SINCE its original conception, the project has grown from an aircraft of 8,600 lb. all‐up‐weight with two 340 h.p. piston engines, to the present turbine‐powered Skyvan of 12,500 lb. all‐up‐weight with two 666 e.h.p. turbine engines. This has caused some minor changes to be made in certain aerodynamic design features of the aircraft but the necessary changes have been remarkably few, as will be seen from the following sections. In spite of the large increase of disposable load (fuel and payload) that became possible with the change from piston to turbine engine power, it was desirable that the wing should be changed as little as possible. Consequently, it was accepted that some revision of the original field performance of the earlier design might be required. The maximum relaxation which could be afforded was represented by a fully‐factored field length of 1,000 yards in hot and high conditions.