Question about dual elevator servos - slight problem...
 

I'm set up the servos on my Funtana 90(Hitec 625mgs) with my Futaba 9C.

The elevator halves end up at the same spot at full deflection, but one rises/falls more than the other throughout the travel; they're not exactly the same until they hit full deflection.

I have an idea why this might be happening:

I had to use a decent amount of subtrim to center one half of the elevator.

If I need to use subtrim to center the surface, then technically the servo isn't at "center" anymore, right?

SO, if this is the case, and I have a decent amount of expo dialed in, could that cause the elevator halves to rise/fall at different rates, and have one catch up to the other at the end of the travel??

Any other ideas are appreciated. I'm using identical horns and arms on both halves, and each pushrod is in the same hole side to side.

RE: Question about dual elevator servos - slight problem...
 

Are the pushrods the same length? If not, then the geometry changes differently on each side as you move through the range.

I don't think the subtrim itself is causeing the problem.

Servos should provide linear deflection through their entire range of operation. By this I mean that for a given change in signal pulse width the servo should move through the same angle no matter where it is in its travel. Due to production tolerance in the feedback pots there will be some error. Less expensive servos -> less expensive pots -> not so good linearity. Even the more expensive servos suffer from this problem. You can minimise the differences by going through all the servos of the same type you have and selecting the pairs that match best.

You could try shifting a servo over by one spline, sub-trimming back to center, then seeing if you happen to end up an a better part of the feedback pot.

I assume that you arranged the pushrod length and servo center positions so that the rods are at right angle to the servo horns and the line from the pivot in the elevator clevises to the hinge line when the elevators are at neutral.

RE: Question about dual elevator servos - slight problem...
 

like phil says take the servo arms off and set subtrims and regular tims to zero and put the arms back on 90 degree angle to servo case and then fit pushrods to go on as close to center as possible and then adjust the clevis to fine tune them to zero.the subtrim will have an adverse affect on travel thru the intire length of travel. the less you have to adjust with subtrims the better the intire setup will be.but you may still need a little subtrim to go righton 90 degrees

RE: Question about dual elevator servos - slight problem...
 

Do you have equal amounts of expo on each channel?

RE: Question about dual elevator servos - slight problem...
 

Yes. The elevator and aux channel have equal amounts. But the subtrim on one is -10 while the other is zero.

It seems like moving servos one spline at a time with subtrim at zero, try to get the surface as neutral as possible might be the solution...?

FWIW, one elevator pushrod is probably twice the length of the other...

RE: Question about dual elevator servos - slight problem...
 

holdup,ijust thought of something.do you have a reversing y in line or did you turn the servo arms one up and one down to get them to both work the same direction? if you did they will never work the same and be right.just an after thought

RE: Question about dual elevator servos - slight problem...
 

Neither.

Plugged into two seperate channels on the Rx.

Both servo arms point up.

One servo's output shaft faces the nose, and the other towards the tail. Thus no need for a reverser or opposite facing arms.

RE: Question about dual elevator servos - slight problem...
 

If you're plugged into separate RX channels, then why the unequal pushrods? You could mount the servos facing the rear for the shortest possible pushrods, put both horns pointed to the top and reverse one of the channels.

Back to your original real question...is it possible that you have positive expo on one channel and negative on the other? That would cause this type of problem. Endpoints would be the same, but displacement anywhere other than the endpoints would be different.

Richard

RE: Question about dual elevator servos - slight problem...
 

thats what is so good with a reversing y.you mount both servos the same way and use two pushrods the same lenght and set the centering with the pot on the reversing y.plugged into one channel and all settings are the same on both servos.can't get much easyer than that

RE: Question about dual elevator servos - slight problem...
 

You may have to go to a better quality digital servo.

Another solution is to swap out several servos to try to find a matching pair.

If you don't want to do this, try putting them on different channels and mixing using a multi-point mix. This way you can adjust for several points between the end-points.

RE: Question about dual elevator servos - slight problem...
 

i now may sound stupid but did you make sure your horns and arms are all the same .measure not just by eye.they are same servo..
if nothing else works ,not all servos are the same speed .
i use hitec diggy servos and a programer..

