Question on Lift.

[fish314]

No, you'd just get another starting vortex, just strong enough to satisfy the Kutta condition, moving the stagnation point to the rear of the airfoil. Real world, you probably have a constant flow of starting vortices downstream, as circulation varies due to minute AoA changes. (Haven't seen that last stated anywhere, but it stands to reason.)

Hmm... That makes sense. Sounds like that'd be really difficult to find, even in fairly controlled conditions. Probably winds up masked by all kinds of other effects and minor turbulences/disturbances.

Actually here's a paragraph from wikipedia's (I know, I know...) entry on the kutta condition that sort of verifies your post:

"Whenever the speed or angle of attack of an airfoil changes there is a weak starting vortex which begins to form, either above or below the trailing edge. This weak starting vortex causes the Kutta condition to be re-established for the new speed or angle of attack. As a result, the circulation around the airfoil changes and so too does the lift in response to the changed speed or angle of attack."

 

[tgrayson]

speed or angle of attack of an airfoil changes there is a weak starting vortex which begins to form,

One thing I read a number of years ago in an old aerodynamics book is that you can stop the float of an airplane in ground effect by pumping the elevator. The book didn't explain how that worked, but I hypothesized that since every AoA change created a new starting vortex, each vortex required energy to be transmitted from the airframe to the air in order for it to form.

 

[PGT]

I'm reading Stick and Rudder, according to the book, we have lift because we have positive angle of attack.

 

[fish314]

I'm reading Stick and Rudder, according to the book, we have lift because we have positive angle of attack.

That's only true for a symmetric airfoil. Most aircraft have cambered airfoils that produce some lift even at zero or slightly negative angles of attack.

AOA definitely plays a part... a big part, actually. But that statement by itself doesn't explain any of the "whys" and "hows."

 

[shdw]

I'm reading Stick and Rudder, according to the book, we have lift because we have positive angle of attack.

Wonderful book. From what I hear, the author was in charge of much of the training curriculum written for the military in WWII. I did not, however, go about verifying this. Either way, enjoy! He was one amazing inspiration for me.

As fish said, only for symmetrical airfoils. Most jets are a great example.

 

[tgrayson]

Most jets are a great example.

I'd be surprised if any non-aerobatic jets have symmetric airfoils.

 

[shdw]

I'd be surprised if any non-aerobatic jets have symmetric airfoils.

Are you sure? I don't actually know, I assumed since the aero professor said jets don't create lift when rolling down the runway that they had to be at 0 AOA with symmetrical wings. Though I guess they could be set slightly negative with non symmetrical wings?

 

[tgrayson]

I assumed since the aero professor said jets don't create lift when rolling down the runway that they had to be at 0 AOA with symmetrical wings. Though I guess they could be set slightly negative with non symmetrical wings?

That reasoning is based on the assumption your professor was correct, and I'm skeptical that he was. According to Raymer, p. 66, the typical starting assumption of the design of a transport category aircraft is that the angle of incidence will be about 1.

Later, on page 564, he states "The lift coefficient is based on the wing angle of attack on the ground (measured to the zero lift angle) and is typically less than .1 unless large takeoff flaps are deployed." Few things about that statement: When he refers to the AoA with respect to the zero lift angle, he's talking about the absolute angle of attack. This is identical to the normal definition of AoA in a symmetrical airfoil, but is different in a cambered one, since you'll need a negative AoA to get zero lift. A positive absolute AoA might still be a negative one using the normal definition of AoA. Second, he says that the lift coefficient during the ground roll is typically less than .1. Of course, zero qualifies as less than .1, but it still rather suggests that he means that the lift coefficient is small, but positive. And finally, he points out that this is the case absent large flap deflections. Since I gather that most jets have some flaps deployed for takeoff, that makes it even more unlikely that the takeoff roll lift coefficient is zero.

 

[shdw]

When he refers to the AoA with respect to the zero lift angle, he's talking about the absolute angle of attack. This is identical to the normal definition of AoA in a symmetrical airfoil, but is different in a cambered one, since you'll need a negative AoA to get zero lift. A positive absolute AoA might still be a negative one using the normal definition of AoA.

Interesting, and leaves me one question. Absolute angle of attack, is that similar in definition to the scientific definition of absolute zero? Basically, with an absolute AOA of zero no lift is created, at least this is what the google sources are saying. In a cambered wing this might be say -2 degrees actual AOA, which means an absolute AOA of 1 in this wing would be -1 actual AOA?

 

[tgrayson]

Interesting, and leaves me one question. Absolute angle of attack, is that similar in definition to the scientific definition of absolute zero?

Yes, except that in this case, it's actually possible to achieve a zero AoA, unlike absolute zero.

Basically, with an absolute AOA of zero no lift is created, at least this is what the google sources are saying.

Correct by definition.

In a cambered wing this might be say -2 degrees actual AOA, which means an absolute AOA of 1 in this wing would be -1 actual AOA?

Correct again. Absolute AoA is often used in stability calculations because it makes the math easier; otherwise, you have to plug into the equations something along the lines of y=mx+b.

 

[Minuteman]

Later, on page 564, he states "The lift coefficient is based on the wing angle of attack on the ground (measured to the zero lift angle) and is typically less than .1 unless large takeoff flaps are deployed."

Is it correct to assume the relationship between the zero-lift AoA and a geometric AoA (e.g. chord line) will vary with the influence of ground effect? I would expect the slope of the Cl-alpha line to change to usually require both a larger geometric AoA and a relatively larger absolute AoA to obtain a given Cl when the ground is not affecting airflow around a wing.

 

[tgrayson]

Is it correct to assume the relationship between the zero-lift AoA and a geometric AoA (e.g. chord line) will vary with the influence of ground effect? I would expect the slope of the Cl-alpha line to change to usually require both a larger geometric AoA and a relatively larger absolute AoA to obtain a given Cl when the ground is not affecting airflow around a wing.

Good question. Yes, I think you're correct. I don't see the zero lift line changing with ground effect, because when no lift is generated, there are no wingtip vortices and thus no ground effect. But when lift is being generated, ground effect essentially increases the aspect ratio of a wing, steepening the lift curve, so for every lift coefficient, you have a smaller required geometric AoA. Assuming the zero lift line doesn't change, then the absolute AoA would be smaller too.