Downwind Turns
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It's all down to relative movement as far as I am concerned.
The Kinetic Energy used in the examples above is all measured relative to a point on the ground. i.e. if you are not moving relative to the ground the Kinetic Energy is Zero. But the ground is moving at 1000 miles/hour (depending on latitude) around the earth.
The earth is flying round the sun at an ever faster rate.
The sun is moving around the Milky Way Blah Blah Blah. You get the picture.
You should only be concerned about the steady state environment the aircraft is in i.e. the air.
That's my view anyway
FIS
The Kinetic Energy used in the examples above is all measured relative to a point on the ground. i.e. if you are not moving relative to the ground the Kinetic Energy is Zero. But the ground is moving at 1000 miles/hour (depending on latitude) around the earth.
The earth is flying round the sun at an ever faster rate.
The sun is moving around the Milky Way Blah Blah Blah. You get the picture.
You should only be concerned about the steady state environment the aircraft is in i.e. the air.
That's my view anyway
FIS
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Bookworm has it right.
I could spout on about Inertial Reference Frames, but let me pose a simple test. In a steady wind of 25 knots or so, trim the aircraft (helo or airplane) into a steady state banked turn, constant power, altitude and airspeed. Watch it for a few circles, and try to see any climbs or descents during its circles.
There are none. None.
The downwind turn is right up there with extra gunmen on the grassy knoll, all myth.
When bad pilots see the groundspeed increase, they pull back on the stick as they turn downwind, they then lose speed and stall, and Darwin gets another point.
I could spout on about Inertial Reference Frames, but let me pose a simple test. In a steady wind of 25 knots or so, trim the aircraft (helo or airplane) into a steady state banked turn, constant power, altitude and airspeed. Watch it for a few circles, and try to see any climbs or descents during its circles.
There are none. None.
The downwind turn is right up there with extra gunmen on the grassy knoll, all myth.
When bad pilots see the groundspeed increase, they pull back on the stick as they turn downwind, they then lose speed and stall, and Darwin gets another point.
Do a Hover - it avoids G
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The downwind turn is right up there with extra gunmen on the grassy knoll, all myth.
Agreed up to a point. The maths/physics is indisputable as you and others have made clear. BUT and it is a big but: Gusts are another matter.
Things change if you are flying low and slow and are hit by a gust up your backside. Gusts are the issue here, not 'steady' winds.
In a strong low level wind there WILL be gusts. How big varies and how bad depends on how slowly your machine normally flies.
When the gust speed reaches say 50% of your normal flying speed then they can be a pretty hairy influence if you take one up the backside following a turn.
Personally I think this is still something for GA pilots to keep in the back of their mind when flying on a very windy (and therefore gusty) day.
JF
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John Farley, I agree with you, but it does raise the question:
Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.
The aeroplane flying downwind would only be worse of if a gust of wind is sharper than a lull. I believe this to be the case, but is there any evidence to support it? Any micrometeorologists here?
Mark
Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.
The aeroplane flying downwind would only be worse of if a gust of wind is sharper than a lull. I believe this to be the case, but is there any evidence to support it? Any micrometeorologists here?
Mark
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Interesting how this always brings up differing opinions from all levels... To me, my simple fizzics say that it shouldn't matter, because my simple brain can cope with the 'parcel of air' theory.
But... there must be something out there that makes us all, including me, wonder. Here's a hypothesis:
Most of the 'downwind turn' bingles that I've heard of have been when someone's turned downwind early after taking off in a strong crosswind. If, when you are turning away from the wind, you are climbing through the height where the wind is increasing as it is no longer being slowed by the ground , then voila, you have a temporary decrease in airspeed. Admittedly, this is a show-off manoeuvre, but it does happen, and it does give the possibility of an illusion that can cause us to doubt the 'don't lose speed/height in a downwind turn' mantra.
If it's not this, then there must be some kind of illusion out there that's keeping us wondering. I used to fly hang gliders, and when showing off, groundspeed could change from 45 to 5 mph (and vice versa) in a trice: you don't go zooming up and down doing that.
But... there must be something out there that makes us all, including me, wonder. Here's a hypothesis:
Most of the 'downwind turn' bingles that I've heard of have been when someone's turned downwind early after taking off in a strong crosswind. If, when you are turning away from the wind, you are climbing through the height where the wind is increasing as it is no longer being slowed by the ground , then voila, you have a temporary decrease in airspeed. Admittedly, this is a show-off manoeuvre, but it does happen, and it does give the possibility of an illusion that can cause us to doubt the 'don't lose speed/height in a downwind turn' mantra.
