# How do helicopters maintain a certain rotor pitch?

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#### Chlomo

##### Well-Known Member
On rotors of large complex helicopters I'm sure there are hydraulic devices that hold the required pitch as dictated by cyclic input by the pilot. Anyone care to share how that works mechanically? Especially with the more modern non symmetrical airfoiled rotors.
How do rotor systems resist such twisting force?
Even curious are those small single place kit copters and they don't seem to accommodate large space for hydraulic machinary.. Since their rotors are mostly symmetrical I assume an angle is needed to generate lift.

Those who are familiar with hydraulics please share with me! My friend Google tells me what hydraulic drift is but offers little explanation as to how they lock and transmit power in one direction only.

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#### Chlomo

##### Well-Known Member
Yes when filled with fluids and valves locked the hydraulic cylinder will resist outside input. My specific question is how is it achieved when the hydraulic circuit is in constant small adjustments somewhat analogous to 'signal debouncing(?)' in electronics.

EDIT: This is apparently very similar to power steering in automobiles I will start again in that direction!

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#### TFF

##### Well-Known Member
Works just like power steering on a car. Each rod connecting to the swashplate has it's own two way cylinder. Collective may or may not have one. Just depends how big a helicopter. Robinson R44Raven II is the smallest helicopter I know that has hydraulics.

#### Blue Chips

##### Well-Known Member
Chlomo,
Exactly how hydraulics are incorporated I can't add anything, sorry.
That said, there is nothing to gain but weight, complexity and expense going to a hydraulically controlled rotor system over purely mechanical on small helicopters. Whether hydraulic, servo or mechanical all conventional helicopters directional and altitude control goes through a swash plate system of one design or another. Large helicopters incorporate other then purely mechanical because of the very high input control forces required. There are a number of YouTube videos on the subject of how a Swash Plate system works.

Ken

##### Well-Known Member
Yes when filled with fluids and valves locked the hydraulic cylinder will resist outside input. My specific question is how is it achieved when the hydraulic circuit is in constant small adjustments somewhat analogous to 'signal debouncing(?)' in electronics.

EDIT: This is apparently very similar to power steering in automobiles I will start again in that direction!
The correcting movement rate is dictated by size of the opening on the servo valve, which controls flow-rate and therefore how quickly the actuator moves. Open the valve a little, the actuator moves slowly. Open it a lot, the actuator moves. This size of the opening, in turn, is determined by the mismatch between the input and the output. In the case of power steering, it's the angular mismatch between steering wheel and steering rack. This is made possible by a torsion bar, which also provides feedback and a way to move the steering rack if the hydraulics give out.

When you refer to "signal debouncing", I'm assuming you're referring to damping the output so the system isn't forever "seeking" the optimal position. In this case, varying the size of the valve opening provides the "gain". In other words, what must be programmed artificially in electronic systems (to prevent electric servos from overshooting the desired position and forever draw electrical power) is somewhat inherent in a hydraulic system.

In any case, you figure out the oscillation in the control system by finding the spring constant (k), damping factor (c), and mass (m) of the control surface...then plugging it into the top equation:

and then solving the differential solution.

In short, hydraulic systems are more inclined to be dynamically stable (control oscillations die out) due to the variable gain of the control valve and the natural deadband in most control systems. At least in my opinion. Thoughts guys?

#### BoKu

##### Pundit
HBA Supporter
...Even curious are those small single place kit copters and they don't seem to accommodate large space for hydraulic machinary...
I could be wrong, but I think the vast majority of small and even medium sized helicopters have completely mechanical controls with no hydraulic augmentation.

Thanks, Bob K.

#### Dana

##### Super Moderator
Staff member
The pilot adjusts the collective pitch in conjunction with the throttle to maintain the desired rotor rpm. It's a completely mechanical system on most smaller helicopters. Later choppers, like the Robinsons, have governors to maintain rotor rpm, though I don't know the details.

Dana

#### TFF

##### Well-Known Member
Small helicopters dont use the flatter bottom airfoils. Remember just like your car, if the hydraulics fail you still have to be able to control. Just more of a workout.

##### Well-Known Member
I could be wrong, but I think the vast majority of small and even medium sized helicopters have completely mechanical controls with no hydraulic augmentation.

Thanks, Bob K.
Only very small helicopters can manage without hydraulics, some examples -

I don't know of any single seat or two seat helicopters that have hydraulics.

The Hughes 300 series of three seat helicopters don't have hydraulics - except a hydraulic damper for each blade in the reticulated rotor system.

The Bell 47/Kawasaki KH4 (3 and 4 seats) do have hydraulics.

The Hughes 500/McDonnell Douglas MD 500 (5 seats) does not have hydraulics.

The Jetrangers/Longrangers (5 seat/7 seat) do have hydraulics.

The Eurocopter Squirrel (5 seat) does have hydraulics.

Of the above there are three different types of rotor systems, the small single seat and two seaters generally have teetering two bladed systems but they are small and light enough to control without hydraulic assistance.

The Bell 47/Kawasaki, Jetranger and Longranger all have two bladed teetering rotors and they are too heavy to control without hydraulics except to land them in case of hydraulic failure.

The Hughes 300 and Hughes 500/MD500 all have the more sophisticated Sikorsky designed articulated rotor systems which are heavy but the blades are individually hinged to allow them to flap up and down and also to lead and lag fore and aft. Because of that freedom they balance themselves by flapping to symmetry of lift at all parts of the circle and also lead/lag to dynamic balance during any form of acceleration so they do not feed much force back into the control system and hence they do not need hydraulic assistance until they are much larger i.e. 8-10 seaters and above.

The Eurocopters have a semi-rigid rotor system where the 'starflex' hub is made from composites and they do not teeter or articulate, instead the blades are 'warped' by the control system for both collective and cyclic pitch changes, they do have hydraulic assistance.