Bending Horiz Stab Linkages

[RicVaughn]

I have a full flying stab 40 span, 230 in area, weight 2.87 oz.. Problem I have is stripped servo gears and now using an HS-81MG it's still jumping gears and bending the control rod. The plane is used in Sportsailplane contest, so the landing are firm at best. Stab sits on a platform with the control horn "hole" 13/16 in from the pivot point. Pivot point for the stab is right at the aero center, 25% area, but the mass balance is at 51% cord. I'm thinking of mass balance or a servo saver to cure the problem, but I can't for the life of me figure out how this can impart so much force to the servo on landing. Any ideas???




Ric

[Andy W]

Ric and I have discussed this at length, and just to provide a little more info, it's not just on his model - another LMR sailplane pilot has the same model/setup/linkage, and although he's also using a beefy servo to prevent the gears from stripping, his 4/40 wire linkage is bending!
Ric, does this diagram show it accurately?
..a

[RicVaughn]

Andy,

Yes, diagram is correct. The 13/16 if not from the mounting plate, but from the piviot point. Only slightly more that a 1/16 difference.


Ric

[Andy W]

I updated the diagram..
Anybody?

[globemaster3c17]

Hmm, sounds pretty odd. Piper aircraft use stabilators, and they have what's called an anti-servo tab. If it weren't for that anti-servo tab moving when you apply pressure to the yoke, you wouldn't have any contol feel. Are you sure 25% cord is the center of pressure for your stabilator? It probably wouldn't need to be off by very much to get a pretty high load on your servo when deflected. I pretty sure mass balancing just helps with flutter issues, but I haven't studied that one much to know if it could help your problem.

[Mike Taylor]

The 13/16" horn seems awfully short for a full flying stabilizer. I had a 30 year old Cirrus upstairs the a full flying stab, and it has a control arm that is drilled at 1 3/4" to 1 7/8". The arm looks like 2/3s of a large U-Control bellcrank. That gave plenty of control for the glider. The radios 30 years ago didn't have EPA adjustments so full servo travel was used...

[Andy W]

That's a good point - I wonder where we could find other full-flying stab geometry pics?
..a

[RB]

Probably need to mass balance at the pivot point. Are you hitting some mechanical limit of travel on the control surface before the servo reaches it's end point ? You probably on use big control surface movements on landing.

RB

[Salto]

A couple of things to consider:

With the stab pivot right at 25%, any error in calculating this position may still result in a force through to the servo during flight. If you've got the pivot accidentally slightly behind the 25% point then you may well be getting flutter issues in flight which may be stripping the gears. For this reason it is common practice to place the pivot point slightly ahead of the 25% point, so that any moment acts to reduce the movement that caused that moment.

With that short 13/16" horn length, and with a normal servo horn length, it's likely you have programmed your servo travel right back to reduce pitch response to a manageable level. This set-up will result in high servo loads and high push rod loads and may have caused the stripping and bending.

A better set up would be to have the servo travel programmed at 100%, use a "normal length" servo horn, then work out what (longer) length of stab horn you need to get the right stab movement. This will result in lower pushrod and servo loads. It will also reduce any backlash due to pushrod slop in the horns. You can reduce pushrod loads and the effect of pushrod backlash further by lengthening both horns further.

If the gear stripping only occurred at landing, and you are sure the stab wasn't hitting the ground, then mass balancing the stab should fix the problem.

Is the stab symmetrical?

Graham.

[RicVaughn]

Hi All,

Yes the pivot is exactly at the 25% aero pioint.
I just measured a control horn from an old Electra, it's 11/16 in to the outermost hole. Not having any flutter or any other problems in flight. Just on landing the force imparted to the servo is stripping/bending the rod.
Right now I don't think I can lengthen the control rod. Also, it is not a symertical stab. At 2.87 oz., I still trying to figure out how 2 oz. max is bending a servo rated at 45+ oz.

Thanks for the input. Keep the ideas coming.


Ric

[Sparky Paul]

It's the sudden stop.. 2.87 oz. times X g, at Y inches...

[Andy W]

The servo is skipping, so that means the stop is about 25G. If that was the case, wouldn't other parts be breaking?
..a

[Sparky Paul]

The servo is skipping, so that means the stop is about 25G. If that was the case, wouldn't other parts be breaking?
..a
.
The plastic gears in Hitecs are made of congealed margarine. I've had them strip -in flight-! Recently had one strip being used as a camera shutter trigger.. all it did was wipe across the shutter button.
Small servos have poor material for the gearing. I found out early to never move a Futaba S-133 by the servo arm. Stripping almost guaranteed.

[RicVaughn]

Hi,

The current servo is a hitec HS81 mg. Another plane has a HS 85 mg and it's bending a 4-40 rod! I figure the max weight the servo sees is apx 2 ozX.80 in. for 1.6 in/oz.. In flight is no problem, just landings.



Ric

[RB]

Get a shock of an RC truck. Leave off the spring. Lite oil.

Only partly joking.

RB

[Ron Laden]

Ric,

One more thing to consider, with your pivot point at 25% mac, then mass balance the stab so it balances just ahead of the pivot, this is said to eliminate flutter and reduce the operating force required from the servo considerably.

Ron.