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My GA theory textbook (which covers fixed wing up to and including EASA PPL) has some suggestions on taking off and landing light aircraft soon after large aircraft (besides avoiding it, if possible), intended to reduce the risk from wingtip vortices. These suggestions are to,

  • when taking off
    • ...after a heavy aircraft taking off, to lift off at a point on the runway before the heavier aircraft lifted off
    • ...after a heavy aircraft landing, to lift off at a point on the runway later than where the heavier aircraft's nose wheel touched the runway
  • when landing
    • ...after a heavy aircraft taking off, to land as early as possible on the runway
    • ...after a heavy aircraft landing, to land ahead of the point on the runway where the large aircraft touched down

When I asked about the why of these suggestions, the answer was that wingtip vortices form because the wings are generating lift, and when the aircraft's wheels are on the ground, the wings don't generate (or aren't generating) lift so no wingtip vortices form.

While that might be true in a "rule of thumb" sense, and a good generalization of these rules to serve as a memory aid, I want a bit more than that. The way I understand it,

  • Any airfoil will generate some amount of lift as long as there is ambient air movement around it; "lift" (as opposed to net positive vertical lift, which isn't being generated when landing anyway because the aircraft is descending) isn't something that suddenly starts being generated when one or more of the aircraft's wheels are separated from the ground
  • An aircraft can move at significant speed relative to the surrounding air even when all of its wheels are on the ground, causing likewise significant speed of movement of the wings through the ambient air

It appears to me that wingtip vortices should form at any time the aircraft (or rather its wings) is moving relative to the surrounding air; whether that's because the aircraft is standing still and there's a wind, or because there's no ambient air movement and the aircraft is moving, or a combination of the two. It also appears to me that the strength of the vortices should be in some way proportional to the amount of lift being generated.

Consquently, it seems to me that wingtip vortices should form at every point during the takeoff or landing roll (as well as taxiing and other movement on the ground) at which the aircraft is moving relative to the surrounding air, and increase in strength with (but not necessarily linearly with) the indicated airspeed of the aircraft.

It seems weird that the air flowing near the wingtips would somehow "know" anything about the position of the aircraft's wheels with respect to the ground, let alone that of any specific wheel (such as the nose wheel).

All that just as the lead-in to my question: am I missing something, or is this a case of the textbook (and teacher) presenting the simple case so as to not overwhelm students?

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You are absolutely right. The vortices form whenever the wings produce lift. Even if the lift is not enough to let the aircraft fly.

Of course it is necessary to have a flow across the wing, and with very low speeds (like taxiing) the vortex is likely to be very small.

But,
it seems your textbook is trying to establish a save way to avoid the vortices of previous aircrafts. And since the strength of the vortex is related to the lift which is related to the air-speed and angle-of-attack this rule of thumb is quite practical. Especially since landing is a highly transient process which is very likely to result in a significant reduction of vortex strength.

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    $\begingroup$ Very close to the ground wing tip vortices are also less important because they dissipate much faster due to the ground effect. The ground basically 'blocks' them. $\endgroup$ – Orbit Jan 20 '18 at 10:45
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You are correct, but your textbook is more correct. Here's why:

Your reasoning is that lift is a function of airspeed, which is true. However lift is also a function of angle of attack. This is why large aircrafts take off the way they do: they accelerate on the ground with a very small AoA, hence little lift. Since lift increases with drag, by accelerating when producing minimum lift, drag is reduced, and the acceleration to takeoff speed can be achieved on a shorter runway length. By pitching up at the right moment, lift is dramatically increased, and lift off is accomplished.

The reverse is true when landing. The purpose of the landing rollout is to absorb the kinetic energy of the plane in the brakes. Having the wings create lift reduces the friction between the tires and the ground, which does no good at all. Therefore lift is dumped as soon as practical during the landing rollout.

It should be noted that, one technique to achieve a short field takeoff in a light aircraft is to accelerate with no flaps. Then, when the takeoff speed is reached, quickly apply a notch of flaps and the plane will lift off the ground. This technique is usually not taught to new pilots because it requires skillful coordination, but it illustrates that lift is suddenly increased at the point of lift off.

Another way to phrase it would be: when the wheels are on the ground, the amount of lift generated is small and usually within the tolerances of a light aircraft. But you are correct in saying that lift is still generated as long as there is relative movement between the wing and air.

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When the aircraft is on the ground, the wheels are supporting it. When it is in the air, the wings are generating the upward force necessary for lifting it. So at take-off, there is a sudden transition from wheels to wings in the lifting function.

Any airfoil will generate some amount of lift as long as there is ambient air movement around it;

That depends on the angle of attack:

  • symmetrical airfoils generate zero lift at zero AoA;
  • a-symmetrical airfoils generate zero lift at a slightly negative AoA;
  • airliner wings have wing twist: the area near the tip can have negative AoA when on ground;
  • the overall AoA of aircraft wings with short nose gear is negative when on ground.

Iron birds at an aircraft factory that go through their fatigue cycles simulate the starting cycle by actuating the wing up/down during taxiing (from bumping on the taxiway), then during the takeoff run the bumping intensifies, then at the take-off nose-up motion the wings bend upwards dramatically - one can hear them groan and creak while they bend.

"lift" (as opposed to net positive vertical lift, which isn't being generated when landing anyway because the aircraft is descending) isn't something that suddenly starts being generated when one or more of the aircraft's wheels are separated from the ground.

Strictly speaking there will be some downward force generated by the wings, but the transition from wheels to wings is so dramatic that for practical purposes we can indeed state that wingtip vortices are only significant with the wheels off of the ground.

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