An autogyro is an aircraft that uses a freely spinning rotor to produce lift while a separate propeller provides forward thrust. It looks partly like a small airplane and partly like a helicopter, but its rotor is not powered in normal flight. This design mattered because it made slow, stable flight possible before practical helicopters became common.
The autogyro helped engineers understand how rotating wings could safely lift an aircraft.
Key Facts
- In normal flight, the autogyro rotor is unpowered and spins by autorotation.
- Forward thrust comes from a propeller, often mounted at the front or rear of the aircraft.
- Lift on a rotor blade follows L = 1/2 rho v^2 S CL.
- Rotor angular speed is related to blade tip speed by v = omega r.
- Weight balance in steady level flight requires Lift = Weight.
- An autogyro usually cannot hover because its rotor is not powered to force air downward while stationary.
Vocabulary
- Autogyro
- An autogyro is an aircraft with an unpowered rotor for lift and a propeller for forward thrust.
- Autorotation
- Autorotation is the spinning of rotor blades caused by airflow passing upward through the rotor disk.
- Rotor disk
- The rotor disk is the circular area swept out by the rotating rotor blades.
- Thrust
- Thrust is the forward force produced by a propeller or engine that moves an aircraft through the air.
- Lift
- Lift is the upward aerodynamic force that supports an aircraft against gravity.
Common Mistakes to Avoid
- Thinking the autogyro rotor is powered like a helicopter rotor is wrong because, in normal flight, the rotor spins freely due to airflow.
- Assuming an autogyro can hover is wrong because it needs forward motion to keep air flowing through the rotor for autorotation.
- Forgetting that the propeller supplies thrust is wrong because the rotor mainly supplies lift, not forward driving force.
- Treating the rotor like a fixed airplane wing is wrong because each blade moves in a circle and experiences different airflow around the disk.
Practice Questions
- 1 An autogyro has a mass of 600 kg. What lift force is needed for steady level flight? Use g = 9.8 m/s^2.
- 2 A rotor blade tip is 4.0 m from the hub and the rotor spins at 35 rad/s. What is the blade tip speed?
- 3 Explain why an autogyro can keep its rotor spinning in forward flight even though the rotor is not powered.