March 1966 - Rotary Wing Forum

KEN WALLIS AUTOGYRO'S.
1
March 1966
Judging by the continual stream of enquiries received on the subject of autogyros, popular
imagination is perhaps further ahead of reality than it is about any other aspect of
aeronautics. This paradoxical state of affairs (a spur to development, but a signal for
caution) arises from man's long-cherished dream of a cheap, simple device to permit him to
fly with the freedom and manoeuvrability of a bird. The autogyro is by no means a new
idea—from the pioneering work of Senor Juan de la Cierva and others in the twenties and
thirties the principle has been known for many years. But only since World War Two (when
both sides in the conflict toyed with tiny autogyros to put a man in the sky for one purpose
or another) has it been recognised that the conception may be the key to the smallest
practical flying machine. So far as small autogyros are concerned, the principle has been
virtually ignored by the main stream of world aeronautical research and development
during the last 20 years. The simple reason is that the VTOL characteristics of the helicopter
have proved more alluring than the mere STOL capability of the autogyro. But, in a number
of countries, protagonists have studied the subject intensively. One of them is Wing Cdr K.
H. Wallis, now retired from the RAF, who for the past eight years or so has delved deeply
into the unique potentialities of the autogyro. No other kind of heavier-than-air aircraft has
2
yet approached the autogyro's power-to-weight ratio or its weight-lifting ability; and the
slow-flying and manoeuvrability characteristics of the type are nearly the same as those of a
helicopter. Also, autogyro handling is hardly more complicated than that of an aeroplane.
Wherever it would help to have a man in the sky—with a camera, a radio transmitter,
weapons or simply to observe—the autogyro offers perhaps the cheapest and best all-round
solution. The first notable product of Wg Cdr Wallis' research was the patented offset
gimbal rotor head. The hands-off natural stability conferred by this clever mechanism was
first demonstrated in August 1961 on the prototype Wallis WA-116—an entirely new
autogyro embodying many Wallis-patented features in the engine, spin-up mechanism and
so forth. As described in this article, development and testing of the WA-116 a US 72 h.p.
McCulloch engine is now complete and within we next few months an important derivative
(the WA-117) powered by a 100 hp Rolls-Royce Continental O-200B. The WA " 117 will be
developed with a view to obtaining worthiness approval for commercial operations. From
the very start of his researches Wg Cdr Wallis has worked to comply with BCAR Section G.
Approved materials are used except in the rotor spin-up mechanism—which, in any case, is
completely disconnected during flight. The only major non-appoved part is the McCulloch
engine, which has not been fully tested to airworthiness standards. WA-116 autogyros
operate under special-category certificates of airworthiness. Because of the small size and
apparent simplicity of the Wallis autogyro there has been much pressure on the designer to
supply plans and kits for the amateur constructor. This need has been given sympathetic
consideration, and further simplification was considered. But, according to Wg Cdr Wallis,
experience has shown that the simplicity is more apparent than real and that amateur
construction is unlikely to produce an aircraft of the necessary standard. Accordingly, his
company cannot entertain requests to supply components for fitting to other autogyros.
There have been many amateur attempts to build other autogyro designs, but they have not
always met with success—owing, for instance, to insufficient understanding of the forces
and dynamics involved. Certain of these machines have experienced rotor system structural
failure following rapid manoeuvres.
The next Wallis development autogyro to fly should be the high-speed WA-118 Meteorite
(Flight last week, page 458). Wallis autogyro research is centred at Reymerston Hall, an
elegant 18th-century house set in the heart of Norfolk. Production autogyros and spare
parts are built in a Cambridge workshop staffed by six craftsmen and managed by Mr
Geoffrey Wallis (cousin of the wing commander) as a sideline to his automobile garage. This
compact little organisation constitutes Wallis Autogyros Ltd. The immediate aim is to start
limited production of the WA-117—enough to support continued research into advanced
possibilities of the principle. The company will remain small and highly experienced, with
the long-term intention of entering into licence agreements with suitable production
organisations. The autogyro is covered by several Wallis patents. The Autogyro in Practice
The limited experimental operating trials of the WA-116 to date have produced mixed
3
reactions. An association was formed with Beagle in 1961 and that company built three
aircraft for Army field trials. The Army assessed the autogyro for its potential as a light
liaison vehicle —a kind of aerial dispatch rider's machine. This possibility was not considered
realistic and the Army then began to look at two-seat light helicopters (the Brantly B2 and
Hughes 200 were the most favoured), but, after lengthy trials, the heavier and higherperformance Bell 47G was ordered. As far as military applications go, the autogyro has yet
to be considered in what would appear to be its most promising role—that of a micro-coin
infantry-style attack machine.
