Reasons for Excessive Pitch Sensitivity? - RCCanada - Canada Radio Controlled Hobby Forum
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Old 07-11-2016, 12:26 PM   #1
Cougar429
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Reasons for Excessive Pitch Sensitivity?


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O/K, with the proviso I have been building and flying for 3 decades have to admit I have never had any craft as sensitive as this one.

Built the now defunct nitro powered Thunder Tiger Rare Bear a few years ago and it has suffered some issues related in this build thread here:

http://www.rccanada.ca/rccforum/showthread.php?t=132690

Anyone with the patience to read through the build thread will see I have addressed most, but one has been an ongoing annoyance and once that I cannot account for under normal circumstances and hope someone may have other ideas or solutions.

Since I built it with the elevator servo in the tail I have eliminated what is by far the biggest problem of tail shedding due to flutter. With this craft the elevator itself is far larger and out of proportion to the stabs themselves and with the idea of solving the pitch control problem I pulled both elevators and cut down their area approx 25%, along with installing some stiffer control wires, (their original stock wires were more flexible and aggravated the problem).

Their elevator deflection is approx 1/4" up or down, much less than anything else I've seen and even with that there is full control, even slowed down for landing.

My problem is twofold. First is my own fault with the attempt to heat and fit the original nylon pivot/hinge setup adding enough drag to prevent the elevators from returning to absolute neutral, even with a servo strong enough to handle any flight loads, (approx 100 oz/in). The very slight residual deflection creates havoc in the air and panic on landing.

So, I realize little travel is required in flight, but every other craft has far more deflection available and is frequently used when doing aeros. With this one, the pitch change can be dramatic when cruising so any more would likely cause an out of control problem or stress the craft beyond tolerance.

Here is where I am hoping someone would have experience to point out some possible corrections or solutions to what has become the least favorite machine in the fleet. With the plane not really short coupled and the CofG repeatedly confirmed bang on am out of ideas.
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Old 07-11-2016, 04:40 PM   #2
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Re: Reasons for Excessive Pitch Sensitivity?

I would guess it's geometry . with a elevator that requires so little movement the resolution of the servo becomes critical . first thing I would do is set up the servo geometry so max required deflection of the elevator requires full travel of the servo . this will maximize your control over the tail . second I would use a large amount of expo 60% or more and thirdly if it's getting better but just not there yet try a higher resolution servo ,assuming you have a transmitter that can utilize such a servo .
You may also want to try a bit more down thrust .

just some thoughts .
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Old 07-11-2016, 05:08 PM   #3
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Re: Reasons for Excessive Pitch Sensitivity?

Most mfg's have a suggested range for the C of G and they are not always bang on and there are many varibles for every plane assembled for how that was done and with what equipment used , I'd be tempted to move C of G slightly forward....would help soften the sensitivity, just a 1/4" or so at a time and see if it helps.
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Old 07-11-2016, 06:38 PM   #4
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Re: Reasons for Excessive Pitch Sensitivity?

Servo resolution is already pretty tight. More so, in fact, than what the original linkage could work with, (their stock linkage flexed far more). That flex is why I suspect the more rigid new setup shows the pitch sensitivity to greater effect.

Still, trying to figure out why this has such low deflection is is the primary aim.

Moving the CofG forward is an option, although with this type of scale it requires a lot for a little. Already have 1 1/2 lbs up there now.
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Old 07-11-2016, 07:15 PM   #5
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Re: Reasons for Excessive Pitch Sensitivity?

Assuming the airframe and control run is solid, a sensitive elevator even with small deflections indicates a low static stability margin. Run it through this spreadsheet to see where the neutral point is and where the CG would need to be for a 10% static margin:

http://www.rcgroups.com/forums/showthread.php?t=1106300

If you want, send me the dimensions and I'll do the spreadsheet up for you. The spreadsheet needs everything measured from one point, the wing LE at the centre is usually the easiest.

I never trust the recommended CG positions, although they are usually far too nose heavy rather than too far aft. That definitely sounds like a low static margin airplane though. I have a 62" Osiris that I fly with a low static margin that doesn't use a lot more elevator control movement than that though.

Full servo travel for the required elevator throw is a good idea, as was suggested.

Send me some dimensions, and we can verify the CG position.

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Old 07-11-2016, 07:27 PM   #6
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Re: Reasons for Excessive Pitch Sensitivity?

I just flew my new 50cc edge and it was pitch crazy . I just moved the push-rod in a 1/4 inch on the servo arm and it changed the plane completely .making the controls tight is a good start but some planes are just more sensitive to the control surfaces ,likely nothing wrong with the plane at all . in those cases you need to make your controls more finesse . if you only need 1/4" throw make sure the servo is moving it's full travel to move the surface 1/4" and you should be in much better shape. this may require a very short servo arm but it will increase resolution and feel to the surface. what is the maximum throw you can get now with your end points and rates set to max. I see in the picture there is no cowl were you flying with no cowl. A true balance check is a great Idea as mentioned above.. cheers Roger.
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Old 07-11-2016, 09:25 PM   #7
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Re: Reasons for Excessive Pitch Sensitivity?

How much expo are you using?
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Old 07-11-2016, 10:32 PM   #8
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Re: Reasons for Excessive Pitch Sensitivity?

Since the control linkages to the elevator use ball links bolted to both sides of the servo arm moving it in then runs into interference with the top of the servo case. Would have to figure out a way around that.

That pic was taken the day I had it out for the first engine run, the best I could find to show the overall planform.

In most cases the first few flights of new craft are made with the cowl off to ensure everything is set and working correctly. It also gives me a baseline to show if I have a cooling problem once installed.

I will try and take measurements later. Busy week with family down, studying for the millwright exam and the Chatham Scale event this coming Saturday.
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Old 07-12-2016, 09:35 PM   #9
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Re: Reasons for Excessive Pitch Sensitivity?

Not to jump on the hype... I have a plane that does exactly what you explain your rare bare does. I spent the better part of 2 seasons trying to sort things out. changing balance further back, further ahead. laterally balancing, thrust changes and nothing seemed to make it better. added an inexpensive 3 axis gyro and all issues disappeared right away..
just a thought. motion R/C has them for under $25'

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Old 07-12-2016, 10:35 PM   #10
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Re: Reasons for Excessive Pitch Sensitivity?

I have one of those and in fact had thought of adding it to this airframe.

May be wrong, but from my heli experience the elevator servo would have to be one that can handle some serious cycles without overheating.
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