The fact that air is moving down doesn't make the wing go up. A balloon moves by the 3rd law as I decribed above. Not "just because air is moving out of the back end".

You are correct in the issue of rotational flow. Camber helps that, as camber could be considered a way to increase the effective AoA, but that is about it. The rotational flow is due to coanda, that is true. However, that's just the start of it. The faster moving flow due to rotation leads to lower pressure on top, which literally pulls air up and to the wing, and accelerates it more, creating higher speed air over the top and more rotational flow. Now, why does the pressure go down? Pressure is a fluid's form of potential energy. Like a roller coaster, at the top of a hill, lots of potential energy and little kinetic, at the bottom, lots of kinetic and no potential. All else equal, you do not gain or lose energy, so one trades with the other. Sub pressure for potential, and you see that the pressure goes down to preserve the same total energy.

Even the lift equation, which comes out of Bernoulli, is just the same as Newton's kinetic energy equation, with different terms. KE=.5*mv^2 as opposted to L=rho/2 v^2 S (surface area). The rho is the same as the "m" term, with S just to get the total for the entire wing section instead of a point result.

Incidentally, the "skipping stone" theory does work, just not much of an effect unless you get to hypersonics and superthin air.

The problem is that most people *think* they know what Bernoulli is, but they only know a part of it, and that part is often wrong (the equal transit idea).
 
I notice that the "Newton" explanation does not explain WHY the air moves up toward the leading edge of the wing. In fact, although that site mentions that it does, it avoid this important aspect after that. Also, in this section:

" Low speed airplanes are effected more by induced drag than fast airplanes and so have longer wings. That is why sailplanes, which fly at low speeds, have such long wings. High-speed fighters, on the other hand, feel the effects of parasite drag more than our low speed trainers. Therefore, fast airplanes have shorter wings to lower parasite drag."

This, while partly true, is very misleading. Sail planes have longer wings because the longer aspect ratio reduces the percentage of the wing that is effected by vorticies, which increases the effective lift. It also is why ground effect works. Winglets, incidentally, use the relative flow at the tip to act as sails, the same way a sailboat tacks upwind. They do reduce the vorticy, but that is a result of their action, and that reduction is not what causes the wing to be more efficient.

Structural design limits are a large factor in wing length in fast airplanes. Very high speed aircraft are also dealing with mach effects, which change a lot of factors. The "fighters" is mentions have shorter wings to stay inside the mach cone, among other things. Also, shorter wings allow for much faster roll rates and other handling quality issues that are vital for a successful fighter. The authors of that site are really talking about things outside of their field of expertise. Reminds me of botantists talking about global warming and saying they're "scientists" when discussing atmospheric physics and clearly not understanding the material.
 
Seagull,

Here's a concept that I've been trying to get a handle on. Some material will say that the lift component acts perpendicular to the chord of the wing. Others mention that lift acts perpendicular to the relative wind.

Which is accurate? Both?

Is one total lift?...and the other effective lift? What causes the difference in the vector between the two?
 
Well, I think that the answer is not so different than what I was getting at above. Vectors are divided up for our convenience in making calculations, and, as such, we can divide them up however is convenient for what we are trying to sort out.

That said, the ACTUAL force is going to be in a direction up (in the direction of positive AoA) and aft. We like to divide it up into a vertical component and a horizontal one in order to quantify lift and drag, but nature doesn't worry about such things.

Here's one for you. How does sweep back allow for higher mach numbers? Very easy, actually, but not easy to find the answer written. Another very interesting topic is handling qualities, which is something I have spent quite a bit of time working on for part 25 transport jets.
 
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Here's a concept that I've been trying to get a handle on. Some material will say that the lift component acts perpendicular to the chord of the wing. Others mention that lift acts perpendicular to the relative wind.

Which is accurate? Both?

Is one total lift?...and the other effective lift? What causes the difference in the vector between the two?

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If you just want a general idea of what is happening then it is accurate to say that lift is perpendicular to the relative wind.

It is more accurate to say that lift is realized perpendicular to the chord line of the wing.

If we take the example of your typical 172 out practicing slow flight while keeping altitude we can look at the difference between the two perspectives.

In both situations the relative wind is coming from directly in front of the aircraft.

If we say that the lift is perpendicular to the relative wind this is true as the aircraft is holding altitude so the lift vector is acting in an upward direction to allow the aircraft to hold altitude. But this does not explain the induced drag that is so prevalent at this low speed.

