Handley Page H.P.20
The Handley Page H.P.20 was an experimental monoplane modification of a de Havilland DH.9A, built to study controllable slots and slotted ailerons as high lift devices. It was the first aircraft to fly with controllable slots. Development Frederick Handley Page obtained his master patent for controllable slots on the edge of an aircraft wing on 24 October 1919. He knew the lift coefficient of all wings increased linearly until the stall was approached, then fell away; he argued that if the stall could be delayed, higher lift coefficients could be reached. In Germany Gustav Lachmann had the same idea, though through a wish to avoid the dangers of stalling. Rather than going to litigation an agreement was reached in which Lachmann, working with Prandtl at the advanced Goettingen air tunnel acted as consultant to Handley Page. Lachmann heard about the Handley Page work when they modified a standard DH.9A with fixed slots and demonstrated it dramatically at the Handley Page airfield at Cricklewood on 21 October 1920. That machine was retrospectively designated the H.P.17. The first aircraft with pilot controllable slots, designed by Handley Page and wind tunnel tested by Lachmann was called the X.4B in the company's contemporary notation