Offset Gimbal Stability

Udi

Platinum Member
No disrespect intended - this discussion reminds me a joke about the difference between theory and practice (a son asks his dad what's the difference... you know the rest). I love theoretical discussions but at the end of the day we have to decide what is the practical approach.

Tim has started this discussion with a very practical question.:
For the stick to require forward pressure for straight and level flight then the rotor thrust vector must be ahead of the pitch pivot. My question is if this is the case, then wouldn't this situation give the offset gimbal head a de-stabilizing effect instead of the desired automatic stabilizing effect?
Unless otherwise proven by flight tests, the only safe answer we can give Tim is yes - the gyro may become G-load unstable at above trim airspeed if the gimbal offset is too small. A prudent designer will design the rotor head in a way that will assure that the RTV is ALWAYS passing behind the pitch pivot.

Furthermore, having little or no spring tension means there is no link for airframe-rotor feedback. The most stable airframe can go wasted if it is hanging from an unstable rotor system without a feedback link. Take full advantage of your well designed airframe by linking it to the rotor with a spring.

Unstable airframe, of course, is better off unlinked from the rotor. This is exactly what RAF have done with the stabilator. The stabilator has replaced the spring trim, which is a good idea for a very unstable airframe as the RAF 2000.

Udi
 

raghu

Senior Member
Oh I agree Udi! With time of work and a couple of days before I take my flight for a long over due holiday, clearly it is time for some intellectual you know what... :)

From a (conservative) design perspective the rules to build a well mannered stable machine ( 1seater or 2 seater) is well documented and is really no secret. In fact it can be summarized in a couple of lines:
1) keep the prop thrustline with in a couple of inches of CG
2) Design a stable airframe
3) Use a offset gimbal with proper offsets (the offsets can be calculated with simple arithmetic using Chuck's spreadsheet.

All other factors such as MOI , various stability derivatives etc. are second order and really can be ignored if you stay within the above boundaries.

But if you are interested in why a certain machines that is closer to the border or just outside conservative design exhibits good or bad stability or simply want a deeper understanding then it is worth talking about some of the more esoteric aspects. Otherwise there is not much else to the stability of a gyro. My discussion of the inertia less rotor was in this vein. There is anecdotal evidance that a few gyros without a spring exhibited ( I think the prototype gyrobee was one) resonable handling.

I guess Tim got more than he bargained for...
 

gyropilot

Only GyroBee Memories
Trim Spring

Trim Spring

C. Beaty said:
The same Pete Johnson that first called my attention to the stabilizing effect of offset balanced with a trim spring experimented a good bit with offset and trim springs of various rates.

His conclusion, as best I recall was that Bensen’s 1” offset was best, combined with a very low rate spring.
Wouldn't a low rate spring exert less influence to couple the cyclic to airframe movement?

If yes, then from what I think I've read in this thread, that could actually be desirable in an unstable gyro, but would be undesirable in a stable gyro.

Do I have this right?

John L.
 

C. Beaty

Gold Supporter
Control feel has a strong influence on our overall impression of a flying machine, John.

The component of rotor thrust fed back in the stick by a Bensen rotorhead provides a signal that guides the pilot in the avoidance of disturbances.

I’ve flown both stable and unstable gyros with helicopter type “dead” controls. This amounts to flying a gyro with stick locked inasmuch as the stick remains in its trim position when flying hands off. The pilot workload flying unstable machines is very high but even a stable machine is uncomfortable at first.

I would be interested in hearing Ken J’s impressions of his transition from the RAF/AAI to the McCullogh J-2. The J-2 has a helicopter type control system.
 

Doug Riley

Platinum Member
A semi-interesting sidenote to all this is that Bensen claimed, and appeared to demonstrate, that his SPINDLE headed gyro was stable hands-off. The spindle unit, of course, has no offset. The teeter bolt is located 2-3 inches above the control pivot axis. Rotor blowback necessarily causes the rotor thrust line to pass in front (relative to flight direction) of the pivot axis any time the aircraft is moving in any direction.

Bensen demonstrated hands-off flight using spindle heads with both the overhead stick (with no trim spring) and with the joystick (with trim springs rigged to create a forward-tipping bias, like the offset gimbal). He claimed that the overhead stick had just enough weight of its own to create the needed bias without the use of springs.

If these two alternative systems worked as they seemed to, then we have two methods OTHER than the flapping/offset-powered gimbal system for generating trim pressure. On the joystick-with-spindle, we have a pure version of the trim spring system, in which the spring tension gets stronger at higher cruising speeds, but G-loading does not affect the trim device's output.

On the overhead-stick-as-counterweight system, we have a trim pressure that stays the same (or even gets lighter -- work out the angles) as you go faster. The overhead stick (since it's a weight) should increase its forward-tipping pressure when G's are pulled. That's an unstable reaction.

I was too green back when I had an overhead stick and spindle head to explore issues like this; the goal then was SURVIVAL. It's intriguing to see that the cat was skinned in other ways, though.
 

Udi

Platinum Member
Doug Riley said:
...The overhead stick (since it's a weight) should increase its forward-tipping pressure when G's are pulled. That's an unstable reaction...
I find it hard to focus after this long weekend, but I seem to think the forward weight of the overhead stick would result in a stable reaction to G load...

