Glider’s Climb in Turbulent Air

Abstract

Abstract

Atmospheric turbulence provides thrust to airplanes.  It is shown that the thrust is equivalent to an energy gain which contributes to the airplane’s compensated climb rate.  Based on the lift-coefficient versus angle-of-attack curve and the force-against-wind energetic interaction of the airplane with its environment, an expression is derived which quantifies the climb rate (“gain”) for a glider induced by the atmospheric turbulence.  It is shown that the gain is proportional to the speed flown, the square of the velocity of the turbulent air movement, and inversely proportional to the airplane’s wing loading.  Quantitatively, the gain for a modern glider flying in turbulent air with alternating gust-induced load variations of e.g. (1 +/- 0.5) ´ g (g, earth acceleration, 9.81 m/s²) amounts up to approximately 0.5 m/s.

We discuss the gust model which is used, the role of the lengths of the gusts, and the influence of the wing’s elasticity.  It is concluded that glider pilots should prefer to fly through turbulent regions rather than through quiet air if they have the choice.  In order to take profit from the gusty conditions they should reconsider the pros and cons of taking water ballast, as the gain due to turbulence is less with the heavier airplane, and they should prefer planes which have rigid wings.

Turbulence is a major contributor to flight energy.  It should be considered in addition to the known updrafts such as thermals, slope updrafts, rotors and lee waves, and as distinct from dynamic soaring using steady wind shears.  The contribution of gusty air is quantified and the theory of the best-speed-to-fly is amended to include the influence of turbulence.