To pushrod or not to pushrod?
 

Ok guys here's the deal. I'm bulding a Midwest Citabria and am up to the point where it's time to set up and install the servos. The plans call for a single elevator servo and a pushrod to the rear of the airplane connected to a Y going out to each elevator half. Here's my issue, I have this fear of pushrods flexing and a fear that pushrods in a Y going to each half of the elevator will cause uneven deflection through imperfect symmatry. I would love to install two elevator servos in the rear of the airplane but would have to add weight to balance that. I've tried pull pull elevators in the past with poor results (I'm redesigning that airplane also). Am I crazy to think that pushrods are going to flex and that a Y is going to induce a rolling moment? What do you guys thing I ought to do? Thanks for the help!

RE: To pushrod or not to pushrod?
 

When I can get them I like to use carbon fiber arrow shafts and make up my own rods with 4-40 threaded ends. I keep the ends as short as I can and to date have never bent or flexed one. I also make up my own light weight pull/pull systems for the rudder. Weigh almost nothing. With the arrow shafts they are stiff enough you need no added support installed for them if you don't want to add any.

RE: To pushrod or not to pushrod?
 

Carbon fiber arrowshafts work well, so do properly braced Nyrods. I'd use two servos (one reversed) in the fuselage cabin area (under the wing saddle). Use two separate pushrods with separate elevators. I've done this many times. It's quick, easy, and strong.

Dr.1

RE: To pushrod or not to pushrod?
 

I usually run a 4-40 pushrod inside a nyrod outer tube for pushrods on larger aircraft, with as Dr.1 suggegsted, 2 servos and pushrods. A reversed servo is not needed, if you have enough room to put the pushrod on the same side of both servos. Just make sure the geometry is the same on both if you use 2 servos.

RE: To pushrod or not to pushrod?
 

Personally, I think your crazy for worrying about it. How will you get uneven deflection? The pushrod can't move side-to-side because the control horns are holding it in place.

I used the "Y" pushrod arrangement many times without any problem whatsoever, including on my Super Skybolt which is easily as aerobatic as a Citabria (If not more).

If you want to get into the geometry of it, then yes, there may be a MINUTE bit of difference between the movement of the two halves, but it will be so small that you'd probably get the same amount or more from using two servos (unless they are extremely expensive servos).

I say go with the planned pushrod and don't loose any sleep over it.

RE: To pushrod or not to pushrod?
 

Hi!
Personally I would never use a Y-coupling!
It is much easier (safer, stronger, lighter) to couple the elevator haves together with an Easton aluminum arrow shaft tubing and use a single (Easton) arrow shaft as pushrod. These arrow shafts are lighter than carbon fiber and just as stiff.
If possible I always use a pull-pull wire system (thinnest fishing wire). Lightest and stiffest method to control elevator and side rudder.

Regards!
Jan K
Sweden

RE: To pushrod or not to pushrod?
 

I use dual servos even on 40 size birds. My nobler uses 2 small 30oz servos on each elevator half. Precise control short rods, minimum flex and redundant. I used the y pushrod once,, never again. Could not match evenly both sides for neutral. Jim

RE: To pushrod or not to pushrod?
 

I have used a coupler from Jamara before that works pretty good. It has spaces in it for three 4-40 rods to slide into and lock in with a set screw. The two outer holes connect to the rods for the elevator halves, the center hole goes to the servo. That way, you can adjust and fine tune each pushrod to get to neutral. There is also a precision bell-crank available, I think I got one through Central Hobbies. Anyway, it mounts in the tail of the plane and basically mounts two "servo" arms onm the outside of the fuselage on either side, with a control arm concealed inside the aircraft. The device is lighter than two servos would be back there, uses ball bearings so there is no slop, and allows for short, firm linkages to the control surface.

I've been through a variety of methods because I never can get a conventional y-rod to work out correctly, and I always like to try and tech my way out of a problem.

RE: To pushrod or not to pushrod?
 

Thanks for the info everybody, most of you confirmed the idea I had planned on using anyway (the two elevator servos with two carbon fiber pushrods). However, I like the idea of the bell crank from Central Hobbies. It's a bit pricey though, is it difficult to install? Here's the link to it [link=http://www.centralhobbies.com/control_linkage/Bellcrnk.html]http://www.centralhobbies.com/control_linkage/Bellcrnk.html[/link]. Do you have any pictures of this installation skydeuce? Is it adjustable in width or will I have to make sure the airplane will fit with it? Thanks for the input everybody!

RE: To pushrod or not to pushrod?
 

I have no quarrel with any of these posts, since I've tried pretty much all of them, and they all work. I do like pull/pull elevator when the configuration of the model favors it. However most models I've done in the last couple years have been with dual elevator pushrods running off of one side-mounted elevator servo. If you're building instead of ARFing, it's a simple matter to brace the pushrods at more or less the 1/4, 1/2 & 3/4 points, and that's plenty. With a model the size of the Citabria, I'd go 4-40 size for sure.

I like the DarrolCady.com pushrod components--CF hollow rods and steel ends. I glue the ends in with JB Weld. (Tried the black CA, had one come loose.)

Side-mounting the elevator servo allows the servo arm to rotate in a vertical plane--no side-to-side motion at all. You can go to Central Hobbies website and look at their description of the DEPS (dual elevator pushrod system) if this concept is not familiar.

But as MinnFlyer says, the Y'd steel ends off a Dave Brown pushrod or some alternate version do work pretty good--they're just a bit inelegant. I guess it depends on how much of a hurry you're in to get the bugger finished and in the air. Here in NH with a slow winter building season, I don't mind taking a bit longer to chase after a little more precision. But I have no doubt at all that the Midwest manual method would work fine. I've had some sweet flying planes done that way.

RE: To pushrod or not to pushrod?
 

I used the Troy Newman / Central Hobbies dual elevator pushrod system (DEPS) on my CA Model Epsilon pattern plane with very good results. With a threaded control horn on each elevator half, it was easy to adjust throws on each side so that they matched perfectly. Extremely light, no slop, keeps weight out of the tail section. Does take a bit of effort the first time to do it right.

RE: To pushrod or not to pushrod?
 

I don't have any photos of the Central Hobbies bellcrank install, but not too difficult to put in to the plane. The sizes are slightly adjustable. You can trim down the center shaft to match your plane's width at the end, or shim the mounts on the outside. It was installed on a Bridi Killer Kaos I had that died due to the firewall leaving the plane during a take-off roll. I have the assembly standing by to put in my next Kaos. I will also buy another one when I put together my CA Epsilon. They are a bit pricey, but I figure the cost is offset by not using two servos in the back, or servo extensions to run back to those servos, and I can put a higher quality, or digital servo to run the elevators.

On the install piece. Measure out on your plans where you want it to go, make sure you have enough room inside and there is notheing that will bind or limit travel. Then transfer your measurements to both sides of the fuse and drill your pilot holes in either side.

RE: To pushrod or not to pushrod?
 

skydeuce:

Depending upon the weight of the engine that you use, you may find that the bellcrank makes the Epsilon tail heavy.

RE: To pushrod or not to pushrod?
 

If you opt to use a single pushrod shaft split into two, make sure to get the pushrod as close to the center of the fuse as possible at the servo end. Usually you have your rudder servo mounted dead-center to use a pull-pull, so position the servo arm on the elevator servo pointed toward the center of the plane, and push the elevator servo as close to the rudder servo as you can without causing mechanical interference.

RE: To pushrod or not to pushrod?
 

Thanks DMANSON,

I'll take that into consideration when I build. I got the Builder's kit and not the ARF. I haven't decided on an engine yet either, it is still a few projects back in the batting order.