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Servo Direct Gear Driven Control Surface

Inq

Well-known member
#1
I'm kind of enjoying some design, CAD work and printing for my Inq'd Trainer. I finally decided to put the servos in the back with the rudder and elevator and was working out how to do it. The easy way (and probably the smartest) would be to just hang the horns outside the fuselage and have short connecting rods to the control horns. Eventually, I want to design and build some war planes and I'd like to experiment with putting all the guts inside so the scale aura bubble isn't burst when I see all those horns and rods flapping in the wind.

At one point, I thought, I could just directly connect it to the surface, but I'd loose any torque multiplication using the control rods. GEARS - I've printed 3D plenty of gears in the past - Involute Spur, Herringbone, Worm... etc... so... why not?

Here's the first attempt. The design starts out using a 9 to 27 tooth ratio for a 3:1 torque advantage. When the servo moves through a full +/- 60°, the control surface will do +/- 20°. Since the gears don't fully turn around I removed the un-needed teeth to help it fit in slender fuselages. It also is custom fit for SG90 servos.

I didn't want to fiddle with critical placing within the plane to mate the gears, so I also designed a gear box that keeps them in contact and aligned. This way the whole unit would just be attached to the control surface first and then slid into place in the fuselage. That's the dream at-least. Here's the CAD drawing...
Servo SG90 Gearbox.png


Total weight of the gears and gearbox using ABS is 4 grams. Here's the first printed version... waiting for the plane to put it in. I used the translucent filament hoping to be able to see the gears in action... but I haven't quite mastered getting it clear enough to be useful.

Gearbox.jpg
 

Foamforce

Active member
#2
Very elegant! Removing the unnecessary teeth is an attractive detail.

Why is the gearbox case necessary? Is it because there is too much flex, so the gears would skip without it?
 

Inq

Well-known member
#3
Very elegant! Removing the unnecessary teeth is an attractive detail.

Why is the gearbox case necessary? Is it because there is too much flex, so the gears would skip without it?
Maybe, it's not necessary. Not sure. I'm just trying to imagine how I'd do it if they were separate. I guess the big gear could be put on say... the rudder and other gear/servo are hard mounted in the plane. Just setting the trasn/recv centered and install the rudder. Should be far easier than trying to get a z-bend through horns in a tight volume.

With the gearbox, it's already aligned and gapped. My initial thinking (as I sit down to CAD now) is that I could simple build a boxed "slot" where the servo would fit to react the torque loads, but it wouldn't even need to be glued or screwed. The rudder hinge would keep it from sliding out and the box would keep it from turning. In theory, I could simply remove the rudder hinge pin and pull the rudder/gearbox/servo out.

Also, the gearbox keeps any debris or even the servo wires out of the teeth. I also packed a little Vaseline inside... well... just because I could. It's not that it needed it. Now... in reality... we'll have to wait a little longer to see if it works as I imagine. ;)
 

skymaster

Elite member
#5
I'm kind of enjoying some design, CAD work and printing for my Inq'd Trainer. I finally decided to put the servos in the back with the rudder and elevator and was working out how to do it. The easy way (and probably the smartest) would be to just hang the horns outside the fuselage and have short connecting rods to the control horns. Eventually, I want to design and build some war planes and I'd like to experiment with putting all the guts inside so the scale aura bubble isn't burst when I see all those horns and rods flapping in the wind.

At one point, I thought, I could just directly connect it to the surface, but I'd loose any torque multiplication using the control rods. GEARS - I've printed 3D plenty of gears in the past - Involute Spur, Herringbone, Worm... etc... so... why not?

Here's the first attempt. The design starts out using a 9 to 27 tooth ratio for a 3:1 torque advantage. When the servo moves through a full +/- 60°, the control surface will do +/- 20°. Since the gears don't fully turn around I removed the un-needed teeth to help it fit in slender fuselages. It also is custom fit for SG90 servos.

I didn't want to fiddle with critical placing within the plane to mate the gears, so I also designed a gear box that keeps them in contact and aligned. This way the whole unit would just be attached to the control surface first and then slid into place in the fuselage. That's the dream at-least. Here's the CAD drawing...
View attachment 231983

Total weight of the gears and gearbox using ABS is 4 grams. Here's the first printed version... waiting for the plane to put it in. I used the translucent filament hoping to be able to see the gears in action... but I haven't quite mastered getting it clear enough to be useful.

View attachment 231984
So you would use this on the elevator right .
 

Inq

Well-known member
#6
So you would use this on the elevator right .
The shaft slides out. There is square keyway that can be seen on the CAD drawing better, so any kind of shaft as needed. For my first plane (link in O.P. above) the shaft will probably be the same for both the elevator and rudder units. I'm currently working on how to parse up the drawing into printer friendly pieces, which way Z goes and so-forth. Once done, I'll be posting the updated drawings there and prints to come.
 

