Conversion of rubber powered models involves four simple steps:
- Installation of a suitable motor.
- Fitting a fuselage-mounted tether hook.
- Adjusting the undercarriage.
- Modifying the fin and tailplane construction to provide easily adjustable elevator and rudder.
Choice of motor depends on the layout of the aircraft type, i.e. where the original aircraft was fitted with a rotary or radial engine or had only a short length fuselage in front of the wings, then the motor can generally be installed directly into the nose and retained, using a rubber band. Inline engine types (long thin nose of model) generally require a motor extension shaft to be fitted to the motor. The motor should be mounted about 25 to 50mm from the nose between two bulkheads cut to accommodate the bearing flanges to either end of the motor. With a short shaft extension a nose mounted bearing will not be required but with a shaft of any greater length a bearing must be built into the nose of the model to prevent vibration.
Fuselage mounted tether hook
A tether hook bent from a paper clip or 20 s.w.g piano wire must be fitted to the fuselage side and approximately in line with the wing leading edge (see photos). The shank of the hook must be at least 50mm long so that the hook may be adjusted by bending the shank, in order to trim the model for best performance. Bending down the hook will lower the outer model wing; bending the hook either forward or backward can be used to trim the fore-aft flying attitude. Most models will also require some ballast to the outer wing tip and a degree of rudder offset to maintain tether line tension.
With a nose-mounted motor in models under about 500mm wingspan, the wheels must be mounted well forward to prevent landing nose-overs. Where an extended shaft motor is installed or where the wingspan is over about 500mm, the undercarriage can generally be fitted as per the plan.
The tailplane should be built to incorporate adjustable elevators, using strips of soft aluminium sheet for hinges (an empty balsa cement tube or paper `twist bag ties’). Semi-permanent adjustments may then be made to obtain results in flight trim. An alternative method is to fit cardboard trim tabs to the rear edge of the tailplane.