Dual-servo elevator linkage geometry
 

What is the best way to setup dual-servo elevator halves? Here's the deal... I'm getting ready to start on a P.A.M. Gee Bee Y and the manual suggests setting up the servos and linkages in a way that looks like it will cause differential movement between the two elevator halves. Take a look at this pic, from the manual, on how they recommend you set it up:

http://astro.umsystem.edu/andy/rc/servos.jpg

On the left side, they have you use the bottom of the two servo-openings for the elevator servo, and put the servo-arm on top. On the right side, they have you use the top servo-opening and put the servo-arm on the bottom. Note the difference in the angle of the pushrods in relation to the stab.

Is this a good setup? They indicate this method doesn't require a reversing servo. Would it be better to go ahead and set the two elevator servos exactly the same (both in the top slots with the horns facing down) and put a reversing-Y on one of the servos?

Thanks in advance!

Dual-servo elevator linkage geometry
 

AndySteere,
My 35% Giles has a similar setup with one servo on each elevator half and 2 servo's on the rudder. All servo's have the servo arms mounted upward. I have one of the elevator servo's reversed (using hitec digital servo's). Ideally you want the pushrod to be at a right angle to the arm and control surface. When setting a plane up for 3D and using 1.25" arms, the pushrods tend to run at an angle. The control horn is located closer to the surface, this allows the exaggerated throws necessary for 3D. I would reverse one of the servos and keep both arms upright. Set the pushrods up so that both side are equal. I tend to build some differential into the surface(not intentionally) because I am not perfect. If your hinge line is not perfectly centered or the control horn is not perfectly set into the surface, you will have some differential. Even if I use a drill press to drill my surfaces, I can never get them perfectly straight. You would probably need a lathe to achieve this level of accuracy. In any event, a computer radio can help to correct a small amount of differential. I try to get the linkages as equal as possible mechanically and then use the radio to fine tune the adjustment. I have not seen a setup like described in the manual but then again I have not seen every type of setup. Most planes that use tail mounted servo's use a reversed servo on one side of the elevator. Hope this helps.

Dual-servo elevator linkage geometry
 

Andy,

No need to worry. Just make sure the servo arms are at right angles to the servo. You don't need to reverse a servo because one is mounted lower than the other. If you use a travel guage you will see that everything works properly.

Dual-servo elevator linkage geometry
 

Well...

It depends on how critical you want to get...

Personally, I would (and HAVE in the past) go with a servo reverser.

There WILL be some deviation. Look at the pic below.

Dual-servo elevator linkage geometry
 

Close up view...

Dual-servo elevator linkage geometry
 

Mike,

Nice picture (you must be in marketing). However, you are starting at the wrong points. The control arm should be almost parallel to the elevator as a starting point(see original picture). Also, in your example you are depicting crazy amounts of throw. I doubt this plane will ever use that much elevator. If the throws are set properly for this type of plane and the controls are attached as pictured, you will NOT see any deviation and you will NOT have any problems in flight. You are more likely to have deviations from mis-matched servos than the linkage. Even then, it most likely wouldn't show up until you are near full throw.


Mark M.

I agree with MMALLORY
 

I had exactly the same linkage geometry on a 1/3 scale Spacewalker..one servo horn up, one down...

With careful adjustments I was able to get perfectly linear responses from each elevator half thru the normal range of travel...in the extremes of throw the elevators would begin to deflect slightly differently. This bothered me, I like perfection and symmetry when possible.

But the Spacewalker wasn't going to be 3D'd much <g> so I used it that way.

I expect the GB won't need 3D throws either...

Dual-servo elevator linkage geometry
 

MM,

I am not starting at the wrong points, The green lines in the picture represent the "Arc" that the pushrod would create if it were only attached at the elevator. I did not draw the pushrod so you could (hopefully) see that the two servo horns were aligned (the red dot indicates where the pushrod would connect and it is perfectly parrallel to the elevator). And yes, the throws ARE exaggerated for clarity, but I intended to show that the more you travel, the worse the problem gets.

Andy's original question was:

"What is the best way to setup dual-servo elevator halves?"

IMO the "best way" is a servo reverser and both servo arms either up, or down.

Dual-servo elevator linkage geometry
 

Mike,

OK, thanks for the clarification. I guess we'll just have to agree to disagree. Personally I think you can't go wrong with most instructions from the manufacturor. This way if there is a defect they will replace the plane without blaming it on your mod. Plus servo reversers add another possible failure point.

Yet another solution would be to go with one "reversed" servo of the same brand/make. I've used digital's (reprogrammed) for this type of setup with excellent results.

Mark M.

Dual-servo elevator linkage geometry
 

Thanks for the input guys! I am interested in both "best practice" AND what is good for this specific model.

ChuckAuger is right that the GeeBee will NOT need 3D throws... as a matter of fact, only 5/8" in both directions. However, because they have you use the inner-most hole in the servo arms (5/16" from servo-center), and a long horn on the elevator, the servo-arm will go through a large arc to provide this (as shown in the diagram below). This should aggravate the problem... as does the fact the pushrod ends don't start at the same point at neutral (as in MinnFlyer's diagram).

