TECHNICAL DETAILS FAIREY ROTODYNE
AEROPLANE JULY 2018 http://www.aeroplanemonthly.com 95Development
Technical Details
Flight Testing
Cancellation
Insights
T
he Rotodyne’s tip-drive system owed its existence
to Austrian helicopter pioneer Friedrich von
Doblhoff, who directed a research programme
instituted in 1942. On his WNF 342 machine, a
conventional piston engine was used to drive a compressor
to supply air mixed with fuel up through the rotor hub and
out through three hollow rotor blades, to be burned in
tip-mounted combustion chambers. The result was that
enough thrust was generated to turn the blades and
provide lift.
The idea was developed through four prototypes
featuring ever-larger rear propellers to provide forward
thrust. But because of the high fuel consumption the third
and fourth prototypes used the tip-drive system solely for
take-off, hovering and landing, with a conventional pusher
propeller mounted behind the pilot’s cockpit providing
forward propulsion.
The third prototype was destroyed during trials but the
V4 model had completed just 25 hours of testing, mostly in
the hands of young engineer August Stephan, by the time
the war ended and it was captured by the Americans. After
the war von Doblhoff joined McDonnell in the USA but
Stephan and a colleague, von Czernin, joined Fairey to
work on tip-drive systems for the Gyrodyne and Rotodyne.
These eliminated many conventional helicopter features
so there was no gearbox, no direct engine input and no
transmission system to power a tail rotor. Yet arrangements
for passing air, fuel and high-energy electrical power for
ignition through the rotor head and blades to the tip-jets
were quite complex. In the Rotodyne Y, compressed air for
the jets was generated by the auxiliary compressors at the
rear of the wing-mounted Elands. From there, the air was
ducted along the wing leading edges and up through the
rotor pylon.The two ducts were kept separate to maintain two
concentric rings around the central shaft housing the rotor
controls and tip-jet and electrical services. The upper part
of the ducting directed air to diametrically opposite pairs of
blades so that each pair received a supply from one of the
two engines. This ensured that if one engine failed power
was maintained by the other.
All the ducting was assembled with the rotor head and
its controls passing down through the centre and the
control runs directed through the rotating swash-plate.
Above the swash-plate a low-voltage electrical supply ran
through the rotor head via a slip-ring assembly to feed four
dual high-energy ignition units in the rotor hub dome. The
impulses they generated were passed along the blades to
the tip-jet igniters, supported by a parallel fuel supply.
The units’ static performance could be measured on a
special spinning rig at White Waltham which tested them
on a slave blade. There was also a rig for noise testing and
developing silencing equipment. In the early days a single
Rolls-Royce Dart provided the fl ow of air for testing, but it
was later replaced by a pair of Avons.
A well-appointed workshop was capable of fabricating
from specialised materials sophisticated jet unit
components like combustion liners and silencers. These
facilities were located close to the main fl ight test unit to
provide a rapid solution to problems uncovered during
fl ight-testing and so reduce delays.
Despite the simplicity the tip-drive system had its
drawbacks. It was always considered noisy in operation.
Despite the attractions of the ‘something for nothing’
system, only one helicopter using it ever achieved quantity
production. The French SNCASO Djinn relied on a cold-air
bleed system and more than 150 examples of the type
were produced.A good view of both the Rotodyne’s beaver-tail, with clamshell doors, and the tip-jet installations. AEROPLANETHE TIP-DRIVE SYSTEM
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