There are only three flight controls: a control stick, rudder pedals and a throttle.
Pitch is controlled by a conventional joystick coupled to the rotor. Pulling back on the stick tilts the rotor back, increasing lift and decreasing forward airspeed. Pushing forward on the stick decreases lift and increases airspeed, as long as it is not pushed much beyond horizontal (danger!).
A simple set of rudder pedals move the rudder on the vertical stabilizer, just as on a fixed wing aircraft. The stabilizer is mounted behind the pusher propeller to maximize stability, and the rudder pedals also control the front landing gear, making it possible to steer the gyro on the ground.
The throttle and choke are mostly mounted beside the pilot for easy access.
To take off the rotor must be accelerated to produce enough lift. This is usually done by using a “pre-rotator”, normally a mechanical device that is engaged to let the engine momentaneously propel the rotor with the aircraft standing still. Once the rotor is up to speed, the pre-rotator is disengaged and the Gyrocopter is accelerated until takeoff can be done in autorotation
It is also possible to propel the machine forward and let the airflow through the blades build up rotor speed. However, this requires a very long runway.
Some few autogyros can ‘jump start’ by using a pitchable rotor, which is accelerated to high RPMs with very low pitch. The drive is then disengaged, and the rotor pitch increased. The aircraft jumps, using the stored energy, and continues then in autorotation.
When power is reduced, the forward speed decreases and the Gyrocopter goes into a steady descent path. The autorotation principle still applies, as the air flowing up and through the rotor maintains the rotor speed. The lifting force is not enough to maintain altitude, but even when the motor is stopped the Gyrocopter will descend and land safely.
In this way the Gyrocopter has an advantage over the helicopter. The helicopter’s pitch angle of the rotors is about 11 degrees, and this would quickly stop them unless the pilot quickly reduces the pitch angle the rotor blades so that the rotor autorotates. No matter how fast the pilot is, potentially essential time is lost.
Dangers of Gyrocopters
Gyrocopters can be dangerous if you try to fly them as if they were aeroplanes.
For two reasons, air must never pass through the rotor of a Gyrocopter the wrong way (negative Gs):
1. A flying Gyrocopter hangs from the rotor like an object hung on a string. As long as the plane is hanging from the rotor, stability is maintained. If air passes through the rotor the wrong way, however, it’s like trying to balance the plane, on its string, upwards. This is what’s called “negative Gs”, which is just another term for upside down.
2. The second air passes through the rotor the wrong way, from the upside down, the rotor slows down, and is also pushed down by the reversed air stream.
The reasons why this can happen are:
Pilot-Induced Oscillation, or PIO
PIO happens when a pilot adjusts his pitch too much too quickly, then makes a countering control input to bring the pitch back. The countering input often overcompensates, and the autogyro begins to buck like a bronco. You can see a similar effect when some learner-drivers are doing kangaroo-hops in a car with a stick shift and clutch. This is most likely at higher engine throttle settings. If the pilot continues to fight the plane, the rotor (which is flexible) can slow down due to the lack of positive G force, and can flop down and strike the spinning propeller, which destroys both and sends the autogyro into an uncontrolled fall. The way to avoid this during an incipient PIO is to apply gentle back pressure on the stick (to raise the nose in pitch) and cut engine power. Note that this is the exact opposite of what fixed-wing pilots are trained to do when in trouble, which has led to some unfortunate accidents and the autogyro’s undeserved reputation for being “dangerous.”
Two factors can lead to pitch instability: no or too small horizontal stabilizers (h-stabs) on too short a tail and high thrust line propeller placement which destabilises the force diagram. A large h-stab, ideally where the propeller blows on it, reduces the tendency of an autogyro to bunt over as a result of improper control input by damping the control response.
Power Push-Over (PPO)
A Gyrocopter’s climb rate or sink rate is directly coupled with airspeed. Higher forward airspeed, higher climb rate. In order to maintain level flight at high engine throttle settings, the pilot must tilt the rotor forward to prevent climbing and maintain level flight. The rotor thus becomes more nearly horizontal, and the control stick becomes more sensitive. Too much forward stick, and the Gyrocopter’s rotor can aim down towards the ground. When this happens, air passes through the rotor from the above, rotor speed drops too low to provide lift, and a high-thrust line autogyro is then pitched forward by the propeller thrust and tumbles end-over-end in a somersault. It is virtually impossible to regain control after a full PPO.
If the propeller power is above the centre of gravity for the aircraft (high thrust line) the autogyro tends to pitch forward when power is increased. If the power is above the centre of gravity for the aircraft, the Gyrocopter tends to pitch up when power is increased, which is harmless. It’s difficult to have a low thrust line without a really tall autogyro however, so most autogyro designs simply try to get the thrust line as low as possible though still being slightly above the centre of gravity.
With proper Gyrocopter training these dangers can be avoided, but they are the reason why both experienced pilots and beginners must get that training!