The Handley Page HP.115 was an experimental delta wing aircraft.
It was built to test the low-speed handling characteristics to be expected from the slender delta configuration anticipated for a future supersonic aircraft.
The HP.115 was designed during the 1950s as a part of the wider supersonic aircraft research program that was sponsored by the Ministry of Supply. At the time, both the delta wing and supersonic flight were both relatively recent innovations.
By 1956, the Supersonic Transport Committee had been deemed necessary to build a demonstrator to prove that the slender delta wing design was not only suitable for high-speed flight but would also be reasonably functional at lower speeds as well.
Initially, work centred around an unpowered glider, but it was determined that a self-powered aircraft would be more economic.
Accordingly, Handley Page was selected to produce its proposal, the jet-powered HP.115, at the company’s Cricklewood facility.
On 17 August 1961, the sole HP.115 performed its maiden flight; flight testing of the wing commenced shortly thereafter.
A separate research aircraft, the BAC 221, was also built to study the high-speed aspects of the wing research.
Over a relatively lengthy period of experimental flying, the HP.115 proved itself to be relatively capable and provided significant data regarding delta wing characteristics during the take-off and landing phases.
The aircraft itself was withdrawn from the test programme in 1974 and subsequently preserved; it is presently on static display at the Fleet Air Arm Museum.
The HP.115 had helped validate the properties of the slender delta wing, leading to a similar wing being adopted for Concorde, the Anglo-French supersonic airliner that entered service during the 1970s.
Through the 1950s, various studies on supersonic transports (SSTs) suggested that the economics of such designs were far too poor to be practical.
Lift is generated in different ways at supersonic speeds and achieving reasonable lift-to-drag ratios requires the wings to have a very short span.
This works well at supersonic speeds but offers very little lift at low speed.
In order to make a design that is able to take-off and land on existing runways, either the aircraft would have to use wider wings and lose supersonic cruise economy, have enormous engine power, or be extremely large.
A way out of this dilemma was presented in Britain by Johanna Weber and Dietrich Küchemann around 1955.
Their team at the Royal Aircraft Establishment (RAE) noticed that delta wings generated large vortexes over the wing when flying at low speeds and high angles of attack (“alpha”).
Specifically, the vortexes increased the speed of the air on top of the wing, thereby greatly increasing lift at low speeds.
This effect was magnified by the length of the wing, and the sharpness of the angle of the leading edge more sweep led to stronger vortex creation, more length gave it more room to operate over.
This suggested that an aircraft with a delta wing running the majority of the length of the fuselage at very great sweep angles, over about 65 degrees, would have reasonable low-speed performance while also keeping the supersonic drag to a minimum through its limited span.
A major concern was the angles needed to generate these vortexes.
The aircraft would have to fly at what would be considered significantly nose-high attitudes, especially on take-off and landing.
They would also need very long landing gear, especially in the nose, in order to keep the wing at a high angle during the take-off roll.
This led to some questions about the handling and control of such a design at low speed.
According to aviation author C. H. Barnes, one source of scepticism for the configuration came from a series of wind tunnel tests that had been performed in America but were later determined to have been misleading.