Use a different motor for the back motor of a tricopter VTOL

Can I use different motors in a tricopter tiltrotor setup? I wanna use a Tmotor MN4012 kv400 for the back rotor of a tiltrotor I’m building. The front motors are Sunnysky X3120s. The back motor is optimized for hover thrust with low pitch and large diameter. The front motors have almost twice as much power as the one in the back and obviously different response to command. Is this allowed by the VTOL code?

Hi Ron,

Yes, no problem. Many of our tricopter VTOLs use a different motor in the rear.


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Hey thank man i’m actually reading through your build log, should’ve checked there first. I’ve done a few trade studies using eCalc and manufacturer test data and found that a pair of X3120 at 1,600w to be sufficient for a 4.2kg nimbus. However, i recall that you and a few others have all chosen the next class up, something like 3520 or even 4112 motors at ~2,000w. What sort of hover lift reserve are folks targeting for Y3? i think i can easily get 100% margin with X3120 KV585, if I keep weight below 4.2 kg or so.

I don’t think you need a 2:1 thrust to weight ratio for success because the hovering is limited when compared to a copter…but it can’t hurt either. Some of my setups have only a 1.5:1 ratio and work fine. It all depends upon your desired payload. My Nimbus VTOL has a 2-axis gimbal camera on the nose and also carries a Cannon S100 for mapping without any lift issues.

My current setup is using the following components.

My Nimbus VTOL components have these weights. I can easily use both payloads together.

  • Multistar 6s 10AH Battery with Plywood Plate = 43.6oz (1236g)
  • 1 VTOL wing = 26.5oz (751g)
  • 1 VTOL wing = 26.5oz (751g)
  • Both tails = 3.35oz (95g)
  • Nose with AS Sensor = 3.45oz (98g)
  • Body with Landing Legs = 44.8oz (1270g)

Total weight without payload = 148.2oz (9.26lb)(4201g)(4.2kg)

Payload 1 (2K camera and gimbal) = 80g (2.8oz)
Payload 2 (Cannon S100 Mapping Camera and Mount) = less than Payload 2
ImmersionRC 600mw, 5.8GHz Transmitter = 18g (0.6oz)
Spironet 5.8GHz.Circular Polarized Antenna = ???

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thanks again Greg. with a X3520 the eCalc site shows approx. 2:1 thrust to weight at hover.

. Not all that is available for lift because the motors are some distance away from the CG, but with about 2kgf available from X4112 in the back with a longer moment arm, overall the lift should be over twice the weight. I am not sure how accurate the site is (although I did some spot check and agrees well with manufacturer data), and the calculation does not take in account loss in thrust from things like the CF tube below the rotors,etc. this calculated margin is why I initially thought i could just use the X2820 and X2220 i had lying around to build this thing, then I saw your post and others who all used 3520 and above…puzzled. the weight difference is significant. With 3120 i shave off 100g from two motors.
I will work my way through your build blog. great community service btw

[edit] my X3120 stats below by comparison. i’ve had a limited selection of prop sizes esp. with the need to buy a pair of matching normal and reverse props. i use 13x6.5E APC. it gives enough thrust at a low current and power absorption to keep the temp down, hopefully.

I have been buying electric props directly from APC Propellers in the U.S. They are inexpensive, high quality, and have both normal and reverse pitch versions. The shipping is reasonably low too.

As an example, I replaced my 13x8 wood props from Foxtech at $29 plus $20 shipping with Black 13x8 e-props at $10 a set (CW and CCW) and a low shipping around $7. They even have 14x8.5 props for extra lifting power, if needed.

i buy from resellers in my home country. Had to pay a premium over the US price but at least I don’t have to wait couple of weeks for shipping or worry about the custom holding it up. I do have the choice of 14x8.5E and EP, but they are only suitable for 3520 and upwards.

I don’t have a problem with ‘de-rates’ as in only using part of the available power of a motor. I just thought that with X3120 the power margin (actual absorbed via props versus available power) is already high enough. I had a quadplane build not long ago and was constantly worried that it’s too heavy and the stall speed is going to be very high. It crashed for a different reason but i decided that my next build is going to be weight-conscious, hence the somewhat more challenging Y3 config.