BDMFC electric flight - motors

For now we will look at basic 'can' motors, so named because of the shape of the motor casing which is a simple cylindrical can shape. These are brushed motors with sizes of 100, 150, 300, 380, 400, 480, 500, 600, 800, 900 to name a few. The sizes relate to the length of the motor casing so a 400 size motor is 40mm in length. These motors are sold under several brand names such as Graupner speed, Permax, MFA Rocket and others followed by the motor size and nominal voltage rating e.g. Graupner speed 600 7.2v.

Voltage ratings for these motors are nominally 4.8v, 6.0v, 7.2v, or 8.4v but it is common practice to use up to 2 battery cells higher than the nominal voltage. Since each cell is 1.2v then a motor rated at 4.8v (4 cell battery pack) can be run with a 5 cell (6.0v) or a 6 cell (7.2v) battery pack (6 x 1.2v = 7.2v). Higher voltage means higher RPM on an unloaded motor so an appropriate propeller must be fitted to prevent damage to the motor due to excessive RPM.

Motors can be run with the propeller fitted directly onto the motor output shaft (direct drive) or through a gear box. Fitting a gearbox allows the use of a larger propeller which is commonly used to power a slower flying model type. At time of writing, a typical price for a simple speed 400 motor with a 2:1 gearbox is around £12.50 This will turn an 8 x 6 APC E prop costing around £2.85.

An MFA Rocked 400 motor fitted with 2:1 gearbox and 8 x 6 APC Electric prop - cost around £15

In addition to driving a propeller, these motors are ideal for use in EDF (electric ducted fan) powered models. In an EDF, the motor turns a multi-blade fan at high speed within a closely fitting plastic duct with a surprising amount of thrust even from the smaller motors.

Small 100 size electric motor fitted with 3 blade fan in an A-10 model, powered by 8.4v flight pack.

As a very rough guide a 600 size electric motor is equivalent in power to a 20 size IC engine although direct comparisons are difficult due to the various configurations of electric motor setups (windings, direct drive or gearbox, propeller, flight pack voltage etc).

These motors are not expensive, starting at around £6-£7 and will withstand a fair amount of mishandling before giving up. Normally reliable and smooth running, if a motor does give up the ghost then it's usually cheaper to replace with a new one than to try to repair brushes etc.

Being relatively inexpensive, the brushed 'can' motors are an excellent way to enter into electric flight without a high outlay of hard earned pennies.

As the Sparkies progress to the more exotic motors this column will be kept updated.

 

Information on brushless motors is in preparation