We are designing a new Fixed Wing plane, all composite again, but this time with no landing gear, using a single peg bungy launch, with a parachute recovery. As w are fitting this Gimbal to this plane, we want the nose clear of motors and props. I wish to use a tail boom mounted co-axial prop, but do not approve of the possible options around - noisy and short lived gears, or Gimler belt and toothed gears, etc.All a bit complex, and awkward to locate and then house in the fuselage.

The very Nice Gimbal - Day and Thermal IR Night cameras, 640x480 Pixels. Fully Gyro Stablised. Serial port to drive and control.


So, I decided to modify a few Brushless motors and make then truly co-axial, with integrated folding prop mount, really beefy bearings, and a nice through hole for tail servo leads and lighting.

I used two HACKER Motors, A40 series, 50amp constant, one of 410kv to spind a 17x15 prop, and one of 610kv to spin a 16x8 prop, and an AXI 4120 series, also 50 amp, of 465kv for a 17x13 prop. Flight test will show which of these is best for our application.

How did I do it? Well, pictures are the best, but essentially I cut of the outrunner bell ends, machine out the inner Aluminium ball race holder, and ream out the hole to 10mm. Then I make a new bell housing, with double beefy bearings at one end, and a folding prop holder. All bonded together with locktite.  

The end results with regard to weights -

The weights are done using a standard motor, fitted with prop holder, fuselage mounting, etc, and then compared to the final weight of a modified motor.

Motor                              Standard weight             Modified weight

AXI-4120-20 465kv                  345g                            342g

Hacker A40-12S-V2-610kv      240g                             243g

Hacker A40-12S-V2-410kv      317g                             310g

As seen, there is no penalty in weight for this mod, even with a 110mm long 10mm diameter stainless steel tube through the motor centers...Through this tube is passed all the wiring for fuse to tail connections. The tube is then fitted to a holder on the fuselage, and the tubular tail boom ( actually an ALIGN 'copter tail boom - 20mm diameter) fits on the other end of the motor shaft, with a small aluminium bushing ( 4grams...)

The pictures:


3689685294?profile=original   3689685478?profile=original








The Nampilot.....

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  • Hey Joe and Gisela,

    as a wildlife biologist, I deeply appreciate your dedication for the conservation of wildlife. As a mechanical engineer I really love your approach of optimizing established techniques and technology by the local means. This "coaxial" propulsion unit looks well designed and well thought through. Do you have pictures or design concept papers of the A/C you want to equip with this unit you might want to share ?

    Cheers, Pascal 

  • Nampilot, thanks

    @Graham, very difficult to NOT fix folding props because of the boom

  • Co-Axial just means 'around the same axis' - so in my case the motor turns around the tailboom tube, ie, a common axis. Counter rotating props can be separate,  two motors with separate props turning opposite, or co-axial, in tandem fashion. 

    Yes, the Gimbal is two axis - 3 axis is not really needed for a nose mounted fixed wing application - you can 'look' everywhere you wish, while the main area of interest is from partially rear, mostly under, and then forward, of course sweeping to port and starboard as well.

    The Nampilot...

  • Moderator

    Looks very good, gimbal is something to drool over, home made or commercial?

    Also why a folding prop, are you planning on doing a lot of gliding? Also my experience is that they are less efficient than fixed props.

  • Forgot to ask, but your gimbal is 2 axis? And if, why?

  • Nampilot, there seems to be misunderstandings about co-axial, what I googled gets me something with 2 conterrotating props. And thanks for sharing your amazing diy knowledge

  • Hi Vladimir - Yes, there have been a few concepts with conventional structures with a prop at the tail. The issues with all of them, in our size planes anyway, is one of portability and location of CG. I need to split the structure - fuselage, taliboom/tailplane assy, wings, sort of into 4 sections, able to be carted in a standard small car. That means either placing the motor in the tailplane assy, which is a CG nightmare, as well as a wiring problem - high amp wires to the tail and esc, connectors for the spilt, etc, or placing the motor in the fuselage, with a drive shaft to the rear, which is not really feasible nor practical. Sice the tailboom is easily supported and implemented using a 20mm Aluminium tube, a coaxial assy make sense, esp if there is no weight penalty.

    The Nampilot...

  • Slow turning, high(er) pitch, large diameter props are much more efficient than multi-bladed smaller diameter props.. Have lots of flight data to prove it too...

    The Nampilot...

  • Moderator

    I build something like this...


  • Why coaxial? Simple really - I want the front free for the payload.  I would therefore prefer a pusher prop type installation, and as I am not partial to flying wing type structures, a non-coaxial implementation would require a raised motor/prop structure, to clear the tailboom or nose, or a twin tailboom with prop between. I find neither of these weight efficient nor aerodynamically sound, especially since I want to use large diameter props, typically from 17inch, up to 22inch. The tailboom tube simply clips onto the motor extension hollow tube, and is easily unclipped  removing the whole tail for transport. 

    Late?? In Namibia??? 02H30 in the morning is maybe a little late...

    The Nampilot...

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