The Hawker P1127 and an improved variant the Kestrel were the 'prototypes' of the famed Harrier. The first prototype P.1127, serial XP831 was delivered in July 1960 for static engine testing, and in October the Pegasus flight engine was made available. The first tethered flight took place the same month and free flight hover achieved on 19 November after which the first publicity photos were released. The second prototype made its first take off conventionally on 7 July 1961. The two aircraft proceeded to "close the gap" between vertical take off and flight, achieved by 8 September. Four more prototypes were ordered. Throughout this period improved Pegasus engines were being developed, with the Pegasus 3 being capable of 15,000 lbf (67 kN) of thrust. Apart from this, the first four aircraft were quite similar, but the fifth, XP980 introduced the taller fin and tailplane anhedral seen on the Harrier. The fourth machine was used, in part to give the Hawker production test pilots P.1127 familiarisation. The first carrier vertical landing was performed by the first prototype on HMS Ark Royal in 1963. The last P.1127, XP984, introduced the swept wing. It was eventually fitted with the 15,000 lbf (66.7 kN) Pegasus 5 and functioned as the prototype Kestrel.
Kestrel FGA.1
Evaluation aircraft were ordered as the Kestrel FGA.1, an improved version of the P.1127, the first flying on 7 March 1964. The Kestrel had fully swept wings and a larger tail than the early P.1127s, and the fuselage was modified to take the larger 15,000 lbf (85 kN) Pegasus 5 engine as in the P.1127/Kestrel prototype XP984.
Due to interest from the US and Germany, the Tri-partite Evaluation Squadron (TES) was formed on 15 October 1964 at RAF West Raynham, staffed by military test pilots from Britain, the US and West Germany. During testing one aircraft was lost;[10] and evaluations finalised in November 1965.
Six of the eight surviving evaluation aircraft (the three allocated to US plus those allocated to Germany) were transferred to the USA[10] for evaluation by the Army, Air Force, and Navy (but not the US Marine Corp) as the XV-6A Kestrel. After Tri-Service evaluation they were passed to the USAF for further evaluation at Edwards Air Force Base, except for two that were assigned to NASA.
One of the two remaining British based Kestrels was attached to the Blind Landing Experimental Unit (BLEU) at RAE Bedford and the other, XS693, went to Blackburn's for modification to take the uprated Pegasus 6 engine. In addition to some strengthening, there were alterations to the air intake, which had throughout the P.1127 and Kestrel series featured an inflatable lip to smooth the intake airflow when the aircraft was almost stationary. There were concerns about the Service life of these devices, so they were replaced with conventional suction relief doors. This aircraft became the prototype for pre-production Harriers.
