Busting an aerodynamic myth -- Planes don't fly due to Bernoulli Effect
 

I think this topic deserves its own thread because it of how widespread the myth is and how important this is to aerodynamics, including car aerodynamics.

It's one of those "facts" that we were all taught in grade school-- that airplanes fly because the wing is curved on top and flat on the bottom, so air has to move faster over the "longer path" along the top and is therefore lower pressure than air flowing below the wing. Well, I've had trouble picturing this in my mind since I was a kid, and I was finally vindicated a couple years ago when I came across an article that explained that the "longer path" theory is a myth, and that airplanes fly due to the wings' angle of attack redirecting air downward.

Suddenly it made sense how a vehicle moving through the air can generate enough lift to hoist itself into the atmosphere. The bottom of the wing directs air downward, applying positive pressure. Airflow along the top of the wing stays attached and is also redirected downward, applying negative pressure to the top of the wing. That's how you get lift. I believe the flow attachment is the source of the "Bernoulli Effect" myth. I think Bernoulli predicted attached flow, which is responsible for the top of the wing directing air downward as the wing moves through the atmosphere.

Anyway-- flow redirection--- it's simple, it makes sense, and there's no hocus pocus about molecules being compelled by magic to meet each other at the trailing edge (they don't).


And here is the most definitive and clear article, complete with animations and a web app, from NASA themselves.

Incorrect Lift Theory

Interesting, considering that I spent several years teaching that to pilots and airplane mechanics

Yeah, I read a forum thread a while back where pilots were complaining about being asked about that on tests, and having to lie about lift being created by "the longer path theory".

And this despite everybody in the aviation industry knowing that a plane has to fly at a pitch greater than zero and that in order to increase lift you increase pitch!

I too have instructed that gem. After many years of aircraft flying and ownership, I can tell you the bare truth:
The 4 forces of thrust and drag and lift and weight, Bernoulli Effect, even your little tale about directing air downward, none of these make a plane fly.
It has been proven that the wallet is indeed the necessary force. Remove the wallet and the plane falls out of the sky like a toolbox with a broken handle.

Skyking touched on what I heard is the current theory. The air is being directed downward, and this "Thrust Vector" is what creates the lift due to the whole action opposite reaction theory. It's tough to visualize it but it has to make sense, if you're pushing air down, something has to be pushing back with equal force.

It's not tough to visualize though. It's the most intuitive thing. You can stick your hand out the window of a car and feel it yourself, or watch a bird take advantage of a wind gust to take off just by holding its wings open at the right angle

An other dificulty with the bernoulli principal as an explination of lift is the ruder, it is semetrical with repect to length of the sides...

1. The way it was explained to me, back in flight training, is that if you "do the math": calculate the total amount of lift from "accelerated air" (Bernoulli) vs the total amount of lift derived from forcing air down (F=MA), you'll wind up with the same number.

2. The air flowing over the upper wing surface is indeed traveling (relative to the airplane) in excess of the surrounding undisturbed air. This is why when planes transition to the "transonic" region of flight (some localized areas of sub- and supersonic flow), they generally go supersonic first on the upper wing surface, causing mainly undesirable flight characteristics (google "Mach tuck.")

2a. Airplanes designed for "high-subsonic" cruise speeds generally have rather convoluted airfoils due to the phenomenon discussed in "2"...to stave off transonic effects as long as possible.

3. The air pressure atop the wing is indeed lower than beneath it. At the tips, the high-pressure air tries to work its way over to the top of the wing, producing "wingtip vortices," which are really cool to look at (when they're visible) and/or disturbing to contemplate (when they aren't visible and you're following a "heavy" for landing.)

Locals in rural Scotland just believe that the big shiny birds are gods and run under cover.

If gravity can be explained by Intelligent Falling, then lift could be the effect of Intelligent Ascending, right?

Mr Adams...

1. The way it was explained to me, back in flight training, is that if you "do the math": calculate the total amount of lift from "accelerated air" (Bernoulli) vs the total amount of lift derived from forcing air down (F=MA), you'll wind up with the same number.

Did the explainer ever do the math?

Yah, this isn't a conspiracy. If you have a curved top wing it will contribute to lift, but pretty much all airplanes have the wing set at a certain angle of attack so it can "redirect" the air downward also.