If we look at lift being perpendicular to the chord line we can see that we are at a high angle of attack due to our low airspeed. By increasing our angle of attack to hold altitude, we have by definition created a bigger angle between the relative wind and our chordline. If we define lift as being perpendicular to the cordline we can see that we have an upward component of lift and a rearward component. This will explain the induced drag.

So you are correct, effective lift is realized perpendicular to the relative wind, total lift can be visualized by realizing it is perpendicular to the chordline.
 
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Bernoulli only produces about 10 % of the airplanes lift. The airplane just needs to be pushing down enough air to support its weight (ie Newton's 2nd Law of Motion.)

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I am not sure if I am understanding this. If you are saying that the bottom of the wing 'beats down air' and forces it downward to create the majority of lift, this is incorrect.

In quite a few large jet powered aircraft, it is permissable to takeoff with up to 1/8 inch of frost on the bottom of the wings in the vicinity of the fuel tanks, which is pretty much the entire bottom of the wing. This does result in a performance loss (max takeoff weight is reduced and climb performance suffers) but you can do it. The numbers are in the flight manual. Taking off with this much frost on the upper surface of the wing will kill you.

I wonder how the 'newtonian' model would account for this?

It is also interesting to note that there is not a very large pressure change required to support the aircraft under the bernoulli model. If you take a typical trainer like a Piper Cadet, it has a max takeoff weight of 2325 lbs and 170 square feet of wing area (24480 square inches). This means that an average pressure differential of only .095 psi is required to create enough lift to stay airborne (note that at sea level standard pressure is about 14.7 psi).
 
Good posts Anon.

Regarding frost under the wing in the vicinity of the fuel tanks, our manuals say it is allowed and there is no performance decrement applied for that. Perhaps there is on other models, but not the aircraft I am familiar with.
 
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Here's one for you. How does sweep back allow for higher mach numbers? Very easy, actually, but not easy to find the answer written. Another very interesting topic is handling qualities, which is something I have spent quite a bit of time working on for part 25 transport jets.

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My understanding, a very basic one at that, is that wing sweep does not encounter the relative free stream "head on", but at an angle. When the airflow "hits" the swept wing "perpendicular" to the leading edge...the vector component actually encountering the leading edge is smaller that the velocity of the relative free stream.

In the ballpark?
 
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Here's a concept that I've been trying to get a handle on. Some material will say that the lift component acts perpendicular to the chord of the wing. Others mention that lift acts perpendicular to the relative wind.

Which is accurate? Both?

Is one total lift?...and the other effective lift? What causes the difference in the vector between the two?

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If you just want a general idea of what is happening then it is accurate to say that lift is perpendicular to the relative wind.

It is more accurate to say that lift is realized perpendicular to the chord line of the wing.

If we take the example of your typical 172 out practicing slow flight while keeping altitude we can look at the difference between the two perspectives.

In both situations the relative wind is coming from directly in front of the aircraft.

If we say that the lift is perpendicular to the relative wind this is true as the aircraft is holding altitude so the lift vector is acting in an upward direction to allow the aircraft to hold altitude. But this does not explain the induced drag that is so prevalent at this low speed.

If we look at lift being perpendicular to the chord line we can see that we are at a high angle of attack due to our low airspeed. By increasing our angle of attack to hold altitude, we have by definition created a bigger angle between the relative wind and our chordline. If we define lift as being perpendicular to the cordline we can see that we have an upward component of lift and a rearward component. This will explain the induced drag.

So you are correct, effective lift is realized perpendicular to the relative wind, total lift can be visualized by realizing it is perpendicular to the chordline.

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Thanks for the reply...this is inline with my understanding of the topic. Is it correct to say that the magitude of induced drag is equal to that of the downwash vector?
 
That is certainly the engineering answer, or pretty much is, and there is some truth in that the air is being pulled inward due to the low pressure above the wing, but that does not really tell you "why" it works.

To head you in the right direction, consider first why a shock wave develops and how a wing near sonic speeds differs from one that is going slower, or, put another way, how the air is reacting to the approaching wing.
 
While you're pondering that one, here is another one that is more simple. Why, in airline ops, does a heavy aircraft need more distance to descend than a lighter one?
 
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....or, put another way, how the air is reacting to the approaching wing.

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I like that: prescient / clairvoyant air.

smile.gif
 
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Thanks for the reply...this is inline with my understanding of the topic. Is it correct to say that the magitude of induced drag is equal to that of the downwash vector?

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I am not sure what you mean by 'down wash vector', but induced drag can be thought of as the 'rearward component of lift'.

