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I'm designing a VTOL based on a conventional airplane design that will use a tilt-rotor to move the motors 90 degrees around a carbon fibre tube that also acts as the wing spar. Each motor is tilted independently using a servo.

The problem I have is what linkage to use to connect the servo to the motor mount. If the servo arm rotates 90 degrees, since it's not on the same axis as the tube/spar, then it won't be in the same final position moving from horizontal to vertical (it will be in a shorter/lower position).

Here are some photos:

View with wing This photo shows you the assembly in horizontal orientation with the wing in white.

View without wing This is the same image as above, with the wing hidden so that you can see the parts. In case it isn't obvious, the servo is blue, control horn white, motor mount black, spar orange, motor red, prop green.

Another angle Same image as above from another angle.

Vertical orientation Here is the assembly with the motor in the vertical orientation.

Another angle Same image as above from another angle.

Any ideas as to the best linkage to use here? Please bear in mind the following:

  1. The servo orientation can't really change as it needs to fit within the wing thickness
  2. I'd prefer to use standard, commonly-available hobby parts
  3. The engine mount is 3D printed so I can modify it if required
  4. I'd like to avoid too much complexity, such as cogs/gears or belts as much as possible

Thank you.

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    $\begingroup$ Ooh, this is a good mechanical engineering question! Have you considered if such a small servo has enough stall torque to keep the motor in place while in flight? I ask because it looks like you've spec'd a fairly small one so that it fits inside the camber of the wing aerofoil. $\endgroup$
    – ifconfig
    Apr 30, 2022 at 5:49
  • $\begingroup$ @ifconfig It's a 34 x 31 x 12mm servo such as the standard ones you'd find in a typical online store. I haven't done the calculations, but I've used these servos in the past and they seem quite good, but as you rightly point out I haven't had them have to support a motor! Given that they're so cheap, I plan to "test" them by building it :) Also, they will be supported by a plastic "rest" when in the horizontal position, so the main torque impact will be when hovering, which will only be a small portion of the total flight time. $\endgroup$
    – Arj
    Apr 30, 2022 at 6:36
  • $\begingroup$ @ifconfig Also, are you suggesting that I move my question to engineering stack exchange and tag it "mechanical-engineering"? $\endgroup$
    – Arj
    Apr 30, 2022 at 6:36
  • $\begingroup$ Fair enough! 😁 We're happy to do our best to help here, but you may find more complete suggestions on a more ME-focused forum. Most of what we help with here is in the "conventional" multirotor category because that's who we have to answer questions :) $\endgroup$
    – ifconfig
    May 1, 2022 at 6:13
  • $\begingroup$ Do you need the servo in that location? Why not put it behind the spar, with a pushrod to approximately the base of the motor? Alternately, you might want to look at the leadscrews used in electronic retractable landing gear. $\endgroup$ May 3, 2022 at 7:59

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I'm no mechanical engineer, but I stumbled across this amazing document which appears to my untrained eye to have basically every mechanism I can imagine:

https://cpb-us-w2.wpmucdn.com/sites.gatech.edu/dist/2/334/files/2017/09/1700-Animated-Linkages.pdf

In it, it mentions the coulisse mechanism as well as some good examples of bars and sliders. This gave me this idea:

Slider

It's a bar and slider, with the servo arm and pin acting as the "bar" with a 3D printed addition to the motor mount acting as the "slider". Here's a top view of the horizontal orientation again which may show it a bit better:

Top view

And here's a view with the motor in its vertical position:

Vertical orientation

And another view of the vertical orientation:

Another view

Finally, an image of just the 3D printed motor mount itself showing the slider:

Motor mount

The only challenge I can see with this mechanism is that the servo arm and pin need to be very strong - a regular nylon servo arm with a steel pin won't do it. I'll need to somehow manufacture an all steel servo arm with the pin as part of the arm (or at least welded to it).

I'd appreciate any feedback or ideas.

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    $\begingroup$ From the looks of it, the servo will need to apply quite a significant torque to get the motor from vertical orientation back into horizontal orientation. This could pose an issue for smaller or weaker servos. $\endgroup$
    – Jacob B
    May 1, 2022 at 1:56
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    $\begingroup$ I second this concern. Building it to test shouldn't be too difficult, seeing as how it's all the same to you, but I also worry that the servo's lever arm will be too small in the "upright" position and it might get stuck at that angle. (this design has a constantly changing fulcrum distance) $\endgroup$
    – ifconfig
    May 1, 2022 at 6:16

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