After our winch clinic this past weekend I stumbled across an organization in Great Britain that you may have already known about, but I didn’t. Gremline.com is a non-government online magazine dedicated to aviation safety. (Why it’s named gremline isn’t clear, but it might be an interesting story.)
John Stewart-Smith is one of the principles, with a heavily stacked resume. Presumably he wrote the essay excerpted below, though there was no attribution. In any case, these snippets from a much longer piece offer further discussion of winch launching to augment the information we soaked up this past week.
A stall during the transition from takeoff to the main climb on a winch launch may result in the glider rolling uncontrollably… A stall during pitch rotation can result in one wing losing lift marginally before the other, causing it to drop. The stalled wing has an increasing angle of attack as it drops, keeping it stalled. The rising wing has a reduced angle of attack, moving it away from the stall and allowing it to produce lift. This induces a rapid rolling moment and can lead to autorotation and a spin.
The stall speed of a glider increases during rotation in pitch as a larger angle of attack is required to achieve more lift. More lift is required to balance the other forces on the glider and to provide a vertical acceleration into the climb. There are three reasons for this:
1. As the nose pitches up the lift force is inclined away from the vertical and must be increased if the vertical component of lift is to balance the weight of the glider.
2. The pull force on the cable is large, typically 80% of the weight of the glider. At takeoff this force is horizontal, providing the glider’s initial horizontal acceleration. As the nose pitches up during rotation the lift force becomes increasingly opposed to the pull force. The lift must therefore increase if it is to balance this pull force and stop the horizontal acceleration.
3. At the end of the rotation the glider is climbing at about 55 kt, which gives a vertical velocity of about 35 kt. The vertical velocity of the glider must therefore increase during rotation from zero at takeoff to about 35 kt. This requires a force which comes from an increase in lift generated by the wing.
The forces on a glider during rotation may be modelled and the load factor (g) estimated for different rotation rates, pull forces on the cable, climb angles and other variables. This modelling shows the stall speed during rotation is very dependent on the RATE of rotation, i.e. the higher the rate of rotation, the shorter the time in which the glider has to be accelerated vertically from zero to 35 kt. As this requires a greater force from the wings, there is an associated increase in the stall speed.
The Safe Winch Launch is an ongoing initiative by BGA. It is recommended that all glider pilots get a copy of the “Safe Winch Launching” leaflet (downloadable from the excellent BGA website) and study it carefully.