Aerodynamic Question

M

meritflyer

Well-Known Member
Doesnt low pressure reside underneath the wing and high pressure above the wing?

(or do I have that backwards)
 
meritflyer said:
Doesnt low pressure reside underneath the wing and high pressure above the wing?

(or do I have that backwards)
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sdrawkcab
 
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.
 
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
 
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...
 
Chris_Ford said:
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...
Click to expand...
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.
 
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.
 
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?
 
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.
 
Chris_Ford said:
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?
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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.
 
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!).
 
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.
 
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?
 
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!
 
seagull said:
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.

).
Click to expand...


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.
 
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