If it's not this, then there must be some kind of illusion out there that's keeping us wondering. I used to fly hang gliders, and when showing off, groundspeed could change from 45 to 5 mph (and vice versa) in a trice: you don't go zooming up and down doing that.
I feel you are perfectly right ZAC21, I don't know the answer neither.
I just try not to be "caught back" by my down wash at low speed...every time I went down by surprise, that was the case....better to learn that fast and remember it when flying logging or crop spraying.
I just try not to be "caught back" by my down wash at low speed...every time I went down by surprise, that was the case....better to learn that fast and remember it when flying logging or crop spraying.
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Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.
Once the airplane is in relatively steady state (straight & level, upwind or downwind), few pilots have problems in this context. The cited problems usually occur while in the turn to downwind, with the pilot trying to force the airplane to fly a specific track over the ground instead of a simple climbing, constant-bank, constant- (though slightly increased) airspeed turn in the air. It is the reduced stall margin in the turn (if the airspeed is not increased), combined with the gust factor, that will get some into trouble.
Do a Hover - it avoids G
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Why should an aeroplane flying downwind at low level on a gusty day, copping a gust up the backside, be worse of than when it's flying into wind and suffers a lull in headwind.
As Intruder says it is the same, providing the shape of the gust is the same. But I never said it was different did I? The thread is about downwind turns isn't it?
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The other point
I am glad that this issue has caused some debate.
I have read the discussion here gusts and ground features, which is a TOTALLY different issue.
The devil here is in the detail. As for the fact the earth is moving though space at a zillion miles an hour, is all quite true. But so is everything around us. For simplicity, we can look at forces require to change things, in relation to the surface of the earth. I know this is simplistic!!!!
I think Book Worm has Newton I & Newton II a little confused. He mentions the momentum, but the aircraft with 60KIAS in the 60kt headwind has ZERO momentum, it only has inertia.
Ascend Charlie is quite right, the aircraft that turns out of wind has overcome its INERTIA and gain MOMENTUM some how. If this was a Super Cub, at 60KIAS a 60-degree bank turn is totally sustainable. This would equate to a rate 4.5 turn, and a 180-degree turn would take 13 seconds. Not even a Super Cub could gain 120KTS in 13 seconds. That bit of science is easy.
The stone on a string example seems to be a question of conservation angular momentum.
The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!
Does anybody have any ideas if the conventional rate/radius maths applies? Conventional logic tells me that with no centrifugal force to overcome, incredibly high rate/low radius turns are possible!!! Also, what effect would these unbalanced forces have on angle of attack?
I have read the discussion here gusts and ground features, which is a TOTALLY different issue.
The devil here is in the detail. As for the fact the earth is moving though space at a zillion miles an hour, is all quite true. But so is everything around us. For simplicity, we can look at forces require to change things, in relation to the surface of the earth. I know this is simplistic!!!!
I think Book Worm has Newton I & Newton II a little confused. He mentions the momentum, but the aircraft with 60KIAS in the 60kt headwind has ZERO momentum, it only has inertia.
Ascend Charlie is quite right, the aircraft that turns out of wind has overcome its INERTIA and gain MOMENTUM some how. If this was a Super Cub, at 60KIAS a 60-degree bank turn is totally sustainable. This would equate to a rate 4.5 turn, and a 180-degree turn would take 13 seconds. Not even a Super Cub could gain 120KTS in 13 seconds. That bit of science is easy.
The stone on a string example seems to be a question of conservation angular momentum.
The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!
Does anybody have any ideas if the conventional rate/radius maths applies? Conventional logic tells me that with no centrifugal force to overcome, incredibly high rate/low radius turns are possible!!! Also, what effect would these unbalanced forces have on angle of attack?
I think Book Worm has Newton I & Newton II a little confused. He mentions the momentum, but the aircraft with 60KIAS in the 60kt headwind has ZERO momentum, it only has inertia.
That is entirely the point, Avi8tor. Newton's laws concern velocity, which is a vector quantity. A 180 degree turn requires the same change in momentum, regardless of the steady wind (and in fact, regardless of the frame of reference).
Newton's laws govern changes in velocity and momentum, not changes in speed.
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The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!
Also, if it has 0 momentum, why does it need power to remain aloft and maintain that "0" momentum? It is because there are external forces acting on it from the airflow.
If you are above a smooth cloud deck and do not have GPS, INS, or DME, you do not know your ground speed, nor does that ground speed affect your flight condition. The airplane also does not know or care about its ground speed -- regardless of cloud cover -- because it can only "see" the air around it. When aloft, the airplane's frame of reference is the packet of air surrounding it -- NOTHING else. All forces on the airplane are with reference to that air packet.
If the earth happens to pull a mountain or building in front of the airplane, that's a whole different story, because the airplane will not be aloft for long...
