Why does an airplane fly?

A) When discussing the stagnation point - when air hits the leading edge of the air foil, I assume it has a higher pressure than above the wing. So, once air reaches the stagnation point, is it the lower pressure that exists on the upper surface of the wing that "pulls" the air over the wing?

B) After the air has moved over/under the wing, at the trailing edge, I'd assume that velocity is somewhat restored to "normal". Does the air aft of the trailing edge play a role on "pulling" the air over the wing to an extent?
 
meritflyer said:
is it the lower pressure that exists on the upper surface of the wing that "pulls" the air over the wing?
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There doesn't need to be a lower pressure over the top to make the air go that way....air will deflect from an area of high pressure. The general idea is that the air will split around the stagnation point, sending a higher volume of air over the top. Due to the law of conservation of mass, when you have a flow into a venturi, the same quantity of matter is flowing in and out. For this quantity to get through the constriction, it *must* speed up. That's when the pressure over the top gets lower.

<<Does the air aft of the trailing edge play a role on "pulling" the air over the wing to an extent?>>

Actually, it impedes it. The air pressure past the "hump" on the top starts to increase in pressure back towards freestream. This is called an "adverse pressure gradient." The air starts slowing down. It may eventually stop and reverse direction. We call that an airflow separation, or "stall".
 
meritflyer said:
B) After the air has moved over/under the wing, at the trailing edge, I'd assume that velocity is somewhat restored to "normal". Does the air aft of the trailing edge play a role on "pulling" the air over the wing to an extent?
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I think you are onto something here. TG and SGull no doubt can expand more eloquently than I, but...the trailing edge is very important.

The sharp trailing edge introduces "circulation" in the airflow around the wing. This circulation effectively "pulls" the air around over the top of the wing contributing to the accelerating characteristic of the air over the top of the wing.

A golf ball spins...introduces circulation over it's surface and creating lift (far too often for me in a direction towards the trees.) It's difficult to get a wing to spin....so the sharp trailing edge effectively introduces spin...or circulatory flow over the airfoil.

The velocity of flow over an object is inversely proportional to it's radius. If we view the trailing edge of the wing as a small radius...the airflow will increase it's velocity towards the trailing edge....but will be unable to complete it's turn around the TE because the centripetal force required it too great...and the boundary layer separates now at the TE.


You talked a little bit about downwash....and it's role in lift. I don't think you want to go there. TG and Realm discussed this a couple of weeks ago...but you don't want to equate the downward deflection of air...with lift development. As I understand...the upwash of air at the leading edge cancels out the downward deflection of air at the trailing edge.
 
tgrayson said:
<<Does the air aft of the trailing edge play a role on "pulling" the air over the wing to an extent?>>

Actually, it impedes it. The air pressure past the "hump" on the top starts to increase in pressure back towards freestream. This is called an "adverse pressure gradient." The air starts slowing down. It may eventually stop and reverse direction. We call that an airflow separation, or "stall".
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TG,

If I understand correctly....separation due to an adverse pressure gradient will begin prior to the Trailing Edge. You can prevent an adverse pressure gradient by "streamlining" the rear of the airfoil...and creating a sharp trailing edge point (or you could introduce a turbulent boundary layer to introduce more energy into the BL for it to adhere longer to the top surface.)
 
meritflyer said:
For anyone having a difficult time with this concept, Schiff says "The rotors of a helicopter create lift identically to the manner of a fixed wing creates lift. The only difference is that helicopter wings rotate to create relative wind without any movement of the helicopter. Fixed wings encounter relative wind only when the airplane is in motion."

Thoughts?
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Sort of... rotary wings do create their own relative wind, so to speak, by rotation. This is how the wings create lift in a hover. But in forward flight (or sidewards or rearward for that matter) the rotor disk starts acting like an airplane wing whereas you add the relative wind from rotation to the relative wind from movement and come up with a "resultant relative wind."

And think about this... in forward flight, the advancing rotor blades have more resultant relative wind than the retreating rotor blades... so why doesn't the helicopter just flip over?

Man, I love helicopter aerodynamics! :)
 
meritflyer said:
A) When discussing the stagnation point - when air hits the leading edge of the air foil, I assume it has a higher pressure than above the wing. So, once air reaches the stagnation point, is it the lower pressure that exists on the upper surface of the wing that "pulls" the air over the wing?

B) After the air has moved over/under the wing, at the trailing edge, I'd assume that velocity is somewhat restored to "normal". Does the air aft of the trailing edge play a role on "pulling" the air over the wing to an extent?
Click to expand...

I think what you're asking has been answered already by B76 and TG, so I'll just add this bit. Again, I'll remind you that things moved because they're pushed. When you breath in, you aren't sucking air in, you are expanding your lungs, reducing the pressure, which results in the ambient pressure to push the air into your lungs.

Ever look at an oxygen duration table for an aircraft using 100% oxygen and notice that it takes MORE oxygen/hour a lower altitudes than higher ones? Ever wonder why? The answer is due to the above. I'll let you have fun pondering that one, but if you think about it, you should sort out the answer with what I wrote above!
 
