Pentacopter - Increasing Power to the Larger Motor vs Outer Motors

I am creating a pentacopter and am having trouble adapting the ardupilot 4.0 to this unique case. The idea is pretty much a quadcopter with a larger motor / propeller mounted in the center to produce most of the lift.

I have adapted the code so the pentacopter is working but would like to make some changes. The problem is that the larger propeller in the middle is getting the same amount of power as the outer 4 propellers. I would like for this larger central propeller to scale such that it provides more of the lift, maybe like 70%.

How can I adapt ardupilot code to allow the larger central motor to have more power when I move the throttle such that the central larger propeller provides most of the lift? I am guessing this would involve some sort of scaling of the motors voltage or a cap on the outer propellers voltage / RPM.


To be more clear, I’d like to scale say motors A,B,C,D (smaller outer motors/propellers) to have less input power compared to the larger motor E/propeller in the center. The concept is that larger props are more efficient, so allowing a larger propeller to do most of the work would be more efficient.

1 Like

Arducopter supports a Boost Motor already with scaling. Is this not what you want to do?

Yes, I did try that already - the problem with the Boost Motor was the control system did not account for the Yaw created by the larger propeller so it was yaw unstable. I would like it to operate like one of the other propellers ideally, but have more power / voltage than the others so it can do more of the work.

OK, so Yaw authority from the 4 motors/props is insufficient to overcome the boost motor? Fiddling with MOT_BOOST_SCALE didn’t work?

I will try that, but from how it responded when we tested it initially, I don’t think that will work.

I looked in the source code a bit too, but haven’t noticed anything promising… Any other ideas?

The example Boost Motor craft was a tri-copter with an IC engine boost motor but it occurs to me that there will be plenty of yaw authority with a tri-copter due to motor tilt. I can see why a quadcopter may be a problem with just differential motor speed for yaw. Interesting case you have there.

Definitely an interesting case…

It works alright with the 5th motor added into the source code as if it were another motor identical to the other standard 4 quadcopter motors, but that does not take advantage of the larger propellers efficiency much because it does not have the ability / power input to do most of the lifting…

Appreciate your help though

So initially with standard current stable Arducopter you had the boost motor on it’s own output configured for “Boost Throttle” (SERVOx_FUNCTION=81) and even adjusting Boost Scale you could not get good yaw control?

Nope, whenever the larger motor kicked in the yaw went instable instantly. I believe the central rotor will need yaw authority

Did the log show the CW or CCW motors, whichever direction counteracts the boost motor direction, going to full output? If so I don’t see how it can work w/o 4 larger motors and/or a smaller boost motor. Well, maybe not smaller or larger but more or less moment of inertia would be a better way to state it.

When I tried the boost motor in that fashion, I had all 4 outer motors opposing the direction of the larger central motor. However, when I adapt the source code such that the the central motor is treated as a 5th additional motor, the torque from the outer rotors was too great, so I had to switch 2 outer rotors to be CCW, while the central rotor and 2 outer were CW to achieve stability.

I’m not following this. Did you try the Boost Motor function as it’s applied in the current Arducopter firmware? Not adapted source code, just current stable code.
Standard Motor Order and Direction for Quad X:
SERVO5_FUNCTION 81 (obviously with the Boost Motor on Servo 5)

I’m simply trying to establish if you have configured this craft as Boost Motor is currently supported and if so what the result was.

I’ll mess around with it more thoroughly tomorrow or this weekend and let you know!

I modified a quadcopter in RF8 to add dual contra-rotating boost motors above the CG, and added a copter frame type to support it. I specified 300x75mm props for the boost motors and 152x152 props for the other 4 motors, so most of the thrust will be generated by boost motors (same kv for the boost motors, but 4 times the available torque). It flies OK, but needs some tuning on the yaw axis.

The RF8 model is here:
and the branch with the new frame type is: