aviation information section

[JIMMY]

ok im confused...

the test question is...

1. At zero degrees angle of attack, a symmetrical airfoil will produce

A. Lift, but less than a positively cambered airfoil.
B. no form drag.
C. no induced drag.
D. no net aerodynamic force.

I chose "D". If its symmetrical and at zero AOA, pressure will be reduced over both sides of the airfoil equally, so there would be no net aerodynamic force right? But the book says answer 'A' is correct. Technically, i can see how answer "A" is correct, because an asymmetric airfoil is still making positive pressure at zero AOA, but would you still consider a symmetrical airfoil as making lift if the forces cancel out?

thanks in advance for anyones help.

jP

 

[TSmyth]

i dont know what the answer is but, suffice it to say, that there aren't any questions on the ASTB even remotely this in-depth!

good luck

 

[IrishNavy05]

The answer is C...induced drag is drag due to lift, and a symmetric airfoil produces no lift at 0degrees AOA. That is one of the points of cambered airfoils- to create lift at zero or negative AOA.

D cannot be correct because no net aerodynamic force would imply no drag,
B can't be right because any object has form drag when moving through air (due to pressure differential)
A can't be right because it doesn't produce lift, as described above

damn i feel like a nerd

also, TSmyth is right, the ASTB is nowhere near this in-depth...go have a beer or ten

 

[Chubby]

Because it is producing lift. The symmetric airfoil will produce identical lift on both sides of the airfoil, but lift none the less. This in effect cancels the lift producing no pressure differential perpendicular to the airfoil, read no net lift but there is lift. It also produce drag due to friction, and since it is producing lift even if it isnt a net increase it does produce induced drag. Thus, 'B' is wrong it has form drag, 'C' is wrong it has induced drag, and 'D' is wrong because the drag is the aerodynamic factor that isn't balanced out. Leaving only 'A'. My take on it at least I could be wrong, either way it's a ****ty question.

 

[fighterpfeif]

It says airfoil. An airfoil is any surface that produces lift (wing, propeller, horizontal stabilizer, etc.). If a any wing that is flying through the air it is producing lift (upward in the case of an aircraft flying through the air), because if it isn't producing lift that aircraft will fall out of the sky.

So it is at a zero AOA, but a symmetrical airfoil (wing lets say) must produce lift in order for the aircraft to fly. A chambered wing will produce more lift becuase fuild (air) is accelerated over the top and .... low pressure .... high pressure .... and so on.

If its an airfoil it must produce lift. All wings on aircraft all produce lift in any range of AOA or flight situation as long as air is passing by them.

That questions stinks and is tricky. It seems that they want to test definitions more than anything. First time I looked at it I had the same reaction, but when I thought about it "A" is the right answer. Good way to test your knowledge it seems.

 

[IrishNavy05]

i'm going to sound like an ass here, but i've got a masters in aerospace engineering...the answer is C

lift is a function of coefficient of lift (Lift=CoefficientofLift*Density*Velocity^2*SurfaceArea/2)

induced drag (drag due to lift) is a function of CL^2, so at CL=0, induced drag = 0


the coefficient of lift of ANY symmetric airfoil at zero AOA is zero, hence, no lift will be produced, and yes, the aircraft will fall out of the sky.

just cuz it's a wing doesn't mean it produces lift....think about horizontal stabilizers, which are generally at negative angles of attack to generate negative lift (produces a moment to create longitudinal stability as the aircraft pitches)

 

[fighterpfeif]

To expand the conversation. What then happens to an aircraft that has a symmetrical wing, lets say an Extra 300. You point the nose down, the point at which the airflow seperates moves from the bottom portion of the wing, through the chord line, and onto the upper portion of the wing. The wing would stop producing lift positively, right? unless you are inverted where you want to have 'negative' lift.

Also the question is talking about airfoils, which I believe by definition means any surface which moves through the air and produces lift, positive and negative, so wouldn't that automatically mean that the object in question is producing lift. The book jpaviator is using might be wrong, C is right like you said no lift no induced drag.

Just trying to learn something new.

 

[kray1395]

i'm going to sound like an ass here, but i've got a masters in aerospace engineering...the answer is C

lift is a function of coefficient of lift (Lift=CoefficientofLift*Density*Velocity^2*SurfaceArea/2)

induced drag (drag due to lift) is a function of CL^2, so at CL=0, induced drag = 0


the coefficient of lift of ANY symmetric airfoil at zero AOA is zero, hence, no lift will be produced, and yes, the aircraft will fall out of the sky.

just cuz it's a wing doesn't mean it produces lift....think about horizontal stabilizers, which are generally at negative angles of attack to generate negative lift (produces a moment to create longitudinal stability as the aircraft pitches)

That's about all I learned at AFIT too. I'm not going to get into the weeds about figuring out which answer is correct, but I believe A is the most correct answer. I kinda like the reasoning given above that both sides of the airfoil will create lift, but will cancel eachother out producing a zero net lift. Since there is lift, there has to be a coefficient of lift. Therefore, there is induced drag. You just gotta make yourself believe that there is lift being produced. That's not hard to do. The air has to move faster over the top and the bottom of the airfoil than the air in front of or behind the airfoil.

And your last statement makes no sense. You said a wing doesn't have to produce lift, then you tried to back it up by mentioning horizontal stabilizers, which are in fact, wings, that produce, lift. Interesting.

