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

[jamesqf]

Originally Posted by winkosmosis

I said they don't create lift, even though flow is fast over the top and nonexistent (or almost) below.

Is the flow almost nonexistent below, though? Thinking in 3D, it seems as though there'd be a lot of turbulent airflow in from the sides.

But instead of arguing about this, why not just go measure it?

[robchalmers]

Wink,

they DO create lift. but not in suficient qunatities to lift it off the ground. the flow over the top of a bonneville camero isn't a as smooth and laminar as that going over a cambered wing section, in fact points like windsheild (base and top) can kill laminar flow, the TT didn't hence the problems.

Take and indycar or F1 car and remove the front and rear wings and I'm DAMN sure it still generate downforce why? the Benoulli effect in the Ground effect venturis of the Indy car 'y axis' no air directing elements under there. Or in the diffuser of the F1 car, again no air directing panels there either. the Accelerated air as it returns to the flow around the car creates low pressure (benoulli) under the car.

I agree - and I've stated earlier - Lift is not 100% benoulli, just just like an egg is 100% white.

[md7989]

Honestly, I was never taught a plane flies by the lower pressure thing, and have always known it was because of the angle of air coming down from the wing.

[Fat Charlie]

It's pretty much the standard that people get taught. I'm not contesting its inaccuracy, it's just what I've always seen presented as fact. A quick google shows how ingrained the "fact" is.

Howstuffworks: How Do Airplanes Fly: Weight and Lift

As for the actual mechanics of lift, the force occurs when a moving fluid is deflected by a solid object. The wing splits the airflow in two directions: up and over the wing and down along the underside of the wing.

The wing is shaped and tilted so that the air moving over it travels faster than the air moving underneath. When moving air flows over an object and encounters an obstacle (such as a bump or a sudden increase in wing angle), its path narrows and the flow speeds up as all the molecules rush though. Once past the obstacle, the path widens and the flow slows down again. If you've ever pinched a water hose, you've observed this very principle in action. By pinching the hose, you narrow the path of the fluid flow, which speeds up the molecules. Remove the pressure and the water flow returns to its previous state.

As air speeds up, its pressure drops. So the faster-moving air moving over the wing exerts less pressure on it than the slower air moving underneath the wing. The result is an upward push of lift. In the field of fluid dynamics, this is known as Bernoulli's principle.

Howstuffworks: Aerial Navigation: Wings, Slats and Flaps

First, let's consider the angle of attack, the angle that a wing (or airfoil) presents to oncoming air. The greater the angle of attack, the greater the lift. The smaller the angle, the less lift. Interestingly enough, it's actually easier for an airplane to climb than it is to travel at a fixed altitude. A typical wing has to present a negative angle of attack (slanted forward) in order to achieve zero lift.

And here's a page from Macaulay's The Way Things Work:

[Otto]

It's both. Higher pressure due to deflected air under the wing + lowered pressure atop the wing.

Try this, ifn you question Bernoulli: Use a soda straw, and blow along one side of and parallel to a vertically hanging piece of smooth paper. Notice that when air is blown, the paper responds and moves toward the straw.

Now blow at an angle against the paper, and it moves away from the straw, due to pressure on one side but not the other.

Aircraft wing uses both sources of lift, in synergy.

Probably, the greater source is deflected air beneath the wing, but Bernoulli helps, too.

[ik04]

Originally Posted by winkosmosis

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.

The de Havilland DHC-7 climbs with a five degree nose DOWN attitude because of the large camber (curve) of the upper surface of the wing. The "downflow" of airflow at the trailing edge of the wing does not exist. Upward pressure at the back of the wing would deform the structure and most of the lift is generated at the center of pressure, usually in the first third of the airfoil.

The airflow on the top surface of the wing does not "arrive later" at the trailing edge. It accelerates due to the longer distance it must travel in sub-sonic flight. The airflow below the wing does not "compress" and is nearly the same as atmospheric pressure. The difference in the pressure above and below the wing is what creates lift. Purely Bernoulli.

The effect of deflecting airflow is Newton's theory. That is how you see RC aircraft with completely flat wings fly. Symmetrical airfoils still use Bernoulli to create lift because the angle of attack causes the airflow to travel faster over the "top" of the wing.

