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TURBOPROP
TURBOPROP

            

TURBOPROP
Wing span: 20 Inches
Designed By: Roy L Clough Jr
Description: Reprinted from Popular Science, September 1953, pp. 193

Gases from a miniature jet engine whirl a tin-can turbine which in turn spins the prop to keep the plane in air.

THE hissing power of a miniature jet engine is harnessed by a turbine to spin the prop on this model plane – just as on a real turboprop airliner. The prop does most of the pulling, but the jet thrust helps, too. Flights of about 150 feet can be expected.

Power plant

The jet power is supplied by a Jetex 50 solid-fuel motor, available in model-supply shops. A small pellet of compressed fuel burns rapidly, emitting exhaust gases under pressure through a tiny hole in the metal case.

Construction

Four 1/16" balsa bulk-heads (stations A, B, C, D) cemented to a flat keel shape the fuselage. The wing, cut in one piece, is laid across the keel and cemented in place. Dihedral is added by scoring, bending and reinforcing the bend with cement.


Power plant

Rudder, stabilizer and wing fillets are cemented in place after the fuselage is covered with tissue; cowl cheeks are added after installing the motor bracket and landing gear. The bracket is set off-center and at a slight angle, to reduce the side thrust of the jet as the gases are deflected by the turbine vanes.

Motor bracket Prop shaft and turbine

Bearings for the prop shaft are cut from tin-can stock and cemented to the top surface of the keel on the centerline. The turbine is made by cutting 16 vanes in a 1 9/16" tin disk and twisting them about 45 degrees. It is then set into an opening in the keel and soldered to the back end of the propeller shaft. The propeller is forced on a short length of rubber tubing slipped over the end of the shaft. This will hold it in place for flying, yet permit easy changing of props to determine the best pitch.

MOTOR BRACKET is set at a slight angle to left (looking from the nose). It also secures the landing gear. Asbestos that comes with motor insulates the keel.

Flying

The trim of this model is unusual because the center of gravity shifts during flight. It should be balanced for a floating glide without a fuel pellet in the motor. A wedge pushed under the stabilizer will keep the nose up if needed.

Flying under power with the weight of the fuel holding down the nose, the model will travel on the level for some distance, then go into a gradually steepening zoom as the fuel pellet is consumed and the motor peaks.

The rudder is bent to the right a bit to counter the jet deflection, which tends to force the plane to the left. Best bet is a straight flight or a gentle left bank as gyroscopic forces of the turbine and prop tend to raise the nose in a left turn and depress it in a right turn.

Cautions

The prop should not be held, once the jet starts perking; heat concentrated on the turbine might melt the solder that holds it to the shaft. If the plane noses in, it should be picked up immediately to let the turbine spin. A thin solution of sodium silicate (egg preservative) or sal ammoniac painted inside the cowl cheeks will keep them from being scorched.
Keywords: turboprop jetex 50
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