My 72" Drifter-II sailplane was successfully launched piggy-back style without incident. The tow-plane pilot was Dominik Eggenschwiler. Despite having only seen it done before, Dominik was confident. At times the tow-plane was partially hidden from low level clouds by mist on a cloud covered day, While there were few thermals, winds were light.
Dominik's trainer was used as the tow-plane. It has plenty of power having a .50 4-stroke. He purchased a launch cradle which is held onto the planes wings with rubber bands. The craddle supports the sailplane, which is also held on with rubber bands. These rubber bands are held in place with lever arms on the front of each side of the cradle. A servo is used to pull pins which allows the lever arm to spin freely, releasing the rubber bands. While these hold-down rubber bands are intended to stay with the plane after release, many were lost. Make sure the realease mechanism works smoothly since the cradle bends a little.
A post flight discussion revealed some issues that others should be aware of when trying this. The first is that Dominik was surprised by how quickly the planes left the ground. Although there is the weight of two planes there is also the lift of two wings. However, the sailplanes flat-bottom wing doesn't like speed and therefore creates a significant amount of drag at the speed the tow-plane flew at. This limited the maneuverability of the tow-plane.
Because of the added lift, Dominik retrimmed his plane with a significant amount of down elevator to maintain level pitch. While this worked well while towing, he was surprised after release when, with only one wing again, the tow-plane pitched down sharply. This may be corrected by adjusting the cradle or sailplane to have a smaller angle-of-attack when attached to the tow-plane.
During flight, there were times when it appeared that one set of rubber bands holding the sailplane had released. Since the sailplane has significant polyhedral, any amount of yaw will cause the outside wing to raise up possibly stretching the rubber bands.. It is this principle that allows a sailplane to turn with rudder only.
The sailplane rudder causes yaw, which turns the outside wing into the airstream, causing the plane to roll, and the rolled wing now produces a lateral force, causing the the plane to turn. Since powered planes typically do not make coordinated turns (i.e. they only use ailerons and side-slip through turns) the yaw in the turn with the sailplane attached causes the sailplane to roll, affecting the flight of the tow-plane. During tow, it may be very important that the tow-plane perform coordinated turns to minimize any yaw.
Piggy-back launches of sailplanes seem very straight forward. It is important that the tow-plane be sufficiently strong. When considering the discussion above, there should be no surprises. Next time, we'd like to experiment more with the control of the tow-plane in flight to test some of the ideas above.