Gyroscopic precession in a tilting rotor?
Thread Starter
Join Date: Sep 2006
Location: Los Angeles, USA
Age: 52
Posts: 1,631
Likes: 0
Received 0 Likes
on
0 Posts
Gyroscopic precession in a tilting rotor?
I was just wondering if the effects of gyroscopic precession exist in a helicopter with a fully gimbal mounted rotor that tilts?
I was looking at the old Hiller Rotorcycle and noticed, much like the suicide machine GEN-4, that they both tilt the whole rotor in the direction they want to go, much like a trike flyer does with his wing.
Hiller Rotorcycle
Are my assumptions correct that 90 degree precession does not play a part in this type of design?
I was looking at the old Hiller Rotorcycle and noticed, much like the suicide machine GEN-4, that they both tilt the whole rotor in the direction they want to go, much like a trike flyer does with his wing.
Hiller Rotorcycle
Are my assumptions correct that 90 degree precession does not play a part in this type of design?
Join Date: Feb 2005
Location: Australia
Posts: 1,957
Likes: 0
Received 0 Likes
on
0 Posts
Are my assumptions correct that 90 degree precession does not play a part in this type of design?
however, have you ever seen a toppleless toy top.
Our toys have the advantage of all sorts of gimmicks
to cancel all sorts of normal flying problems.
What do you reckon?
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
Adam,
You are correct about the gyroscopic precession, in that the rotor responds flapwise ~1/4 resonant cycle behind the forcing input (which is close to 90 degrees for a "soft" low hinge offset rotor). Don't forget that the rotor has aerodynamic forces which generally act in a similar direction as the inertia forces, but will provide damping to the gyroscopic response (thus are altitude dependant).
So the pilot has direct control over pitch and roll rates, just like a fixed wing. The problem comes from the servo effect of the rotor being above the centre of mass - required for teetering rotors. This means there is a 0.25 to 0.5 second delay from stick input to machine response. This is why you have to "damp down" cyclic input for a stable hover - the machine is statically stable, but dynamically unstable.
You are correct about the gyroscopic precession, in that the rotor responds flapwise ~1/4 resonant cycle behind the forcing input (which is close to 90 degrees for a "soft" low hinge offset rotor). Don't forget that the rotor has aerodynamic forces which generally act in a similar direction as the inertia forces, but will provide damping to the gyroscopic response (thus are altitude dependant).
So the pilot has direct control over pitch and roll rates, just like a fixed wing. The problem comes from the servo effect of the rotor being above the centre of mass - required for teetering rotors. This means there is a 0.25 to 0.5 second delay from stick input to machine response. This is why you have to "damp down" cyclic input for a stable hover - the machine is statically stable, but dynamically unstable.
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
Adam,
I realise that i misunderstood your post - you are asking if gyroscopic precession is still relevent for a rotor with direct control.
The answer is still yes. Instead of a cyclic the pilot directly controls the pitch of the blades, but it is still 90 degrees lead. By angling the teetering pivot angle to less than 90 degrees you can also compensate for coning and wee-wa.
A search in google should find the various threads that discuss those.
PPRuNe search never seems to work that well.
I realise that i misunderstood your post - you are asking if gyroscopic precession is still relevent for a rotor with direct control.
The answer is still yes. Instead of a cyclic the pilot directly controls the pitch of the blades, but it is still 90 degrees lead. By angling the teetering pivot angle to less than 90 degrees you can also compensate for coning and wee-wa.
A search in google should find the various threads that discuss those.
PPRuNe search never seems to work that well.
Thread Starter
Join Date: Sep 2006
Location: Los Angeles, USA
Age: 52
Posts: 1,631
Likes: 0
Received 0 Likes
on
0 Posts
Thanks Graviman.
I don't understand this however. In both the Hiller Rotorcycle and the Gen-4 one can clearly see that by tilting the rotor disc forward, the heli moves forward. This seems to indicate no GP. It would be almost impossible to fly such a thing if you had to re-program your brain to give left cyclic to move forward.
