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Mar 31, 2016, 10:41 AM
Registered User
The extra centripetal force combined with larger hinge offsets would make it hard for the spindle to tilt. Slower body response.

Remember when the spindle tilts it has to pull the blades in essentially making your rotor disc shorter. The more offset the more it pulls thus the more the rotor fights the spindle tilt.
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Mar 31, 2016, 11:38 AM
I'm not as bad as they say.
Quote:
Originally Posted by grant31781
It seems like the main rotor would not need to teeter since the fly bar can. How does this differ from the gws Dragon fly head?
It doesn't need to, but early ones did. Very soon they adopted a snubber or damper to restrict the teetering motion and get more control power.
The GWS didn't teeter, but the blade holder arms were pretty flexible so they acted like a flapping hinge.
Note that when you make a rotor really, really rigid the phase angle changes to something less than 90, so you have to adjust the swashplate phasing and/or the flybar phasing angle. This is readily apparent in the small rigid rotor helicopters and the small ones with a flybar where the flybar is not 90 from the main rotor.
The bigger the rotor gets, the harder it is to make it truly rigid, but still the phase angle gets less than 90 and so the controls have to be adjusted accordingly, even on full sized helicopters.
Full sized gyrocopters have free teetering rotors with no snubbing so the phase angle is always 90. This is what makes the teetering spindle work in what looks to be the "correct" direction to apply cyclic in the right place. Unfortunately this leads the uninformed observer to incorrectly assume that the spindle tilts is actually just forcing the rotor in the desired direction by force. (A little thought about a teetering head will explain why this is impossible as no torque can be placed on the rotor through the teeter.)
This mistakenly leads to metal gear servos, when in fact the problem is the flapping hinge is too stiff and the centripetal forces in the 6/9 position (as you described, 90, 270 azimuth positions for the roll example) are too high for low power servos.
To add to this saga, when the flapping hinges are made flexible enough the coning gets higher and the following rate goes up, and the correct adjustment should be adding tip weight to reduce coning and following rate, but most people do something else, not understanding the root causes.
Fortunately some combinations can be made to work, so the exact blade/rotor/head/servo combination gets copied as it is known to work.
Latest blog entry: AIrcraft I've built.
Mar 31, 2016, 11:57 AM
I'm not as bad as they say.
Quote:
Originally Posted by chadandrews
Thanks Mickey (or do you prefer Mick?). Couple of questions regarding the offset hinge.

1. Am I correct in thinking the heaver the blades, the more centripetal force will be created thus more feedback from the rotor disk is transferred to the rotor mast?
Yes

Quote:
Originally Posted by chadandrews
And thinking oppositely more control from the mast to the actual rotor plane.
No. This assumes that direct force on the blade is what is causing the rotor to move, which it is not.

Assume the blade at the 90 position in a CCW rotation rotor. If the mast is tilted left right at this position, this applies an upward downward force on the blade root only, as the tip will stay in plane. Due to the phase lag response of the blade, it will rise up fall a small amount, 90 later over the nose. This is not what you would expect for a left right spindle tilt. However. Now that the blade is over the nose, the left right spindle tilt increases the AOA of the entire blade producing significant up force. 90 later over the left hand side that blade has now reached its maximum change in position above the previous rotor plane position. For a teetering system the rotor is now tilted so that the rotor thrust is not aligned with the CG and a roll occurs. For an offset hinge system, the CG is not aligned with rotor thrust, AND the offset hinge arrangement applies a rotational torque to the shaft and the model rolls to the right.
Note that the response of the rotor blade to the cyclic input is governed by the blade mass, rpm and the instantaneous change in lift of the blade (the forcing function).
So the assumption you implied that tilting the shaft applies force to the blade to effect the control is not correct.


Quote:
Originally Posted by chadandrews

2. Am I correct in thinking the distance from the center of the mast to the hinge point acts like a lever for the above control, i.e. longer distance more control (longer moment?), shorter distance, less control. Control in this case being a function of the centripetal force the blades are putting on the offset hinge to pull the hub back into alignment with the plane?
Yes, ultimately with no hinge offset you have a teetering system, which is the least control authority (but also the highest following rate which leads to an aircraft with a very high reaction to the controls, but no real authority in the controls, making it difficult to fly, especially as the reaction rate (called the following rate) gets higher as the model gets smaller).
But the caveat is that the thing that makes the blade go out of plane to exert this torque is not direct input, but cyclic pitch.
As had been stated in the last couple of posts, in spindle tilt gyros, the flapping hinge is basically needed to decouple the centripetal force of the blade from the servo so the mast can be tilted.


