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Solved Is there any better elevator control?

Monte.C

Well-known member
#1
Working on a design right now, the full size original had just a bit of dihedral to the horizontal stabilizer. I can build this dead flat but would love to build in some dihedral. I could see using two servos for the elevators but this is a pretty small build, and besides in general terms that's an overkill solution to something that looks like it should be simpler. I could see running two pushrods tied together from one control arm, and there are those really lightweight flexible pushrods, I believe they call those specific ones Snakes. I guess there's no mechanically correct way to tie the two sides to each other, right? They hinge on different axes. What's the most elegant solution?
 

Merv

Well-known member
#3
I agree with @Grifflyer, two pushrod is the way to go. I would like to suggest a slight improvement, that will accomplish the goal. One long pushrod bent into a V, with the point of the V attached to the servo in one hole.

I use this method in my F22 to control one aileron and half an elevator for increased roll.
 

Hai-Lee

Old and Bold RC PILOT
#5
If you have sufficient fuselage room where the tail attaches you could use an aileron torque rod arrangement driven by a single pushrod with a forked end near the tail. You could also use the same torquerod setup being driven by 2 smaller servos mounted side by side and connected back to the receive by a single "Y" harness arrangement.

There are many possible solutions/options

Have fun!
 

Monte.C

Well-known member
#6
If you have sufficient fuselage room where the tail attaches you could use an aileron torque rod arrangement...
Ah - I wasn't aware this mechanism could be created in a relatively simple way. A bit of searching shows me how to do it. This guy says that's how it was done from the late 70s into the 90s. Brilliant!
DSC_0225.jpg
Only thing is that he's running this inside the wing, and the rod wants to be in alignment with the hinge axis.
But what I see here is a way to offset the connection from being on the control surface to being in or alongside the fuse, and to allow for any reasonable difference in hinge axes of the two surfaces. You're still using two connections from one servo. So this could be used shorter and with just the wire to bring the lever rod (control horn) in towards the fuse. Love that. Keep it tight.
Good to have the idea in my toolbox. @Hai-Lee is a genius. Thank you!
 

Piotrsko

Well-known member
#7
If you use ball cap rod ends, alignment is a not issue. If youre using one servo, use a long pushrod end so the wires flex easily. We did all sorts of goofy stuff back in the 70's.
 

Monte.C

Well-known member
#8
We did all sorts of goofy stuff back in the 70's.
You guys invented most of what we do in the 70's!
I think you mean rod ends for the aileron torque rods. I think I got you.

When the elevators are co-planar and on the same hinge line we can cut them out of one piece, or tie them together with a solid connection like stiff wire or a wooden coffee stirrer. (Hey Starbucks, I'm still open to sponsorship.) For dihedral elevator surfaces or for swept hinge lines my first thought was for a ball joint/universal to tie the elevators together. That would be super clean. But as they swing the connection would want to change in length just a tiny bit, enough to always be trying to force the glue joints or the foamboard or warp the surfaces or strip the paper from the foam. This problem wouldn't exist if these "tie rods" were on the hinge axes or just if the joint itself was located at the intersection of the two hinge axes.

I guess that's the most elegant solution. Each elevator has a stiff wire "let in" to the foamboard, glued solid, set at an angle from the hinge line such that where the two meet on the centerline with a universal joint, that joint is located at the intersection of the two hinge lines. If you follow. I can see it but I don't want to draw a diagram. But this looks so fiddly to build in a tiny foamboard plane.

Oh well, this is hurting my brain. More & more I think horizontal tailplanes are just SO awesome!
 

Namactual

Well-known member
#9
I feel your pain brother.

I went through this on a few of my builds. I did the single servo tied to dual control rods on the Phantom, and just decided to go single axis elevator on the Guardrail. I hated to make that non scale compromise, but it just was not worth the added weight and complexity on a mini build like that.
 

Hai-Lee

Old and Bold RC PILOT
#11
As it seems that 70s solutions are suddenly new again I wonder if 60s solutions might be applicable and new again also.
When servos were very heavy and extremely expensive it was common to mount them close to the CG point and run a system of pushroad and BELLCRANKS to operate any control surface and any wing or tail geometry.

With bellcranks mounted on each of the tail pieces and a central bellcrank to split the mechanical input you can operate any tail geometry be it dihedral, anhedral, a sweep angle, or even an asymmetrical tail simply and reliably ALL from a single servo pushrod input.

Just a little prehistoric information.

Have fun!
 

Monte.C

Well-known member
#12
As it seems that 70s solutions are suddenly new again I wonder if 60s solutions might be applicable and new again also.
When servos were very heavy and extremely expensive it was common to mount them close to the CG point and run a system of pushroad and BELLCRANKS to operate any control surface and any wing or tail geometry.

With bellcranks mounted on each of the tail pieces and a central bellcrank to split the mechanical input you can operate any tail geometry be it dihedral, anhedral, a sweep angle, or even an asymmetrical tail simply and reliably ALL from a single servo pushrod input.

Just a little prehistoric information.

Have fun!
Reading you loud & clear. Ain't happ'nin on THIS build!
 
#13
The best design for the linkage depends on where it has to fit. Possible solutions:

Separate control horns for each surface and a single pushrod that splits off into a Y at the end. Because of the misaligned rotation axis, it will need a little bit of flex somewhere, but that is easy to do with a wire/dowel hybrid pushrod.

Use giftcard control horns that flex a little bit to connect to the same pushrod.

Create a torque tube linking the sides together with a universal joint.

Connect the sides together with a torque rod that engages a slot on one or both sides.

Use redundant servos. (less likely to crash if you have a servo or linkage failure in flight)

Build an internal lever arm that takes input from one pushrod and drives two pushrods.

Change the design to a full flying H-stab so both sides pivot on one hinge line.

Put the movable elevator on only one side and correct for the roll input with an aileron mix.