RE: Question about dual elevator servos - slight problem...
 

It doesn't matter how I move the servos, it won't be possible to get pushrod length the same or even close.

I am going to start over with it, it was setup rather hastily, and I found out that I accidentally programmed some of the Funtana stuff while still in the UCD model memory. Basically had to do all the UCD's linkages at the field today.

RE: Question about dual elevator servos - slight problem...
 

IMHO, this is your main problem. If you can't get the same geometry with both these servos it will be almost impossible to get them to match. Both of these servos need to be installed the same way -- both facing the nose or both facing the tail. Then you need to be electrically reverse the direction of one of them, can be done with a special Y-connector, rewire servo leads, or within your TX (what TX are you using?). After that you need to make sure your pushrods are identical length and that each servo arm is exactly 90 degrees to the case. If your using 2 separate channels make sure your subtrims and end-points are the same for both channels. If you can't get both servos to setup with 0 subtrim, then try other servo pairs to get as close as you can and finish with the absolute minimum use of subtrim.

An alternative to the above is the use of a programmable digital servo and/or a JR matchbox or Futaba servo synchronizer.

RE: Question about dual elevator servos - slight problem...
 

Like I said before, even with the two servo's output shafts facing the same way, it will still not be possible to get pushrod length the same.

RE: Question about dual elevator servos - slight problem...
 

OK, call me thoroughly confused here??? You clearly state you have the servos oriented differently so you would not need to reverse one of them (look at the last line in your post #7). If that is the case the shafts would not be facing the same way. One shaft will be closer to the elevator control horn than the other.

It is possible to have unequal lengths of pushrods and still have matching control movement. The only way this can happen is if the distance between the pushrod clevis and the rotating servo shaft is identical to the distance between the pushrod clevis and the hinge line and they are perfectly parallel to each other. This will take some very fine exacting measurements. The neutral position imaginary shape formed by the servo shaft, hinge line, control horn pushrod connection, and the servo arm pushrod connection must be a perfect rectangle with perfect 90 degree corners. Both sets of elevator controls will have to be perfect rectangles. This can be done because the servo movement is linear.

RE: Question about dual elevator servos - slight problem...
 

Yes. Per the directions and pics in the manual.

This is how the F90 servo setup is in the fuse. One servo slot is cut farther forward than the other, or the cases will hit each other inside the fuse.

Currently, the way I have it, the two servos' output shafts are as close to each other as they're going to get. And there is still a decent difference in pushrod length.

Unless I'm missing something here, I'm right. Here are a couple pics to clarify the situation.

RE: Question about dual elevator servos - slight problem...
 

Your pictures clearly show what your problem is. The pushrods do not form a parallel connection between your servo and control horn. The angle formed by the pushrod and the control horn must be exactly 90 degrees. The angle formed by the pushrod and servo lever must also be exactly 90 degrees. You do this by making sure the pushrod connections are exactly the same distance away from the pivots on both the servo lever and control horn. If you cannot achieve these 90 degree connections then the length of the pushrod effects the degree of deflection of your control surface.

Maybe this will help:

RE: Question about dual elevator servos - slight problem...
 

From looking at your pictures, you need to rotate one of the servos so it matches the other one. Then make another pushrod to adjust for the change in distance. It shouldn't really matter if the pushrod is 90 degrees to the control/servo horn as long as both sides are the same. I would say rotate the servo on the left side then all you have to do is shorten that pushrod. take the servo arm off and turn it so it is pointed up like the right side. If you have a Tx that will do it,reverse the servo that needs it to match stick movement. If your Tx doesn't do this, get a reversing Y connector and plug the servos into it and they should both operate in the same direction.

RE: Question about dual elevator servos - slight problem...
 

Look at his pictures a little closer. Notice that the placement of the elevator servos are not the same distance from the tail. One servo is ahead of the horizontal stabilizer, the other just under the leading edge. This is due to the design of the airplane. There is not enough depth to the fuselage to have 2 servos back-to-back. He has the servos oriented the way his assembly manual instructs him. What he does not have is equal distance tween servo and pushrod and control horn and pushrod. He has to deal with the unequal lengths of pushrods and the only way to do that is to make sure the servo lever and control horn connects are identical in length so that the angle formed with the pushrod on both the servo side and control horn side is exactly 90 degrees. The length of control horn and servo lever must be identical for both servos. It sets up a simple reverse parallelogram; move the servo lever +10 degrees results in moving the elevator -10 degrees.