I expect the contributions from bernoulli create less drag for the lift they create than changing the direction of the airflow. Hence why commuters use asymmetric airfoils, and I don't recall seeing a sailplane with a symmetrical airfoil (they are especially efficient).

Plenty of airplanes however, (i.e. stunt planes) have symmetrical airfoils and fly well rightside up or upside down. Now the angle of attack still makes a bit of bernoulli effect in reality, but air is being directed downward and newton takes it from there.

Having an airfoil also allows a steeper angle of attack without the flow seperating (stalling). So an airplane with just a curved top wing can nose up with less chance of a stall, a stunt plane can nose up or down and not stall (i.e. if (s)he is upside down and shoves the yoke forward)

It's worth noting that arched wings are much stronger than flat ones due to curvature. They flap less. Nature exploits curvature in many ways. There is more to learn from birds and fish. :)

exactly. it is possible to design a plane that flies solely on the Bernouli Effect, but it would take an immense amount of power to over come the drag of the airfoil design.

I think the main reason the top of wings are usually curved is so flow attaches and stays attaches, then departs with a steeper angle. If the leading edge is razor sharp, air will hit it and never attach unless the angle is really shallow, so it has to be radiused. And then after the leading edge, the slope is shallow and gradually gets steeper so flow can remain attached.

http://i56.tinypic.com/qobypz.png

http://scienceray.com/physics/wind-tunnels/

^^^^^^^^What he said

A factual title for this thread would be, the bernoulli effect isn't the only contributer to a plane flying.

However, it does not change that the bernoulli effect DOES contribute to a plane flying, so it not a MYTH. The fact that the OP has difficutly picturing it is not uncommon, may people have problems with the principal the the FASTER something moves, the less pressure it has, it seems contradictory. However, the speed of the plane, in combination with the redirection of air downward + decreased pressure above it causes it to lift.

If the bernoulli effect was A MYTH, it would take far more power then what is used to lift standard designed aircraft into the air.

Read the NASA article. The Bernoulli Effect is not what makes the plane fly. The air flows faster over the top of the wing, yes, because of the angle and the downward curve at the rear-- airflow on the bottom is compressed because it hits the underside and slows down relative to the plane. But the air speed difference is a tiny tiny contributor. It's not the majority or even a major portion of lift.

Redirection of flow is what creates lift.

If you know of a plane that can fly at 0 degrees wing pitch, post it.

I'm not quite ready to discount the bernoulli principle as being a significant contributor to lift.

One of my favorite RC sailplanes in my little fleet uses a SD7037 airfoil (pictured below). It is fast and has good penetration upwind (important when you don't have a motor), but it doesn't like slow very much.

But what I want to point out is that here it is drawn in normalized position, the leading and trailing edges are on the 0.00 line. So that when the air splits at the leading edge at this angle of attack, it should "rejoin" at the same exact height at the trailing edge. If there were no bernoulli, this would not create lift.

But it is creating quite a bit of lift. The zero lift angle of attack for the SD7037 is actually -3.29 degrees, meaning you have to lower the leading edge to get this to stop making lift.

http://www.worldofkrauss.com/foils/d...s=true&chord=5

My statement is that the Burnulli principal come into play, it is the principal that creates the upwash.

I think you are confuning the Bernoulli effect with the longer path effect. Bernoulli states that the faster it moves (air or water for example) the less pressure it has.

Even in the article it states-
""

When in effect the Bernoulli effect causes, among other things:
The air is bent around the top of the wing, it pulls on the air above it accelerating that air down, otherwise there would be voids in the air left above the wing. Air is pulled from above to prevent voids. This pulling causes the pressure to become lower above the wing. It is the acceleration of the air above the wing in the downward direction that gives lift.

You are confuning the "equal transit" theory with Bernoulli, Equal transit used the Bernulli principal, but when that was found to be false, what they found was the principal still held true, just in a different way.

No, I understand that part. That's what I said in my original post.

My point is that it's the pulling of air downward and the resulting low pressure at the top that creates lift, not the speed difference itself.

I'm emailing the NASA guy listed at the bottom, because I think section 3 of that article is unclear. In section 1 he specifically states that it's the turning of flow that creates lift, then he says you can explain lift with the air speed difference. How can it be both exclusively? You should have to add those two together.

I'm not sure the normalized position based on the leading edge and trailing edge makes sense. We know that aerodynamics at the rear is more important than the front right? So why can't the rear-facing slope of the wing be more important than the front-facing slope? That is the part where flow gets pulled downward to meet the wing, which I think is how most lift gets created.

