Autogyro and helicopter have an almost identical appearance: both have a large diameter main rotor, which produces the lifting power, on the top. However, if we ignore their similarity in appearance, autogyro and helicopter are two different aircrafts.
As the matter of fact, autogyro is an aircraft between helicopter and airplane. Apart from the main rotor that generates the lifting power, a small propeller is also mounted on the tail of autogyro, enabling it to move laterally. In general, autogyro has a relatively smaller fixed auxiliary wing that provides the helicopter some lifting power. The biggest difference between the autogyro and helicopter is that autogyro’s main rotor and engine transmission system are not connected. The lifting power of autogyro is not generated by the engine, but, like the windmills, the airflow in the front that blows the main rotor and rotates it to create the lifting power. As helicopter’s main rotor is connected with the engine transmission system, it can, like an electronic fan, create a lifting power while powering the helicopter. Furthermore, as the main rotor can rotate by itself, only a small amount of torque is transmitted to the body of autogyro. Therefore, unlike the helicopter, autogyro does not need a tail rotor to fight against the torque of main rotor. However, most of the autogyroes have installed a tail stabilizer and control unit to control its flight attitude.
The biggest difference between the autogyro and helicopter during the flight is that the main rotor of helicopter tilts to the front, whereas the main rotor of autogyro tilts to the back. However, it is important to specify that, for some autogyro models, their main rotor is connected with the engine transmission shaft through the clutch when taking off. That is, they rely on the engine power to rotate the main rotor and further produce the lifting power. This can shorten the take-off distance, enabling the autogyro to climb up in a high angle of attack that is close to 50 degrees. However, the autogyro cannot lift in a vertical direction or remain still in the air like the helicopter (that is, “hovering”). Instead, the autogyro shall firstly reach an altitude before release the clutch and let the main rotor freely rotate in the air.
As autogyro’s lift power is mainly produced by the main rotor during the flight, the fixed auxiliary wing provides little lifting power. Some autogyroes even do not have a fixed wing as their lifting power is produced only by the main rotor.
As the main rotor of autogyro is powered by the aircraft’s forward moving speed, if its engine suddenly turns off in the air and the propeller that enables the helicopter to move laterally also stops, the autogyro will continue to fly due to inertia, and then gradually reduce its speed and height. When the flight altitude decreases, a relative airflow from down to up will also be created, enabling the main rotor to continuous rotate and keep its lifting power. In this way, the pilots can continue to control the aircraft and safely land to the ground. Even if the pilots are unable to control the autogyro or the autogyro is losing control, it can still land like a parachute. Although the aircraft will unavoidably land to the ground heavily, the autogyro will not spin around after the landing.
When there is an accident, helicopters can also safely land to the ground due to the inertia of main rotor. However, the main rotor of helicopter requires some times to transit from automatic force to rotation and will lose some height during the transition. If the helicopter’s flight altitude is not high enough, it can crash to the ground during this transition period. The autogyro, on the other hand, is powered by the rotations of main rotor. Therefore, it requires no transition and is not bounded by the flight altitude in emergency cases.
Besides, as autogyro’s main rotor is not powered by the engine, it does not have issues of big amplitude and noises created by the engine’s power driven rotor system. In other words, not only have its main rotor, body and other components relatively longer lifecycle, but also the crew members are less tired as the noises created by the autogyro’s engine when powering the propeller are relatively smaller.
Another advantage of autogyro is that its landing distance is bigger than the take-off distance. Pilots have excellent skills can immediately land it to the ground without sliding. As the matter of fact, it can be landed to rugged terrain or deck, and requires only a small space that is slightly bigger than the diameter of main rotor for the landing.
The angle of attack of the main rotor’s blades can be automatically adjusted to keep good pitch, roll and yaw stability. When the rotation hub rotates, it looks like a large inertial wheel and the rotor does not have the change of periodic cyclic pitch. Furthermore, as the installation angle of main rotor is bigger than that of the helicopter, it provides better gyroscopic effect and higher stability. Therefore, in the private autogyro flight training manual of the US, it is said “autogyro has the best stability among all aircrafts”.
Autogyro also has better anti-wind capability and needs wind when taking off. For regular autogyroes, wind can help to start and accelerate the main rotor and shorten the sliding distance of autogyro. When the wind speed reaches a level, normal autogyro can climb vertically into the air. In general, the anti-wind capacity of autogyro is better than the fixed-wing airplane of the same level and is roughly equivalent to that of the helicopter. Even more, when there is a turbulence or big wind, the autogyro can have better performance than helicopter.
There are two types of autogyro. The first one has a relatively simpler structure, and it needs to slide in order to take off and has been largely produced at the moment. The second one can climb vertically into the air in three ways: first, having its main rotor driven by the engine. Second, rotating the main rotor first until it reaches a multiple of its normal rotational speed and then releasing the clutch, meanwhile changing the pitch using the blades of main rotor to have bigger lifting power and then jumping to take off. Third, using the small rockets mounted on the main rotor tip to rotate the main rotor in order to climb up vertically. However, this is often done by automatic programs.
Autogyro has a high performance-price ratio. Despite its precious performance, its price is only about one fifth or one tenth of the helicopter at the same level. Therefore, it is very cheap, which is about the same price as the Chrysler Jeep. The autogyro that has its main rotor driven by the small rockets as described before is also known as the “tiltrotor”. Having its vertical lift capability firstly triggered by the small rockets, its fuel engine and propeller will then drive it to fly laterally. Its payload price per pound is only one third of the cheapest helicopter.
As autogyro does not have the boom, tail rotor transmission system and drop down gearbox as the helicopter, and most of them do not have the main rotor transmission system and drop down gearbox, autogyro has a relatively simpler structure, budget price and low defective rate. Besides, as it is easy and convenient to use and maintain autogyro, relevant expenses are relatively low as well.
Sources: “Flying Around the World – Blog” Autogyro.