The AIR-BRITAIN Militarv Aviation Historical Quanerlv

A view of a P.74 model shown at a Farnborough Show, with a fin (incorporating a rudder) in lieu of a tail rotor.
(Percival Aircraft PY3509 via Mike Hooks)
The P.74, Percival's first attempt at a helicopter design, was
schemed to meet Specification EH.125D, which called for an
experimental helicopter to conduct ground and flight tests of a
rotor driven by jets at the blade tips. The jets were to be
supplied with gas from a fuselage-mounted gas generator.
The Specification, which was issued on 08May1952, called for
the prototype to be fitted with stub wings to relieve the rotor of
aerodynamic loads in flight, but the final design did not have
wings. Conveniently, the chosen power plant consisted of
two Napier N.Or.1 Oryx (Napier .E.164) gas generators
(convenient because Percival's Luton works was immediately
adjacent to D. Napier & Sons' Flight Test Department, where
Napiers had already built a helicopter-rotor test facility). This
facility was also used to try out experimental gas generator
systems, and was also used for ground running of the
prototype P.74. The single prototype was issued with the Air
Ministry serial number XK889 on 120ct1955, under Contract
No. 6/Aircrattl7054/CB.9(b).
XK889 was rolled out May 1956 for engine runs without the
rotor, and later tests were made with the rotor fitted . The
Specification called for an extensive ground test schedule
prior to any flight tests. When the flight test stage was
reached, the aircraft failed to fly due to the un-powered cyclicpitch
controls being unable to overcome system inertia,
despite the heroic efforts of two pilots attempting to obtain liftoff.
A 'P.74 Mk.II' with an RB.108 engine and redeSigned rotor
Aeromilitaria - Winter 2010
system (with hydraulic power for the cyclic pitch controls) was
proposed as the Percival P .113, but this came too late to save
the project, which was cancelled and the prototype sent for
scrap. As originally schemed, the P.74 had a large
conventional fin, but it was found necessary to provide the
complication of a tail-rotor on the aircraft, thereby partly
negating one of the claimed design benefits, which was of the
system simplification to be achieved by eliminating rotor drive
shafts and gearboxes. As already mentioned, the rotor was
driven by hot gas ducted from the engines through to the main
rotor blades and ejected at the blade tips. The rotor blades
were of a laminar-flow cross-section.
Doubts about the success of the original concept had led to
earlier revised proposals under the design numbers P.104 and
P.105, but these did not proceed because of confidence -
misplaced as it turned out - that the P.74 would fly
successfully. The P.104 was essentially the P.74 with
conventional (non-Iaminar-flow) rotor blades and powered
cyclic-pitch controls, while the P.1 05 was a more fundamental
redesign with a different arrangement of the gas-generating
units and gas vents. The P.74 gas vents, shown in one of the
photographs, ejected hot gases at ground-level while the
engines were running but when no gas was being directed to
the rotor - very hot and dangerous for the ground personnel -
while the proposed P.105 would have had the Oryx engines
and gas vents above the cabin.
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