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I think there are 2 things here
1. Newton didn't lie. I think everyone agrees that if you are doing 60 kias with a 60 knot headwind and that INSTANTLY changes to a 60 knot tailwind, by either turning the wind or turning the aircraft then you have a little problem. Thats the theoretical application.
2. The practical application during most flight is that if the aircraft is flown properly during the turn then it will have time to overcome its inertia during the turn.
You can't argue with Newton but with airmanship you can out manouver him. I think you guys are aguing two sides of the same coin.
1. Newton didn't lie. I think everyone agrees that if you are doing 60 kias with a 60 knot headwind and that INSTANTLY changes to a 60 knot tailwind, by either turning the wind or turning the aircraft then you have a little problem. Thats the theoretical application.
2. The practical application during most flight is that if the aircraft is flown properly during the turn then it will have time to overcome its inertia during the turn.
You can't argue with Newton but with airmanship you can out manouver him. I think you guys are aguing two sides of the same coin.
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And again
I will tackle this all one point at a time:
Bookworm: -
Newton I concerns inertia (state of rest or state of motion) and Newton II covers momentum. Sorry, you are wishing away the physics by saying the momentum is the same regardless of the wind!!
We all agree that the momentum with 60kt headwind/60KIAS is ZERO. If the wind was zero the aircrafts momentum would be the mass X TAS (groundspeed). These are clearly different situations!!!
Intruder: -
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!!!
(Quick explanation for those not in the picture: By 90-degrees through the turn the super cub will have zero headwind component therefore TAS = GS = 60kts. Also it would have had to attain 60kts in the direction of the wind. These two are at vectors are at right angles therefore 60/.707 = 85. If you don’t believe me, draw the picture.)
Totally agreed, all normal thrust/lift/weight/drag remain the same. I think we are in agreement that angle of attack and aerodynamic issues are NOT groundspeed dependant.
Explain your statement!!! We have CLEALY demonstrated that the Physics is different, and it cannot be wished away!!
If I am wrong, show me the maths!!!
Bookworm: -
That is entirely the point, Avi8tor. Newton's laws concern velocity, which is a vector quantity. A 180-degree turn requires the same change in momentum, regardless of the steady wind (and in fact, regardless of the frame of reference).
We all agree that the momentum with 60kt headwind/60KIAS is ZERO. If the wind was zero the aircrafts momentum would be the mass X TAS (groundspeed). These are clearly different situations!!!
Intruder: -
First, as soon as the airplane starts the turn, groundspeed will change. But that doesn't matter anyhow...
(Quick explanation for those not in the picture: By 90-degrees through the turn the super cub will have zero headwind component therefore TAS = GS = 60kts. Also it would have had to attain 60kts in the direction of the wind. These two are at vectors are at right angles therefore 60/.707 = 85. If you don’t believe me, draw the picture.)
Also, if it has 0 momentum, why does it need power to remain aloft and maintain that "0" momentum? It is because there are external forces acting on it from the airflow
The airplane also does not know or care about its ground speed.......
If I am wrong, show me the maths!!!
Last edited by Avi8tor; 22nd Feb 2004 at 14:09.
With regard to gusts and turbulence, into wind or downwind, what you are discussing can also be experienced in a boat. Try rowing (at the same speed) into the waves or with the waves There shouldn't be any difference in the ride. But there is.
OK, one more time.
Newton's first law is a special case of the second law, which says that the rate of change of momentum of a body is proportional to the net force on it. No force, no change in momentum.
When you talk, rather vaguely, about a body "overcoming its inertia", you mean changing its momentum, which requires a force, according to Newton's second law.
In the former case (with the headwind) the change in momentum in a turn downwind is zero to 2 X mass X TAS, i.e. - 2 X mass X TAS.
In the latter case (still air) the change in momentum in a turn similar 180 degree turn is mass X TAS to - mass X TAS, i.e. - 2 X mass X TAS.
What the aircraft has to do to "overcome its inertia" in each case is to change its momentum by 2 X mass X TAS. This requires the same aerodynamic forces.
Physics works in any inertial frame of reference. You can consider the problem in a reference frame moving with a jet on an airway flying overhead if you like. The absolute values of momentum are different but the change in momentum is the same. There is no absolute value of momentum, the only thing relevant to Newtonian physics is its change.
Newton I concerns inertia (state of rest or state of motion) and Newton II covers momentum. Sorry, you are wishing away the physics by saying the momentum is the same regardless of the wind!!
When you talk, rather vaguely, about a body "overcoming its inertia", you mean changing its momentum, which requires a force, according to Newton's second law.
