Zen and the art of the autogiro


I've always been fascinated by autogiros, even when I didn't know what they were. The film 'Shape of things to come' made circa 1936 (I wonder if it influenced Ken Wallis?) has a wonderfully modern electric autogiro that someone should model.

Yes, an electric autogiro in 1936. But then 'Aeromodeller' actually flew an electric ducted fan in 1946. Does the word 'Vintage' have any real meaning?

Sometime in the late 1940's I built an autogiro published in 'Indoor Flying Models' by Ron Warring (one of those large Harleyford books). Not really knowing what an autogiro was I added another rubber motor to drive the rotors. Of course it didn't fly!

When the Rodart was published in 'Aeromodeller', I built one for a Mills 0.75. It would loop (into the ground), roll and try to bunt, but never flew. I got fed up of repairing it and turned the rotor pylon into a wing pylon, built a pair of wimgs and flew it as a pylon model. With the help of the then-fashionable pendulum elevator, it would prop hang. So much for modern R/C.

In the 60's I was a member of the Larks club and witnessed the many attempts to get the prototype Chippiwa to fly. It left the ground many times but invariably crashed. It would complete most of a circuit but, on turning into wind, would rear up and crash. Tilting the rotor to get the nose down simply didn't work, but it did tend to stop the rotor. I was amazed when the design was published. It's worth explaining here that lateral control was effected by having two large horizontal tail surfaces working in opposition and fore/aft control was acheived(?) by tilting the rotor backwards and forwards. In the light of hindsight, the other way around might well have worked.

When I was working for 'RCM&E' in the early 80's I had the chance to review the Micro-Mold Wallis, designed by Roy Sturman. I saw Roy fly his many times and noted that he always reduced the throttle after take-off. When I flew the review model I realised why. It needed every last scrap of power available to get the rotor speed up and get into the air (a 60 size engine turning an 11x7 pusher prop AND spinning up a 63" rotor). Once it was flying it was dramatically overpowered and highly unstable in pitch. It also had the Chippiwa characteristic of rearing up uncontrollably. I now realise that the solid obeche blades were a liability and a set of helicopter blades with a forward CG might well have cured it. Take-off to crash ratio was about 3 to 2.

Whatever, controlling an autogiro with a fixed pitch heli head does not give enough control power and every control input moves the paddles, adds drag, and slows the head down.

When the electric RTP craze hit us, I built a half-size version of a C/L giro published in an 'Aeromodeller Annual' which flew quite well. It would tear around on the floor until the rotors wound up and would then leap into the air and stop! It then flew very slowly in a nose high attitude.

Bill Hannan has published several F/F giro designs usually around peanut size. One of these is the Tyro-Gyro. I found that it was a pig to trim but very consistent once trimmed (it needed a lot of lateral offset on the rotor head). It was always flown indoors and on one celebrated occasion hit the rafters and dived vertically into a cardboard box containing Mick Wilshere's EZB. There were lots of turns on the motor and it produced lots of thumping noises accompanied by bits of Mick's model flying out of the box. The gyro was unscathed but Mick wasn't happy!

Nowadays, in the USA, you can buy an autogiro conversion kit for just about anything from a Lazy Bee to a Sig Rascal. So, thought I, what about a Pico-Stick? I made a typical tail assembly from 3mm depron to replace the original tail and also made a short set of wings to make it look like a typical full-size machine. Apart from rudder and elevator controls, it had lateral tilt on the rotor to give roll control. The rotor blades were also of 3mm depron with ply root reinforcement and attached to the hub via 16 SWG wire to give some flexibility.

Pico-Gyro Pico-Gyro

The intention was to fly it with the standard wings, then switch to the smaller wings and finally to attempt to fly it with just the rotor. It flew beautifully with the standard wings - why not, it was a standard model plus a rotor. It could be turned with both the rudder and the rotor tilt and would do vertical descents. With the short wings it steadfasttly refused to fly. I tried adding power by changing from the GWS ILS unit to a geared 280 motor, without success. Needless to say it didn't fly with just the rotor.

I used the conventional modelling approach of having negative pitch on the blades and raking the rotor axis back to give easy rotation of the rotor, knowing that this produces a lot of drag. I was aware that the full=size machines actually used a symmetrical blade set at zero pitch. This setup does not 'autorotate' easily but gives less drag and reduces the rolling effect. I tried a new rotor with this setup. It needed a preliminary spin and then wound up so hard in a light breeze that it took the tail off! I decided that a much taller rotor pylon was needed and converted the model back into a standard Pico-Stick.

Pico-Gyro Pico-Gyro

Now, the original full-size machines gave/had lots of problems but, once sorted, could be flown by any pilot with a few minutes instruction. Most early examples used an existing fuselage/tail unit plus a fixed rotor. Some retained the wings and used the ailerons for roll control, some dispensed with the wings and used ailerons on outriggers. The next stage was to tilt the head both laterally and fore/aft for control.

With a rotating wing, forward flight causes a rolling effect due to differential lift - the forward moving blade producing more lift than the aft moving blade. Various means were tried to combat this, including making one side of the tail an inverted section. The eventual solution was to make the blades flap up and down so that the advancing blade would flap up more and dump lift. Originally, this was done very simply with a spar which bent - it didn't last long!

Moving back to models, single rotor machines have always been regarded as 'difficult'. The basis of most of the available kits is to have conventional tail controls plus lateral tilt of the rotor. There are lots of videos to be found on the web showing that they do work.

When I saw the Monotwirl in a recent magazine, I had to build one. It is all depron in construction and very similar to the current 'Shock Flyers'. Mine came out at 165 g (5.9 ounces) with a CD-ROM motor and 1200 mAh 2 cell Li-Poly battery. A power/weight ratio that will prop hang and doesn't need the rotor. I have to say that it is almost the worst plan I have ever seen with some inexplicable notations and some curious construction features that seem to deny the existence of friction. So, I had to build it more or less exactly as the plan to prove that it couldn't fly.

Monotwirl Monotwirl

On the first flight, it took off into a near vertical left bank (the rotor is anticlockwise) but was controllable via the rotor tilt and I managed a left circuit holding on lots of right aileron and got it down in one piece, merely knocking the motor off.

The second flight resulted in a low level circuit with the model refusing to climb, which I put down at the time to low temperature and an old Li-Poly battery.

More flying revealed serious problems with the structure of the model and a marked reluctance to climb or even raise its nose. It flew very fast and all landings were 'arrivals'. The general impression was of a model which had very small wings. I remain convinced that any successful model autogiro needs a ballraced rotor hub. After many adventures it actually managed to consume four rotor assemblies, including throwing a blade in flight. My original misgivings about gluing a spruce spar to a depron rotor hub were entirely justified.

As to that 'well-known' rolling tendency, if only poor Cierva had known. All he had to do was tilt the rotor to one side and his troubles would have been over - I think.

It was never intended that this page should concentrate on one model, so more on the Monotwirl can be found on its own page here.

My fascination with autogiros has now been reawakened and I intend to continue my attempts. Watch this space - but don't hold your breath.

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