Here is an illustration of what I see to be the problem:
http://astro.umsystem.edu/andy/rc/gb-elev-servos.gif

The black pushrod stays pretty much parallel to the stab, where the red pushrod's angle increases the further off center you go. The red pushrod will not push the horn as far as the black pushrod, because of this geometry. Playing around with the drawing program, which lets me rotate and move lines and groups of objects around, seems to confirm this. In the bottom section, I would have to increase the length of the red pushrod to deflect the surface as much as the black pushrod does... with the same degree of servo movement (shown at 45 degrees in the diagram).

This specific model has been reported to have a tendency to "tuck" in high-G maneuvers. Could this be caused by this slight difference in elevator travel?

Any furthur thoughts?

One up, one down
 

Just use the outer most hole on the servo horn and control horn. Make sure the servo hole you use is the same distance from the stab and the rudder on both servos. Use your travel in the radio to adjust your throw. How complicated does it have to be?

Dual-servo elevator linkage geometry
 

Regardless of where the pushrod is located, if one servo arm is up, and the other is down, there is going to be SOME (even if minimal) difference in throw. I'm not even saying that it will be enough to make a difference. I'm just saying if you really want it to be the "Best" way of doing it (Andy DID ask what the best way was), then you want to have both servo arms facing the same way.

Dual-servo elevator linkage geometry
 

Andy,

Try the diagram again with both servo arms parallel to the elevator at neutral position. You may need to move a servo to make this happen.

The model may be tucking due to one wing snapping.

I have a spreadsheet which will calculate and diagram your servo/arm/surface movements to great detail. I can email if you like. I forget where I got it. It's rccalc95.xls

Are you selecting matched servos for the elevator pair? (speed and throw)

Dual-servo elevator linkage geometry
 

Andy, the difference in your diagram is due to the different servo locations, not the direction of throw on the control horn. If you superimpose the servos over one another, the difference will be very small. Smaller yet, if you simply make the pushrods a little longer (i.e. mount the servos farther away from the elevator).

Often times the problem occurs because the tail section of the fuselage is very narrow, which require the dual servos to be mounted one over another (similar to what you have draw, Andy). That offset is what creates the bulk of the linkage geometry difference, but folks often blame the different servo horn throw direction instead. Heck, I have a plane with dual elevator servos mounted under the wing with long pushrods, and people still insist that I should get a reverser.

I don't like exceedingly short pushrods, as they create more geometry problems than the little more rod stiffness is worth. I don't like eletronic reversers either. Even the best ones introduce some non-linearity that defeats the purpose of what they try to solve.

If you're still worried about the difference in servo throw direction, you can use a comparable servo from a different manufacturer which turns the opposite direction given the same input.

Servo Reverse
 

Andy - I have run into this also. I just put a reversed servo on the other side.

Dual-servo elevator linkage geometry
 

Thank you all for the great suggestions!

The GBY is an ARF, with all the servo openings pre-made, and I really don't want to tear into it to move them. I don't have any digital servos (nor do I want to spend the money on them at this time). The servo's are the same model from the same batch, but not "matched" (though I do have four of them that I can play with).

So, I think I'm going to put them both in the top location, with the arms down, and run the other elevator half to an unused channel of my JR8013DT. I've already setup the mix and I think it will work just fine.

Thanks again guys!

Dual-servo elevator linkage geometry
 

Andy - I was going to suggest exactly what you decided on. I had the same issue and originally used the reversing Y harness. It turned out that extreme weather conditions caused the Y harness to change my elevator trim. It was as someone else mentioned an uneeded point of failure. With all the computer radios out there, I'd think we'd all do the mixing solution. Good luck

Dual-servo elevator linkage geometry
 

Your going to have problems if you slave one servo into a diferent port on the reciever and mix then together.

You will only be able to trim the one hooked up to the elevator port on the reciever. The other servo will not move with manual trim inputs.

As others have said, you will get a difference in control travel if you use the horn on opposite sides of the servos. Even if you mount the servos on top of each other and the horns appear to be swinging in the same arc, they will move differently.

I know this because I tried it on a Sig Cap 231 Breitling ARF.

NOT PRETTY!!!!!! Especially when you HOG back on the elevator and the plane goes into a corkscrew loop.

Use a REVERSING Y-harnes!!!!! They cost $8.00. Thats right; eight bucks. If you can't afford $8.00, get outta the hobby. I don't mean to sound condescending or like a jerk.

Get the y-harnes with an adjustable pot and dial it in. Then "FUGETABOUTIT"

You'll be happier if you do.. No sense putting yourself through a headache for no reason.

Dual-servo elevator linkage geometry
 

Rcpilet, thanks for the comments!

I already solved the manual-trim issue. On a JR 8103DT, mix 5 and 6 both are linked to the digital trims. They're not multipoint mixes, but they'll work just fine.

Dual-servo elevator linkage geometry
 

On my GBY I needed to move everything forward as much as possible because of using a glow G-23, its much lighter than gas!
So I ran pushrods instead. Works great!

On other birds that needed rear servo mounting the use of a reverser worked just fine and as you see there are many ways to skin a cat :p

Dual-servo elevator linkage geometry
 

My GBY came in tailheavy too... even with the gas engine and large ignition battery. If I had known that ahead of time, I would have gone ahead and moved the rudder servo forward and run it pull-pull like the full-scale.

Here are some pics of what I ended up doing (click on images for larger versions):

http://astro.umsystem.edu/andy/rc/gb...uild28_320.jpg http://astro.umsystem.edu/andy/rc/gb...uild29_320.jpg http://astro.umsystem.edu/andy/rc/gb...uild30_320.jpg