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Old 01-15-2021, 10:30 PM
Brushless motor guy
Join Date: Jan 2021
Location: Kawerau, Bay of Plenty, New Zealand
Posts: 5

Originally Posted by quorneng View Post
That looks a very interesting layout but it does raise the question as to whether any 9x6 prop is good for the sort of power you are considering.
Even if you find a suitable prop the aerodynamic efficiency converting the shaft power into thrust would not be good at the rpm you mention, There will also be some additional mechanical losses of the extra bearings and the tooth belt.
Note also all the 8 bearing in your set up will be under a side load from the belt. A direct motor drive has only 2 bearings and no side load.
I have suspicion a belt ratio reduction (2:1?) would make more sense
It will be very interesting to see how it pans out.
1) APC tables say the 9x5 is good for 16,000, and the power levels come straight from the tables. I suspect the prop tips may bend forward a bit under static load at 13,000 rpm.

Shaft power to thrust is included in the tables. - I have found that rpm levels are about 1.5 times what you would expect from basic theory for that thrust, with 2-bladed props. Power is not up 1.5 times, though.

2) I would like to find info on drag from belt and the 2 bearings.
I picked the wider ones for side and end loads, but the motor bearings will not last well with the side loading.
They do withstand quite a bit of out-of-balance (vibration) loading, but that is only milligrams.

3) The aluminium motor shafts could also snap off-due to fatigue.

There is a 20-odd kg rotating bend side load part of the time. (11 mm offset).

1 n-m acting at 7mm, = 142 N, or 14.2 kgf.

On a round 5mm shaft, this is 13.3 kPa, so not at all critical.

Putting the motor elements in a stronger housing may be required in the long run. We shall see.

4) Building a big bell-type quadcopter motor may be simpler.

I am sure doubling the diameter would make for a much more powerful and torquey motor- you double the circumference, and the radius, and the depth.

I need to find out about number of poles, number of turns, wire gauge , insulation grade, and source, etc, and a source for the barrel and stator core.

Each stator pole would be about 8 times the volume., but magnets only 4 x the area.

- making the motor over 4 x power would cause cooling problems, though.

Last edited by OwenB; 01-15-2021 at 11:13 PM.
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