Inq

Well-known member
#7
Here is the tail section without the surfaces. I couldn't figure out a easy way of making them serviceable, so I just glued screwed them in. The surfaces will be glued over these. Another good thing about the "gearbox" versus being loose, they're already aligned and ready to go.
Tail.jpg
 

Inq

Well-known member
#8
The Good, Bad and the Ugly...

First, the pictures...
Top.jpg

Side.jpg


The Good
  1. Nothing is in the slip stream! Speed or Scale aficionados celebrate.
  2. The elevator is rock solid, with no play.
The Bad
In both of my careers, it was my job to make things (sometimes literally) fool proof. I also recognize, I could never go into business, as my designs are NEVER perfect, NEVER complete. I'm always see something wrong and want to fix it.
  1. The Rudder Sucks! Way to much play to be usable.
  2. No adjustability.
  3. No disassembly.
  4. Although, not pertaining to the gearbox, my Empennage is too heavy. I need to use more drastic measures of lightening.
The Ugly
This tail section isn't going into my plane and I have to tear it apart to retrieve my servos. I see some real promise, but there too many problems to go into my first trainer. I will re-explore this when I start getting into making some speed or scale airplanes.
  1. The gears in them themselves, don't have any measurable backlash especially when printed in Nylon. But...
  2. In the current gearbox design the output gear was left floating. My assumption was that, I'd have the airplane structure enforce the location, giving a modicum of external (to the gearbox) adjustability. In the elevator, the geometry forced the gears into contact and thus work great... the rudder didn't.
  3. A new design would have to make the gearbox the primary gear aligner.
  4. Find creative ways to make them internal, but still adjustable and be able to disassemble it.
Final Analysis: Big fat fail, but with some glimmer.
 

Piotrsko

Master member
#9
Eh, rudder isn't the issue here. Too much control surface and definitely adverse control yaw. And it's going to take a *pot load of ballast to get the CG correct. Worst case with the rudder is it will fly sideways unless it flutters then everything is off the table until it falls off. You could glue it straight and fly with just the elevons. Probahly not a beginner plane, but it does look rather airworthy. A hole saw surgery might be appropriate, with tape over the holes.

Btw tailwheel is in slipstream.
 

Inq

Well-known member
#10
Eh, rudder isn't the issue here. Too much control surface and definitely adverse control yaw.
The experiment I was attempting is for a plane using rudder and lots of dihedral for roll... From my original learning experience (40 years ago) on a SIG Kadet Senior with no ailerons was that it needed a seemingly huge rudder. Also, can't have it slopping around for fine control.

Btw tailwheel is in slipstream.
:ROFLMAO: Big and ugly isn't it? 2" Tundra Tail Wheel. If the plane promises to stay above ground for several flights, I'll gift it with some wheel pants.
 

Bricks

Master member
#11
One of the reason to keep servos forward is CG, you might be better off coming up with a push pull system that is hidden, or at least a thought.
 

Inq

Well-known member
#13
One of the reason to keep servos forward is CG, you might be better off coming up with a push pull system that is hidden, or at least a thought.
I had one forum member recommend them going in the back. Also among the 3DLapPrint plane models I have, two have them in the back and two up front with push rods. I should have simply done the Moments calculation... Totally irresponsible of me! The CG is far more important... see update at https://forum.flitetest.com/index.php?threads/inqd-trainer.71134/post-734052
 

Inq

Well-known member
#14
U could give RDS a try. I made myself some 3d printed prototypes that worked pretty well. It is a very strong and secure method to hide cotrol linkages in scale or high performance gliders.
View attachment 232220

Here is a pretty good description on how RDS works:
https://augiemckibben.tripod.com/g2_instructions.html
I remember those now. Very pricey back when. Simple concept, lots of friction that I guess a dab of Vaseline will handle readily. Have you (or anyone else) ever use these? I might have to use those some day.

BTW, I noted that they actually used this same principle in a real (kind of) plane that I want to model some day...
avia537.png
 
#15
I remember those now. Very pricey back when. Simple concept, lots of friction that I guess a dab of Vaseline will handle readily. Have you (or anyone else) ever use these? I might have to use those some day.

BTW, I noted that they actually used this same principle in a real (kind of) plane that I want to model some day...
View attachment 232224
that's a very simple concept that works pretty well even in most demanding plnaes like acrobatics slope soarers or gliders. They use a piece of fiber glass or carbon for the rod sockets which helps a lot to reduce friction and also make them super strong and light.
I made a POC and it worked, also i was working on a 3d printed prototype that worked well, it needed a feew adjustments but it works.