You misunderstand how electric models work!
VERY FIRST - get yourself a wattmeter. They aren't expensive - about £10, I don't know what they will cost over there. Unlike with an i/c engine, you cannot tell by ear whether you motor is overloaded or not.
1). An electric motor will not go any faster than it's designed speed - the Kv number.
With your wattmeter, you will be able to SEE if you are asking more of the motor, prop, speed controller, and battery combination - than they are capable of delivering. It will show how many watts, amps, volts, and a few other things, all at once - on an lcd screen. It will then give you a guide to how long that flight pack will run the motor for - pack amps /motor drain.
The meter does not fly in the model, it is only a ground-testing tool, and connects between the battery and the speedo.
They used to call the transistor a three-legged fuse - and if you put too small a speedo into your model, you have put a fuse in after the battery. If it goes phut, EVERYTHING goes off, unless you have used a separate UBEC.
-
2). Motor
Disregarding throttle setting for the moment, an electric motor wants to run at it's designed speed, and will suck more and more AMPS from the speed controller (and battery) to achieve this, up to the point where something either melts or bursts into flame!
You can safely shaft-run an electric motor - in fact that is advisable whilst setting up, then you can't accidentally redesign various bits of your anatomy if the motor starts unexpectedly!
It can/will not run any faster than it's Kv number. But if you ask it to turn a full-size aircraft prop, it will bust a gut trying!
Also, unlike an i/c engine - it will not stop! It will keep chopping away until the power is removed, or it bites into something that physically stops the prop, making the motor suck more amps - see above!
It normally states in the motor specifications how many cells(volts) it can handle, and what prop sizes are recommended, power draw, motor shaft diameter, mounting hole spacing, and other relevant bits and pieces.
-
3). Speed controller
You need 50% more power handling in the speed controller, than the motor can draw. (We call it headroom). That goes towards NOT blowing up the speedo if you momentarily snag the prop in a tuft of grass, on take-off, etc.
30 amps seems very small for that prop and battery - but you haven't said what power the motor draws, or what it is, so it can be looked up.
That speedo, 30 amps, sounds about right for a small foamy model that weighs in at around 1k max. (Tundra, Acro-Wot, etc).
-
4). The prop.
The difference between an 11 x 8 and an 11 x 7.5 is negligible. The diameter - 11 remains the same, so only the pitch has changed. Picture it as a screw (properly named, it IS an airscrew). The pitch says how far into an object the screw will go in one turn - measured in inches, and disregarding slippage - (air isn't very stiff!). So - the smaller pitch prop will go half an inch less per rev. This will use LESS power. (It also means the model speed through the air will be half an inch per rev less - you will hardly notice, unless you are flying pylon racers!)
Increasing prop diameter increases the load on the motor, as does increasing the pitch.
If needs must, you CAN put an i/c prop on an electric model, but not the other way around. An electric prop is much more slender and if you think how an i/c engine works - very roughly, the rotating mass of the prop forces the piston to compress the cylinder content, then the detonation forces the piston to turn the prop, so you are getting alternate direction twists applied to the hub. This doesn't happen with an electric motor, which also why the airframe can be built much lighter, there are no constant torque stresses twisting everything back and to.
if you are calm and collected when all about you are going berserk - you've missed something important!