B767Driver said:
TG,

If I understand correctly....separation due to an adverse pressure gradient will begin prior to the Trailing Edge. You can prevent an adverse pressure gradient by "streamlining" the rear of the airfoil...and creating a sharp trailing edge point (or you could introduce a turbulent boundary layer to introduce more energy into the BL for it to adhere longer to the top surface.)
Click to expand...

I'm not aware of any way to prevent an adverse pressure gradient, although you may reduce it. Consider: the point of minimum pressure will occur towards the first 1/4 of the airfoil. Since that is the point of minimum pressure, it follows that the pressure after that point is higher. Air generally wants to flow from high pressure to low pressure, yet we have an airflow moving from low pressure to higher pressure, solely due to its momentum. It will be slowing down. Eventually, it will stop, although on low AOA airfoils that will be towards the trailing edge, so who cares?

True, you can direct some higher velocity air into the boundary layer, which will give it some extra energy to overcome the adverse pressure gradient. Vortex generators, various blowing mechanisms, slots, etc. can produce these effects.
 
ChinookDriver said:
And think about this... in forward flight, the advancing rotor blades have more resultant relative wind than the retreating rotor blades... so why doesn't the helicopter just flip over?

Man, I love helicopter aerodynamics! :)
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I've only skimmed a book on helicopter aerodynamics, but don't the rotor blades change AOA as they rotate, which is why the backward moving blade can have a retreating blade stall?
 
tgrayson said:
I've only skimmed a book on helicopter aerodynamics, but don't the rotor blades change AOA as they rotate, which is why the backward moving blade can have a retreating blade stall?
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Yup, they change AOA due to what's called blade flapping. The retreating blade flaps downward creating an increased angle of attack to compensate for the dissymetry of an increased relative wind on the advancing blade.

At some point, the retreating blade simply cannot keep up lift-wise with the advancing blade, and retreating blade stall occurs. This usually occurs at high fwd airspeeds and when one or more of the following conditions occur:

- High gross weight
-Low rotor RPM
- High density altitude
- Steep or abrupt turns
-Turbulent ambient air

And now everyone knows WAY more than they ever wanted to about helo aerodynamics!
 
B767Driver said:
You talked a little bit about downwash
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Realm pointed out that he thought that the "Understanding Flight" people were using the word "downwash" incorrectly. I've borrowed the book from a friend and I think he's right. They use the word to mean the downward momentum created by the production of lift. "Downwash", in the aerodynamic literature, is used solely to denote the downward momentum produced by wingtip vortices, which don't exist in 2-D aerodynamics.

Downwash in that context is unrelated to lift, although you will see people bringing it up. Max Munk, a noted aerodynamicist, had written an article earlier this century where he took pains to point out the vortices had nothing to do with producing lift, so apparently it's a common thought. An engineer of my online acquaintance, when pondering a technology designed to kill wingtip vortices also wondered whether it'd be a "lift killer" as well.

Barry seems to use "downwash" in the same way that "Understanding Flight" does. Misusing words in this way really makes it difficult to clarify the issues, because you first have to redefine the words and then put them in context. When someone is stuck on the definition in their first encounter, it's difficult to get it out of their minds. Law of Primacy in action.
 
Merit, have you read "Stick and Rudder?" If not, its a great book that explains the component of lift. I have to go re-read the first few chapters, this thread has been really informative.
 
N57, have you read the Hardy Boys? Very imformative on being a sleuth.

The problem with text on lift today is its like women, no two are alike.
 
meritflyer said:
N57, have you read the Hardy Boys? Very imformative on being a sleuth.

The problem with text on lift today is its like women, no two are alike.
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I can tell you like Nancy Drew more. don't lie.

I agree, I havn't read The Proficient Pilot, but I have an old copy that a friend of mine gave me. I'll wait till I finish Stick and Rudder.
 
meritflyer said:
Nancy Drew was a skank.
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Your just jelous because she was a better sleuth than you, which was your original career goal. Then you found out the Hardy Boys flew aeroplanes and didnt understand how they flew. Then you became a CFI and still didn't know how they flew. So then you came on JC and posted. Now you know its because of magic. Then you found out Frank was going out with Nancy. And Joe wasnt gay.
 
meritflyer said:
Ask 10 different CFIs the reason an airplane flies and you'll get 10 different answers all containing bits and pieces of the real reason.
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That part is true. But, understanding how an airplane flies isn't on the flight instructor PTS. :) A flight instructor, as you know, doesn't have to know much more than a Commercial Pilot, according to the FAA. So, where exactly is the flight instructor supposed to get this knowledge?

I have my CFI students read Skip Smith's book, "Illustrated Guide to Aerodynamics", and we go through the bulk of it together. It doesn't go into circulation theory, but otherwise is pretty complete. I'm not sure exactly how much they retain after the training is over, but they understand the basics and can recognize BS when they hear it.

A thorough understanding of this stuff isn't easy, but you're now taking the right steps by moving on to more advanced material. Read Skip Smith's book and read Carpenter's book and spend some time thinking about this stuff applies to real world flight. Things will start to click.
 
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