From : Don Stackhouse

Ah, yes, the incredibly persistant myth of the downwind turn again! No matter how many stakes are driven into it's heart, it somehow manages to regularly arise from the grave. I guess this proves that vampires don't do downwind turns!

I know, I could give you another one of my in-depth technical treatises to try my hand at mathematically dispelling this myth, but I'm not going to do that this time. (The rushing sound you hear is the collective sigh of relief from all the members of RCSE who are familiar with my previous technical treatises!) Instead I will offer only this:

One of the standard tests we do with our sailplane designs in the development phase is to trim the model into a fairly tight thermal turn and let it fly itself, hands completely off the sticks, for MANY turns. This is a test of spiral stability and dutch roll characteristics, as well as pitch damping. The Monarch 'D-lite' is particularly good at this. If we have everything set just right, the model makes a perfect circle in the air, with a constant bank angle and turning radius. When viewed from the ground, the model will appear to fly in a perfect circle if there is no wind at the model's altitude. If there is a wind, the model will drift along with it, so that its path through the air is a circle, with no change in pitch attitude, bank angle or glide path (remember, the model is completely hands-off during this, so any accelleration or decelleration would be accompanied by responses in those parameters), but its path viewed from the ground will appear to be a series of connected ellipses due to the drift over the ground. There is a definite optical illusion there, but if you watch the bank angle and pitch attitude you will see no wind induced (other than turbulence, which is another matter entirely) changes in a properly set-up sailplane.

If there really was a significant wind-induced effect on airspeed in a turn, this would be more pronounced at very high altitudes above the ground where the wind is generally much stronger. In actual practice, the thermal circles you see at a contest at high altitude (strong wind) will generally be rounder and smoother than at low altitude (less wind speed), the opposite of what the myth of the downwind turn would predict.

I believe what you are seeing is a combination of pilot error due to the optical illusions present when viewing a drifting turn from the ground (much more pronounced at low altitudes and close visual ranges) plus the smaller thermal diameter (with the increased probability that the model will see stronger lift on one side of the circle than the other) and greater turbulence and wind shears present near the ground.

At cloudbase, winds of 25-50 knots are not uncommon. This is well above the stall speed of most model sailplanes. If turning in the wind really had inherent airspeed changes associated with it, we would see a severe stalling tendency with virtually all sailplanes (including even full scale ones!) nearly every time we made a thermal turn at high altitude on a windy day. I've flown models and full scale sailplanes many times under these conditions (for fun, try flying a full scale Schweitzer 2-22 in 40 kt. winds sometime!) and I can assure you, no such phenomenon exists.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net