Sabre (oz7245)

About this Plan

Sabre. Scale model jet fighter for .049 ducted power. This is an Uncle Willies plan and as such has few remaining identifying features, in fact nothing except that it is from MAN.

Quote: "The man who first proved that scale jet jobs can be flown with ordinary engines, details construction of his successful F-86D for Half-A engines. The Sabre by Thomas H Purcell Jr.

The F-86-D was chosen for the ducted fan power system because its parameters best favor stability in flight. One unusual development was that the full scale prototype set a world speed record after the model had been completed. This phenomenon is probably unique in the history of model aviation.

The model flies very realistically and is easily controlled by the tab settings. It weighs 6 oz and has a static thrust of 3 oz. This 50 per cent thrust to weight ratio is probably better than that of the full scale Sabre. The scale ratio is 15.8 to 1. The full scale Sabre flew almost 700 mph. Thus, if the model should fly 44 mph, it would move as many plane lengths per hour as the prototype. The flight speed of the model has not been measured; but in normal sport flying the model appears to be going about 35 mph. This compares with 550 mph full scale, which is a good cruising speed for such an aircraft.

If trimmed for left circle, the model so flies regardless of power conditions. The low torque makes the model easy to trim for powered flight and greatly reduces the stability requirements as compared with conventional propeller-driven models.

The use of moderate dihedral was predicated on the need for some control over the well known tendency of models to spiral under power. A distinct advantage in a swept wing is variation of effective dihedral with lift. This means that, if the model tends to spiral, it is necessary only to trim the model for slower flight, which makes the wing operate at higher lift. This gives the airplane more effective dihedral and eliminates spiral tendency. The builder is warned to trim the model very carefully by test gliding before attempting powered flight.

The plans presented are intended to cover the main details necessary for the construction of the ducted fan and its components. Details of the wing, tail surfaces, and scale trim are noted briefly, as space permits.

Sabre construction starts from the inside because that is the simplest way to install the duct. Select a medium thick sheet of 1/32 in balsa for the fuselage bulkheads. Trimming is facilitated and structure is strengthened if the bulkhead sheet stock is covered with a cross grain layer of tissue on both sides before the bulkheads are cut out. The bulkheads are best outlined on the sheet stock by tracing over carbon paper. Then they should be trimmed to size on the inner contours, but oversize on the outer. Make two of each and butt cement the halves together.

The duct material is selected, thin 1/32 in sheet balsa. Form this sheet stock by wetting it and wrapping it around a quart size soda bottle or a similar sized tube. This should follow edge cementing enough sheets together to form the largest diameter in the duct.

The duct sheet should dry on the forming cylinder while it is held in place by rubber bands. The duct walls, from bulkhead No. 5 to the tail, form a truncated cone. Therefore, the sheet which forms these walls must be tapered. This is best accomplished by rolling an undersize cone from the duct material and slipping bulkheads Nos. 5 to 12 over this cone. Then the cone is expanded by pushing crumpled newspaper into the larger end until the cone completely fills the circles inside the bulkheads.

Spaced properly, the bulkheads are then cemented to the duct walls except near lapped wall material. When the cement has dried, the wall, slit lengthwise in the lapped portion, will have perfectly mated edges. When the excess sheet material is removed, the remaining edges should butt together closely. A seam of cement at this joint should dry while the expanding paper is still inside the duct.

Upper and l0wer keel strips which form the structure backbone forward of bulkhead No. 5, cut out and attached to bulkhead No. 5, permit location of the remaining bulk-heads in their proper places. The general arrangement of these bulkheads can be seen on the plans and in the photograph of the fuselage internal structure.

After the bulkheads in the forward fuselage portion are in place, the bond paper duct walls are installed by 'cut and try' procedure. The intake duct walls need not be very smooth, but sudden changes in the dart internal section area should be avoided. If installation of the intake ducts generally follows the photograph of the fuselage internal structure, performance will be satis-factory.

Additional pieces corresponding to the upper halves of bulkheads 5 and 7, and the strips which form the horizontal frames and edges of the access door should be cut out and cemented in place according to the plans, then the nose carved to approximate shape and cemented in place. When all cement has dried well, fuselage bulkheads should be trimmed to their proper contours and the whole assembly sanded to make a faired body when covered.

The fuselage is covered with strips of thin 1/32 in balsa sheet, though if tissue is preferred, the established practice with stringers should be followed. The Sabre was originally covered with tissue and was only 2/10 oz lighter than when planked. Some ballast may also be necessary at the tail..."

Direct submission to Outerzone.

Supplementary file notes

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