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Cyclic/Swashplate Position


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While the whole idea of lift remaining the same and the actual rotor disk changing it angle took me a while to grasp I do understand it. However if you input forward cyclic what side of the swash plate actually is pitched up. The book I am reading says that if forward cyclic is input 90 degrees left of the nose the blade is at its greatest pitch angle. Wouldn't this mean that the left side of the swash plate is raised?

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That sounds all wierd to me.

 

If you want to raise the nose, then you need to increase the pitch approx. 90 degrees before the nose. (Over the right side in a ccw rotating heli.)

 

If you want to bank left (raise the right) then you need to increase the pitch approx. 90 degrees before the right. (Over the tail boom in ccw rotating heli.)

 

\Joker

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Ok so say you are raising the nose. This is aft cyclic correct? If you are increasing the blade angle over the right side of the aircraft isnt the swash plate raised on the right side? Perhaps I am poorly wording my question, if you give aft cyclic does this raise the aft part of the stationary swash plate or the right side of it?

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Ok so say you are raising the nose. This is aft cyclic correct? If you are increasing the blade angle over the right side of the aircraft isnt the swash plate raised on the right side? Perhaps I am poorly wording my question, if you give aft cyclic does this raise the aft part of the stationary swash plate or the right side of it?

Yes, in a counter clockwise rotor system aft cyclic will raise the right side of the swash plate which will increase the blade angle on the right side. However, the extra lifting force of the increased blade angle takes effect 90 degrees later in the rotation so the nose will pitch up in this case. This is called gyroscopic precession. If you have a clockwise rotor, aft cyclic will increase blade angle on the left side.

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It isn't gyroscopic precession.

 

That is the poor man's way of describing it, to give some way of understanding it for non-rocket-scientists.

 

The advance angle varies from aircraft to aircraft, and is only approximately 90 degrees. You put in a control input, the blade angle changes to generate a lifting force, the force takes time to act (f=ma), and the blade finally moves. It takes around 90 degrees of rotation from the place where the input is generated until the place where the maximum deflection has happened. Gyroscopes ain't got nuttin' to do with it. But it helps to understand it. :rolleyes:

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Another way to think of it is that it will take time to fly that blade to that position. While we think of it as instantaneous inputs, it has been a progressive input over the course of several revolutions of the rotor system. Because of the phase shift between an input and the max deflection that results from that input, pitch change tubes lead the blades in the rotation (like leading an animal on a leash), that also gives the swashplate a near normal looking input for two-bladed rotor systems.

 

"Poor man's way of describing it..." I liked that, EH! :)

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But the RFH uses Gyroscopic Precession, so, will you ever escape the poor man's way of describing it? Probably not, but you can escape that understanding of it.

 

Gyroscopic Precession, or rather just precession, describes inducing a torque at one point on a rotating body and having it manifest 90 degrees later. Phase shift, or phase lag, describes two cycles that are not simultaneous but that mirror each other; commonly used in electronics in the discussion of sine waves. One leads the other through the cycle (cyclic pitch change, anyone?). And that is all that the use of "gyroscopic precession" is attempting to get at, the idea that it takes time to make the change happen so the input has to come early enough in the revolution to give the blade the time to fly to the place where you want that change to happen in the circle.

 

The argument against the term "gyroscopic precession" is that the rotor disk is not a solid rotating body, that the phase shift occurs due to an "internal" change and not an interaction with an external force, and that the phase shift does not always occur 90 degrees later.

 

Well, my paraphrase may not be entirely accurate, also being a poor man's understanding of the situation, so I will welcome any correction.

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And not all pitch horns are 90 degrees ahead of the blade.

 

Most helos will tilt the swash plate in the same direction as the cyclic input, and let the advance angle on the pitch horn sort out the phase lag. But machines like the BO105 (if I recall correctly) have the control rods running up the left side of the cabin (instead of to the front of the swash plate), so to make it easier, they tilt the swash plate 90 degrees from the cyclic input and the pitch horn works directly off that. Others have a 45 degree lead on the swash plate and then a 45 degree lead on the pitch horn.

 

Think about a complex head like on those multi-blade CH53s. It would be impossible to have a pitch horn operating 90 degrees ahead of the blade, as it would cross over the next blade and be in the middle of the one before that! So, the horn reaches as far ahead as it can, behind the blade in front of it, and the swash plate tilts at an odd number of degrees to make up the required lead angle.

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Perhaps the wording has changed over the years in terms of gyroscopic precession. Perhaps it was originally worded "think of it in terms of..." rather than "due to the effects of..." ?? Just food for thought. Maybe someone that trained on the first helicopters can fill us in. :P

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If you want forward motion you put in forward cyclic and doing that requires the stationary swash plate to tilt forward. So saying that it means that the front of the stationary plate is low in the front (noise) and high in the back (tail). To lift the tail you need gyroscopic procession because to lift the tail you need maximum deflection at the tail. In a counter clock wise rotor system the leading blade right after the noise (90 degree's before max deflection) will achieve maximum pitch and will start to rise and hit its max rise at the tail witch is your maximum deflection. With maximum deflection achieved you will have forward motion.

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