Pull-Pull screaming servo! Help!
 

Was setting up the pull-pull rudder system in my H9 1/4 Cap last night. Using stock set-up...I had read on these boards that the stock wires were weak...so I soldered them up, and put them in a vice...hooked a fish scale up to each wire and tested 'em both to 25-30 pounds indicated on the scale. Seems OK to me.
Installed a new Hitec 645MG servo...This is a hi torque, metal geared servo. Yeehaaa. Fed the wires through the fuse, dinked around with everything until the wires were fairly tight when the horn is attached to the servo. Got the radio out, centered everything up, and wow! What a lot of noise! The servo groans every time it is released. I decreased the tension on the wires a little bit, and that helped some...but there was still a decent amount of noise as the servo tried to center itself. (when the rudder control was released) I've never set up a system like this before...in my other planes, I would have serious issues with a servo groaning like this.
By the way...in my opinion, I really can't loosen the wires much more...any ideas?
Thanks.
Tracy

Pull-Pull screaming servo! Help!
 

mmm,

I have the same servo in a pull-pull set up on my yellow extra. I'm not getting noise. Make sure the width of the holes you are using on your control horn (on rudder) and servo horn are the same. Maybe the cables are not the same tension?

mmmm????

Pull-Pull screaming servo! Help!
 

I had a hitec 225 make noise when I checked for control surface slop, ya know like when you grab and wiggle the control surface and the servo was missing a lower bearing, so the shaft was rock'en around

Pull-Pull screaming servo! Help!
 

I have one on pull-pull that sounds something like a carpenters saw on thin hard wood when going through the motions. The servo is a High Tork ball bearing from Tower. I was told this is normal by another modeler. No, it doesn't center as fast as a standard.

Pull-Pull screaming servo! Help!
 

Your servo shouldn't scream at you. Make sure you don't have any binding. Check that the surface moves freely (no hinge binding), that the cables don't touch anything inside the fuselage, that they aren't suffering from "stiction" if you have them inside cable guides, and that you don't have any binding where they pivot on the control and servo horns.

Also make sure there's no force exerted on the surface from the tailwheel on the ground or tailwheel springs.

Pull-Pull screaming servo! Help!
 

If your servo is a 5645 that would be a normal digital servo squeal. If it not then I think you have a problem. Does it make that noise when it's not hooked up to the pull-pull wires? If yes, its a bad servo, If not then it's one of two things; 1. Excessive friction or 2. The geometry of your pull -pull linkage isn't right.

Oh yea, make sure your tail wheel is not sitting on the ground...

Pull-Pull screaming servo! Help!
 

Hey guys...thanks for the great suggestions and ideas. The airplane is laying on it's back on the bench, and there is no part of the tail that touches the ground. (or anything else for that matter)
When I get home I'll have to make sure that the wires aren't hitting anything inside the fuse. I'm pretty sure they aren't, but hey...
Also...The servo sounds fine everywhere through it's range except when it is released...It's like it has a problem centering.
When I release the rudder/throttle stick, the servo will hum loudly for a couple seconds and then eventually center and shut off.
My NEW thoughts (hahahah) are
1) Friction along the setup somewhere (servo arm, wires rubbing the fuse, etc)
2) Would a low battery cause this? I was using my bench battery...???


To answer some of the questions...I'm not entirely sure if the cables are the same tension....how perfect does this have to be?
The servo is a brand new HS645MG. Not a digital servo, but tough.
The flight pack is a 2700mAh NiMH 4 cell flat pack.
The servo sounds fine all through it's motions, and only hums near center, OR sometimes when it is held in position, although I think that is normal.
Again...thanks for all the help.
TM

Pull-Pull screaming servo! Help!
 

Look at this thread and check out the web links.

What is proper Pull Pull cable installation
 

Ok, this may be a dumb question, but are you suppose to criss cross the pull pull cable? It seems to me that it would be less likely to bind that way. What is the proper install method of running the cable thru :confused: the fuse? Thanks in advance!

Pull-Pull screaming servo! Help!
 

Also check to see if the cables hold the exact same tension when you go each way.If not you have a probable have a control horn misalighned.If the atachments on the servo are perfectly strait across they must also be across the hinge line on the rudder stait across.

Pull-Pull screaming servo! Help!
 

If the pull pull cables are attached to the servo arm that is directly attached to the servo any cable tension is going to pull on the servos output shaft. Thats probably whats causing your buzzing.
I have seen installations where the servo arm has a short pushrod on it that connects to another arm. That arm is where the pull pull cables attach and that arm takes all the pulling load. Not the servo.
Heres a pic to help visualize this,

Pull-Pull screaming servo! Help!
 

it's ok to cris cross the cables if it means they are going to exit the fuse with less binding.

life is good!!!

Pull-Pull screaming servo! Help!
 

Servos do make noise. Do a radio check. If it is OK fly it. Too much thinking may give you a heart-attach. Relax.

Pull-Pull screaming servo! Help!
 

<<Servos do make noise.>>

Indeed. But sometimes they are talking to you, and you had best listen. We use a lot of closed-loop control systems in helicopters. These are push-pull, using solid pushrods instead of cable like a pull-pull, but the end result is the same. If the arrangement is in a bind, or loading the servo assymetrically, premature failure can, and usually does, result. Timbersnake has a valid concern.

Thanks for the photo, Wayne. I was thinking about doing just that on my H9 CAP. But why so much arm moment?

