Trim Spring

[ahancock]

Does anyone have info on the effects of different strength rotor trim springs.
What is the difference between a strong spring stretched a little compared with a weaker spring streched a lot. The force at one point would be the same, but different as the rotor moves fore or aft.

[Chuck Roberg]

Chuck Beaty answered this question within the last couple of weeks. I don't remember enough of what he said to post it here. I would suggest you do a search on "trim spring" or "C. Beaty"

[Doug Riley]

I don't know of any quantitative studies of trim springs.

Springs are rated by their "spring constant." This is a number, expressed in pounds per inch or the equivalent, that describes how hard you have to pull to get a given amount of stretch. A stiff spring has a higher spring constant than a soft one. McMaster-Carr has pages of springs in its catalog, all rated by spring constant.

A low-constant spring will allow the gimbal head to move more in response to a disturbance (well, at least if you hold the stick lightly enough to allow the head to "float" in response to spring and rotor forces). You'd like the head to move just enough to minimize the effect of the disturbance -- neither more nor less than that. Therefore, spring selection has an effect on static pitch stability -- too much, and your gyro under-corrects for disturbances; too liitle, and it overreacts.

The spring system only works if the airframe itself either sits still or reacts in a stable direction to the disturbances. If the frame pitches down in a downdraft, it will slack the spring and tend to neutralize the stabilizing effect of the spring.

(In effect, the new RAF mast-mounted HS provides a more stable platform to which to attach the south end of the spring. The airframe itself -- in its no-HS version -- is too unstable to provide a good attachment point.)

[gyropilot]

(In effect, the new RAF mast-mounted HS provides a more stable platform to which to attach the south end of the spring. The airframe itself -- in its no-HS version -- is too unstable to provide a good attachment point.)
Doug,

That's an excellent way to explain the new RAF stabilizer... easy to visualize.

Possibly a lot of unneeded and expensive new hardware if only a proper horizontal stabilizer is installed in the prop thrust, thus not only stabilizing the rotor, but the airframe as well.

Regards,

John L.

[Doug Riley]

Yeah, John, the "whale tail" is in some ways a classic "work-around" -- a way of accomplishing a mechanical task when the obvious way to do it is forbidden. It happens in the patent context, but here, it's necessitated by RAF's threat to yank the franchise of any instructor-dealer who uses a conventional HS.

[KenSandyEggo]

(In effect, the new RAF mast-mounted HS provides a more stable platform to.......put your can of beer on while it's parked.) That's the only use I see, other than a place for your wrenches when you're working on the rotor head.

They only have 3 U.S. dealers now, don't they? I assume Dofin's certificate suspension is over.

[Doug Riley]

The key fact to remember about trim springs, gimbal heads, swivelling masts and any other stabilizing scheme that manipulates the rotor head* is this: All these devices rely on rotor thrust as a stabilizing force. Rotor thrust essentially disappears in zero G flight. Zero G flight is a routine occurrence on a sunny, thermally day. Therefore, these schemes don't work in a commonly occurring flight condition... in fact, in precisely the flight condition where we most need them, and where PPO accidents are likely to happen.

*Including really good strick skills on the part of the pilot!

[Aussie_Paul]

....I have tested the Raf "stabilator". Most people on this forum would not have had that opportunity. The one thing it does really well is to reduce the pressure to hold the stick forward when on the ground. As some of you will know, this becomes an increasing problem with the heavier 2 place machines unless you are "super muscle".

The other thing it does is to make the control pressure more severe at high airspeeds. This will stop the inexperienced from "really over controlling" but for an experienced pilot it reduces the manoeuvrability at speed, a pain in the you know where!!!!! :eek:

IMHO it does nothing for pitch stability at all!!! :eek:

The only advantage that my testing revealed was the "next to no trim spring stick pressure while on the ground". This is really helpful to the female or less muscular endowed male Raf pilots.

I give credit where credit is due, and hang s**t when s**t is due!!! :D

Aussie Paul. :)

[birdy]

Not wanting to split hairs Doug,but isn't it the variation in rotor thrust that activats the controling movements on the trim spring?

"Rotor thrust essentially disappears in zero G flight."

And this is when it has it's strongest corrective power,init?

[Doug Riley]

Birdy: If the rotor thrust disappears, then, yes, the spring may* pull the head back (as will a pilot who's on top of things!). The problem is that pulling the head back doesn't accomplish anything during zero G. You're pointing a non-existent rotor thrustline farther back. If the aircraft has a high thrustline and the throttle is high, the weak/nonexistent rotor thrust won't be enough to produce a recovery. You'll PPO despite the movement of the head.