Quote:
Originally Posted by chadandrews
Just want to understand cause and effect before I experiment.

Thanks

Chad
Understood. Although lack of understanding hasn't stopped experimenting before. Even the pioneers built stuff before they understood it. But I think it makes experimenting easier with at least a cursory understanding of what's going on.
Last edited by mnowell129; Mar 31, 2016 at 01:53 PM.
Mar 31, 2016, 12:01 PM
Registered User
I found this about phase angle

http://blog.aopa.org/helicopter/?p=678

Some time in the past, it was decided to draw an analogy between a rotor and a gyroscope. Unfortunately, this analogy is incorrect and has caused confusion ever since. A rotor is not a gyroscope, due to the articulation, real or virtual, of its blades.

A rotor is a system of whirling pendulums, and its reaction to displacement inputs is governed by the laws of vibration. In accordance with those laws, the only time a rotor blade responds with a 90 degree phase lag to a cyclic control input is when that blade is hinged at the axis of rotation and does not include any pitch-flap coupling.

As the flapping hinge is displaced away from the axis of rotation, or as pitch-flap coupling is applied, the phase lag steadily reduces below 90 degrees due to the increase in natural frequency of the blade. In a given helicopter rotor, the phase lag also changes with the Lock number, i.e. increased aerodynamic damping reduces the blade natural frequency and thus increases the phase lag, such as would occur if flying one day in freezing conditions in Death Valley, and another day in hot weather at the top of Pike's Peak.

In summary, a rotor is a dynamic system in resonance, i.e. an oscillating system responding to periodic disturbances. All else follows.

The phase lag may be referred to as "precession", but this is a less than rigorous - and confusing - use of this term.
Mar 31, 2016, 12:02 PM
Registered User
Hi Grant,

I appreciate your feedback, its really making me think. I still feel we are a little apart on timing though, but I do see what your explaining. And great point on pulling the rotor inwards, this would actually increase the rpm too. How interesting.
Mar 31, 2016, 12:13 PM
Registered User
Not sure I got the quote function right below, but that was my ah-ha moment. Thanks Mickey


Quote:
Assume the blade at the 90 position in a CCW rotation rotor. If the mast is tilted left at this position, this applies an upward force on the blade root only, as the tip will stay in plane. Due to the phase lag response of the blade, it will rise up a small amount, 90 later over the nose. This is not what you would expect for a left spindle tilt. However. Now that the blade is over the nose, the left spindle tilt increases the AOA of the entire blade producing significant up force. 90 later over the left hand side that blade has now reached its maximum change in position above the previous rotor plane position. For a teetering system the rotor is now tilted so that the rotor thrust is not aligned with the CG and a roll occurs. For an offset hinge system, the CG is not aligned with rotor thrust, AND the offset hinge arrangement applies a rotational torque to the shaft and the model rolls to the right.
Note that the response of the rotor blade to the cyclic input is governed by the blade mass, rpm and the instantaneous change in lift of the blade (the forcing function).
So the assumption you implied that tilting the shaft applies force to the blade to effect the control is not correct.
Mar 31, 2016, 01:54 PM
I'm not as bad as they say.
Quote:
Originally Posted by chadandrews
Not sure I got the quote function right below, but that was my ah-ha moment. Thanks Mickey
You're welcome.
Note that I mixed some right/left/up/downs in the original post, I've since edited so it matches itself.
Sorry if this causes confusion.
Latest blog entry: AIrcraft I've built.
Apr 12, 2016, 07:14 PM
Registered User

scratch build


Hi and thanks to everyone who has contributed to this thread on autogyro.
I'm going to scratch build an autogyro and have been mulling over the concept of using a cyclic pitch head from a 450 helli. I have a 2 blade flybar head and a 3 blade fbl head to choose from and will use aerobalsa blades for this build.
The post about gyros by M. Nowell, and the head used there is really good but its a fixed pitch head and I'm not sure how to transform that theory into a cyclic pitch head version.
Has anyone tried a cyclic pitch head?
Has anyone got any ideas or theory they can share?
All and any help will be much appreciated and combined with the info that has already been posted in this thread I hope this build will fly successfully.
Nov 18, 2016, 11:36 AM
Registered User
I just have to say, I LOVE this thread! I have probably read the original posts about 6 times now, and as I bounce around the hobby and come back to things with rotors I end up back here refreshing my memory. it is so well written, it's like reading something I wrote and almost forgot. (I am not saying that I could write that well.)

I am really glad it is "sticky".

Thank You!