ORIGINAL: IFlyEm35
Ok here is the deal with the downwind turn. Its more about momentum then airspeed. I TOTALLY agree that the airplane doesn't feel the wind when its in the air. And I made that argument with an astrophysicist. His thought here is when you have a airplane with a lot of mass and turn from a headwind to a downwind it has to accelerate. e.g. You take off at 100 kts indicated and there is a 20 kt headwind thats 80 kts groundspeed and instantaneously turn downwind you will still have that 80kts momentum pushing you and would have to accelerate to 120 kts groundspeed in the opposite direction to maintain that 100 kts of indicated airspeed. Grant it the wind does the acceleration but it can take a period of time which is increased the faster you whip the airplane around. Now I know that this can't be done "instantaneously, but it still shows what forces are acting on the plane. Point is that this is more pronounced in a 747 then it would be in an R/C model because of mass and surface area.
To summarize: The radius of a constant altitude turn has nothing to do with aircraft mass or size. True airspeed and bank angle are the only inputs to the turn radius equation (r=V^2/11.26*tan(phi), V = TAS knots, phi=bank angle in degrees). The only time the relative motion of the earth has any direct bearing on an aircraft's performance or behaviour is when we want to know something about it's ground track (like speed and/or direction), or when the airplane is in contact with it. Either during takeoff or landing roll, or when making for example a crash landing (upwind please). The airplane acts on the airmass and only the airmass (not considering ground effect). The apparent difficulties with making an upwind or downwind turn with a model have nothing to do with aircraft performance and everything to do with pilot perception. When you or I make constant-altitude and bank turns IN an airplane, we are not interested in drift over the ground, only in making a circular path with respect to the airmass, and we can do that if we maintain constant altitude, airspeed and bank angle, whether the wind is calm or blowing steadily at 200 knots. Either case makes no difference whatsoever to an airplane's performance or the way the turn is performed, even one just lifting off, ignoring wind shear and gradient effects. There is no such thing as absolute velocity. When we want to make a constant radius turn, then bank angle is varied as groundspeed changes in order to maintain the desired radius.
I'm not sure I understand what you mean by, "80 kts momentum pushing you". The instant the wheels leave the ground, the ground's relative motion becomes irrelevant to the airplane, as far as it's performance is concerned. It's only relevant with respect the the pilot's desire to position the aircraft relative to the runway or a departure course, etc.
So-called 3D airplanes doing "walls", "waterfalls" and other maneuvers with very high pitch rates follow exactly the same path through the airmass whether performed upwind, downwind or crosswind, if of course they are performed with the same inputs under the same conditions, at the same entry speed, etc. They look very, very different, however when performed upwind and downwind because the airspeeds are so low that windspeed (and the resultant groundspeed differences) causes large variations in the paths relative to the "stationary" observers on the ground. This is true of foamies, 40% planes, full scale acro planes, fighers, and it would be true of 747s if they could turn tightly enough. Put another way, if an observer was watching the airplane perform one of these maneuvers while moving at exactly the same velocity as the wind the airplane was experiencing, then the airplane would scribe the same path relative to that observer as it would relative to a "stationary" observer watching the same maneuver in calm wind.
A bit wordy, I'm sure and sorry for that.