Possible Problems with an Autogyro
This is caused by an interaction of the rotor and the undercarriage. If the rotor is brought out of balance (for instance, because the blades are not properly aligned), and if the undercarriage's natural frequency is close enough to the rotor frequency, the undercarriage will amplify the periodic rotor movement and also aggrevate the unbalance of the rotor.
- Before flight: check if the blades are exactly opposing each other. Badly opposing blades do not give you ground resonance, but they will start the phenomenon if it is present.
- In general during take-off or landing: Remember that it is the rotor speed that amplifies the effect. So change the rotor speed as soon as possible (using the rotor brake or throttle).
Ground resonance was a problem during the first days of autogyros and helicopters, but it is now fully understood. If your autogyro suffers from this phenomenon at normal operating rotor speeds, something is definitely wrong with it!
Design the undercarriage in such a way that its natural frequency will differ enough from the operating rotor frequencies.
Pilot induced oscilation is a result of the "response time" of the autogyro. If the autogyro is brought out of balance (for instance, by a gust), the pilot will try to bring it back into the original flight position. However, the autogyro does not react immediately. Because of the lack of immediate response, the pilot might over-react this correcting movement, causing an even greater unbalancing movement at the moment the autogyro does react. The pilot might overcorrect again and again, causing the oscilation to grow to fatal magnitude.
Avoid sudden movements. Remember that a rotating object always reacts a quarter of a cycle later (see "How to behave if you are rotating
"). This rule applies not only for the position of the blades, but also in time. If you pull the stick back and forth within a quarter of a rotor cycle, your autogiro won't probably even notice.
When you encounter this phenomenon during flight, the most difficult part is to recognise that you, the pilot, are an active part in it. Pilots always want to be in control (which is good!) and therefore automatically counteract all distubances. This is clearly the time you should not
do that. It sounds very dangerous to let go of the controls during a growing banking oscilation, but it really is the solution! Your autogiro is a stable aircraft, so it will find a right flight path by itself.
It is not the full solution, but it is wise to make the combination of fuselage and tail just stable for banking (not too stable). The effect of this is that PIO can not be started by a severe gust. Mounting the tailplanes in the propeller slipstream will make them more effective.
A power pushover is a situation where the rotor is, in effect, uncoupled from the aircraft. What really happens is that if you unload the rotor (by making a zero-g manoeuvre) the rotor drag almost vanishes and the thrust of the engine will make the aircraft duck. This will only happen if the center of gravity lies below the line of thrust. In the figure, the center of gravity is denoted by the circle-with-blocks symbol.
- Avoid flying at zero-g! It is dangerous! You can get in a situation where you have little or no control over your aircraft. On top of that, you can encounter some serious mast-bumping, which can damage your rotor head.
- If you are in a PPO: reduce power immediately.
- Take great care that the center of gravity of the whole aircraft remains somewhat above the line of thrust at all loading conditions. This is not as simple as it sounds! It means the designer (and the pilot) should know the vertical position of the center of gravity and should be able to adjust it (or to adjust the line of thrust).
- A horizontal stabiliser only helps at high speeds. As the phenomenon is largely speed-independent (the drag forces are small with respect to the thrust), mounting a large horizontal stabiliser does not overcome the problem for moderate or low speeds. Horizontal stabilisers should only be mounted to correct the aerodynamically unstable shape of the fuselage.
This results from the gyroscopic effects of the propeller, which is not articulated.
If you are not familiar with gyroscopic effects, please read the chapter "How to Behave if you are Rotating
Be careful with sudden banking movements, especially during a landing.
Mount the rudder in the propeller's slipstream, so that remains controllable at all flying speeds.
You know by know that the rotor has built-in hinges to work properly. These hinges give the blades some freedom, but not unlimited. If the blade movements are too large, the blades will hit the stops. This is called mast bumping. Some autogyro pilots call it "blade flapping", but I'd rather use that term for the normal flapping movements of the blades. Normally, the blades are held outward by the centrifugal force acting upon them. They are also "held back" by the load acting upon the rotor. It is therefore that mast bumping occurs at low rotor speeds and/or unloaded rotor situation (zero-g manoeuvre).
keep your rotor loaded and spinning during flight. On the ground, avoid low rotor speeds. Have you ever wondered why people getting in or out of rotorcraft keep their heads down although they could easily stand upright? On the ground, with the rotor partially unloaded, the blade flapping movements are much larger than in flight and the blades could easily hit somebody! In this state, the rotor is very sensitive to cross-winds.
- Give the blades enough room to perform their normal movements.
- Choose the disc loading high enough.
- If all else fails: try using tip weights.
|the power curve|
Well, here it is: the power curve. There are two lines in the power curve:
- The line of the available power (continuous line) and
- the line of the required power (dashed line).
This chart is drawn only for stationary horizontal flight
, which means that the pilot keeps on flying at the same altitude with the same speed. The chart shows that both the power available from the engine and the power required to overcome the drag increase with increasing airspeed. Yes, the engine becomes more effective when flying faster! Sadly, the power required to fly at higher speeds increases more rapidly, due to the increase in drag.
At the maximum speed, the two lines intersect and it is clear that the engine cannot develop enough power to go any faster.
But what happens at minimum speed? To maintain a slower speed at a given altitude, the pilot has to reduce the power and increase the angle of attack (pull back the stick). At very low speeds, the angle of attack of the rotor becomes so large that it is mainly giving drag instead of lift. In fact, the engine is taking over quite a portion of total lift of the aircraft. This also means that the pilot is unloading the rotor, which is really unwise. Unloading the rotor will reduce he rotor speed. If the pilot could go to zero airspeed, you would see an autogyro standing on it's tail, without the rotor turning! Did I say autogyro? It is a helicopter by now! Obviously, the pilot can't get that far.
At speeds that are less then the minimum speed, it is impossible to maintain both speed and altitude. At the point of minimum speed itself, the pilot is flying with full throttle, just as if he were flying at maximum speed! The region of speeds that are less then the minimum speed is called "behind the power curve". Flying behind the power curve is a phenomenon that does not only occur with autogyros; some fixed-wing aircraft have it as well. However, the minimum speed for fixed-wing aircraft often nearly coincides with the stall speed. As autogyros simply do not stall, the phenomenon is better known with autogyros.
Push the stick forward to gain speed. It will make you loose altitude, but you will loose altitude anyway. As you are probably already flying full throttle, your engine power can't help you anymore at this speed. (Some fixed-wing airliners have automatic "stick-pushers" to prevent the pilot from flying behind the power curve.)
I think the solution is worse than the problem here. You could design an automatic stick-pusher, but it will probably be activated in every good landing. It is normal to fly behind the power curve during the last phase of the landing, just as it is normal for fixed-wing aircraft to stall during the last phase of the landing.
Back to the table of contents