The Handley Page HP.115 represented an experimental leap in aviation, a delta-wing aircraft meticulously crafted by the renowned British aircraft manufacturer Handley Page.
Its primary purpose was to serve as a testbed, investigating the low-speed handling characteristics anticipated for a slender delta configuration, a crucial component envisioned for a future supersonic airliner.
Conceived during the 1950s, the HP.115 played a vital role in the broader supersonic aircraft research initiative, generously sponsored by the Ministry of Supply.
At that point in history, both delta-wing designs and supersonic flight were nascent innovations, ripe for exploration.
By 1956, the need for a demonstrator became evident, one that could substantiate the feasibility of the slender delta-wing concept not only for high-speed aviation but also for practical functionality at lower speeds.
Initially, the focus was on an unpowered glider, yet it became apparent that a self-powered aircraft would be a more cost-effective choice. Consequently, Handley Page was chosen to bring its proposal to life, resulting in the jet-powered HP.115, manufactured at the company’s Cricklewood facility.
On August 17, 1961, the solitary HP.115 took to the skies for its inaugural flight, marking the commencement of flight testing for its unique wing design.
Simultaneously, another research aircraft known as the BAC 221 was constructed, dedicated to exploring the high-speed aspects of this innovative wing configuration.
Read More: Handley Page HP.42 “Giant British Airliner”
Through an extended period of rigorous experimental flying, the HP.115 demonstrated its considerable capabilities and contributed invaluable data on delta wing characteristics, particularly during takeoff and landing phases.
Played a Pivotal Role
In 1974, the aircraft concluded its tenure in the test program and was subsequently preserved. It currently resides as a static exhibit at the Fleet Air Arm Museum.
The HP.115 played a pivotal role in validating the attributes of the slender delta wing, setting the stage for its adoption in the iconic Concorde, the Anglo-French supersonic airliner that entered service in the 1970s.
During the 1950s, extensive research into supersonic transports (SSTs) highlighted a significant challenge: the economic feasibility of such designs appeared quite unfavorable.
Supersonic flight introduces unique aerodynamic principles where lift generation differs from subsonic flight. To attain reasonable lift-to-drag ratios, supersonic aircraft necessitate wings with notably short spans.
While this configuration excels at high speeds, it yields minimal lift during low-speed operations like takeoff and landing.
To develop an aircraft capable of using existing runways for these crucial phases, designers faced a conundrum. They had to choose between broader wings, sacrificing supersonic cruise efficiency, deploying exceptionally powerful engines, or creating exceptionally large aircraft.
Produced Substantial Vortexes
A potential solution to this predicament emerged in Britain circa 1955 through the efforts of Johanna Weber and Dietrich Küchemann and their team at the Royal Aircraft Establishment (RAE).
They observed that delta wings produced substantial vortexes over the wing when flying at low speeds and high angles of attack, commonly referred to as “alpha.”
Specifically, these vortexes had the remarkable effect of accelerating the air above the wing, resulting in a substantial increase in lift at low speeds.
The magnitude of this effect was closely linked to two factors: the wing’s length and the sharpness of its leading-edge angle. Greater sweep in the wing’s design led to the creation of stronger vortices, while increased wing length provided more space for these vortices to function optimally.
This revelation hinted that an aircraft featuring a delta wing running along a significant portion of the fuselage, at extreme sweep angles exceeding 65 degrees, could achieve satisfactory low-speed performance while simultaneously minimizing supersonic drag due to its compact span.
Handling and Control
However, a notable challenge surfaced concerning the angles required to generate these beneficial vortexes. Such an aircraft would need to adopt what might be considered notably nose-high attitudes, particularly during takeoff and landing.
Furthermore, it would demand extended landing gear, particularly in the nose section, to maintain the wing at a high angle throughout the takeoff roll. These considerations raised questions about the handling and control of such a design during low-speed maneuvers.
C. H. Barnes, an authoritative figure in aviation literature, noted that skepticism regarding this configuration emanated from a series of wind tunnel tests conducted in the United States. These tests were initially viewed as misleading, casting doubt on the feasibility of the delta-wing concept.
Handley Page, the British aircraft manufacturer, received authorization to proceed with the construction of a single aircraft based on their HP.115 proposal. This construction primarily took place at the company’s established facility in Cricklewood.
The HP.115 was characterized by a delta wing with an exceptionally low aspect ratio, featuring a sweeping angle of 75°.
This wing design incorporated various elements for optimal control, including trailing edge elevons, spring servo-tabs, and anti-balance tabs for enhanced lateral and longitudinal sensitivity.
Additionally, the aircraft featured infinitely adjustable perforated air brakes configured as split flaps, pneumatically activated using a pre-charged air bottle, positioned at 50% chord.
The airfoil section utilized was a modified bi-convex type with maximum thickness at 4% of the chord.
