Originally Posted by
pattern_is_full
Of course, any frontal airflow over a wing will produce lift (assuming the wing is cambered and not at a negative AoA). The lift produced scales with half of the square of the airspeed (the equation is a bit more complicated, but L = (1/2 v^2 ) is still in there).
A little messing around with a decent aerodynamics sim (which has the advantage of being able to give me "weight on wheels" as a value) shows me that with a B717 at MTOW 110,000 lbs, flaps 13:
Lined up in a 9-knot headwind, the wind alone was producing 200 lbs of lift. (the weight on wheels is 200 lbs less than actual weight).
Accelerated to 50 kts IAS, the lift was 7000 lbs
At 100 kts the lift was 17000 lbs
At 150 kts the lift was 52000 lbs
And of course at lift-off the lift must be at least the weight of the aircraft - ~110,000 lbs minus a little fuel weight burned on the TO run. I say "at least" because at rotation and shortly therafter, when the vertical speed is accelerating from, say, zero to 1800 fpm, lift > weight.
One can say the lift is increasing rapidly with speed - but equally, will still be a rather low percentage of the final lift needed to get off the ground - until AoA is increased in the rotation, which at least doubles the lift (and thus the force behind the vortex, although there may be a proportional factor in there as well).
Clear a helicopter to pass behind that (hypothetical) B717 at the point the B717 was passing 100 kts, and the vortex may be only about the same as that generated by a B1900 turboprop at lift-off (~17000 lbs, also at ~100 kts, coincidentally).
Note also that ground-effect has a suppressing effect on wing vortices - they are generally about 30% less forceful than at the same speed/weight/etc. in free air.
you also have a fraction of total lift as a result of the downward deflection of the thrust vector. During rotation you will definitely need more lift than the weight of the aircraft due to the down force at the tail.
You can can actually see the starting vortex by moving your hand through the bath. You will see a little whirlpool emerge in the opposite direction to the bound vortex on your “bath hand-wing”.