ferranrosello
Member
- Joined
- Nov 14, 2005
- Messages
- 398
- Location
- Madrid
- Aircraft
- Ela 07
- Total Flight Time
- FW: 600, HELOS: 3550, GYROS: 3020
I’ve read a lot of threads about gyro’s stability. Some of them are really good, but it is very difficult to explain gyro’s stability and control by moments acting on cg.
I’m a pilot, not an engineer, and I’m more interested in understanding physics of flight than in accurate calculations involving cg’s moments and arms. I think there is an easier and more understandable explanation of gyro’s stability and control. But, it is needed a whole picture vision of gyro’s flight. Please, don’t focus just on cg alone.
Control
We all are used to study gyro’s control by moments acting in the three axis (pitch, roll and yaw). This is no bad, but it is FW culture, and gyro’s control does not have the pitch and roll axis so neatly independent as planes do.
The way in which a rotary wing aircraft is controlled in pitch and roll is by changing rotor’s aerodynamic force direction (by moving the rotor disc). The fuselage, which is hanging from rotor head by a U joint, will follow the rotor’s new direction until it aligns the resultant vector acting on cg (weight plus fuselage drag and power thrust).
In this way it is easy to understand the control mechanics. When you tilt the rotor disc you are tilting the aerodynamic force vector. The fuselage will react aligning the weight plus thrust and drag vector. In simple terms, it will try to maintain the cg aligned with the rotor thrust vector.
Stability
What will happen if a turbulence creates a sudden change in the rotor aerodynamic force?
This is the main question when talking about stability (but not the only one). The good reaction happens when an increase in lift produces a reduction in the rotor disc AOA (this is call statically stable).
A helicopter is not statically stable. After a change in relative speed, the aerodynamic force vector will change and an acceleration will occur. The rotor will move in the space and nothing is going to reset the original trimmed condition (unless you have a wonderful AFCS fitted).
But it doesn’t mean that the helicopter is unstable. But, what happens to an isolated fixed wing?. When the wing’s AOA is increased (because of a wind gust), the wing will suffer a positive pitching moment. So it is not possible to fly a plane without an appropriate horizontal tail surface.
And an autogyro? This is a different story.
Because of the gimbals between pitch and rotor axis, the gyro’s rotor head is positively stable versus AOA. That means that the autogyro’s rotor is statically stable. But it does not mean that the whole aircraft will be stable in all conditions.
The cg position versus rotor thrust is not important for gyros stability. Cg will tend to be aligned with rotor thrust. But cg position is important in order to ensure full control capability. Longitudinal extreme cg locations may reduce control authority, but not stability.
The real problem in a teetering rotor is flying in sustained (several seconds) low g environment. Then the rotor aerodynamic force vector is not there any more. And without this vector the cg has no reason to keep aligned. And then is when the high thrust line and fuselage drag becomes alive.
If this is the case, the only way to stop a sinking nose is an appropriate horizontal surface. Two additional benefits are an improvement in pitch stability and a reduction in the time lag between control inputs and fuselage attitude. Makes flying an autogyro easier and safer.
I hope this lines can help in understanding gyro’s control and stability.
Regards, Ferran.
I’m a pilot, not an engineer, and I’m more interested in understanding physics of flight than in accurate calculations involving cg’s moments and arms. I think there is an easier and more understandable explanation of gyro’s stability and control. But, it is needed a whole picture vision of gyro’s flight. Please, don’t focus just on cg alone.
Control
We all are used to study gyro’s control by moments acting in the three axis (pitch, roll and yaw). This is no bad, but it is FW culture, and gyro’s control does not have the pitch and roll axis so neatly independent as planes do.
The way in which a rotary wing aircraft is controlled in pitch and roll is by changing rotor’s aerodynamic force direction (by moving the rotor disc). The fuselage, which is hanging from rotor head by a U joint, will follow the rotor’s new direction until it aligns the resultant vector acting on cg (weight plus fuselage drag and power thrust).
In this way it is easy to understand the control mechanics. When you tilt the rotor disc you are tilting the aerodynamic force vector. The fuselage will react aligning the weight plus thrust and drag vector. In simple terms, it will try to maintain the cg aligned with the rotor thrust vector.
Stability
What will happen if a turbulence creates a sudden change in the rotor aerodynamic force?
This is the main question when talking about stability (but not the only one). The good reaction happens when an increase in lift produces a reduction in the rotor disc AOA (this is call statically stable).
A helicopter is not statically stable. After a change in relative speed, the aerodynamic force vector will change and an acceleration will occur. The rotor will move in the space and nothing is going to reset the original trimmed condition (unless you have a wonderful AFCS fitted).
But it doesn’t mean that the helicopter is unstable. But, what happens to an isolated fixed wing?. When the wing’s AOA is increased (because of a wind gust), the wing will suffer a positive pitching moment. So it is not possible to fly a plane without an appropriate horizontal tail surface.
And an autogyro? This is a different story.
Because of the gimbals between pitch and rotor axis, the gyro’s rotor head is positively stable versus AOA. That means that the autogyro’s rotor is statically stable. But it does not mean that the whole aircraft will be stable in all conditions.
The cg position versus rotor thrust is not important for gyros stability. Cg will tend to be aligned with rotor thrust. But cg position is important in order to ensure full control capability. Longitudinal extreme cg locations may reduce control authority, but not stability.
The real problem in a teetering rotor is flying in sustained (several seconds) low g environment. Then the rotor aerodynamic force vector is not there any more. And without this vector the cg has no reason to keep aligned. And then is when the high thrust line and fuselage drag becomes alive.
If this is the case, the only way to stop a sinking nose is an appropriate horizontal surface. Two additional benefits are an improvement in pitch stability and a reduction in the time lag between control inputs and fuselage attitude. Makes flying an autogyro easier and safer.
I hope this lines can help in understanding gyro’s control and stability.
Regards, Ferran.