In order to reach zero lift, you have to angle that rear-facing slope so that it's balanced by downforce on the rear-facing bottom surface.

...does that make bouyancy the "toilet neutrality"?

I'm a little confused. You specifically asked for 0 degrees here:

then you say that position doesn't make sense...

My point is that this, and most asymmetrical airfoils, make lift at 0 degrees, so it is simply a matter of going fast enough and/or being light enough before you have a plane that can fly at 0 degrees wing pitch.

What do you think is redirecting the air on top of the wing?

The Coanda effect is redirecting air, and as I understand that is related to the Bernoulli effect. But it's not the Bernoulli effect itself that is creating the lift. If airflow left the back of the wing at 0 degrees, you could attribute lift to Bernoulli, but lift requires airflow leaving the wing at a downward angle.

That's why the 0 degree plane you use doesn't apply IMO. Because what matters are the angles at the rear of the wing.


You said zero lift happens around -3 degrees. Would that happen to be the angle at which the top and bottom trailing edges are equal angles from horizontal?

No, that is the definition of zero degrees angle of attack. The same amount of air is going over the top as the bottom (more or less)

yes, that is (about) the zero lift angle of attack for that airfoil.

you need to get your terms straight. His diagram shows a wing with a line through it. This is the chord line and that is what is used to define angles of attack and angles of incidence.
By definition that wing produces lift at a 0 degree angle of attack.
Many sailplane wings will fly at 0 and slight negative angles of attack.
The angle of attack is the angle formed by the chord line and relative wind.
The angle of incidence if formed by the the chord line and the longitudinal axis of the fuselage.

If Bernoulli was that powerful, you could take a thin flat plate, pitch it a few degrees so that there would be no attached flow on the bottom rear-facing side, only the top front-facing side. The huge difference between flow speed on top and bottom would let Bernoulli exceed the downforce from turning the flow upwards.

Yes I know the definition, but what I'm saying is that the effective "flat" angle is different.

I've read elsewhere that most lift, something like 80%, comes from the top of the wing pulling air downward, and remaining lift from the bottom pushing air downward. That makes that rear-facing surface the most important part of the wing. With the wing "horizontal" per the chord line, that surface is still turning flow downward.

An analog would be a car... Remember the Audi TT lift fiasco? Even at zero chord angle, a car can have dangerous lift, necessitating spoilers. It's not because air flows faster over the top of the car, but because of the angle of the sloping windshield and trunk. If it was Bernoulli, the little spoiler that they added to the trunk wouldn't have solved the issue.

http://image.importtuner.com/f/slaug...02-audi-tt.jpg


Heck... if Bernoulli was that powerful, cars with air dams preventing much airflow underneath would lift off.

The "effective flat angle"? Where did you find that engineering gem?

Are you really not understanding my point? Look at the wing. See how the top slopes backward? That sloping surface is what creates most of the lift. The chord angle is arbitrary in this discussion if that slope is the most important ****ing surface.

http://www.worldofkrauss.com/foils/d...s=true&chord=5


You can't say that since the wing is creating lift when the chord is horizontal, that must mean that the Bernoulli effect is the source of lift. The chord angle and the lift generating surface angle are two different things.

Just because the wing chord is an engineering metric doesn't mean you can't apply common sense and understand how the wing actually works and why a "horizontal" wing is creating lift by pulling air downward.

now, with what you are saying, the tail being the important part and all, wouldn't you think the center of lift for this airfoil in the newtonian only lift theory would be at the trailing edge of the airfoil?

IF the airfoil is "horizontal" like your diagram, the center of lift would be somewhere between the highest point and the trailing edge

I thought all this flying business was resolved about 100 years ago. Now I'm starting to wonder if airplane indeed exists.

Stand on your bathroom scale.
Hold hair dryer in your hand and point it at your forehead.
Run blow dryer on cool setting(don't fry your brain).
Check weight before and after.
If you get lighter with wind blowing over your head,you are developing lift!

Sailboat sails develop lift .
Airplane wings develop lift.
Cars develop lift if they are shaped like a wing.

Tape a sheet of paper to a table edge with most of it hanging down.
Blow some air over the table in the direction of the paper.
Shazam! The paper lifts up!
Lift caused by the airflow faster over the top than the bottom.

I rest my case........

Phil