Wingtip Vortices

[juxtapilot]

Hi all,

Right to the point: If a wing is generating any lift then it is producing wingtip vortices. Is that correct? So if an airplane is on the ground taxiing, the wing is producing some lift and thus generating wingtip vortices. To go just a litte farther; if the aircraft is tied down and it has a little wind blowing over the wings (even 5 knots) then the airplane wing is generating wingtip vortices. I understand that the vortices are very weak, but they are there. That is my understanding. Is it correct?

Thanks!

[Crockrocket94]

From what I know yes.

[SmitteyB]

Absolutely. You're right about them being weak, probably hardly noticeable in a wind tunnel, but they're there.

[mhcasey]

Unless the wing is infinitely long.

[juxtapilot]

Thanks!

[B767Driver]

Originally Posted by juxtapilot

Hi all,

Right to the point: If a wing is generating any lift then it is producing wingtip vortices. Is that correct? So if an airplane is on the ground taxiing, the wing is producing some lift and thus generating wingtip vortices. To go just a litte farther; if the aircraft is tied down and it has a little wind blowing over the wings (even 5 knots) then the airplane wing is generating wingtip vortices. I understand that the vortices are very weak, but they are there. That is my understanding. Is it correct?

Thanks!

At five knots I doubt that the boundary layer can adhere throughout the chord of the wing...therefore no vortex would be created. The boundary layer would separate early and any bound vortex would disintegrate.

At some airflow velocity this would change....I'm guessing near the stalling speed of the airplane. This would ensure that the boundary layer doens't separate and the bound vortex makes it all the way to the trailing edge...where it can morph into a trailing vortex at the wingtips.

Basically, I'd think the airflow would need more energy than 5kts to produce any defined vortex action. Remember from your wake turbulence avoidance procedures....vortices are not produced until the airplane lifts off.

[tgrayson]

Originally Posted by B767Driver

therefore no vortex would be created.

I agree with your conclusion, but via different reasoning. Reference Carpenter on p. 173, who discusses the rear stagnation point on the top of the wing. At low velocity, the airflow can negotiate the trailing edge; without the separation, there's no circulation and hence no lift.

vortices are not produced until the airplane lifts off.

Seems unlikely, as long as the wing is at some positive lift coefficient. It's probably quite small during the takeoff run and hence the vortices are weak, but it's not due to a velocity effect.

[B767Driver]

Originally Posted by tgrayson

, but it's not due to a velocity effect.

I agree about the lack of trailing edge separation. At 5 kts, my point of contention was that I wasn't sure that the energy of the airflow could even make it that far due to skin friction. Maybe it can.

I didn't follow you on the above quote. Seems that the ability to generate a vortex would be highly dependent upon the velocity/energy of the airflow.

[tgrayson]

Originally Posted by B767Driver

I agree about the lack of trailing edge separation. At 5 kts, my point of contention was that I wasn't sure that the energy of the airflow could even make it that far due to skin friction. Maybe it can.

I don't know. There might be different speeds at which both happen. Separation will be caused by adverse pressure gradients and a very low speeds, I don't know how severe the gradient would be compared to the momentum of the airflow.

I didn't follow you on the above quote. Seems that the ability to generate a vortex would be highly dependent upon the velocity/energy of the airflow.

Agreed, but for an aircraft at rotation velocity, the airflow has quite a bit of energy, particularly a transport category aircraft. The insignificance of the vortices must be due to the lack of lift being generated due to a very small AOA, producing very small pressure differences between the bottom and top of the airfoil.