<u>Oops! I accedently recreated a thread. How can I delete this one? Sorry!
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Opjose,

I understand you're the one that can answer my question. I need help sizing a prop for my electic motor. Here's what I have:

Plane: Sky Raider Mach II, wing span: 54", weight: 5lbs (maybe less)
Motor: Monster Power 46
ESC: 60A Brushless [Volcano Series]
Battery: 4S 5000mAh 25C

Do you know what size propeller I need? Do you need anymore info?

Thanks for the help!

Check your motor manufacture for prop data specs. Want a prop that has efficiant marks yet high thrust . Raider i believe is a trainer. Not ment to fly fast.

Looks like you are a local guy. FRM member? If so talk to John L or Tom F. I know they have both used this motor before and would have some good input.

The Monster Power .46 is an 800 watt motor.

So that is what you want to prop for.


The motor is rated for either 4S or 5S LiPo's.

Since you are using 4S, 14.8v LiPo's the motor will produce less RPM's than it would with the 5S LiPo, so it is best to adjust accordingly.

So 14v x 670kv = 9380 RPM's at max throttle.

Assuming 20% reserve or so, that gives you an operational RPM range of about 7600 RPM.

ThrustHP predicts that an 8 pitch prop will produce about 60mph in this RPM range which is significantly OVER the stall speed of this plane, which is around 22mph.... so far so good.

At max RPM's with the plane somewhat unloaded you'll get about 72 mph.

Assuming some inefficiencies and allowing for some prop loading, let's assume that the max RPM's you'll see NOT in a dive will be around 8900 with a 13" prop...

8900 RPM with a 13" prop translates to 1.15 HP

1.15HP x 748 watts = 860 watts.

So this is right in line with the capacity of the motor... also remember that under load your battery voltage drops a bit...

That works out to about 58A BURST with this setup in line with your ESC.

BTW: I find the Monster Power motors sustain higher than the rated wattage without damage, and that the specs are purposely conservative, so don't worry about running right up to the motor limits.

In actual practice you don't want to run the plane at full throttle all the time, but it is nice to have the extra power on tap so to speak.

So a 13x8E prop will maximize power and thrust.

You also may want to pick up a 12x8E prop. You may elect to use the latter after testing, to get more flight time out of the plane at the expense of thrust.

All of this falls in line with my own experience with the MP .46 motor as well....

All-righty-then! They were correct, Opjose, you're the man!

I've got one more question to add to this then: Hobby Partz recommended the 40A ESC. With the 13x8E prop this is way undersized. At full throttle we measured close to 70 amps at the battery with a clamp-on DC amp meter (not an inline). I was originally thinking of down-sizing the prop and upsizing the ESC to 60. After going out to the club races today I'm considering a 80A ESC. I understand that the size of the ESC won't drain the battery any faster. Is this correct? And what size ESC would you recommend?

Thanks Opjose!