Originally Posted by peteflys
Ok, so read a lot on here about getting the right prop/ESC/battery combo for my plane. I get the part about over stressing the equipment. But I'm coming from the glow/gas crowd where I see guys that have installed too small of a prop on a glow motor and it's just screaming.
I would think it's possible to under prop an electric motor, but does it hurt anything?
I have a 60 size Mustang. Installed an E-flight 60 motor, Castle 100 ESC, 6C 5000mah 35-45c battery. I have read several combos like this, only some guys run 15x10 and others run 15x8 prop. With a 15x8 I'm pulling 36 amps and 850 watts. Is that "not enough" draw? Haven't tried the 15x10 prop yet, I bought both.
There is no such thing about damaging a brushless electric motor by under-propping it. You are way ahead of the game by having the ability to measure the current draw of your power system. To many folks put on a big prop, have spectacular performance, then wonder why their motor quits with that magic smoke coming out of its windings.
The maximum RPM your motor will turn with no prop is determined by the motors "KV" rating, and the battery voltage. Your motor will not turn faster than that limit.
That said, putting to small of a prop on your motor, or to low of a pitch on your prop can drastically affect how your model will fly. And, on the other hand, putting to large of a prop on your motor can drastically affect how long the motor will run before you get magic smoke out of the motor windings.
You will quickly find out that the useful propeller diameter and pitch range for an electric motor is quite limited, as compared to a glow or gasser engine. As an example, I've got three models equipped with the Castle Creations ICE series of ESC's. These ESC's have built in data recording ability. On my Hacker A60-16M motor with its 19X12 APC-E wide blade prop, that motor turns over at 6800 RPM on the ground, and in a fairly steep dive, the motor unloads to 7200 RPM. At the same time, the ICE ESC shows that the current drops from about 70 Amps on the ground to about 25 Amps or so on the full power steep dive.
These motors are more of a "Constant RPM" power source, as compared to a gasser or glow engines sort of "Constant Torque" power source.
So, flying with a big diameter, low pitch prop on a gasser/glow engine might work. But doing the same with an electric motor might wind up with the model accellerating like gangbusters on takeoff, but struggling to maintain flying speed.
Programs such as www.motocalc.com
can really help sort out what different prop diameters, pitch, model size, wing area and weight all work together on determining how your model will fly.
However, modelcalc is only as accurate as the motor specs. I've found that on at least one E-Flite 60 motor a few years ago, the factory specs were way the heck off.