The propeller of a powered airplane is also an airfoil and it produces thrust (lift) based on exactly the same prinicipal.

Drag producing devices such as trailing edge flaps and spoilers use Newton's law to redirect airflow for increased lift during takeoff and landing. They are still increasing the speed of the upper airflow and thus, creating lift.

I can go into much more detail with regard to rotary-wing aerodynamics, but that is not germaine to this thread.

[winkosmosis]

I got a response from the NASA guy. Here is what I sent:

Hello- I just found NASA's page debunking the "equal transit time theory". Thank you for posting this. Since I learned about "how wings work" in elementary school it just didn't compute in my mind, and now NASA backs me up. A wing producing lift by being angled to turn flow downward is intuitive and matches what we experience when we stick our hands out a car window or swing a rigid plate through the air.

I have some questions though, and I'm guessing you're the person to ask since you're the editor and and NASA official listed (Incorrect Lift Theory)

1- The airflow speed over the top of the wing isn't higher because of equal transit time, but for some other reason. Is it because in flight, a wing is pitched upward, which causes air hitting the underside to compress and slow down?

2- It says in section 1 that it's not the distance that matters, but it's the turning of airflow that produces lift. But then in section 3 it's vaguely implied that the Bernoulli effect can explain lift if you use the real speed instead of the "equal transit time" speed. I don't understand how both can be correct. You'd have to add the Bernoulli lift due to speed difference to the lift created by the airflow turning.

3- If the air speed difference creates X amount of lift, and airflow turning creates Y amount of lift, what are the relative sizes of X and Y?

4- Is the "Bernoulli effect" here the phenomenon of airflow over the top of the wing being pulled downward to match the curve of the top wing surface? Is that what creates lift, the redirection (which I think Bernoulli predicted), and not the speed difference itself? That's known as the Coanda effect isn't it?

His reply:

Glad you are taking a careful look at all this.



Lift .. and drag are created when a solid object moves through a fluid (gas or liquid). The fluid can’t go through the solid. So it goes around the solid. To move the fluid around the solid you have to subject the fluid to forces. As the fluid goes by the body, there are three conservation laws that must be observed; conservation of mass, conservation of momentum, and conservation of energy. These conservation conditions must be satisfied at every point in the flow, at all times, at the same time. Newton’s laws of motion are expressions of the conservation of momentum. Bernoulli’s law is derived from the conservation of energy. So in any flow problem, Newton’s laws and Bernoulli’s law are satisfied at the same time … they aren’t added together.



Tom

[robchalmers]

THANK YOU MR NASA !!!!!!

You STILL seem to be hooked up on Longer line theory or equal transit time being Benoulli law- IT ISN'T!!!!!

Benoulli IS required for flight

to quote
"Bernoulli’s law is derived from the conservation of energy. So in any flow problem, Newton’s laws and Bernoulli’s law are satisfied at the same time … they aren’t added together. "

I'm sorry if I seem a little het up about this maybe its because I too have a degree in Aeronautical engineeering and spent a ****load of time in a wind tunnel. but reading what he says is just dispelling Long line / ETT and not the fact that flight comes the principles of Benoullis law.

Next go to this Nasa link
http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html

and put in 0 degree angle of attack and mess with the camber and chord%

See what happens when you try a flat bottomed wing, -3.5deg AoA,8%camber, 11.5% chord thickness

a) flow lines (deceptive because they seem to show ETT) - but there is UPwash
b) lift generated (1239lbs)

[Quazar]

Thank you, as this is what I said.

The problem here is a fundumental misunderstanding of scientific principals. This thread is called "Planes don't fly due to Bernoulli Effect". This statement is false. It is based on a limited understanding of the Bernoulli principal and its application to a concept that has been debunked for over 20 years.

What isn't debunked is the Bernoulli Effect.

What is debunked is the equal transit time, or longer line theory.

The OP stated in his first post he had problems with the concept that wind traveling faster created lift. Which used the concept there was less air pressure on top of the wing, creating upwash. This seems to still be a concept he is struggling with, which is why he is confused by the effects of the Bernoulli principal.

[UFO]

Originally Posted by winkosmosis

I got a response from the NASA guy. Here is what I sent:


His reply:

Excellent questions, and an unexpected set of answers, but it makes sense too. Thanks for running this down.