The Art of the Helicopter John Watkinson - Karatunov.net

Other types of rotorcraft 355
and control systems then this is it. A tail-mounted pusher propeller for forward flight
would be naturally complemented by a NOTAR system for hovering, leading to a very
clean external appearance with low drag.
9.4 The convertiplane
A convertiplane changes its operating principle according to the flight regime, most
obviously between hover and cruise. The Fairey Rotodyne (Figure 1.11) is a convertiplane.
This machine has two turboprop engines that in addition to driving variable
pitch propellers can also produce significant amounts of compressed air. In the hover
the compressed air is fed along the rotor blades and fuel is burned at the tips to drive
the rotor. There is no torque reaction and yaw control is obtained using differential
propeller pitch.
The tip jet drive is inefficient, but is only used for take-off and landing. Once forward
speed has been obtained, the rotor drive is switched off and the rotor is tilted back so
the machine becomes a gyroplane. The rotor is partially unloaded by the wing. Landing
is achieved by reigniting the tip jets and reverting to helicopter mode, although landing
on a runway in gyroplane mode is also possible.
The winged McDonnell XV-1 has a single piston engine driving either a propeller or
an air compressor. In order to hover the compressor output is fed along the blades to
tip jets as in the Rotodyne. No yaw control could be obtained with a single prop, and
instead a pair of small fixed pitch tail rotors is installed, powered by small reversible
hydraulic motors. The XV-1 can then take off as a helicopter. In forward flight the
engine power can be diverted to the prop so that the machine becomes a compound
gyroplane.
The XV-1 has a substantial wing area and full conventional aircraft controls and in
cruise these will become functional. The fore-and-aft cyclic control is then automatically
adjusted to control rotor speed. This is reduced to 50% of the hover RPM, so
that the wing is carrying about 80% of the machine weight. Thus the XV-1 can fly as a
helicopter, a gyroplane or an airplane.
Following the progression of unloading the rotor in forward flight, the next step in
the view of some designers is to dispense with the rotor altogether. There have been
numerous proposals. A two-blade rotor is stopped transversely in flight to become a
wing, a four-blade rotor becomes an X-wing, a rotor folds and retracts into the hull.
All of these proposals suffer from the same practical difficulty – as the rotor slows
down it loses the centrifugal stiffening effect. As has been seen earlier in this book, as
a rotor slows down it ceases to be gyroscopic and the phase lag starts to reduce from
90 ◦ . This causes a control problem. A more serious difficulty shown in Figure 9.6 is
that at some stage a near-stationary rotor blade will have significant sweep forward and
this is an unstable condition. In conventional wings and blades flutter is avoided by
bringing the mass centroid towards the leading edge. However, at low rotor speed the
reverse flow region would envelop the retreating blade such that leading and trailing
edges interchange so this technique cannot be used. The blade and hub would need to
be immensely rigid in torsion and bending to survive the loads. As a result no practical
machine has emerged and it is unlikely that one will.
The only in-flight rotor-folding technique that appears viable is to tilt the rotor backwards
and to feather the blades. As the rotor slows down it will cone up 90 ◦ and
ultimately stop with the blades trailing in the slipstream. However, if the rotor is going
to be tilted, why not use it as a propeller?