Private operators of WA-116s are Mr Ray Wijewardene, who has demonstrated one for
several months in Ceylon, and the Norfolk and Norwich Aero Club at Swanton Morley, who,
under an Air League scheme, are introducing a wide crosssection of the private flying
community to rotary-winged flight (Flight, August 12, 1965, page 264). At such an early
stage in the development of an advanced new concept in flying machines it would be
surprising if there had not been operating difficulties and in fact both the privately owned
WA-116s have been damaged in ground accidents at various times, however it is significant
that in several thousand flights in under five years by a total of over 80 pilots of every
conceivable kind of previous experience, there has not been a single accident associated
with the flying characteristics of the aircraft.
STOL take-off attitude (above) at full-power, stick hard back and rotor drive disengaged.
Hands-off landing (below) shows WA-II6 docility.
4
Invariably the trouble has been incorrect pilot action on the ground: stick too far forward on
take-off; taxying too fast downwind and trying to turn; and too much speed on the ground.
With correct handling, it is claimed, anyone of average skill should be able to fly a WA-116
from any surface over which it can be taxied. The WA-117 could well be the smallest aircraft
ever to get a C of A, but it will not be the cheapest; there may not be much substance to an
autogyro, but it needs to be made with watch-like precision. The WA-116 with a permit to
fly sold for £1,950, but WA-117s will cost nearer £2,750 apiece. Although autogyros are
expensive compared with ultralight aeroplanes, their slow-flying ability, manoeuvrability
and STOL performance (better than that of any fixed-wing aircraft) are key factors for many
aerial-work applications. In the first instance, Wallis Autogyros Ltd are concentrating on
single seaters, since it is the ability to put a man in the sky as cheaply as possible that seems
to offer the greatest unchallenged market.
The WA-116 Most pre-war autogyros had tractor engine-propeller arrangements. A pusher
system was chosen for the WA-116 on the score of compactness, because it gave the pilot a
better view, and because the noise and smells of exhaust fumes were behind him. The
consequent difficulties of engine cooling and the effect of power variations on heading
(because the force of the slipstream blows over the rudder) were considered less important.
Very little can be done to cut the overall height of any autogyro as it is largely fixed by the
propeller diameter, the pusher arrangement is not the best in this respect because of the
necessary rearwards inclination of the rotor disc. This factor is really troublesome and leads
to inefficiency when it comes to installing a more powerful engine. A four-bladed propeller
is regretfully envisaged for the WA-117.
One-, two-, or three-bladed rotor? A rigid hub or an all flapping arrangement? All of these
alternatives were considered before it was decided to aim for absolute simplicity —
whereupon the choice fell on a two-bladed rigid rotor arrangement with a simple teetering
action. Pre-take-off spinup of the rotor normally implies mechanical complication but this
highly desirable feature was arranged in an ingeniously simple manner. A rigid rotor tends
to have vibration problems but Wallis considered that simplicity was more important— in
5
any event the problem has yielded in the face of careful design and construction. Structural
testing has included strain-gauging the main fuselage and control tubes for the
measurement of in-flight loads. Rotor blade functions have been assessed photographically
as described below.
Rotor Head: The rotor head is at the heart of any rotary winged aircraft and, together with
the control system, is the outstanding feature of the WA-116 design. The gimbal head and
the control system are so proportioned that rotor drag and dynamic movements of the nonrotating structure counteract each other throughout the flight regime. For the moderate
speed range envisaged in this case, a simple fixed-geometry offset gimbal is sufficient. The
suspension geometry is the result of considerable trial and error—inherent stability is
displayed at all times and the "stick force per g" characteristics follow normal aeroplane
values. The Timken taper roller main bearing is of the "dead-axle" kind to avoid the fatigue
problems of a small-diameter revolving shaft, the axis of which is located some 2" aft of the
point where the main suspension plate is pivoted from the supporting pylon. The twin
control rods are linked through self-aligning bearings to the suspension plate a further 2" aft
of the main axle. The roll spindle axis is below that of the pitch spindle.