I reviewed the instructions for the plane (its available as a PDF file on www.horizonhobbly.com) and the illustrations in the instruction shows the pushrod being exactly parallel with the horizontal stabilizer; it lies exactly in the middle between the hinge line and the servo shaft.

RE: Question about dual elevator servos - slight problem...
 

Thanks for pointing that out, didn't really see that, just the servos were opposite each other. Sounds like a screwy settup from the git-go. I'd be tempted to reposition the servos on the fuse.

RE: Question about dual elevator servos - slight problem...
 

I see hilleyja's reasoning, and it makes the center position the same for both elevator halves. But the fact is that the two servos are not the same exact distances from their respective control horns. The end result is you will never get a true exact identical positions for the fully deflected and center unless you play with the ATV's. And still they might never be both perfect at center and both ends. Mechanically and geometrically they are different. There will always be a slight difference in their mechanical throws. Even if they are mechanically identical, then you have the difference in matching of the servo throws and center positions.

The end result is that it really does not matter, it will fly fine and we are probably splitting hairs here.

Joe

RE: Question about dual elevator servos - slight problem...
 

That is not the case if you can form a true parallelogram between your servo arm/pushrod connection and your control horn/servo connection. The length of the pushrod will not impact the deflection of the control surface. The length of the servo arm and control horn do, however, impact the deflection.

Actually, the point I made above is 100% dependent upon not making any ATV or subtrim adjustments. Once you do that your geometry becomes skewed. You are right though, that you can probably get the adjustments close enough to satisfy most requirements -- the highly competative pattern flyers would have a problem getting the precision they need.

Here is another stab at my explanation with a graphic representation. A1 and A2 represent the servo arms, the circles at their base is the servo shaft. B1 and B2 represent the control arms, the circles at their base represent the hinge pivot point. Note that the pushrods are of different lengths. If you were to deflect A1 and A2 to +10 degrees then B1 and B2 would also deflect to -10 degrees. This is because the As and Bs are the same length and the angles made by their pushrod connections are 90 degrees at neutral.


BTW, I am having some doubt about my reasoning. I can't pinpoint it right now but my parallelogram arguement is probably flawed.

RE: Question about dual elevator servos - slight problem...
 

I ran this by a mechanical engineer at work and agreed with your reasoning. I told him to make a 2D model of it to see what would happen at full deflections. It's kind of hard to imagine this stuff.

Joe

RE: Question about dual elevator servos - slight problem...
 

fiveoboy01, Your setup is correct as designated in the manual. Delete the model from your radio and start again. That way you will have no left over mixes or whatever to cloud the issue. Make sure you have installed the correct pushrods for each side. The 3 pushrods for the tail are all different lengths, the longest one (I believe, I am at work) is for the rudder, not an elevator half. When you hook up the pushrods, use the hole furthest from the elevator surface on the elevator horn for that end and the hole on your servo arm that comes closest to being square (90degrees). On mine it was the 3rd hole out from the servo center. With the servos powered up, ensure that both elevator halves are at neutral. If you have an 8 channel reciever use channels 2 and 8 and activate the ailevator mix. If you only have a 7 channel receiver, the you must use a mix. When you slave the left elevator to the right using a mix, you are going to use channels 2 and 5 and compensate using a different screen to get the halves to move equally. Look at the faq on the Futaba web page and it will help you a lot. Sounds to me like you may have the push rods mixed up a bit and the the radio setup is compounding the problem. Wish I could help more but as I said I am at work and besides, I use the 9Z on mine. The different functions are approached differently on the two radios.

RE: Question about dual elevator servos - slight problem...
 

Actually, the split elevator function for the 9C is channels 2 & 8. You then activate AILEVATOR and reduce the airlon percentage within the function to 0.

Actually, the longest pushrod is for one of the elevator controls. It is connected to the single servo on the left side of the fuselage.