WASHING LINE?

AUTOGIRO WASHING LINE THEORY!

STEVE MIDSON SHARES HIS THOUGHTS ON DEVELOPING A SMALL FF AUTOGIRO

The Covid 19 shut down was supposed to give us modellers a fantastic opportunity to make the models we have been meaning to for years! Along with others have spoken to, with no fl ying to work towards I had lost a lot of enthusiasm and done nothing, until the May 2020 issue of AeroModeller arrived with a feature on a control line autogiro and I started experiments with a FF electric version.

I have always been intrigued by autogiros but apart from a very heavy autogiro kite that quickly shook itself into parts, I had not done anything serious. I have seen powered C autogiros at fl ying displays, also a successful FF model of a Pitcairn at the Crawley meetings. A 'Google' study of autogiros was fascinating (including a hang glider version), all of which showed the rotor blades up at an angle – the so-called coning angle.

We are used to having propellers that are rigid and strong enough to transmit their thrust to the hub without bending forward, but we all know that swinging a weight in a circle produces a pull at the centre – centripetal force (you can call it centrifugal if you like but that is not strictly correct!) It appears to me that the coned angle autogiro utilises this pull to lift the autogiro, like a model hanging from a washing line! Hence the article title! Indeed, it is this pull which lifts the autogiro, the aerofoil blades only lift themselves and e ciently create a windmill!

How does the rotor turn? The rotor disk is set back about 10 degrees so the airfl ows upwards through it, and the blades are set at a negative angle of attack causing them to pull forward. The advancing blade at o clock position has an airfl ow at right angles to the blade, and sees the addition of rotating speed plus air speed, similarly at 9 o’clock it sees the subtraction but rotation speed is faster than airspeed so the blade is still pulling. At both the 12 and 6 o’clock the airfl ow across the blade is at an angle which eff ectively thins the aerofoil section, but it is still pulling forward.

This variation in airspeed across the blade causes the advancing blade to lift more and for lift to reduce at other positions, hence to stop the model rolling over the blades must be free to move up and down – be hinged. Any hub design must support the blades from drooping and hitting the tailfi ns!

Many years ago I had a toy with 4 blades that was catapulted up with the blades folded then as air speed dropped they sprung open and it autorotated down. I clearly remember that the hinges were at an angle which meant the blades laid fl at at launch then took on a negative angle of attack when opened. The same principle can be used here. By hinging the blades at an angle the negative angle of attack will increase as the blades move up. Similarly if the rotor slows up, as when the pulling power runs out, the increasing negative angle of attack will speed it back up again. Indeed, the amount of angle will have an eff ect on the normal running speed of the rotor.

How much total lift will each rotating blade produce? I now have to apologise to those (like me) who were brought up with feet, inches, and pounds. Modern mechanics calculations produce an answer for force in 'Newton'. The centripetal pull at the hub can be calculated with the formula: pull (N) = mass (kg) x velocity of the centre of gravity (m/sec) squared, divided by the radius of centre of gravity (m)

And the velocity is 2 x the radius of the centre of gravity, times rotations per second

You do not have to do the calculation but it can be seen that the weight of the blade is best at the tip. The 'Lift' produced by this

Steve Midson likens the lift of an autogiro to suspending the fuselage by its rotor spindle from a washing line! pull is of course reduced by the 'sine' of the blade coning angle.

I discovered long ago that a simple rubber powered helicopter was unstable until tip weights were added at the blade ends, which produce a gyroscopic/stabilising eff ect and moved the centre of gravity outwards. 2, 3 or 4 blades? 3 blades are obviously going to be di cult to balance. The majority of full-si ed autogiros have just 2, but 4 means more pull/lift!

So to the model design. Early full-sized designs had small wings with excessive dihedral tips – not seen on modern designs. What is common to all is an eff ective tail fi n often in the propwash of the pusher propeller, but if our model is to be front propeller, big tail fi ns will be needed for directional stability.

P.S. Later trials were with the hinges of the rotor blades set at an angle, so called Delta Hinge.

This gives the eff ect that the blade angle reduces as it lifts up, and hence accelerate the blade thus improving starting run up. Unfortunately, the blades also move back unbalancing the disc which vibrates horribly anyway! More model details will follow when experiments, have been completed, and success or failure discovered! (Editors Note: My experience is with the much easier twin rotor autogiros which have no coning angle. Can anyone share their road to success with single spindle FF ‘giros?) ■

Steve’s latest is a contra-rotating autogiro where

the tor ue e ects cancel out promising but

more development required as the lower blades hit the upper!