The rotation plane of the blades is displaced by parallel movement of the control rods
(pitch) and by opposite movement (roll) or any combination of the two. Stick loads are
alleviated by springs attached to the control rods. Ball, roller and self-aligning bearings are
used throughout the control system and there is no lost motion. To compensate for the
natural unequal lift distribution between the advancing and retreating blades the rotor is, of
course, free to pivot (teeter) in relation to the rotor head disc. Teeter action is limited
during pre-take-off up to about 200 rotor r.p.m., whereupon two spring-loaded limit-stops
fly out under centrifugal action to give full flight-teeter freedom. Re-engagement of the
stops occurs at around 180 r.p.m. This prevents any possible damage to the rotor head
while the aircraft is being taxied over rough ground. If there was ever an in-flight tendency
to exceed the teeter limits, the stops would take the initial load—providing some warning to
the pilot —before the intentionally weak pivots would shear. Thus, there is some additional
movement provided, but so far the stops have never been touched in flight. The patented
teeter stop arrangement is fail-safe—should a bob-weight or lever arm break free in flight
the pivot bolt would shear to prevem engagement of the limit stops. A simple handoperated frictionstrap rotor-brake works on the main suspension plate. A high-speed lowtorque flexible-shaft system weighing a mere 5 lb total, transmits engine power to the main
suspension plate to spin the rotor to about 280 r.p.m. prior to take- - considerably more
spin-up is available for occasional very short take-offs. A commercial epicyclic gearbox is
used to reduce the high shaft-rotation speed at the rotor head. The output pinion engages
on an internally toothed wheeI on the suspension plate/rotor spindle. At the engine end
the drive is made by running a plain rubber wheel on to a drum at the propeller hub.
6
There is no risk of torque reaction affecting control of the aircraft if the rotor drive is
inadvertantly left connected during flight, since the gearing is such that the rotor wiII always
just free-wheel even at maximum engine r.p.m.
A small electric tacho-generator is run from the suspension plate through a Tufnol gear
wheel—the only gear wheels in the entire aircraft that mesh during flight.
Rotor Blades: The 20ft 2in diameter rotor blade assembly is composed of identical halves,
each weighing 17.5 lb, rigidly connected. There is very slight washout and the aerofoil
section is flat-bottomed, with a reflex trailing edge. Streamlined mass-balance weights, to
compensate for a robust trailing-edge structure, are fitted over a leading-edge metal glove
which is bonded to the blade; careful design is needed to compensate for the high
centrifugal loads in the balance weight attachment There is also considerable internal massbalancing of the blade. As long as autogyro production is in limited quantities a wood-andsteel composite form of construction will be used. Two craftsmen make a finished rotor (a
pair of blades) from scratch in around three weeks at a total cost, including materials, of
about £350. The blades are built on a flat-surface jig in controlled conditions to ensure
complete interchangeability. For each rotor a record is kept of all material release-numbers,
of all weight and balance checks during construction, and of workshop humidity and
temperature. The assembly process begins by laying a thin birch-ply skin; on this is placed
the UHT-steel tapered insert, which is bonded into place with Aerodux 185 glue. The steel
insert is carefully balanced before assembly into the blade. Successive strips of Hydulignum
are glued down to form a solid leading-edge "D"; spruce strips act as spacers at the rear of
the box. Multiple veneers make up the root section, which is tapered into the lifting portion
of the blade. Shaping of the built-up box is performed by hand. Every set of blades is
balanced and flight tested before being covered with madapolam and doped to a high gloss
finish. The hollow portion is not directly vented—breathing through the skin is quite rapid.
