Expert: Paul Soderman - 11/11/2009

Question
Hi, my research group and I are trying to build a glider, but we have run into a few problems. The glider is supposed to be initially thrown, but it will climb higher using thermals. I need to figure out the maximum amount of weight our glider can hold and still climb on thermals. I can figure out the amount of lift our glider can get when it's just coasting using the lift equation and plugging in air density, head wind, cross-sectinal area etc... But I can't seem to find out how to find out the amount of lift generated by a thermal. Suppose we took an average thermal of temperature and velocity, etc, can anybody tell me how much lift will be generated on the glider due to the thermal???

Answer
When a glider enters a thermal updraft there will be an increase in angle of attack on the wing and an increased lift.  The new angle of attack can be easily calculated from the vector sum of the forward speed and updraft velocity.  However, soon after the initial jolt, the glider will settle down and fly as it did in still air.  It will not know it is in an updraft, but will actually glide downward as before.  But seen from the ground the glider will be climbing if the thermal is strong enough.  So all you have to do is calculate the initial descent rate and subtract that from the thermal velocity to get a climb rate.  So, to answer your question - after the initial entrance to the steady thermal, the lift on the glider will return to be the same as in still air. But relative to the ground it will be gliding at a different angle.

The same thing happens in a steady head wind or tail wind.  The airplane just flies relative to the local air and only goes faster or slower relative to the ground.  It doesn't know it is in a head wind or tail wind unless there is turbulence that shakes the plane.
Paul