Boulton Paul P.111 and P.120

Single seat experimental aircraft to Specification E.27/46 to meet the requirements of OR.241, designed to explore the characteristics of tailless deltas. It was designed to be the smallest possible airframe that would accommodate both a single Rolls-Royce Nene turbojet engine, a Martin-Baker-built ejection seat and a delta wing. Because of the need to internally house the relatively wide Nene engine, which used a centrifugal compressor, the fuselage of the P.111 had a rather tubby appearance; air was supplied to the engine via a flattened oval nose intake. The inward-retracting landing gear had a noticeably wide track and a tall nosewheel leg placed it at 17° on the ground; sizable doors covered the recesses of the undercarriage while retracted. All-metal construction was used throughout the aircraft, with the exception of the wing extensions and the fin tip, which were of glass-reinforced plastic; the exterior skin of the aircraft typically used a stressed-skin approach. The delta wing of the P.111 was relatively thin in comparison to contemporary wings and was swept at an angle of 45°. When not fitted with tip extensions, this wing was strongly cropped at about 75 percent of the pure delta shape, while the installation of the larger extensions resulted in an almost unbroken pure delta shape. The P.111 could be flown with these wings or with one of two pairs of extensions. The purpose of these tip extensions was to investigate the different aerodynamic effects that their fitting would have upon the delta wing without having to replace or reshape the whole wing to do so. The fin, which had an unswept trailing edge, also featured a removable and adjustable tip, though it is believed that the aircraft was always flown with the particularly pointed tip fitted. The fin accommodated a VHF aerial at its tip as well as a camera installed in its leading edge. Control of both the roll and attitude of the P.111 was achieved via a pair of elevons, which were designed to also function as ailerons where appropriate, and could be controlled both electrically and manually. Fuel was accommodated across six flexible fuel tanks, three within each wing. Flight information was continuously recorded by instrumentation installed just aft of the cockpit, parameters captured typically pertained to either the controls or the engine. The P.111 was fitted with one of the first fully-powered control systems, which were hydraulically driven and accompanied by electrically actuated trim tabs. Traditional push-pull rods were used for all controls. One Rolls-Royce 5,100 lb st Nene 3 RN.2 turbojet.

P.111 Specification
Span	Length	Height	Wing Area	Empty Wt	Max AUW	Cruise Speed	Maximum Speed	Range	Service Ceiling
25 ft 8 in	26 ft 1 in	12 ft 6 in	269.25 sq ft	7517 lb	10127 lb		649 mph/ 564 kn		35000 ft
7.82 m	7.95 m	3.81 m	25.01 m2	3410 kg	4594 kg		1044 km/h		10668 m

P.111 rebuilt after an accident; the modifications included the addition of four petal-shaped airbrakes upon the fuselage, which reduced the landing speeds considerably, while the undercarriage doors were changed to reduce the trim change incurred. A lengthy nose-mounted probe carrying a pitot head was also added at this time.

Produced to the same specification as the P.111, the primary difference between the P.120 and P.111 was in the tail unit. The new aircraft featured a swept wing fin and rudder, while the horizontal tail surfaces were moved upwards, roughly at the upper two-third area of the fin, as a means of improving both longitudinal and directional stability. For the purpose of recording airflow behaviour, a cine camera was embedded into the tip of the fin, while a drogue parachute was housed within a bullet between the rudder and the jetpipe, the latter being lengthened somewhat from its P.111 counterpart. The horizontal tail surface was swept at an angle of 45-degrees and was not a tailplane in the traditional sense, as pitch control was performed using the elevons instead, yet it was all-moving and adjustable for trim tab purposes. The wing, while highly similar to that of the P.111, it featured noticeable bulges of the outbound landing gear doors, while semi-elliptical wing fences were also addted at the junction between the wing and wing tips. Unlike the P.111, the wingtips of the P.120 were permanent, not removable or replaceable; they could be rotated either differentially or together for lateral or longitudinal trim. Electro-hydraulic actuators were used to move the wingtips at the direction of the pilot via a control panel on the port side of the cockpit. While the fuselage was generally similar to that of the P.111, and virtually identical forward of frame 290.88, the rear fuselage featured considerable revision. It was strengthened considerably via the use of additional formers, which was deemed necessary to properly account for the different tail unit fitted. The end of the rear spar was directly attached to the powered rudder. Further minor changes included the addition of a VHF radio aerial upon the spine of the fuselage, just aft of the cockpit, and a repositioned pitot tube. One 5,100 lb st Rolls-Royce Nene 3 RN.2 turbojet.

P.120 Specification
Span	Length	Height	Wing Area	Empty Wt	Max AUW	Cruise Speed	Maximum Speed	Range	Service Ceiling
33 ft 5.5 in	29 ft 7.5 in	9 ft 6.5 in	290.13 sq ft	10656 lb	12580 lb
10.2 m	9.03 m	2.91 m	26.95 m2	4833 kg	5706 kg