I was thinking of cobbling a system together out of some bearings, pushrods, and large servo wheels/horns I've got lying around. No sense in buying something when you've got the stuff on hand to make it.

But if there is a good reason for the size of those components, I'll buy such a set-up with no grief. Where did you get it?

And how is your heli-quest coming along?

Steve

Pull-Pull screaming servo! Help!
 

Steve,

I wish that was a pic of one of my planes. Its my buddies Carden 40% Edge.

The reason the moment on the pull pull arm is so large is because that is the same distance as the horns are on the rudder. Distance at the arm and at the surface have to be the same to keep the geometry correct. What you cant really see too well in the pic is that there are actually two servo's driving that pull pull arm.

As far as the heli goes,,,,,I never pulled the trigger. I wound up getting an Aeroworks 29% Edge kit. I am going to put my BME 44 in it.

Pull-Pull screaming servo! Help!
 

Okay, got it. Thanks.

I hear you re the new plank. These larger (than common "sport" models- the really big ones like your pal's are TOO big for my situation) airplanes have really got my interest up.

I haven't flown my 1/4 scale CAP yet, but am already thinking about the next one. What to get???

It is all about diversions. After over ten years of being very heavily into helos, and owning more than a dozen different ones, I've settled on two nice ships that I fly when I get the urge to. My main focus is back on airplanes; but the little nitro stadium truck I bought for my grandson is a kick, too.

I'm about to get him a new one, and keep the first one for myself. Ah, toys...<G>

You'll get another helicopter one day. They're too much fun.

Steve

Tiller and bellcrank
 

Wgeffon is correct, He is using a tiller arm bellcrank, If you care to see my Edge with pull-pull on the elevator and rudder here are the links. I used two digital 8411 servos, 1 for the rudder and 1 for the elevator. I have a carbon fiber rod running from the servo to the bellcrank, then the pull-pull wires connected to the bellcrank, servos are slilent in the neutral position. I know it is hard to tell how the wires are connected but I can't get a better shot because of the location in the fuselage.

http://users.ev1.net/~scoplin/eservo.jpg

http://users.ev1.net/~scoplin/ebell.jpg

http://users.ev1.net/~scoplin/etail.jpg

Pull-Pull screaming servo! Help!
 

I got the picture; very nice work.

Thanks for the idea.

I assume one finds this stuff (bellcranks, etc.) at Central Hobbies, or one of the more specialty-oriented shops?

And while we're on this topic, I would like you guys' opinions on braided cable vs single-strand, soldered vs crimped connectors, for the pull-pull mechanism.

This is new territory for me; I'm proceeding with caution...<G>

Steve

Bellcranks
 

I got mine from Don' Hobbies 800-972-6273 or you can order direct from Nelson hobbies, mine are Nelson Bellcranks. I have had good luck with the Du-Bro coated pull-pull cable and 4-40 threaded ends, crimped not soldered. If you call Dons you will find that he recommends coated Kevlar and doesn't use wire at all.

Pull-Pull screaming servo! Help!
 

I checked out the Nelson site; looks like just the ticket. And Don's is the place to get this stuff, eh? Is that pull-pull kit he sells with kevlar wire(?), connectors and ball links any good? I was going to go with the DuBro standed cable, but if this kevlar works good...

Any other "specialty shops" with this sort of goodies?

Steve

Pull-Pull screaming servo! Help!
 

Timbersnake,
Quick question: Did you exercise the servo before hooking up the cables? Did it/does it make the same noise?
I have found that the higher torque servos tend to make a little more noise than standard servos. Don't know about digital though...
With the little extra noise the high torque servo makes together with the resonance chamber, called a fuselage, and the guitar strings attached to the servo running the length of the resonance chamber, you will have a louder sounding noise.
That is what I came up with regarding my Cap 232.

Just a thought...

Jeff

PULL,PULL
 

If you really want to set up a pull,pull system right you should us a bellcrank in the middle. Use short push rods to the bellcrank and then from the bellcrank to the control horns. By using this method you can make the pull wires as tight as you want and take the stress of the servo bearings. I suggest that you use ball links from the servo to the bell crank for positive movemnt and no slop whatsoever in the transition of force. But thats just my opinion. Ken....The "BALSA BANDIT"

Pull-Pull screaming servo! Help!
 

<<...bellcrank in the middle...>>

Uh... that's, like, what we've been talking about for a half dozen posts or so...<G>

New stuff to me; I'm getting quite an education here.

So, anybody have any suggestions besides Don's, for a vendor?

Not looking for a manufacturer, but a shop that sells a variety of this specialty big-airplane goodies. I'm sure Don's is great, but I like several back-ups.

Steve

Kevlar
 

I have never used the Kevlar, I have always used the Du-bro stranded.

Steve C.
 

What you are talking about is not really a "bellcrank", but rather a tiller bar. I have used Sig heavy duty tiller bars for years in large (1/4 up) models and have never had a problem. To see a breakdown of the tiller bar go here:

http://www.sigmfg.com/cgi-bin/dpsmar...nuF.html?E+Sig

You have to click on catalog at the top left, then on hardware at top right, then hardware again on the middle left. The heavy duty tiller bar is the first item listed.

Hope this helps some. I am going to start using the Kevlar cable in all my large models. It is NOT wire and will not cause interference with your radio gear like the wire cables will.

Take care, stay safe, and happy flying! :cool: