Chapter 3

 

Chapter 3

Choosing the MOTOR

With such a profusion of different types of slot car electric motors on sale in the shops today, the newcomer to electric R.T.P. flying could be excused for feeling somewhat bewildered and hesitant in his choice of a suitable power plant for his model.

This is not a treatise on the subject of electric motors, but simply an attempt to save the "fledgling" electric flyer both time and unnecessary expense by discussing one or two fairly inexpensive and quite adequate motors.


The direct drive use of a standard can motor, and at right, conversion with a tube bearing epoxied to the case to carry a layshaft and 4:1 gears. This enables one to use a more efficient, larger propeller.
The direct drive use of a standard can motor, and at right, conversion with a tube bearing epoxied to the case to carry a layshaft and 4:1 gears. This enables one to use a more efficient, larger propeller.
 

Considerations to be borne in mind from the outset are, the size of the model the motor has to cope with, the probable weight and drag of the model, and the power available. Generally speaking, there is not a great difference in the overall size of most slot motors, but the amount of power they produce in relation to their weight (power/weight ratio) varies considerably.

Certain motors have the ability to "slog" comparatively large propellers around hour after hour using direct drive from the spindle without showing any signs of overheating or failing, whilst others are strictly "high speed" units, only satisfactory when spinning small propellers at high revolutions. In many cases it is possible to adapt the latter type of units to drive larger propellers by the use of simple reduction gearing.


Geared 26D motor mounted on a sub-frame for Corsair.
Geared 26D motor mounted on a sub-frame for Corsair.
 

At the high revolutions and often higher voltages required for electric flying, many motors, quite suitable on the race track, tend to overheat, shed parts, or "throw" their armature windings when airborne. In many cases the more sophisticated and expensive motors prove to be less suitable than the cheaper "cooking" variety.

If it is your intention to construct a small light model aeroplane with a wingspan of up to about 24 inches, then provided the design of the nose area is reasonably slim or streamlined to prevent "blanketing" of the propeller, a good choice would be either a "Rikochet" Mk. l, a Riko "Rocket"or a Riko "Wildcat" (not generally obtainable now, but a really excellent little motor), matched with a Cox 3in. x 1 3/4in. propeller any one of these combinations are quite suitable for speeds ranging from slower "scale" type flying to high speed racing. The Riko "Rocket" is more suited to moderate currents not exceeding 12 volts so care would have to be exercised not to overload this otherwise satisfactory little motor. All three motors are quite light, just under 1 1/2oz. in weight.


DC Motor Drawings.
DC Motor Drawings.
 

A disadvantage with this combination is that the very small propeller is quite unsuitable for use with scale model aircraft with radial engines and cowlings (e.g. Gloster Gladiator). The small Cox propeller blades do not extend beyond the cowling and are thus rendered inefficient by being "blanketed" by the large nose area, besides looking sadly unrealistic. If larger, more suitable propellers are to be used with either the "Rikochet," "Rocket"or "Wildcat", then the use of reduction gearing is necessary.

The modification to carry suitable reduction gearing is quite straightforward. Two miniature ball races are soldered or epoxy glued, one on each end of a 3/4in. length of suitable i/d metal tube. The assembly is soldered or epoxy glued to the casing of the motor, with additional support provided by locking wire wrapped around the whole motor casing and metal tube. A layshaft is fitted through the tube supported by the ball races. Two thrust washers are soldered or epoxy glued to the layshaft. one each end of the tube assembly to prevent movement longitudinally. Ripmax 4:1 ratio nylon gears are fitted, the smaller gear being locked on to the motor spindle by its grub screw, the larger gear to the layshaft which in turn acts as the main propeller mounting shaft (see photo).

Other makes of gear wheels may be quite suitable at the same or similar ratio, the reduction assembly should be quite suitable for driving 5 1/4in. x 4in. (Topflite) propellers or even larger.

For aeromodellers who like to specialise in the construction of miniature or sub-miniature model aeroplanes with wings of under 15 inches span, a suitable lightweight electric motor is essential. The Rikominx R1017 motor is highly recommended for this purpose. It is a very powerful little motor, particularly good for use with a Cox 3in. x 1 1/2in. propeller, light in weight (under 1oz.) and has a cylindrical casing measuring 3/4in. (dia) x 1 5/16in. (length).

With all electric motors it is important that adequate provision is made for air cooling when the motor is mounted within the model. The Rikominx should be mounted with the spindle and brush casing assembly facing forwards, that is, towards the oncoming relative airflow. The free unrestricted passage of air both into and out of the engine compartment is of great importance.


Electric motor data table.
Electric motor data table.
 

The power/weight performance of the Minx is so impressive that it presents the ideal power plant for many of the lightweight flying scale kits available in so many model shops.

For larger and somewhat heavier models of between 24in and 36in wing spans a natural choice would be either a Mabuchi FT-36D (22120) or a "Rikostreak" Mk. 11 (RI014). These motors are larger and in the case of the FT-36D proved to be rather loo bulky and slow for slot racing, but are both admirably suited to driving propellers of from 4.6ins. diameter x 4in pitch to 5.5ins. diameter and 3in pitch (direct drive).

Combined with suitable reduction gearing both of these motors are quite capable of swinging much larger propellers. 7ins. x 4ins. for example. The FT-36D weighs 38 grammes and produces 0.01 h.p. at about 10.000 r.p.m.

Model aircraft spanning 40ins. or more are well within the capability of these two motors. Fortunately both motors are cheap to buy and are available in quantity in this country. If you decide that you require a good all round "slogger" one of these should be your first choice.


Comet plan.
Comet plan.
 

A very useful hint from American modeller Pat March to readers is to take note of the identification numbers when purchasing Mabuchi motors as they are of definite significance. For example, the Mabuchi FT-36D 22120: the first two digits after the D indicate the wire size in hundredths of a millimetre, and the following digits show the number of turns per pole.

Other good motors for electric R.T.P, are the FT-36D 27110, and FT-26D 2675 and the smaller FT-I6D 18110. Pat March warns us that there are at least two versions of each of these motors, indistinguishable by inspection, but the lower powered versions are quite worthless for R.T.P. work. The only test is by using an ammeter. Power units named Miura Group 20, Champion Black Power, and Big Chief have all been well used for R.T.P. work, with success.

 
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