I am building a plane that has a split elevator. The two elevator halves are not connected to each other. Each has a push rod. The two push rods are connected together inside the fuse and then to a single servo. This is essentially a "Y" with the two upper sections of the "Y" going to the elevator halves (about 12 inches long) and the bottom portion (2 inches) going to a single servo.

My questions are:

1. Does the part that connects to the servo have to be in the exact center line of the plane.
2. If it is off center by about 1/2 inch, how will it effect elevator deflection. Will one elevator deflect more than the other (major, minor, none at all)

My crude math tells me that the effect will be minimal. Please advise...
Thanks,

What matters is the travl direction of the pushrod system. If the servo pushes te rd directly parallel to the centerline of the airplane... the rod can be off to the side any amount you want. the man rod can be a straight line from the servo to one elevator half... then add the fork to the other elevator half and it will give equal throw.

If the main pushrod is pushed at an angle to the fuselage centerline... then the throws will not be equal. It won't matter if the forks of the rod are identical or not.

How unequal? that depends on the angular devitation. A long pushrod with a low angle and close spacing between the control horns at the elevator... you may have trouble measuring it with a micrometer. A short rod at high angle and wide spacing between the control horns you can get one side to throw double what the other side does...

The ideal, direct parallel situation s virtually impossible (the servo arm moves the connectin in an arc... the control hors move the connections in arcs...) But you are normaly going to be closer to the un-measureable difference than the outragous difference.

You building a GP RV-4? They use this setup. I must admit I didn't give it much thought, but I guess you could have a point. Either way, it worked fine. The differences between a left, middle or right mounting are minimal (If you have any trig background consider the small angles case where sinA ~ A).

Not an aerodynamic comment, but more of a structural one.

I noticed in the picture that the two elevator pushrods are held together by wheel collars. Personally, I'm not sure I'd trust a critical flight surface like the elevator to wheel collars. My GP Giles 202 uses this same method for elevator actuation, and where they connect, I soldered them togther. First I wrapped the joint (about 3/4" in length) with electronic hook up wire (the solid stuff) and then soldered the whole shebang... She prolly has 300+ flights on her with no problems. Might be a better idea than the wheel collars...

Just my $0.02

Tom

Thanks for the advise. I am looking into another method of connecting the push rods. I bolted the servo tray in place with the push rods and there is vertually no difference in deflection between the two elevators. My meter is only acurate to 2 degrees but I can see abotut 1 dgree difference at the full up position. The connection at the servo arm is about 3/8 inches off center.

In an elevator setup which has large throws - ANY offset is easily seen
A really easy fix is to make a fork- identical in distance from center when measured side to side .
Then, put the center of the arc on c/l of the fuselage.
If possible - put servo on it's side - then there is theoretically (Love those theories), no parallex.
Another easy fix is to tilt the servo to 45 degrees -
This will negate a lot of the side to side swing and you can still easily reach the mounting screws.
My elevators typically deflect up to 45 degrees each way -so any goofup is very easy to see.

And place a fairlead around the pushrod near the y-joint to support the pushrod against any forces moving it up or more importantly sidewise, motion which WILL result in a mismatch of displacements.

I've just had to make a Y rod for my split elevators. I taped the two pusrods together at each end then bent them together, it means I now have two 100% identical rods. To find where to mount the servo, I moved the elevator pushrod by hand, then moved it form side to side, while checking the throws. When I found the exact spot where there was no diff, I marked it and mounted the servo so that the arm was perfectly in line with the mark.

Dick Hansen nailed the problem. Here's what can happen...

As the servo rotates, the deflections will not be the same in the middle due to the rotation of the pushrod - even if the neutral point and full throws are the same for both halves. The longer the pushrod, the smaller the difference will be. personally, I gave up on using forked pushrods because I'm anal-retentive and even if I couldn't see a difference, it bothered me to know that there is one.

Now I go with two servos.

Another way, as Dick said, is to mount the servo sideways or have the servo go to a bellcrank mounted vertically. I've done the bellcrank thing, but it's more weight, more slop and more things that can go wrong. So the two servo method works for me.

One way to connect the two wires is to wrap the copper wire and rods together then solder over the wire to secure them! The trick is to get both of the end wies the same exact amount BEFORE you apply solder!!! AS for the question about if the servo is off center, you are sort of correct but the amount that it will matter will be very small! In a .40 size airplane it won't make much difference, unless it is extremely off center. Even then, if you attach the pushrod to the servo arm then bend the pushrod for it to travel down the middle of the fuse, then no problem at all!!! More care should be taken to ensure that the 2 "Y" end rods will make both elevator halves travel the EXACT same distance from center.

Not only does using two servos get rid of the "Y" pushrod problem, having separate servos for each elevator half is safer too.

I'm a fan of using two servos for the elevator.

Well folks, I thank you all for your replies. All considerations taken, I think the proof is in the pudding - meaning, if it flies ok then who cares if there is a one or two degree difference from full down to full up. Unless you are flying a pattern ship in a contest, I don't think you will ever notice the effects. For my flight patterns and airplanes, this minor deflection difference will be less than the effects of a warped wing. For most airplanes and Sunday flying, we correct for these things without wondering why the plane does not fly like an arrow. As a matter of fact, most of us fly with slightly warped wings, uneven ailerons, CG off specification, and other abnormal construction problems. These slight imperfections only rear their ugly head if we are expert pilots who are use to flying "perfectly built" airplanes.

I will live with the problems associated with the split elevator and fork push rods until I build a ship that needs the accuracy of dual servo driven elevator halves.

Again, Tanks for the tips and replies.

What would all you guy's say is acceptable? A few degress, a few mm or what?

thanks

If the difference is big enough that I can measure it then it's not acceptable. For me that's about 1/64" - 1/128". But sometimes no matter what you do, the servo and geometry can only get so good and you have to live with it unless you want to buy a servo matcher and digital servos.

WOW!!!
Does that 1/64 or 1/128 of an inch translate into measurable differences in flight?[:-]

It might to someone who is a better pilot than I am. But I figure why not build as accurately as possible? Those numbers were specifically in regard to matched control surface movement.

I always TRY to build as accurately as posible. But my question is How accurate do you have to be with the split elevator differences before it shows up as adverse flight characteristics?

I have a Great Planes Cessna 182 that uses this particular Elevator control. I ended up using a Sullivan Elevator splitter. So far so good and it affords some travel adjustments.




[link]http://www.sullivanproducts.com/ContSysAccMainFrame.htm[/link]

Here's a part from MK, available through most Pattern suppliers.

Can you believe these things cost 40 dollars?

http://www.centralhobbies.com/contro.../Bellcrnk.html

Use pull-pull instead of push rods. Much easier to set up and throws are always exactly the same.