If the airframe is stable enough to continue flying level thorugh the zero g event, and if the zero G doesn't last long enough to cause a catastrophic loss of RRPM, then you should come out OK on the other side.

My comment was directed mostly at those who continue to believe that gadgets that manipulate the control head can prevent PPO. If you have a high prop thrustline (above CG), something OTHER than the rotor must be used to cancel out the nose-down force created by the proper thrust. The cancelling force can be from a H-stab (the simple choice), or some other device that produces a down-force in the tail -- but whatever the device, it must be independent of G-loading in order to be reliable.

* It may not, too. If the frame pitches forward very rapidly, the spring may go slack. The rotor may, in any case, lag behind and slam the rear teeter stops as the frame pitches over... a type of damage often seen in post-PPO examinations of wrecks.

[John_Read]

But, Doug... It would have to be a very substantial disturbance, and a downdraught at that, to cause zero or negative g. wouldn't it? My experience as a glider pilot taught me that the updraughts are strong and tightly located, the sink is widespread and relatively slow on those thermic days. Of course, if you were flying in a thunderstorm ...... :eek:

But in normal weather you would have a reduction in G, certainly ... but nowhere near zero. I am talking about the benign weather in England of course. Maybe it is different elsewhere.

[John_Read]

The original question is still an intriguing one. How do you determine the best strength of spring for a given set-up?

[Doug Riley]

John: A couple things in reply.

First, I'm surprised that you haven't experienced zero or negative G on a thermally day in hill country. I have, on various occasions. I recall one flight, especially, in my Kolb ultralight on a lovely sunny summer day in which I hit such strong negative G's that my head smacked against the cockpit framework overhead. I think I might have gotten at least a good lump, if not blacked out, had I not been wearing a helmet and shoulder harness at the time. This took place in the Southern Tier hills of New York State -- not exactly the Alps, although the area is good soaring country and is the long-time home of the Schweitzer sailplane company.

I flew my no-HS, early model Air Command in the same area one year. I felt very glad to get back in one piece. I set the throttle to below cruise and wallowed along; fortunately there was more lift than sink and I ended up gaining altitude even at reduced power.

Second, you don't have to go all the way to zero G to get into PPO trouble. A significantly reduced G-loading can be enough to leave the rotor with insufficient thrust to hold the nose up, in the case of a badly misaligned thrustline. The dipping of the nose further unloads the rotor and can finish off the PPO job without further help from turbulence.

You may recall that, back in the Bensen era, PPO crashes were typically preceded by a number of cycles of porpoising, or PIO. When engine redrives came into fashion in the 80's, they allowed us to go from 4-foot to 5- or 6-foot props. This required that the engine be raised to preserve ground and tail clearance. Unless the seat also was raised (they WEREN'T raised, at first) the result was a drastic increase in thrustline misalignment.

Suddenly, PPO accidents began to happen without any preliminary porpoising. The gyro would be flying along, apparently under control and -- BANG -- the rotor would hit the prop and down it would tumble to a fatality. Some of these accidents, I think, were attributable, in whole or in part, to low-G flight caused by downdrafts.

Again, the point about the gimbal head is that control and stabilizing forces in the case of a teetering rotor are purely a function of G-load: no G, no control; reduced G, reduced control. An adequate frame-mounted HS or equivalent device that's independent of the rotor and isn't G-dependent is helpful in a centerline thrust machine and a life-critical in a high-thrustline machine.

[birdy]

I understand wot your saying Doug ,but you'v confused me a bit with,

"The problem is that pulling the head back doesn't accomplish anything during zero G. You're pointing a non-existent rotor thrustline farther back."

I thought that pulling the head back/tilting the rotor bolt back,was moving the rotor thrust forward of the COM.???[lifting the nose.]

[Doug Riley]

Just terminology, Birdy.

Say you look at the left side of a gyro. I think of a clockwise rotation of the head as "tipping the rotor thrustline back." Yes, this back ROTATION moves the thrustline forward (in the LINEAR sense) relative to the CG. And, yes, this movement creates a relative nose-up torque IF the rotor is creating any thrust. If it isn't making thrust, then tipping the rotor about doesn't accomplish anything.

This last point is what the guys who advocate "training, training and more training" as a substitute for proper design just can't seem to get.

[quadrirotor]

So, Doug, what do you recommend?