We all agree that the momentum with 60kt headwind/60KIAS is ZERO. If the wind was zero the aircrafts momentum would be the mass X TAS (groundspeed). These are clearly different situations!!!
In the latter case (still air) the change in momentum in a turn similar 180 degree turn is mass X TAS to - mass X TAS, i.e. - 2 X mass X TAS.
What the aircraft has to do to "overcome its inertia" in each case is to change its momentum by 2 X mass X TAS. This requires the same aerodynamic forces.
Physics works in any inertial frame of reference. You can consider the problem in a reference frame moving with a jet on an airway flying overhead if you like. The absolute values of momentum are different but the change in momentum is the same. There is no absolute value of momentum, the only thing relevant to Newtonian physics is its change.
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The bit I can't get my head around, is the forces in the turn. The Super Cub at ZERO groundspeed has NO momentum to overcome. Here centrifugal/centripetal forces don't apply. Therefore, with the aircraft banked, the horizontal component of the total reaction is unbalanced!!!
Remember that acceleration is changing velocity. Velocity has speed and direction components. Therefore even turning at a constant speed implies an acceleration.
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The energy came from the wind...
Avi8tor - reading between the lines of your Cub example, your point seems to be "don't confuse me with energies, forces or momentum; we all know a Cub hasn't got the horsepower to accelerate that quickly, so it MUST drop out of the sky".
I believe that the answer to your confusion is that, in the ground-based frame of reference that you are considering, the "horsepower" is in the 60 knot wind that's blowing! Remember, your little Cub is blatting away as hard as it can, indicating 60 knots airspeed into wind, even though it is stationary. Pretty much any manouever that stops it pointing into wind will immediately "accelerate" the airframe down-wind, at least from the point of view of a ground based observer. For example, if you close the throttle and pull the nose straight up to the vertical, the subsequent stall and tail-slide will appear - to the ground observer - to be travelling down-wind at the aforementioned 60 knots. For our hapless aerobatic pilot it's going to look pretty bizarre out of the window, and she's probably going to crash, but that's where we came in on downwind turns.
So, back to your example, as the Cub pilot starts the 180 degree downwind turn, they progressively stop fighting the wind. As they turn away from the wind, the wind itself contributes the 60 knot component so that, yes, when we are 90 degrees around the turn we will be going sideways at 60 knots over the ground (but perfectly balanced in the air).
This explanation certainly works for me. In fact, now I'm just waiting for a strong steady wind so that I can practice flying backwards
I believe that the answer to your confusion is that, in the ground-based frame of reference that you are considering, the "horsepower" is in the 60 knot wind that's blowing! Remember, your little Cub is blatting away as hard as it can, indicating 60 knots airspeed into wind, even though it is stationary. Pretty much any manouever that stops it pointing into wind will immediately "accelerate" the airframe down-wind, at least from the point of view of a ground based observer. For example, if you close the throttle and pull the nose straight up to the vertical, the subsequent stall and tail-slide will appear - to the ground observer - to be travelling down-wind at the aforementioned 60 knots. For our hapless aerobatic pilot it's going to look pretty bizarre out of the window, and she's probably going to crash, but that's where we came in on downwind turns.
So, back to your example, as the Cub pilot starts the 180 degree downwind turn, they progressively stop fighting the wind. As they turn away from the wind, the wind itself contributes the 60 knot component so that, yes, when we are 90 degrees around the turn we will be going sideways at 60 knots over the ground (but perfectly balanced in the air).
This explanation certainly works for me. In fact, now I'm just waiting for a strong steady wind so that I can practice flying backwards
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Folks,
A lot of brain cells are being extended to prove a Newtonian result in an earth-referenced scenario.
The "v" in all of the physical equations is TAS and is airmass-referenced! All of your extensive arguments only reinforce the pitfalls and inherent dangers of flying an airmass-referenced machine by reference to the ground.
But if you wish to discuss windshear, then you are dealing with the physics of a machine that is operating in an unstable medium - one that changes far faster than the relatively miniscule changes in lift, drag and thrust can compensate.
Stay Alive,
A lot of brain cells are being extended to prove a Newtonian result in an earth-referenced scenario.
The "v" in all of the physical equations is TAS and is airmass-referenced! All of your extensive arguments only reinforce the pitfalls and inherent dangers of flying an airmass-referenced machine by reference to the ground.
But if you wish to discuss windshear, then you are dealing with the physics of a machine that is operating in an unstable medium - one that changes far faster than the relatively miniscule changes in lift, drag and thrust can compensate.
Stay Alive,
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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
>>
It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant.
>>
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.
>>
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!!!
>>
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.
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
>>
It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant.
>>
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.
>>
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!!!
>>
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.