Aerodynamic Question

[meritflyer]

Doesnt low pressure reside underneath the wing and high pressure above the wing?

(or do I have that backwards)

[pilot602]

Backwards ...

[JEP]

Originally Posted by meritflyer

Doesnt low pressure reside underneath the wing and high pressure above the wing?

(or do I have that backwards)

sdrawkcab

[CoBuilder]

It wouldn't make sense if high pressure was on top since high always moves towards low.


Take a piece of paper and hold it on one side, then blow on top of it and it'll come up meaning high pressure moves toward lower on top giving it a shove up.

[rasec]

just remember... high always seeks low

[FlyingNole]

just think of a soda bottle.....I always thought of it as pressure against the bottom of the wing pushing it up. If it was on top, it wouldnt fly but thats just the layman's way of thinkin of it

[bwade210]

Yeah...what everyone before me said! What I get for showing up late.

[Chris_Ford]

Let me branch this off into a broader question... How well does that theory actually explain lift? I honestly don't feel that I have a 100% certain answer for "What makes an airplane fly" if I'm asked...

[EatSleepFly]

Pfm

[n57flyguy]

Originally Posted by JEP

sdrawkcab

LOL

[MidlifeFlyer]

Originally Posted by Chris_Ford

Let me branch this off into a broader question... How well does that theory actually explain lift? I honestly don't feel that I have a 100% certain answer for "What makes an airplane fly" if I'm asked...

Stick with "money" for the answer.

Both theories likely apply to varying degrees, but Bernoulli v Newton arguments tend to cause more trouble than they are worth if you're just explaining it for the benefit of non-technical students and lay people.

[B767Driver]

I read something a while back written by a physicist... he said the lifting force in pounds due to the pressure differential over the top and bottom of the wings is not sufficient to support the weight of the airplane.

He says lift is primarily the result of the amount of air being diverted over the wings and deflected downward.

Not being a physicist or an aero engineer...I'm not really sure what is accurate. It sure seems that the professionals are all over the map when trying to explain the production of lift. Pick up a few different textbooks...it's sure to leave one confused.

The one thing that all will agree on, however, that the theory of Equal Transit...where the air molecule flowing over the top of the wing meets a like molecule flowing under the bottom of the wing at the trailing edge at the same time...thereby causing an increased velocity over the top of the wing. This is definitely not true but is proliferated quite frequently among pilots and instructors.

[Chris_Ford]

Well I've also heard of the "rotational" theory or something like that... I'm not quite sure about it, has anyone else heard of it?

[n57flyguy]

Read Stick and Rudder. My dad is an aeronautical Engineer, so he has been telling me this stuff since I was 5, of course it was a engineers perspective not a pilots.That does take guts to come and ask that though.

[B767Driver]

Originally Posted by Chris_Ford

Well I've also heard of the "rotational" theory or something like that... I'm not quite sure about it, has anyone else heard of it?

In my previous post, I should've said that the pressure differential due to path length alone was insufficient to cause lift. Obviously, the pressure differential above and below results in lift production.

That's how I understand it...due to circulation, and the Coanda Effect, the air is accelerated and diverted downward at the trailing edge (downwash), and this forms a low pressure bond above the wing. Of course the more air that is diverted over the wing and diverted downward...the stronger the lifting gradient will be. I believe the magnitude of the downwash vector is directly related to the induced drag...so the more air diverted...the greater the lift...the greater the drag. Engineers? Am I close?

But I should stop here...as I'm getting outside of my knowledge base...and let the professional engineers take it from here.

[seagull]

Actually, you are not correct. Although you can use the amount of air deflected downward to MEASURE lift, it does not CAUSE lift, as the downwash itself is a RESULT of the rotational motion of the air around the wing, and that rotational force leads to the differential in relative flow velocity around the wing, and THAT is what leads to lower pressure over the wing that actually is the lifting force.

There is no "bernoulli vs. newton" in actual aero, as the bernoulli flow and equations are based off of newton, essentially.

Saying that lift is caused by downward deflecting air is a true misapplication of, and misunderstanding of, Newtons laws. It is rather like saying magnetism is a result of iron being attracted to a magnet and that explains magnetism. While that might show how a magnet acts, it is far from how a magnet is actually creating that force to begin with!

Objects move because they are pushed, regardless of what causes that pushing force (can be differential air pressure, magnetic force or even gravity!).

[Propilot]

http://www.grc.nasa.gov/WWW/K-12/airplane/short.html



use the links down the right side.



The rotation you speak of is "flow turning".


Essentially, F=MA

A= Acceleration

Acceleration is Veloctiy over Time.

Velocity is a vector quantity with both a magnitude and direction.

Thus, if you change the direction, you change the velocity, changing t he acceleration, changing the force.



The theory of equal and opposite reaction, from the air molecues hitting the bottom of the wing and propelling the wing upwards, will not produce accurate numbers at low speed flight BUT, it does predict lift values accurate at hypersonic speeds (ie, the space shuttle re-entry).




