Is there a general rule of thumb when choosing motors for a twin?

I've kinda gotten the hang of choosing a motor for a single. What baffles me is how to factor-in the second motor.

The only factor that comes to mind is a twin has a second prop wash over the wing. I'm certain there are other factors, but have no idea what they are and what to do about them.

Hoping for some guidance!

Unless you like to fly in circles, I would recommend two fairly close matching motors systems, unless you are talkig about an inline pair, (push-me-pull-you_).

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Originally Posted by eflightray

Unless you like to fly in circles..

That's funny!


My goal is to properly size twin motors for a given airframe.

Example: (I'm making this up)
- Piper Comanche vs Twin Comanche
- 65" wing
- 750 in2 wing area
- 9 lbs auw
- 28 oz/ft2 wing loading
- single motor is a Power 60

Is there a rule of thumb to go from that single motor to a twin setup on the same airframe?

I would think your single rating could be divided by 2 for the twin motors. If you need 100watts to fly each motor should be at least 50 watts each. IMHO

I've always thought that it made sense to split the power as well. Especially with electric motors where it's still the total watts per lb provided. One factor I think is important would be the Kv values of the motors. I feel that having a similar total prop disc area for moving the air is going to be needed to keep the climb for the power used similar. So we can't replace a big slow turning prop that has a good bite with a couple of small high RPM toothpicks. That's certainly going to affect the climb rate if things are way out of whack.

Just use your normal calculation for a single motor, then use two motors each with half the power capability.

Then design the motors the way you would for a single -- i.e. what prop size can you handle, do you want speed or thrust, etc.

Battery is calculated as a single. 2 ESCs are each handling 1/2 the power / current of a single engine.

Thanks guys... that's what I'll roll with.