Rear Mounted Servos

If you're gonna cut two holes in the fuse sides, why not just cut one in the fuse bottom, make a small hatch to cover it and gain access to the servos, and install the servos upside down internally using shorter internal pushrods...

cheers astroflyer

Park flyer plans

Why not make them out of carbon fibre? It is a lot stiffer than equivilent length wire.

On my Ultra Stick 120 I've got the rudder and elevator servos mounted externally just in front of the tail. The pushrods are 12" long, are 4-40s, not 2-56s, and are NOT threaded all the way (my LHS pointed out that all-thread rods are weaker because of the metal removed to make the threads), and haven't had any problems with them. I used Dubro p/n 2139 swivel ball links to make the connections at the servo arm and control horn on both.

With the smaller tail surfaces of the US 40 vs. US 120 I'd think 14" of 4-40 would hold up fine.

My 2 cents worth.

Jim

It's hard to imagine an uglier way to install servos than to have them sticking out the sides of the fuselage. I would rather put the battery in a compartment accessible through a hatch on the bottom of the fuselage, under the tail. Then you can run an extension to the radio. I would expect the servos to get really oily hanging out in the wind like that. That's the problem with ARFs. They don't necessarily balance where you want them to.

I'd make it from carbon fiber arrowshaft or 1/4" square bass.

Dr.1 Driver "There's a Hun in the sun!"

I mounted my elevator servo in the tail as you mentioned on my UltraStick

1. The 440 rod I used came to about 11 inches, and it vibrated excessively when the engine idled. If I were you I would use a carbon fiber tube for

14" Can't you move the servo any further back?

Not if you use a proper pushrod. Make one using a 5/32 carbon fiber rod with about an inch of 2-56 threaded steel rod on each end.

John VB

Normen,

What engine are you using on your US 60, and did you use a anti-vibration mount? I've got a anti-vibe mount on my US 120 and that may be why my 12" elevator & rudder 4-40 pushrods don't vibrate excessively.

Jim

Yes! This works! With the servos back in the original place and the battery where I was thinking of putting the servos, the plane comes nicely into balance. I'll just need a 10 inch extension. This forum is sure a great resource for us uninformed!!! Jerry

Why not do an experiment. I gather the reason that you are worried is because you're unsure on whether this rod would buckle or not. Given the max torque of the servo & its arm length, you can determine the maximum force on the rod (well your surface load could be higher but I'm assuming that the servo is properly matched for the control surface). Anyway you can set up a simple jig & put this load on the control rod to see whether it buckles or not or how much margin you have till it does.

MN

I second that emotion. If the original poster has already cut holes for servos halfway between wing TE and stab, then he's stuck with that decision; if he hasn't, he should cruise

for long-running threads on "ultra stick 60" to see some good examples of tail-mounted servos. I have one of these planes under construction, and am using the 5/32" carbon rod for the elevator servo, mounted just behind the last fuse former before the stab. The rod is plenty stiff for the short span involved. The rudder servo is mounted just forward of the same former (to stagger the holes) and inline with the vertical stab. I'll use pull-pull on that. Would use pull-pull on the elevator (others have), but the stab is at the bottom of the fuselage; the servo/surface geometry isn't great, and having a horn hanging out there bothers me.

Someone else has complained of oily servos. Well, a servo with its mounting ears may be inserted through a hole large enough for its case bottom. I've put basswood 1/4 x 3/8 bearers inside the fuselage, and if I mount the servo with the mounting tabs inside, it's recessed perfectly so that only the output arm is exposed. I can cover the hole completely with a thin ply or CF plate. Neat, concealed and clean.

I've got a Saito 100, and I'm using the Sullivan Dynamount anti-vibration mount.

The only caveat to this is that if you meet the ground unexpectedly, that big heavy battery's gonna act like a battering ram inside your plane and trash your servos, receiver and whatever else is in its way. I'd externally mount the servos farther back with shorter rods and move the battery to the firewall if you have to.

-- Morris Lee snipped-for-privacy@verizon.net

I found a sure cure for that: make the battery box in the rear with some birch ply in the front of the box. Angle the ply so that the battery is ejected out the bottom if you hit that hard. Personally, I not hit that hard without damaging everything anyhow! Mine seem to be a light bounce or complete destruction!

I am a proponent of the philosophy that a plane should be built to fly, not to crash. You can brace the servos so that they won't shoot forward in a crash and get their gears stripped by yanking on the pushrods, and you can brace the battery so that it doesn't act like a missile. But I'm not going to uglify the entire plane just because I don't want the battery to potentially be able to damage something. If you don't want crash damage, don't crash!

Better yet, build your plane from scratch and make sure while you are building it that it is going to balance properly with the equipment that you would like to put in it.

By the way Jerry (the guy with the original question), if you decide to put your battery in a compartment in the back, make sure that the door can support the battery pushing on it with its weight multiplied several times in a loop or fast turn. A guy told me once that he put a small biplane with a chin hatch into a hard turn and saw the battery shoot out and land about 30 feet away, and the biplane went into a spin ending in a violent crash (at full throttle).

Robbie

Easy for you to say :-) :-) I've only been flying models for about 6 months and have been through 3 trainers. I'm hoping the Ultra Stick will last a little longer than the others :-) :-)

I put the door on the port side just behind the fuselage former that supports the rudder/elevator push rod tubes. Added a couple of strips of plywood to the former to strengthen and block the opening. Also added attachment supports for the plywood cover and screwed it on. The battery is encased in foam and the additional space above and forward of the battery is foam filled. Should be pretty strong and looks good too. Covered the door with black MonoKote and it looks like the baggage door of a full sized plane. I appreciate all the good advice provided. Maiden flight this afternoon..................... Jerry