Defininatly dont teach/tell anyone that equal transit BS that someone mentioned earlier. total bs.

[greaper007]

As long as we're talking about aerodynamics.... What about the forces on a plane in a zero sideslip configuration. Does weight have any place in it or is it just the horizontal component of lift?

[seagull]

The NASA site says what I did above, essentially. It doesn't get that deeply into it, just as most college textbooks don't really explain magnetism, but stop at the point where they have explained how a magnet is made.

Using the "Newton" method alone stops short of the "why", which is the fundamental problem with using the explanation. Of course, the simplistic approach to Bernoulli that most aviation books use (even ones like Aero for Naval Aviators) falls even shorter!

So, as long as you don't care about "why", you're fine. It's like explaining how an untied balloon will shoot around the room. Can you explain where the force that is actually moving that balloon comes from? It's surprisingly simple and does not even require someone to know who Newton was!

[B767Driver]

Originally Posted by seagull

Actually, you are not correct. Although you can use the amount of air deflected downward to MEASURE lift, it does not CAUSE lift, as the downwash itself is a RESULT of the rotational motion of the air around the wing, and that rotational force leads to the differential in relative flow velocity around the wing, and THAT is what leads to lower pressure over the wing that actually is the lifting force.

).

I don't see how this is not similar to my previous post regarding circulation and the coanda effect....maybe you can expound on that a bit.

If I stated that downwash causes lift...I either miscommunicated it...or you interpreted it incorrectly. As you said...the magnitude of the downwash vector can be used to 'measure' lift production...and presumably drag as well...I believe that was my intent of the post. The circulation introduced by the angle of attack of the wing and the resultant acceleration towards the trailing edge should be a sufficient rudimentary explanation of lift production.

If this is not correct...please add further.

[seagull]

I think that the explanation that it is the acceleration of air downwards that causes lift is ok for someone that does not want to dig too deeply, however, it is, in the end, the differential pressure around the wing that pushes the wing upwards, just as it is the pressure of the Earth against your feet that pushes you in opposition to gravity. The forces that keep an airplane in the air are really no different than the force that is keeping my coffee cup from falling through my kitchen counter at the moment!

[chris]

Here's another way to think of it...

It is the pressure difference between the upper and lower surface of the wing which is responsible for the lifting force. This pressure difference causes the air to "push" up on the wing. In accordance with Newton's 3rd law (action=reaction), the wing will push down on the air. In a way, the downwash is the result of the lifting force.

Some questions that may be asked:

1. What causes the pressure difference in the first place?
The pressure difference is caused by the difference in velocity between the air molecules on the lower surface of the wing, and those on the upper surface. Bernouilli's theorem shows the relationship between velocity and pressure.

2. Why does the air travel faster on the upper surface?
This can become pretty complicated, but the crucial reasons behind this have to do with the shape of the wing itself and to the existence of viscosity.
If you were to expose a non-rotating cylinder to a stream of air, the pressure pattern above/below the cylinder would be identical, and thus, there would be no net lifting force. If you were to rotate the cylinder clockwise (assuming the stream of air is coming from left to right), the air would travel faster above the cylinder than below it, and this difference in velocity would cause a difference in pressure, and thus, a net lifting force would be produced.
The air travels faster above the cylinder when it is rotating because of the existence of viscosity between the air molecules and the cylinder. In a way, the cylinder is pulling on the air as it rotates, causing it to travel relatively faster above it than below it.

The important point here is this: Due to the shape of the wing, it can generate a "circulatory flow" like the rotating cylinder example above, without actually having to rotate.

3. How does a wing generate this circulatory flow?
The circulation is able to be produced because of the wing's shape. An important reciple is to have a streamlined leading edge, and a sharp trailing edge. The sharp trailing edge causes the air to "separate" from the wing at the trailing edge, and this separation occurs, once again, because of viscosity.
Imagine what would happen if the leading edge were very sharp. The airflow would not be able to remain "attached" to the wing as it flows over it, and the result would be a large amount of drag. This will destroy the pressure differences between the upper and lower surfaces, and this will destroy any lifting force.
This is similar to how a stall occurs. Once you exceed a certain AOA, the airflow can no longer maintain its smooth, attached flow, and it seperates at the leading edge, causing a complete brakedown of airflow and pressure distributions.


So, overall we see:
- Due to the shape of the wing and the airflow, the air travels faster above the wing than below.
- This causes a pressure difference in accordance with Bernoulli's theorem (higher pressure below, relatively lower pressure above the wing).
- The pressure differences causes a net force which acts upwards. The higher pressure below the wing is pushing up on the airplane.
- In accordance with Newtons 3rd law, the reaction is that the airplane pushes down on the air (downwash).


Now I personally don't fully understand how the shape of the wing truly causes the circulatory flow, nor do I fully understand why the air has to seperate at the trailing edge, but from my research, they are extremely important to the production of lift.

[seagull]

Chris,

Good post. The answers to your questions at the end aren't hard, but hard to explain with just text and no drawings (or maybe I'm just too tired!). Flightwise has some good drawings for it, pick up a copy.