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 Sir Frederick Handley Page (15 November 1885 – 21 April 1962) was an English industrialist who was a pioneer in the aircraft industry and became known as the father of the heavy bomber. His company Handley Page, Handley Page Limited wa ...

obtained his master patent for controllable slots on the edge of an aircraft wing on 24 October 1919. He knew the

lift coefficient In fluid dynamics, the lift coefficient () is a dimensionless quantity that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a co ...

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 Gustav Victor Lachmann (3 February 1896 – 30 May 1966) was a German aeronautical engineer who spent most of his professional life working for the British aircraft company Handley Page. He was, with Frederick Handley Page, the co-inventor of the ...

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 Ludwig Prandtl (4 February 1875 – 15 August 1953) was a German Fluid mechanics, fluid dynamicist, physicist and aerospace scientist. He was a pioneer in the development of rigorous systematic mathematical analyses which he used for underlyin ...

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 Cricklewood is a town in North London, England, in the London Boroughs of Camden, Barnet, and Brent. The Crown pub, now the Clayton Crown Hotel, is a local landmark and lies north-west of Charing Cross. Cricklewood was a small rural hamlet ...

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 but became, retrospectively, the H.P.20. The

Air Ministry The Air Ministry was a department of the Government of the United Kingdom with the responsibility of managing the affairs of the Royal Air Force and civil aviation that existed from 1918 to 1964. It was under the political authority of the ...

met the cost. Like the H.P.17 it used a D.H.9A engine, fuselage and empennage, but fitted with an entirely new wing.''Flight'' 10 November 1921 p.732-3
/ref> The H.P.20 was a high-wing monoplane, using a thick wing with a straight leading edge but taper on the trailing edge. It was a semi-cantilever structure bolted to a small cabane on the fuselage and braced to the lower fuselage longerons with a pair of steeply rising struts on each side. The heaviness of early cantilever wing structures is shown by a comparison of the loaded weight of the H.P.20 (6,500 lb) with that of the loaded standard biplane DH.9A (4,645 lb including fuel for over 5 hours of flight and a 460 lb bomb load).
/ref> The undersurface was flat and the front edge cropped to allow the full span slats, when closed, to form the true leading edge. The slats were hinged ahead of the wing and at their leading edges; their rotation formed the slots. In addition, slots opened in front of the ailerons when they were lowered. This was done via a groove in the wing just in front of the aileron hinge, narrowing towards the top surface. The spans and wing areas of the DH.9A and H.P.20 were about the same, so the wing chord of the monoplane was about double that of the biplane. As a result, the tailing edge extended aft beyond the DH.9A's pilot's cockpit and so the H.P.20 was flown from what had been the gunner's position. There was a small cut-out in the trailing edge to enhance the pilot's view. As there was no upper wing to house the fuel, the H.P.20 had a slightly round-ended cylindrical tank mounted high over the centre section to provide gravity feed. Because high angles of attack were used in landing and take-off with the slots open, the undercarriage was lengthened, as it had been on the H.P.17. There was an airflow direction indicator mounted on a little boom which projected forward of the starboard wing. The H.P.20 was first flown with slots closed on 24 February 1921 at Cricklewood. About a month later it was flying with controllable slots. In one test it landed at 43 mphh (69 km/h) at a

wing loading In aerodynamics, wing loading is the total weight of an aircraft or flying animal divided by the area of its wing. The stalling speed, takeoff speed and landing speed of an aircraft are partly determined by its wing loading. The faster an airc ...

of 11 lb/ft² (54 kg/m²), about the same as a

Cessna 152 The Cessna 152 is an American two-seat, fixed- tricycle-gear, general aviation airplane, used primarily for flight training and personal use. It was based on the earlier Cessna 150 incorporating a number of minor design changes and a slightl ...

. This corresponds to a lift coefficient of 1.17. Aerodynamic loads made the slats hard to operate reliably. After manufacturer's tests, the Air Ministry agreed to take on the aircraft, but the Ministry's pilot made a heavy landing at Cricklewood during acceptance flights and the H.P.20 remained there for repairs until final delivery in February 1922.

Specifications

References

Bibliography

Note

* * {{Handley Page aircraft 1920s British experimental aircraft H.P.20 Aircraft first flown in 1921