IC engine to motor convertor

[Ben Mullins]

Hi all,

  I was just wondering if there is any formula or magical saying which tells you how to convert IC engine size (like .40) to the same on a motor (like Max watts, Kv etc...). Can anyone clear my head of this wonder!!

Ben. 

[Tim Mackey]

Unfortunately there is no easy answer as there are many many variables - however read both parts of my article "Electrickery" and I give some guidelines. One "system" that works reasonably well in many ( but not all ) cases is the "doubleup-add zero".

EG: double the IC numbers ( .40 becomes 80.......and add a zero = 800 )

So for a good .40 size replacement look for around 800 Watts of input power.

Read this

[Ben Mullins]

Thanks timbo
 

[Mike Hawkins]

I know that

1 H.P. = 746 Watts

An OS 40 LA develops 1.0 B.H.P. and an OS 40 FX develops 1.36 B.H.P. So the 40 LA is 746 W and the 40 FX is around 1000 W.

Also 46 FX is 1.62 B.H.P. and 60 LA 1.6 B.H.P. So it really depends on the particular engine you are looking at replacing.

The doubling it and adding zero works great for working it out roughly, ideal at the field. Using the B.H.P. also has its own problems in that while they develop the same power, they more than likely swing different props.

I dont fly electric, I much prefer IC, this is just my understanding.

Mike

[Nick Rigg]

As Timbo explains in Electrickery, the weight of your model is also a factor

 e.g. my Pulse XT is designed for a 40-46 size IC engine or EP motor equiv. But as the model's AUW including battery is only 5.5lbs, I can get away with 550watts for decent performance

[Steve Hargreaves - Moderator]

Personally I don't think its worth converting the ic equivalent power to electric as the characteristics of ic & electric are so different......yes your OS40LA may develop 1bhp or 746 watts but it will do this at about 15-16,000 rpm. The prop size needed to achieve these revs will be so small that it will be very inefficient. Say you use a 10x6 or 11x7 as a useful size prop what rpm will it get to on these props...9,000? 10,000? How much power is it producing at 10,000 rpm.....? 0.5bhp? 0.6bhp? Who knows......!!!

With electric you get max power (wattage) that the motor/prop/battery combo is able to deliver & this is pretty much controlled by the prop size fitted.

So probably best to forget the comparison & just go off all up weight of the intended model. Aim for 70-80watts per lb for Piper Cub style flying....100-120 watts/lb for general flying...150-200w/lb for aerobatics...250w/lb plus for 3D stuff.....always remember you do have a throttle if you have too much power.

[Tim Mackey]

I agree....but people keep asking ! It is I guess an obvious question for many, and one which we try our best to simplify, but as pointed out....not easy. There is also of course the higher performance 2T engines such as a nice Rossi I had one many years ago rear induction/ slide valve etc, and although only a .61 - it was producing over 3 BHP!

[Steve Hargreaves - Moderator]

........And would probably need a 6x4 prop to get to the 30,000 rpm it needed to turn at to produce it!!!!!!

Not much use in yer Sopwith Camel methinks!!!!!!!

Getting power out of an engine is never a problem....making it useable is!!!!!!

[Tim Mackey]

Absolutely, although on a 9 x 6 the tips went supersonic, and the noise was fantastic ( but not to the neighbours )

I had it in a Mirage F1, tuned pipe, air retracts. the works...man it was fast

[Steve Hargreaves - Moderator]

Wow....to go supersonic you'd need to turn the prop at about 25,000 rpm!!!!

 Flippin' 'eck!!!

[Tim Mackey]

yep... and it was in the days of graupner nylon, all bendy and rubbishy things - or wood of course.