wjcalhoun

Gentlemen:

I need some advice setting up the pull-pull system for my 1/4 scale WH extra. I am using a Hitec HS-7955TG rudder servo. The geometry is square - control horn pivot points exactly on the hinge line at the rudder, and the pivot points are even with the axle of the servo at the fore end. Arm length (not that it matters) is 2 1/4", and matches for the servo arm and the control horns.

The problem is that the servo sings quite a bit when energized at neutral rudder. I have loosened the tension on port and starboard cables to the point that they have perceptible sag (perhaps 1/4-3/16") back through the fuse. The servo does not sing if the H9 arm and cables are not connected, so the servo itself seems OK.

What actually is the proper tension for pull-pull cables? Any suggestions for eliminating the singing? Obvioulsy, the servo is drawing higher current while singing, so I'll have a negative impact on battery life if I cannot solve the problem.

Any ideas?

Thanks in advance.

JOHNS3D

My Feedback: (5)

on the 30% extra you should using at least 3" arm on your servo, it will help a bit to get more deflection. My cables doesnt have huge tension, just enough to avoid any lose point. Any pics?

wjcalhoun

Thanks. Can get some pics tomorrow. It is a 25% plane (baby extra from wild hare), with phenolic control horns that span 2.25 inches - that's why i used the 2.25 inch arm, but take your point on the longer arms having more deflection, particularly with larger planes.

JOHNS3D

My Feedback: (5)

yep should be fine, cause you are using 7955 servo, is the same that i use on my 30% extra with 3" swb arms.... you can hear the servo sing cause is trying to center all the time....digis servos does all the time. You can hear the servo bunzzing when it hit 40 deg. of deflexion on each direction, but just that. You can recognize when the servo is under huge load, and also when is just trying to center again.

propchief

My Feedback: (65)

I would make sure there is no binding in the rudder system. With the cables disconnected from the rudder move it back and forth the full travel and ensure it's smooth with no binding. Then work the cables by hand and make sure they move freely. That servo should not buzz unless it's under load. Something is preventing it from going to neutral.

wjcalhoun

Thanks propchief

I pulled the cables off the control horns, checked the cable motion, checked the rudder travel, etc, and saw no obvious problems, but when I reassembled, the singing is very much toned down - just a normal digital centering sing, not a buzz. The only thing I noticed was that the cables had a bit of torsion, probably from adjusting the cable length with the adjustment screws. Perhaps that did it. Although I did not specifically find the initial problem, it seems fine now. Thanks all.

smarks

The 7955's will buzz and is completely normal. If you want to find out if there is a load on the servo, connect an AMP meter to it and then you can be 100% sure. I will not setup a big plane anymore without using an AMP meter. It makes it very easy as well when setting up ganged servos.

ec121

My Feedback: (2)

Was the plane propped up to get the tailwheel off the ground? If it continues, push the rudder each way to stop the buzz and that will tell you which part to look at.

JoeAirPort

My Feedback: (41)

The cables only need to be tight enough to take up the slack....not so tight that they sound like guitar strings. So if you pluck them they should just make a thud sound not a "boing". You can hold the rudder at center with your left hand, then move the servo arm with your right to see how much slack the cables have. They definitely should not droop at center. This will cause the tail to wag at higher flying speeds. The servo should not sing at all at center. I just tested my 88" Yak with the same servo and it's silent. The elevators buzz a little due to the servo holding up their weight.

wjcalhoun

Tail was off the ground, so the tail wheel was not the problem. I now have the cables about as Joe suggests. No buzz, only a trace of singing. I think it may have been uneven tension on the cables from torsion. When I detached the adjusting clevis from the control horn, there was a fair amount (perhaps 8-10 turns) of twist in the cables. Problem solved now, I think. I appreciate all the advice.

Thanks all.