Can anyone answer this question?

[pezzer]

Hi guys, I was explaining my quad to someone and they asked why are the stator motor mounting screws are cross uniform? As in, they do not lie on the same circle, two of them have a smaller diameter and two have a larger one.

I of course could not give and answer, and my dad who has flown all his life could not give an answer either...why are they different?? Why are the mounting screws into the stator different dimensions from each other?



Thanks for the help
Pezzer

 

[Craftydan]

Simple -- so they can mount on two different bolt patterns. Unless the airframe is a electronics-included kit (and sometimes even then), the bolt patterns on firewalls and motor mounts are not standard. While mounting all four is best, most motors with these layouts can get away with only two (opposite each other). A manufacturer adding in a bit of versatility in their mounts makes their motors directly compatible with more frames.

Granted, on the frame side, many have switched to slots for the bolts rather than holes (to accommodate different patterns on motors), but in those cases you can use all four on the odd-ball bolt pattern in the slots without a bit of trouble.

 

[Konrad]

The 16mm and 19mm bolt patterns are the toy airplane modeling industry standard to allow the interchangeability of motor mounts and gearboxes.

For an understanding of why one needs to look back into history to the grand old days of the brushed Mabuchi 380 and 550 canned motors.
https://www.mabuchi-motor.com
These are industrial motors found in a huge array of consumer products.

In the 80’s Graupner was very successful at marketing these to us modelers under the name “Speed”.
https://www.graupner.com/Accessories/Engines/Electric-Engines/?p=2
These canned motors lacked the torque and power we now associate with brushless motors. As such gearboxes were very popular for getting the torque we needed to turn our props. As a result of need to fit the graupner “Speed” motors firms settled on making gearboxes and motor mount that fit Mabuchi motors without adaptors. As a result most motor accessories than need to bolt on to the motors have ether or the 16mm and 19mm bolt pattern. Conversely any motor that wanted to be interchangeable with the established motor mounts and/or gearboxes had the 16mm or 19mm bolt pattern.

P.S. The Mabuchi 550 (Speed 600) look to be using the 25mm bolt circle