If you are maintaining straight and level flight with a 15 degree angle of attack, then your chord line is at a 15 degree angle to the relative wind. This tilts the lift vector back 15 degrees from vertical. From this we can visualize a vertical component of lift supporting the weight of the aircraft. Connect the top of the vertical component of lift with the original lift vector and this gives you the 'rearward component of lift'. It equals induced drag.

What you have actually done here is draw a triangle, with the total lift created by the wing being perpendicular to the chord line and the vertical component of lift being perpendicular to the relative wind. If you connect the ends of these two vectors you finish the triangle.

From this we can see that at high speed and low angle of attack, induced drag is very small and parasite drag will predominate. As angle of attack is increased due to flying slower or due to the increased load factor in a turn, induced drag will increase.
 
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[I like that: prescient / clairvoyant air.

smile.gif


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Yep, I suppose the key is "how do it know?"!
 
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Here's one for you. How does sweep back allow for higher mach numbers?

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I have not though about this much other than to look at the standard answer illustrated by a vector diagram. But this is an interesting question, so here goes.

To fly at high mach numbers it is desirable to have a very thin wing without much camber. Since camber increases the velocity of the air flowing over the wing it is possible to encounter mach effects (shockwaves) on the upper surface of a thick cambered wing while an aircraft is flying significantly below the speed of sound.

So, it is obvious we want to reduce the camber of the wing and reduce the acceleration experienced by air traveling over the top of the wing. The easiest way to do this is to make the wing thinner.

Thin straight wings can be used to fly at supersonic speeds and have been used successfully on designs like the Bell X-1 and the F-104. The only problem with a very thin straight wing is it is hard to make them strong and light weight due to their very low internal volume. There is also the little problem of where to put all the fuel.

You can mitigate the problem somewhat by increasing the chord line. The longer the chord line in relation to the thickness of the wing, the less camber it has. This would allow you to build a thicker, lighter, stronger wing with adequate space for fuel. Our only draw back is that now we have a wing with a long chord and a short span, giving us a low aspect ratio. Our hypothetical design has a very large wing tip that is exposed to efficiency robbing wingtip vortices.

If you sweep the wing, this increases the 'effective' chord of the wing. The air flowing over the top of the wing must travel over a longer distance. This has the effect of reducing the camber of the wing. The designers are able to increase the thickness of the wing, without encountering mach effect until a higher speed. You are now able to build a longer, thicker wing with a higher aspect ratio, while minimizing mach effect at higher speeds.
 
Anon-

All true and great observations, however, sweep does more than that, and the key to that extra benefit is referenced in the post I made just prior to this one....

Incidentally, once you have sorted this answer out, you'll see why aircraft such as the F-104 don't gain any advantage from sweep back.
 
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That is certainly the engineering answer, or pretty much is, and there is some truth in that the air is being pulled inward due to the low pressure above the wing, but that does not really tell you "why" it works.

To head you in the right direction, consider first why a shock wave develops and how a wing near sonic speeds differs from one that is going slower, or, put another way, how the air is reacting to the approaching wing.

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Wow...okay....let me take a shot. The air approaching a subsonic wing in basically incompressible...the air has no advance warning that the wing is approaching...and the air is able to maintain a laminar flow over the wing. A wing near sonic speeds now is able to begin compressing the air ahead of its path...giving the air "advance" warning that it is approaching. Due to the wing sweep...the inboard section of the wing "disturbs" the advancing air and introduces "span wise" flow of air accross the leading edge. The spanwise flow meets the relative free stream of the air and produces a perpendicular vector that is reduced in magnitude. This acts to delay the onset of mach effects to a higher speed.
 
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While you're pondering that one, here is another one that is more simple. Why, in airline ops, does a heavy aircraft need more distance to descend than a lighter one?

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The angle of attack for economy descent speed is greater for the heavier airplane.
 
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The air approaching a subsonic wing in basically incompressible...the air has no advance warning that the wing is approaching...and the air is able to maintain a laminar flow over the wing. A wing near sonic speeds now is able to begin compressing the air ahead of its path...giving the air "advance" warning that it is approaching.

[/ QUOTE ] The air is actually able to start moving out of the way when a subsonic aircraft is approaching. Due to air's incompressible nature at low speeds, a pressure wave travels slightly ahead of the aircraft. Think of this as the 'bow wave' of a boat.

When an aircraft is supersonic, then the aircraft 'sneaks up' on the air molecules it encounters and has to force its way through. (Remember, pressure waves are transmitted through air as sound waves. If you are faster than the speed of sound, no pressure wave can stay ahead of you.)
 
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