A BASIC GUIDE TO SUCCESSFUL
ELECTRIC FLIGHT

Simply ... VOLTS x AMPS = WATTS

That's it really, as simple an equation as you are ever likely to see but very useful in explaining some of the effects you will discover as you start to experiment with electric flight.

VOLTS... Supplied by the battery pack according to the number of cells. (1.2v per cell for NiCad) Assume 1 volt per cell in a working system. AMPS... The rate at which the motor draws energy from the battery, affected by how 'hot' the motor is. (Determined by the thickness of the wire forming the windings on the motor armature), and other factors. WATTS... Power produced or work done. (gulp... school physics).

We can easily measure the amps being drawn from the battery (the current in the circuit) with an ammeter connected such that it breaks the motor supply wire (see drawing coming up).

Consider the following examples with reference to the formula above...

Put a larger propeller on the motor and the amps will increase. The volts are the same so the watts increase and more energy is available to power the model. But the battery will be drained more quickly.

Add an extra cell to the battery pack and the voltage will increase, the amps will also increase because the motor and propeller will spin more quickly in response to the extra voltage input. The watts will therefore increase again providing more energy to power the model at the expense of motor on time.

Now fit a gearbox, the motor spins more quickly, working less hard to turn the propeller and consuming fewer amps. The volts are the same so the watts reduce, less energy available to power the model..... So increase the diameter of the propeller, this increases the load on the motor again increasing the amps. If the amps return to the level seen before fitting the gearbox, restoring the watts also to their previous level, where is the extra energy referred to in the gearbox section coming from? Answer... in the extra efficiency of the larger propeller turning more slowly and driving the air backwards with less "slip".

Try reading that again! You should be seeing possibilities. As mentioned in the gearbox section if this extra energy is not needed to fly the model the amps can remain a little lower thus increasing the motor run time.

Another angle of attack becomes evident when we discover that all motors have a limit to the amps that they can withstand. The SPEED 400 size has a limit of 9 amps, the SPEED 600 size should not draw more than 20 amps.

Fit a gearbox and increase the propeller diameter a little in the name of efficiency, the amps and hence the watts have decreased.

Now add a couple of cells, maybe three to the battery increasing the volts. The amps will have returned to their previous level (perhaps just under the motor's limit if you are pushing for maximum power) due to the extra load caused by spinning the propeller more quickly. But lo... the watts have increased dramatically so there is more energy available to power the model, without decreasing the motor run time.

It's a cracking little formula really!

chapter 9 >>>