[Doug Riley]

Andre, with regard to overall design:

I like the thrustline very close to the CG. The craft should have a HS with partial immersion in the prop wash. The HS should be sized to comply roughly with the Cierva formula, IF there is no thrustline offset above CG. In the case of thrustline offset above CG, the HS should be sized to the greater of (1) the Cierva formula and (2) the size that allows full compensation for the thrustline offset at no more than 3 degrees of negative incidence. If the craft has an enclosure, the HS sizing, placement and incidence must also fully compensate for the pitching moments of the enclosure, without a need for the HS to operate at angles of attack more than 3 degrees in normal flight at any airspeed or power setting.

These are rules of thumb, based on experience and not on exhaustive analysis or methodical testing. I've found that craft set up according to these criteria will either fly through short duration low-G flight with no pitch excursions, or will nose up in such conditions. Either type of reaction is OK IMHO, as long as any nose-up reaction is not excessive. A nose-DOWN reaction is NOT OK. It indicates that the "full compensation by the HS" criterion hasn't been met.

As far as trim springs go, once you find the tension that produces hands-off flight, you then can experiment with different lengths of spring (in the same guage and diameter as your original) to vary the spring rate for best behavior in turbulence. I don't know of a numerical way to do this. If you start with a spring of known spring rate, though, and stay with the same guage and diameter, you will at least eliminate some of the trial and error as you try different spring lengths.

[MichaelBurton]

What if we changed the arm length insted of the spring force. could the spring retain tension on the cable at all times and give more reliable trim?

[Doug Riley]

Michael: Not sure what you mean by "more reliable" trim. The spring system isn't UNreliable, in the sense of something that quits on you when you need it. If the spring has the right spring constant, then it will either have some tension at the extreme stick-back position, or it will have none -- but that amount of tension (even if it's zero) will be the RIGHT amount for the particular rotor and gyro. IOW, if the spring is the right one, then the fact that it goes slack with stick back doesn't mean the system is defective. it just means that particular machine doesn't need any spring tension with stick back. (You should be extra careful to safety the spring in place if it's very slack with stick back. I've had them fall off -- now THAT's unreliable!)

It's true that you can trade off arm length against spring constant as an additional variable to complete the same tuning task. You'd do it by changing the location of the spring's attachment to the torque bar.

[MichaelBurton]

yes but what I realy want to do is allow the arm length to change instead of the spring pressure. I dont like the trim cable out there flying in the wind. Is there a better way than the one we all know and use? :rolleyes:

[Doug Riley]

Michael: If what you object to is how slack the spring gets with the stick aft, with cable dangling and all, then you could go either to a spring with a lower constant (longer spring with thinner wire) or the same spring attached closer to the pitch pivot. These two choices amount to the same thing mechanically. With either method, once you make the right change, you can end up with a spring that's got some tension left even when the stick is all the way back against the rear stop. No more floppies.

HOWEVER, doing either of these things will reduce the amount of centering pressure in your controls. For example, when the stick is full forward, there will be less spring pressure pulling it back toward neutral. Depending on how much of this centering pressure you give up, you can end up either with a pleasantly light stick or with one that lacks "feel" and loses a sense of "solid stability."

[Ron R]

Mine doesn't have one at all. It doesn't seem to be too much of a project to make an assembly for it. How critical is it to have one on your machine? Thanks!

[birdy]

Mine doesn't have one at all.

Dosn't have one what?? A spring?

[Ron R]

Sorry Birdy, yes, I meant a trim spring. But I have a few plans to make the assembly. Plus, Dan Leslie's book gave some good advice on how to set it up. I'll do it. Thanks Ron R.

[Udi]

What kind of gyro is this, Ron? Do you have to hold back pressure on your stick when you are flying? Can you post pictures of your rotor head?

Udi

[Ron R]

Udi, it's a stock Bensen/Brock head on a KB2. I don't know about back pressure, I'm still in transition from a Parsons trainer to my single seat KB2. Any input from you Bensen/Brock people out there? Thanks everyone for your interest and input! Ron R.

[Udi]

If it's a standard KB head than you have to have a trim spring. Did your gyro ever fly without it???

Udi

[Ron R]

Udi, I bought it used, but the guy I bought it from didn't have it on there. He said he flew it. I've only "hopped" in it, so I can't say how well it flys. Thanks, Ron R

[birdy]

Hmmmm..........,I'd get an experianced pilot to do some short hops init firstup Ron.

[Brent_Brown]

I bet it needs one you can fly without but your arm is now the spring not fun I know