The prototype blades are still in excellent condition after nearly five years of arduous
development and demonstration flying. Airframe The autogyro airframe is built from 2in dia
13- gauge (2.413mm/0.095") aluminium tube. A main tube extends from the nosewheel to
the fin and tailwheel. The seat fits conveniently forward of the vertical rotor mast, which is
braced by light alloy T-section struts. The engine is cantilevered aft to counteract the weight
of the pilot. The wide-track main undercarriage is plan-braced by trailing links and landing
decelerations are absorbed by simple coil-spring struts. The nose leg has rubber in
compression suspension. The handbrake works on to all three wheels in order to resist pretake-off spin-up forces. The nosewheel is steerable. Accommodation is decidedly minimal.
At first, pilots were seated rather like the witch on a broom stick; but a neat glass-fibre
cowling was envisaged from the start to enclose the lower part of the pilot—mainly in the
interests of aerodynamics.
Power plant: The WA-116 is powered by an extensively modified McCulloch 4318 twostroke engine of the sort originally built to power drone target aircraft.
7
Nominal power is 72 b.h.p. at 4,100 r.p.m. When it was thought that the McCulloch would
be the power unit for production autogyros, much work was done to improve the engine
from the reliability and vibration points of view; carburettor icing was also cured, and the
cooling problem was tackled to good effect. Modification work has reached "phase 5,"
where virtually only the nuts and studs remain unchanged. Full modification has been an
expensive and involved process, and at the end of it the engine is still not certified and
remains unpleasantly noisy. Though a degree of silencing may be possible, the company
feels that the work necessary would not be justified. The McCulloch-powered aircraft is
therefore seen as a special order machine which may suit certain operators.
Fortunately, the WA-116 layout is not too sensitive to the choice of powerplant and three
other types of engine have already been installed, as described in the development section.
Performance: By any standards the WA-116 has an extremely favourable power-to-weight
ratio of 7.65 lb/h.p. at gross weight!! This is compared with over 12 lb/h.p. for most
light aircraft. Speed and climb characteristics may be appreciated from a glance at the chart
below. In terms of airfield performance the low-speed capability of the WA-116 permits it to
achieve a minimum take-off distance (to unstick in still air) of about 75ft at low weight off
concrete. At gross weight off grass the distance increases to about 200ft. The subsequent
climb gradient is steep. Therefore, although the autogyro is not VTOL, its STOL performance
is superior to any fixed-wing aircraft so far demonstrated and turns many small open spaces
into potential airfields. As a weight lifter the autogyro is quite remarkable. With an empty
weight of 2341b, it has comfortably flown at an all-up weight of 6551b. With a suitably clad
pilot (say 195 lb) aboard and full fuel (58 lb), there remains some 62 lb for equipment even
before the normal 550 lb gross weight limit is reached. With a fuel consumption of 3 Imp
gal per hour, the WA-116 has an endurance of 2hr 27min—equivalent to a range of around
200 miles—assuming the pilot can tolerate the noise and buffeting.
8
Typical photo-recording of rotor blade behaviour and the information deduced—from which
blade loading is calculated and teeter action checked. initial orientation is by reference
shadows.
Wool tufts provide a visual indication of the airflow; development of the inner blade profile
might lead to useful gams in rotor efficiency. Deflection of the blade is measured by
reference to the top edge of the frame, and teeter angle from the upright rod.
For short observation missions the autogyro really comes into its own. With just a pilot, a
video camera and fuel for an hour aboard, a WA-116 has climbed from brakes-off to 1,000ft
in 56sec.
An aircraft as basically simple as the Wallis WA-l 16 obviously requires less dissection than
do most others accorded the "Flight" technical-artist treatment. But the dynamic principles
of the autogyro are perhaps as complicated as those of any other kind of flying machine.
The WA-116 embodies eight years' painstaking research and is built under several Wallis
patents. Sample blade sections and the mass balance attachment details are shown above
the control circuit schematic diagram.
9
Above is a three-view drawing of the WA-l 16.