What am I missing?
I don't understand this however. In both the Hiller Rotorcycle and the Gen-4 one can clearly see that by tilting the rotor disc forward, the heli moves forward. This seems to indicate no GP. It would be almost impossible to fly such a thing if you had to re-program your brain to give left cyclic to move forward.
What am I missing?
Join Date: Apr 2003
Location: Vancouver, BC, Canada
Posts: 1,635
Likes: 0
Received 0 Likes
on
0 Posts
Adam,
The first two examples on this web page may help with the following.
Disregarding its paddles, the Hiller has a 'Direct Control Hub'. It is the rotorhead control method used by gyrocopters. Operationally, is essentially the same as a conventional 2-blade helicopter with its teetering hinge and pitch-change bearings. It operates with a 90-degree aerodynamic [gyroscopic] precession.
The GEN H-4 has a 'Weight Shift Hub'. This flight control is very similar to that of the trike, in that it operates with, essentially, a 0-degree precession.
The 'Direct Control Hub' pulls the top of the mast [a force] in the given direction; after the rotor disk has tipped. The 'Weight Shift Hub' pries [a moment] the mast in the given direction.
Dave
The first two examples on this web page may help with the following.
Disregarding its paddles, the Hiller has a 'Direct Control Hub'. It is the rotorhead control method used by gyrocopters. Operationally, is essentially the same as a conventional 2-blade helicopter with its teetering hinge and pitch-change bearings. It operates with a 90-degree aerodynamic [gyroscopic] precession.
The GEN H-4 has a 'Weight Shift Hub'. This flight control is very similar to that of the trike, in that it operates with, essentially, a 0-degree precession.
The 'Direct Control Hub' pulls the top of the mast [a force] in the given direction; after the rotor disk has tipped. The 'Weight Shift Hub' pries [a moment] the mast in the given direction.
Dave
Join Date: Nov 2004
Location: Cambridgeshire, UK
Posts: 1,334
Likes: 0
Received 0 Likes
on
0 Posts
Adam, now it's your turn to misunderstand me! 
Dave's website is an excellent resource for the many possible variations in design. It is worth understanding the mechanics of those different heads.
The pilot only has direct control of the hub (or servo paddles in Hiller mechanism), but the blades are still free to flap as they wish. This means that if the pilot pitches nose down to gain speed, then so does the hub - ie no cross coupling. The teetering blades want to stay in the same plane that they were before, but the new hub attitude forces a cyclic pitch change (hence relative angle of attack) on the blades. Thus the blades respond ~90 degrees later by nutating at a constant rate in pitch, or roll if the pilot desires. The heli shaft plane then follows the rotor plane, with a small delay due to the finite angular inertia and seperation between hub and mass centre. This servo mechanism will keep going as long as the control plane differs from the rotorplane the control plane. Thus it is still gyroscopic precession.
You need to think this one through to get it - it is not immediately obvious.
Dave's website is an excellent resource for the many possible variations in design. It is worth understanding the mechanics of those different heads.
The pilot only has direct control of the hub (or servo paddles in Hiller mechanism), but the blades are still free to flap as they wish. This means that if the pilot pitches nose down to gain speed, then so does the hub - ie no cross coupling. The teetering blades want to stay in the same plane that they were before, but the new hub attitude forces a cyclic pitch change (hence relative angle of attack) on the blades. Thus the blades respond ~90 degrees later by nutating at a constant rate in pitch, or roll if the pilot desires. The heli shaft plane then follows the rotor plane, with a small delay due to the finite angular inertia and seperation between hub and mass centre. This servo mechanism will keep going as long as the control plane differs from the rotorplane the control plane. Thus it is still gyroscopic precession.
You need to think this one through to get it - it is not immediately obvious.
Last edited by Graviman; 2nd Apr 2009 at 11:29. Reason: Tried to make explanation a little clearer - if that helps.
