Expert: Paul Soderman - 6/15/2011

Question
Good day Sir,
I use a plane shape glider.I throw the plane at an angle of 30 degree from horizontal(just as assumption).The plane fly at that angle and after a distance it rise and stopped in the air before fall(free fall).Do you agree if I say the in term of velocity vector that the at the flight direction the speed decrease?And when it stopped in the air with critical angle of attack exceeded the lift is zero?I don't have anymore idea to explain to my lecturer since he is fussy japanese.He keep asking about velocity vector just before the stall.I read your answer I need to confirm it with you whether my explanation is correct.Thanks a lot.Have a nice day.

Answer
Without seeing your experiment, I can't be sure of the velocity history.  But it sounds like during the glide, the glider experienced a nose-up pitching moment, slowed and stalled before falling.  The glider probably slowed before pitch up but I can't be sure.  If the glider actually stopped in the air, the lift would be zero at that instant. But it is more likely that it had some forward motion and some small lift that was less than the weight of the glider.  As I said before, the velocity vector is always in the direction of the flight path no matter where that flight path goes.