View Full Version : Off set sensitivity
08-29-2004, 12:54 AM
Dose the amount of off set,1/2 inch or 3/4 inch ect affect the responce/sensitivity of it's reaction?? :confused:
08-29-2004, 06:55 AM
Offset does not affect the way in which a gyro flies, Birdy.
But it does affect our perceptions; stick feel and how it talks to us. More offset; more talk.
The teeter bolt height also affects offset
08-29-2004, 08:15 PM
Not sure if you understood me point Chuck.
If the offset was decreased,would the effect be decreased.??
You say "More offset; more talk.".I assume less offset means less talk ,or positive reactionary effect.IOW,Dose the greater offset make the rotor bolt shift further forward in reation to the same increase in load as compared to a lesser offset??
Different offsets won't change the distance moved by the stick in offset reaction.
A larger offset requires a stronger spring to trim,but would a larger offset also present a stronger/greater corrective reaction to load changes??
In other words,in identical situations of load increase on the rotor[from an updraft],would two offset settings[say 1/2 inch and 3/4 inch] apply the same amount of degrees pitch change??
08-30-2004, 06:39 AM
Let's begin at the beginning, Birdy.
No offset; no trim spring; no feedback.
Say ½" offset requires 10# of pull from the trim spring and you are using a spring with a rate of 10#/inch. The spring has to be stretched 1" to trim.
With a 1" offset and the same spring, you'd have to stretch it 2" to trim.
But the same "G" load increase would apply twice the nosedown force with an offset of 1" as it would with an offset of ½".
With identical spring rates in both cases, the head with the 1" offset would tilt at twice the angle with a given "G" load increase as would the ½" offset head. Hands off, at least.
It's something most people don't even think about; just throw on any old spring that happens to be within reach, but trim spring rate is equally as important as offset.
08-30-2004, 06:48 AM
Thanks Chuck. Aussie paul.:)
...With identical spring rates in both cases, the head with the 1" offset would tilt at twice the angle with a given "G" load increase as would the ½" offset head. Hands off, at least.
Chuck - what is the desirable (or ideal) rotor tilt response to G load? Should the tilting merely cancel out the blowback response, or should it actually cause the airframe to pitch down into an updraft (or up for a downdraft)? Is the main objective to maintain a constant airspeed? What parameter can we use to fine-tune our offset/trim spring system?
08-30-2004, 09:27 AM
I suppose, Udi, desirable rotor response Vs. offset/trim spring depends on the stability you begin with.
If a gyro is stable stick fixed, then you obviously don't need any more rotor response than blowback compenstaion, if that much.
Perhaps the best examples are both of Dick DeGraw's partially powered rotor gyros. Both use cyclic control methods that preclude rotor thrust feedback into the cyclic control system and both are normally flown with locked cyclic controls via a spring detent mechanism. The detent positions are trimable via handwheels.
When Dick and wife Karol fly these machines cross country, it's generally hands off once trimmed and on course.
On the other hand, to fly an unstable machine such as a stabless RAF-2000 hands off, you probably want all the help from rotor feedback you can get, much more than blowback compensation.
08-30-2004, 09:30 AM
Can this offset feed a porpoising tendancy? more than a spring don't we need a damping device (other than your arm)????
08-30-2004, 09:50 AM
Too much feedback increases the restoring force and the likelihood of overshoot.
08-30-2004, 08:12 PM
So you can have too much offset??
What,governs the amount of offset when building a rotor head.??
If you double the offset you should double the spring strength.
This doubled offset and doubled spring strength would have the torque tube react exactly the same rate of degrees as the 1/2 inch one with half strength in the identical condition???
08-30-2004, 09:04 PM
Nooooo..... It depends upon spring rate, Birdy.
If you need 30# of spring pull at the back of the torque bar to trim, you can get it in an infinite number of ways.
A spring with a 30#/inch rate would have to be stretched for 1 inch.
A spring with a 10#/inch rate would have to be stretched 3 inches.
For the same "G" load, the 10#/inch spring would allow 3 x as much feedback into the stick as the 30#/inch spring.
Spring rate is something that Bensen seems to have experimented with. He used double springs which doubles spring rate whereas he could have used a single spring and stretched it twice as far. I assume he was very aware of the effect of spring rate on stick feel.
I suspect the 1" offset that Bensen used is something that just happened. The first offset gimbal heads used wood blades which didn't require trim springs with a 1" offset; depending on the accuracy with which the blades were built.
He probably stayed with the same offset after going to metal blades which required a trim spring, discovering there is a stability advantage to having excess offset balanced out with a trim spring.
08-31-2004, 03:09 AM
So, would you advise me to shorten or lengthen my offset from a safty aspect in rough conditions on my ferel ??
IOW,make it more or less responsive.
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