I better go grab another cup of coffee.

Udi
 

Doug Riley

Platinum Member
Apparently I need the coffee more than you do, Udi. I shouldn't try to say anything of consequence before 1700 Zulu.

You're right; the stick will pull down-and-aft once the G's actually take hold (the centering/breakout forces on spindle heads are REALLY light, however). This tends to tip the spindle forward and reduce AOA and therefore G-load. Interestingly, the weight of a pump-handle stick does the same thing, despite the voodoo theories that such sticks are de-stabilizing.

We need more coffee all 'round, I guess.
 

Udi

Platinum Member
gyropilot said:
Wouldn't a low rate spring exert less influence to couple the cyclic to airframe movement?

If yes, then from what I think I've read in this thread, that could actually be desirable in an unstable gyro, but would be undesirable in a stable gyro.
I am very interested in this subject. I agree with your conclusion, John, but I think there would be an "optimal" spring rate for every gyro. Greg G. likes to talk about the "harmony" between the different parts of a gyro. Like any other control system, the best "tuned" gyro is one in which the different control mechanism mesh with each other to form one, well-tuned system. The fact that most gyros fly "ok" with nothing but some random parts slapped together, shows how forgiving the gyro really is.

I would be interested to know how the designers of some of the more famously stable gyros came up with spring rates for their designs. I think most designs are completely empirical, but the SH may have a little more "engineering" in it.

Udi
 

rtfm

Gold Member
Spreadsheet of offsets?

Spreadsheet of offsets?

From a (conservative) design perspective the rules to build a well mannered stable machine ( 1seater or 2 seater) is well documented and is really no secret. In fact it can be summarized in a couple of lines:
1) keep the prop thrustline with in a couple of inches of CG
2) Design a stable airframe
3) Use a offset gimbal with proper offsets (the offsets can be calculated with simple arithmetic using Chuck's spreadsheet.
Hi. I was reading this old thread with some interest, and came across mention of a spreadsheet developed by Chuck to calculate gymbal offsets. Is this generally available? If so, I'd love a copy. Can someone point me to it?

Regards,
Duncan
 

Hot Wings

Newbie
Dragging out an old thread:
I started to do some calculating on control pressures for my part 103 tractor gyro project so I could start picking hardware for the expected loads. When I got ~500 ft lbs of torque being needed to move the rotor 9 degrees at a 2 "G" load it was obvious I had made a bad assumption. That led me in a round about way to this thread. I was looking for specifications for Bensen tension spring constant.

After reading about half of this thread I realized I'd seen this set of vectors before. If you think of the rotor as nothing more than high inertia wing with the aerodynamic center at 50% this all looks exactly like a Spratt/Zuck wing. Unless I'm missing something the two systems control is almost identical?

Working the control forces back from the offset gimbal head I now see control forces an order of magnitude smaller.

Thread drift: Has anyone ever built a small gyro with a side stick?

Doug Riley;n173751 said:
If these two alternative systems worked as they seemed to, then we have two methods OTHER than the flapping/offset-powered gimbal system for generating trim pressure. On the joystick-with-spindle, we have a pure version of the trim spring system, in which the spring tension gets stronger at higher cruising speeds, but G-loading does not affect the trim device's output.

On the overhead-stick-as-counterweight system, we have a trim pressure that stays the same (or even gets lighter -- work out the angles) as you go faster. The overhead stick (since it's a weight) should increase its forward-tipping pressure when G's are pulled. That's an unstable reaction.
 

Vance

Gyroplane CFI
I have flown some light gyroplanes with very light cyclic control forces.

I see no reason they couldn’t use a side stick.
 

Hot Wings

Newbie
That is welcome input!
Will save me about 3 or 4 pounds and get rid of one more hazard in the cockpit in the event of a crash.
 

Jazzenjohn

Gold Supporter
I have no experience with the side stick like on the black, but on my second gyro I had an offset overhead stick and one issue I had with it might be of some relevance. I used the Overhead stick to train myself on flying by taking off with the low (regular) stick and switch to the O/H stick in the air. It took some time and I eventually learned to fly it but my particular stick was offset from center so I didn't have to look at it. Inflight, with the ground and horizon, there wasn't much problem with holding it level, but after landing the offset didn't allow for the easy visual ability to know when the stick was centered and the disk was level. Once when there was some wind and after I had landed but before the blades had slowed, I inadvertently let the disk drift off from level and when I turned I could feel the wind coming up under the blades and get my right wheel light. I corrected and everything was fine but I then knew what Doug Riley said about most ground tip overs are most likely "Fly overs" from lack of positive rotor control until the blades slow to an insignificant speed. When first flying with an offset stick whether high or low, it might help to keep that in mind.
 

Gyro28866

David McCutchen
In regards to a "Trim Spring".
On my Tandem Dominator, it had a trim spring of about 12#'s of resistance. I replaced it with 2 springs with approx. 6#'s of resistance each. One springs ID is slightly larger than the OD of the other spring.This allowed me to pass one spring inside of the spring. The combined resistance is still approx. 12#, But if one spring should break - the Pilot is only catching half of the load, not al of it. This arrangement also "cages" the 2 springs into each other. In the event of a spring breaking, it stays put and doesn't end up going through the prop.
 
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