Opjose, I here you're the one that can answer my question. I'm trying to determine what size of propeller I need for my current setup. I am new to electrics and have actually been out of the hobby for 30 years! Here's what I've got.

Plane: Sky Raider Mach II, 54" wing span, 5lbs (or less)
Motor: MonsterPower 46 (670kv) OutRunner
ESC: 60A Brushless Speed Controller ESC [Volcano Series]
Battery: Blue Lipo 4-Cell 5000mAh 14.8v 4S1P 25C

Can you help me out? Thanks!

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....

c-h-e-e-s-e AND CRACKERS,,, Opjose......
WHAT kindova dohickey / program / background do you have ANYway?

I swear.... I bet I could suggest the overall general physical dynamics of an Experimental 1/4 scale Electro-Snoopy-Doghouse, and I bet you'd come back with a bunch of detailed figures for exact selections of ESCs, UBECs, Batteries, Props; Servos, ie: the whole propulsion requirements plotted in Relational Parabolic 3D-curves.
(disclaimer: Altitude variables N/A, as it's an 'Electro')


-unbelieveable-



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Should I put an 80A ESC in this thing just so I don't have any heating issues, no ducting needed, etc.? What size would you recommend?

While an 80A ESC is probably not needed, it does afford you an extra margin of safety.

Since the motor can briefly handle relatively higher loading, the higher capacity of the ESC might be a good idea.

Also if you later decide to use the power system on another plane, you'll have less worries.

With a similiar setup I used a 70A ESC with success and both the motor and ESC remained fairly cool in operation.

Try to keep the ESC somewhere where it will get adecuate airflow.

Air flow is what I'm concerned about. The ESC is, of course, behind the firewall and under the battery (which I have suspended, no contact between the two). Do I need to, or is it advIsable to slot the firewall and the bottom of the plane (Sky Raider Mach II) at the back end of the fuel tank location? Or, will a 70 or 80 amp ESC not going to get hot enough to require those mods?</p>

Most planes designed for electric power have provision to allow air to flow through the fuselage to keep the ESC, and battery packs cool.

Given that the Sky Raider II was designed for glow, it does not have the openings.

You can either open an intake oval on the bottom of the firewall, and then a larger exhaust opening further back on the fuselage to deal with all of this.

Some people also resort to placing the ESC right under the motor, exposed to the airflow, on planes like these.

A larger ESC will still build up heat, but it is not likely to get AS warm for a given power level, because of larger components and heat sinks that take longer to heat up...

However you are best off treating it as if it they were all the same... all requiring a degree of cooling airflow.

You don't have to go nuts about it, but do something.

If you cannot expose the ESC, I've used clear material taken from commerical package "bubble" packaging... the stuff you normally have to open with scissors to get to the merchandise.

You can often cut one or two suitable clear intake scoops from a package.

Then I can open a hole in the covering or fuselage, and put the 1/4 egg shaped plastic scoop over the hole, wide side forward, to pull in air.

Remember though that the exhaust needs to be 2 to 3 times LARGER than the opening or pressure will build inside the fuselage permitting no airflow.

Better to have a small intake and a large exhaust, than a large intake and a small exhaust.

Wow! You ARE the guy with all the answers!

Thanks! I'll take all that into consideration when completing this project.