Lone Star Rotorcraft Club
We are Chapter 62 of the Popular RotorCraft Association, located in Houston, Texas.
This page is not intended to be a text book lesson or description of aerodynamics, it is just meant as a quick, maybe even oversimplified, view of two things that you will hear about as you become involved with gyros. There are books that go into great detail about these subjects on our Books page. This particular page is meant only to give you an idea about what these terms refer to.
PIO and PPO
All aircraft have some flight characteristics that a pilot must be mindful of in order to stay out of trouble. In fixed wing aircraft for instance, one of the concerns of a pilot is a stall. A stall occurs when there is not enough clean air flowing over the surfaces of the wings to produce lift. So fixed wing pilots are aware of their particular aircrafts stall speed, and are careful during turns and landings to not allow the airspeed to drop too low.
Gyros have things to be aware of too. Pilot Induced Oscillation, or "porpoising" as it is sometimes called, is one of them. It is caused by delays in the pilots reaction time and the over controlling of the aircraft. It can be likened to an automobile that has lost traction at the rear wheels in a cornering situation. First, the back end swings one way and then the other. As the driver attempts to fight the skid he unwittingly contributes to the severity of each successive skid because his responses are "out of sync" with the vehicles movements. Rather than correct the "fish-tailing" the actions of the driver actually help the car into a spin.
In a gyro, the movement is up and down not side to side, and that is why it gets referred to as "porpoising". But the result is the same, except instead of a spin, he causes the aircraft to flip over. The pilot thinks he's making the proper corrective control inputs, but he's late and too exaggerated with his movements. At this point he actually helps the aircrafts oscillation increase until it rolls forward, or the rotor blades make contact with the propeller or tail and cause the aircraft to break apart. A gyro pilot will learn all about PIO and how to keep it from happening during training. And just like the stall of a fixed wing aircraft, it's just a characteristic of the aircraft to be mindful of.
Power Push Over, or sometimes referred to as a Buntover, is a characteristic of pusher type gyros that have a thrust line above the centerline of drag of the fuselage. PPO occurs to gyros when the lifting force and the drag of the rotors is no longer sufficient to keep the thrust of the propeller from pushing the top of gyro into a forward roll.
When designing a pusher gyro, one of the problems is how to apply thrust from the propeller to the center of the mass (and drag) that hangs below the rotor. The propeller has to be up high enough to clear the ground and the pilot usually sits low. So most designs started out with the center of thrust above the center of mass (and above the center of drag of the fuselage). This is no big deal most of the time because the lifting force of the rotor disc is angled forward and combined with the drag of the rotor itself, the top of the gyro is held back in normal flight.
The times it does cause a problem though, is when the rotor blades are unloaded and their lift and drag is greatly reduced and the engine thrust is at a high setting. This can happen when you are making a steep climb under high thrust. The pushing force on the back of the gyro is high and the lift and drag of those big rotor blades keep everything balanced during the accent. Now, if the pilot quickly pushes the stick forward before he throttles back the engine, he can unload the rotors and the thrust of that hard pushing propeller can tumble him forward. This is considered an unrecoverable situation.
As written above, PIO can unload the rotors too, and so can wind shear. Any thing or action that causes the rotors to unload can cause PPO. With proper training a pilot can avoid making the maneuvers that can cause the power push over and be ready to react to turbulence.
In addition to training the pilot to avoid these problems, designers are helping too. Early gyros did not have a horizontal stabilizer, but many people now agree that having a horizontal stabilizer greatly reduces the risk of these problems due to the engine thrust moving over the stabilizer, and have now incorporated them into their designs.
Also, most Gyro manufacturers are now producing "Centerline Thrust" models. And some even sell upgrade kits to raise the pilots seating position on older designs so that the thrust line is even with or lower than the center of drag of the fuselage.
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