This selection mirrored the profile expected for a supersonic transport, offering an advantageous distribution of cross-sectional area along the chord and thus minimizing wave drag during supersonic flight.
Notably, the aircraft featured a distinctive plywood leading edge that allowed for the substitution of new sections with varying degrees of camber, although this capability was never employed in practice.
The cockpit of the aircraft was designed to house the pilot, who sat on a Martin-Baker-built ejector seat beneath a sliding canopy.
The instrumentation in the cockpit included essential instruments such as an airspeed indicator, altimeter, yawmeter, electric turn-and-slip indicator, artificial horizon, directional gyro, and a standby compass.
Notably, the cockpit did not include any lighting, which allowed for a relatively small battery to power the turn-and-slip indicator.
Read More: The Air Force’s 50,000 Ton Press
Manual flight controls were utilized, incorporating a differential gearbox arrangement. The aircraft also featured hydraulically-actuated wheel brakes operated by foot pedals in the cockpit.
Additionally, a drogue parachute, stowed at the base of the rudder, could provide extra deceleration when needed. Various flight parameters, including airspeed, altitude, angles of incident, and other data, were primarily recorded by a pair of synchronized flight recorders.
The aircraft was equipped with a fixed tricycle undercarriage, which was adapted from the main gear of a BAC Jet Provost Mk 1 and the nosegear from a Jet Provost Mk 2.
Bristol Siddeley Viper Turbojet
The fuselage, mainly constructed from conventional aluminum alloys, consisted of a shallow rectangular section girder.
A nacelle at the nose housed the cockpit. The power source for the aircraft was a single Bristol Siddeley Viper turbojet, with the wing’s interior capable of accommodating up to 150 gallons of fuel distributed across three separate tanks.
The engine was positioned above the wing and integrated into the base of the tailfin. The tailfin featured a bullet fairing at the top to accommodate a cine-camera for recording airflow visualization experiments.
Some of these experiments utilized smoke generators mounted on the wing leading edges. Both the tailfin and rudder were swept at an angle of 60°, with a slight upturn near the engine exhaust to mitigate thrust-related pitching movements.
During the inaugural gathering of the Supersonic Transport Committee in 1956, a crucial decision emerged: the necessity for a specialized aircraft dedicated to low-speed testing became apparent.
Initially, the envisioned test aircraft was considered to require minimal power, leading to the initial consensus that an unpowered glider would suffice.
This notion prompted the creation of an official specification after an initial concept for such an aircraft was proposed by Slingsby Sailplanes. The task of developing this glider was assigned to Slingsby, who subsequently initiated the Slingsby T.48 project.
However, upon a thorough evaluation of the operational expenses associated with the test program, it became evident that a powered version would offer a significant advantage, offering 200% more flight time at a 95% reduction in hourly costs.
The glider necessitated each flight to be towed by another aircraft, such as the English Electric Canberra bomber, to reach a relatively high altitude of approximately 30,000 feet (9,140 meters).
Read More: Do 31, a Great Idea, so what Happened to it?
Additionally, according to Barnes, certain officials, including Godfrey Lee and Charles Joy, advocated for metal construction to enhance structural strength, while others sought the ability to explore phenomena like Dutch roll and perform takeoffs, both of which required a powered aircraft.
Due to a combination of these factors, development of the T.48 project was discontinued, and a revised specification centered around a self-propelled aircraft was issued in December 1959.
Extensive Ground Taxying Trials
On August 17, 1961, the solitary HP.115 aircraft, bearing the designation XP841, embarked on its maiden flight at the Royal Aircraft Establishment in Bedford.
The experienced pilot, J.M. Henderson, who conducted this historic flight, expressed his enthusiasm and satisfaction with this inaugural journey. Prior to this flight, extensive ground taxying trials were conducted to ascertain the optimal takeoff trim.
Henderson had also undergone thorough simulator training, which, as described by Barnes, had portrayed the aircraft’s handling characteristics in a more pessimistic light compared to the actual experience in flight.
All pilots chosen to operate the HP.115 had received extensive simulated flight training before taking control of the real aircraft.
Merely a month after its maiden flight, the HP.115 showcased its capabilities in an aerial display at the 1961 Society of British Aerospace Companies (SBAC) airshow. By September 29 of that same year, the contractor’s trials were successfully concluded.
The aircraft swiftly transitioned into its primary role of conducting low-speed research, a vital contribution to the supersonic transport development program that would eventually lead to the creation of Concorde.
In parallel with the HP.115, a separate aircraft, the BAC 221 (a modified Fairey Delta 2), was tasked with high-speed flight research.
The HP.115 proved to be an exceptionally capable aircraft, with its pilots demonstrating the ability to execute rapid bank angle changes while maintaining safe control even at remarkably low speeds, as slow as 69 mph (60 knots, 111 km/h), which was approximately one-third of the contemporary Lockheed F-104 Starfighter’s speed.