10
11
1. Pitot-static head
2. Wool tuft drift indicator
3. Adjustable rudder pedals
4. Rudder cables
5. Self centering steerable nosewheel(bungy rubber)
6. Nose-wheel rubber shock absorber
7. Nosewheel - internal expanding brakes (all wheels)
8. Cylinder head temperature guage
9. Control stick lock
10. Rotor rpm guage
11. sensitive airspeed indicator reading accurately down to 10 kts
12. Sensitive altimeter
13. Glass fibre cowling
14. Control stick
15. Throttle
16. Fuel pressure gauge
17. Fuel tank pressurisation pump (2 Ib/sq in)
18. Seat pan
19. Simple lap-strap
20. Rotor spin-up drive control—connected by double (push-pull) Bowden cables to engine
and rotor head engagement mechanisms
21. Knock-off handbrake working on all three wheels
22. Gimbal-type control assembly
23. Undercarriage trailing link
24. Pressurised fuel tank (8 Imp gal)
25. Coil-spring strut
26. Rubber block rebound
27. Glass bowl fuel filter
28. Master ignition switch
29. Bulkhead
30. Headrest
31. T-section light-alloy braces
32. Control tubes
33. Pylon
34. High-speed flexible drive
35. Commercial epicyclic gearbox
36. Tachometer generator
37. Rotor brake
38. Teeter roller-bearing housing
12
39. Bearing adjustment
40. Rotor brake handle
41. Roll spindle
42. Pitch gimbal yoke
43. Rotor head fixing plate
44. Steel engine bearers
45. Magneto
46. Rubber insulated engine mounts
47. Spin-up disengagement springs
48. Cylinder-cooling baffle
49. Modified McCulloch 4318 72 b.h.p. flat-four two-stroke engine
50. Carburettor air intake
51. Rubber spin-up drive wheel
52. Spin-up drive drum
53. Laminated beech propeller
54. Steel mass-balance
55. 1/16" birch ply covered with fabric
56. Spruce ribs
57. Blade-lock attachment fixture
58. Tailwheel
59. Main frame 2in dia aluminium tube
60. Undercarriage pivot
61. Main landing gear
62. Teeter-stop balance weights
Future Autogyro Developments: The most important development of the WA-116, and one
that is due to fly within the next few months, is the WA-117. A prototype has completed
ground-running checks of the installation. Although the -117 will be heavier and slightly
larger it will have much the same useful load and performance as the -116. With the private
owner in mind, Wallis has plans for an economy machine (the WA-119) powered by a
modified 40 h.p. Hillman Imp engine. An airframe has already been modified for the watercooled engine, and a few tentative hops performed. The results are promising. From the
technology angle the most exciting development is the WA-118 Meteorite. This high-speed
research vehicle is about to fly, powered by an Italian 120 h.p. Meteor Alfa 1 supercharged
two-stroke four-cylinder X-engine originally built for target drone aircraft but now under
test for full certification. The Meteor Alfa employs a conventional pressure-fed oil system,
and hence does not require a petrol/oil mixture. For take-off and landing the pilot sits
upright with the canopy slid forward; for cruising flight the seat moves forward to put the
pilot in a reclining position with the bubble canopy making a cosy closure. The radial engine
is a snug fit behind the minimum-area cockpit. Most of the external structural tubing is
13
streamlined. For the first flights a standard WA-116 rotor head and blade assembly will be
used. As the speed trials progress, a variable-geometry head will be fitted which has a
sliding roll-spindle to increase the offset of the gimba! as rotor drag increases. Blade
diameter will also be progressively reduced, and the last ounces of speed potential may be
realised by retracting the undercarriage to form stub wings. The photographic analysis of
blade performance will be an important part of the high-speed research.
WALLIS WA-116: Most first reactions at the sight of an ultra-light autogyro are of horror
that anyone should fly upon so unsubstantial a piece of machinery. With the prospect of
flying one of these devices, apprehension rises to a degree which is perhaps determined by
the general level of individual inquisitiveness, at the two extremes, one is either faithfully
content to accept the instructions—taking heart from the remark that it behaves much like
an aeroplane—or one tries to understand what is happening and (may it be admitted?)
losing just a little faith in the process. Autogyro dynamics are complicated as those of any
other kind of flying machine and even to understand why the rotor blades go round in the
direction they do and not in reverse, requires faith and an involved explanation.
The rate-of-climb versus speed chart on this page gives one of the best impressions of what
an advanced autogyro the WA-116 will do. Airborne handling is more or less conventional
and far more like an aeroplane than a helicopter.
Wallis WA-116 climb and descent performance against speed for various power settings.
The curves were deduced from flight trials of the prototype G-ARRT, and were conducted at
about 1,000ft at medium weight, and in rough air.
