[Ron Marwood]

If you can picture the MK kit of the Cessna 336 wing span of 74 inches ( I don't know what the weight will be). I plan on installing a nitro 46 or 60 in the nose and an electric motor in the rear (it calls for a nitro 40 there). I fried my OS 40 (poor cooling)
I need help determining what motor / ESC / Battery would be best for this application.
I am not familiar with motor numbering so I need generic values that I apply when I select components.
Thanks

[Cougar429]

General rule of thumb is 100 Watts for every pound on a fixed wing. Since you already have a nitro up front I would make the assumption you can cut the weight calculation by 1/2 the total.

Once you have the watt requirement you need to determine what battery you will use, (3S, 4S, etc.). That decision will also determine your max ESC current rating.

The following are simply examples and may not be accurate:

What that means is that if you want to pull 600 Watts out of a 3S battery you would need an ESC of at least 80A to stay reliable. For a 4S pack to supply the same output power the ESC could theoretically be 60-70A. This would follow that the higher the cell count for the same output power you could lower the ESC amp rating. Part of the problem is that ESC's for higher cell counts also become more expensive.

Now on to the battery. The battery has not only to be able to supply the amps required, but you will have to decide on the MAh capacity to set the amount of flight time for the power system.

All this math can really get confusing, but at least now there is enough background experience to help lead the way. One thing you can do is look at other aircraft that are in the calculated weight range and see what they have for an electric power system.

NOTE: You will likely be able to run the nitro motor far longer than the electric power system. This risks the chance to discharge the battery beyond the minimum cell limit, which can damage the pack even the first time this occurs. I just read in the MAAC mag tonight there are systems that will cut it off once a fixed number of MA are discharged. You might want to look into that bit of equipment.

Waiting for the motor to sag is NOT a good way to tell the battery has discharged as by then you have dropped the cell voltages to or beyond that lower limit.

Generally I start with a new and untested power system very conservatively. This means I do light power flying for a short time. Not only does this help work the pack into shape it allows me to check the total MAh it takes to fully rewind the battery. I can then slowly work my way up in power demands and/or flight time to finally set the timer on my Tx and know I can land before risking that damage.


Yours sounds suspiciously similar to the real aircraft, (or any pusher for that matter) in that rear engine cooling is a difficult aspect to get correct. Unlike tractor mounting where you have that prop blast forced through the cowling rear engines require enough airframe speed to provide adequate airflow.

p.s. Don't forget you will need adequate cooling for the electric motor as well.

[Ron Marwood]

Hi Cougar,
I found that my 60 will not go in so I tried my OS 46AX and it just fits.
The plane weighs 11 pounds with enough components laying in the belly to approx.
the flying weight less fuel.
I downloaded Ohms law and came up with the following figures using your suggested watts per pounds.
11 pound twin engine plane
6 pounds per engine
100 watts per pound = 600 watt requirement
4s battery = 14.6 volts
amp draw = p/v =42 amp
safe bet ESC should be 60 amp.
I have not put any thought on amp hour rating yet but that comes next.
Now I have research what motors have 600 watt ratings.
Thanks for your help
Ron

[Cougar429]

Don't forget to look for one with the correct Kv rating, as you want to turn the prop the right speed, (avoid one for a heli as it will be too high and will likely not have the mounting and prop adapter). There will be a lot out there with prop recommendations.