I am flabbergasted that this debate still continues in the new millenium.
The simple fact is that you fly in air. In a steady state wind (whether that wind is zero or 1 million km/hr) turning in any direction does not effect airspeed.
Think of it this way.
You are sitting 100 feet above the earth in a hovering helicopter.
Someone puts some cloud below you, a big cloth below you, stops you from looking down whatever.
Is it possible to determine your groundspeed?
Could you turn your aircraft in such a manner as to be able to determine the wind?
The answer is no.
Now to one HUGE misconception
from avi8tor
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It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant.
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This is simple hilarious.
For a start let's since this is supposed to be a technical forum let's get the terminology correct.
Inertia
1) Inertia is the property of bodies that have mass to resist acceleration. F=MA is a direct consequence of inertia.
Inertia is not dependent upon anything except your mass. Your ground speed being zero or 1 million km/hr does not effect inertia (in classical physics - let's forget about relativistic effects and relativistic mass).
Maybe avi8tor is thinking momentum - which is dependent upon velocity and mass. Velocity is a relative quantity (i.e. relative to what you are measuring it against). As we all know our airspeed and groundspeed are different.
However even in that case the important quantity is airspeed. Your momentum relative to the air you are flying in is the important quantity.
Now it makes no difference AT ALL to your performance what speed the fluid you are travelling in is doing to some other observer. If the fluid is NOT accellerating (there are no bumps, no turbulence etc) it is in fact impossible to tell what speed that fluid is doing to someother observer.
Remember all motion is relative. Your flying performance is purely a function of your speed in that fluid, the fact that that fluid is moving with respect to something else is completely irrelevant.
Avi8tor, why on earth would you think your inertia or even your motion relative to the ground is more important in flying performance terms than say your motion relative to pluto, or sirius. The air around you is going at about a million miles and hour relative to the sun - why isn't your inertia dependent upon that?
The only time your motion relative to the ground matters in any shape or form is when you want to stop on it, or leave it.
Now two other points.
1)It is rarely a steady wind (no acceleration in any direction) close to the ground.
2)When close to the ground perception often over rules.
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COURSE IT MATTERS, that the whole crux of the question!!! The Super Cub will have to attain a groundspeed of 85kts by 90-degrees through the turn!! IF the conventional rate/radius maths applies, it would have had to accelerate from 0 to 85kts in 6,5seconds!!!
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It will also have attained a plutospeed of around 55 000km/hr in the same time. My god it must have a big engine.
Groundspeed no more effects your flying performance than does your pluto speed. Why would you think your change in momentum with respect to the ground is any more important than a change in momentum with respect to the sun?
btw a 90 degree turn in 6.5 seconds is a bit quick.