 

[brd2881]

I think about this stuff every time I fly...gives me a warm and fuzzy.......no, really...I use it all the time...

 

[kray1395]

OK, let me put it to rest. The answer is A, and here's the reasoning given by a dude who knows what he's talking about.

- Cambered airfoils produce lift at zero AOA, which proves that the
"Newton" explanation is wrong? INCORRECT

Incorrect. Air has mass, and this means that it has inertia. Because
of inertia, an exhaust port can produce a narrow jet of air, yet an
intake port cannot pull a narrow jet inwards from a distance. This
concept applies to wings. When a cambered airfoil moves forwards at
zero AOA (Angle of Attack,) air moves up towards the leading edge, and
air also flows downwards off of the trailing edge. The air which
flows downwards behind the wing keeps moving downwards, and so the
rear half of the wing controls the angle of the downwash, while the
leading edge has little effect. (In aerodynamics, this is called the
"Kutta Condition.") In a cambered wing at zero AOA, the rear half of
the wing behaves as an airfoil with positive AOA. On the whole, the
cambered airfoil BEHAVES as if it has a positive AOA, even though the
geometrical angle of attack is zero.

 

[Oh-58Ddriver]

I'll premise this by saying I also have an MS in Aero, and my initial reaction was C. But I think I talked myself out of it, and here is why:

The confusion lies in the question - it states a symmetrical airfoil, not a symmetrical wing. Induced drag is drag due to lift mostly caused by wingtip vortices. Picture a wind tunnel with an airfoil that extends from one side to another, so it essentially does not have wing tips. It is an airfoil section, not a wing.

We also must assume the question means "a symmetrical airfoil at zero angle of attack moving forward through the air."

But, A is also the wrong answer. For the geeks out there, think of the graph of coefficient of lift versus angle of attack for a symmetrical airfoil: it crosses through (0,0). Meaning, no angle of attack, no coefficient of lift. And, obviously, no coefficient of lift, no lift.

Irish guy is right, B is incorrect because form drag would be present if the airfoil was "moving." Form drag is caused by flow separation creating a pressure differential. On a symmetrical airfoil, it is very small as the flow normally does not separate until very close to the trailing edge. But, its still there.

D is wrong because there is a total aerodynamic force caused by said drag. In this case, it would be directly aft.

So there is no right answer on that list. :banghead_

Your book sucks.

EDIT: Unless of course we are not supposed to assume it is moving. In that case, the only force acting on it is gravity. The total aerodynamic force would be directed downward as weight. Weight is opposite of lift, and can also be considered negative lift. So, if it was not moving, A could be the answer: "Lift, but less than positively cambered airfoil." It would be a lot less, because it would be negative...

If there were questions this hard on the ASTB, there would not be a lot of pilots...

 

[Chubby]

Induced drag is drag created as a result of the wing developing lift. A wing like that at a zero degree angle of attack would produce no lift, so no induced drag is possible.

Spanwise flow will create induced drag. Maybe?

 

[IrishNavy05]

The confusion lies in the question - it states a symmetrical airfoil, not a symmetrical wing. Induced drag is drag due to lift mostly caused by wingtip vortices. Picture a wind tunnel with an airfoil that extends from one side to another, so it essentially does not have wing tips. It is an airfoil section, not a wing.

That is exactly the reason that no induced drag is created. BECAUSE it is an airfoil, no wingtip vortices are created so there is no induced drag. there is no lift vector for the vortices to shift backwards

Everything else you are right on, but I am convinced C is the correct answer...

also, to the guy who talked about my horizontal stabilizer example, the existence of negative lift proves the existence of zero lift, because, as OHdriver said, the CL versus AOA curve goes through (0,0)

 

[sevenhelmet]

BECAUSE it is an airfoil, no wingtip vortices are created so there is no induced drag. there is no lift vector for the vortices to shift backwards

So your argument is that a wing isn't an airfoil?


This is a semantics issue. I believe the answer is "A", although the question is very badly written. The ONLY type of airfoil that doesn't produce induced drag w/ lift is an 'infinite' or 'endless' airfoil as in wind tunnel testing. There isn't enough information in the question to assume it is infinite, so you aren't able make any assumptions about induced drag. That rules out choice 'C'. An airfoil is not necessarily a wing, but it might be; all wings are airfoils.

Symmmetric airfoils do produce lift at 0 AOA, but the NET lift is zero because the upper and lower surfaces produce equal amounts of lift. A positively cambered airfoil will produce some amount of NET lift at 0 AOA because the top surface produces more lift than the bottom surface. So a symmetric airfoil DOES produce lift @ 0 AOA, it's just not useful or measurable as a net upward/downward force. There WILL be a backward force on the airfoil (drag), some of which is caused by the lift produced on the top & bottom of the airfoil (induced drag.) This all is true WHETHER OR NOT the airfoil is "infinite." That's why I picked 'A' although I reiterate, that question is crap and should be reworded or thrown out.

 

[kray1395]

also, to the guy who talked about my horizontal stabilizer example, the existence of negative lift proves the existence of zero lift, because, as OHdriver said, the CL versus AOA curve goes through (0,0)

That is an assumption you can't prove at this point. CL must go through 0, I agree. But you can't assume that it equals 0 at the same point AOA equals 0. You guys are arguing against an expert, who has already proven that a symmetrical airfoil, at a zero angle of attack, does produce lift. It is due to the flow of air off of the back of the airfoil flowing downwards. Just google it.