14
IN THE AIR . . .
Beset by a powerful impression of precariousness, heightened by the unfamiliar buffeting,
the ab-inito pilot is nevertheless subconsciously relaxed—though there is no question of
releasing the grip of iron on the throbbing joystick. The airfield performance is more STOL
than that of any aeroplane, but only in a very strong wind does it approach the VTOL
capability of a helicopter. Almost as important to autogyro practicality as the patented
Wallis offset gymbal rotor head is the neat spin-up mechanism that has been developed and
patented by the company. The McCulloch two-stroke engine is hand swung; getting going
can be a one-man operation. Giving the warmed-up engine a few bursts of throttle to clear
any plug oiling, the throttle is returned to the idle position prior to beginning a gradual
upwards pull on the spin-up-drive control lever, the rotor blades having previously been set
in motion with a few hand pushes. The planet gear drive at the rotor head engages, the
rubber wheel starts to bite on the engine drum, and the airframe shudders gently as the
rotor gathers speed with continued movement of the engage lever. Spin-up drive fully
engaged, the throttle is slowly opened to bring the rotor up to 280 r.p.m. for a normal takeoff. Above 200 r.p.m. the joystick retaining catch is released and the control is pulled back;
approaching 280 r.p.m. the autogyro may slide a little on its locked wheels as engine torque
eases the foot pressure on one side. At 280 rotor r.p.m., with the stick hard back, the
handbrake is knocked off and the autogyro leaps forward as the throttle is pushed fully
open—automatically disengaging the spin-up-drive control. On concrete, unstick appears to
occur instantaneously (actually in a mere 25yd, even in still air), but on grass there is more
time to concentrate on the optimum handling and to ease the stick forward so as to balance
the aircraft on the main wheels for an ideal lift-off (this can take up to 70yd in still air on
long grass). Nothing like full power is needed to achieve a spectacular climb gradient; the
variation between the nose-up attitude at low-speed and the nose well down at high speed
is quite marked, and beginners tend to fly fairly fast. Forty knots is a good speed to do
circuits since the rate-of-climb potential is best, and, in the event of engine failure, rate of
descent is a minimum. Only a whiff of throttle is needed for level flight at 40kt. Directionally,
the WA-116 is very sensitive to the lightest toe pressure on the rudder pedals, power
variations, too, start things swinging, together with a rolling twitch from torque reaction if
the throttle is blipped on the approach. Pitch control forces immediately feel right although
roll is, not surprisingly, on the heavy side—especially to the right. The aircraft seemed
promisingly stable during a few daring moments of hands-off flight; there was barely any
disturbance on a day when it might have been bumpy in an ultra-light aeroplane. The
vibration level was also extremely low; very clear photographs have been taken with
cameras bolted to the airframe—an important factor for the kind of operations envisaged.
The WA-116 has been successfully flown in cloud as has been demonstrated during several
high-altitude flights—on one occasion to over 10,000ft. At high speed in level flight the nose
tucks well down and the pilot is pressed hard against the backrest by the slipstream —rotor
15
speed is highest in this condition, around 450 r.p.m. To reach the back of the low-speed
drag curve, power is initially reduced (to avoid climbing) and then, past the hump, the pilot
must restore engine thrust to stay level; a fair proportion of the gross weight is then
supported by thrust, and rotor speed drops to perhaps 380 r.p.m. with the nose high in the
air and forward view limited. The WA-116 will fly level at around 10kts on full power. With
power off at that speed descent rate is over 3,000ft /min—a signal for caution. There may
be some difficulty in learning the autogyro's low-speed characteristics with the limited
instrumentation fitted (sensitive a.s.i., altimeter and a wool-tuft drift indicator) and in the
absence of outside visual reference at a safe height. There is no chance of re starting a
stopped engine in flight, although there is no problem to a deadstick handling but, of
course, the flare must be precise and is complicated by the approach angles involved—
much steeper than for most light fixed-wing aircraft. Normal touchdowns from a power-on
approach at 40kt are perfectly simple —just keep the aircraft level and the ground cushion
will do the rest, as Wg Cdr Wallis has demonstrated during countless hands-off landings.
Night landings can be performed with a minimum of ground lighting.