Avia BH 7b (oz16919)

About this Plan

Avia BH 7b. Free flight scale model for CO2 power. Wingspan 24 in.

Quote (google-translated): "The original was a racing aircraft. Built in 1924 and equipped with a supercharged 280 kW Hispano-Suiza 8Fb engine, it reached speeds of 300 km/h during test flights. However, the actual race for the President of the Republic's Prize ended badly: the Avia "stumbled" over a forgotten survey marker during takeoff and crashed.

The model's racing performance is more modest; nevertheless, it completed flights of approximately 80 seconds. For a small semi-scale model, this is quite respectable and certainly exciting.

The model's scale corresponds almost exactly to the original; only the vertical stabilizer and the wing dihedral are larger. A special feature of this Avia is the extensive use of rigid foam as the construction material. This is either ROHACELL 51 in thicknesses of 1.3 mm and 6 mm, or the similar rigid foam called DEPRON, which is available in thicknesses of 3 mm and 6 mm and from which the necessary 1 mm thick sheets can be cut with a polystyrene saw. ROHACELL can be bonded with virtually all common adhesives, while DEPRON is susceptible to solvents and can therefore only be worked with white glue or two-component adhesive.

Both materials have advantages and disadvantages. ROHACELL has a coarser structure than DEPRON, but is also significantly stronger. The best way to find these rigid foams is to consult your local phone book (Yellow Pages). While DEPRON is occasionally available in hardware stores, you're more likely to find ROHACELL at a plastics supplier.

Building the Wing: The 1 mm rigid foam sheet for the wing skin must first be sanded. To do this, we place the sheets on a flat surface and sand them with a sanding block. (Later, on the finished wing, we would sand the skin along the ribs.) Following the drawing, we glue the wing spars together from hard balsa and mount the ribs from 1 mm balsa (root and tip 2 mm). Now the lower skin can be fitted (three parts: both wing halves and the center section).

The leading edge is sanded at an angle so that the upper skin can sit flush. We make this skin with a 5 mm oversize; the trailing edge is sanded to a point.

We must incorporate the wing warping into the skin. For this, each wing half is supported at the trailing edge with a 5 x 10 mm balsa strip, and this is further supported with 3 mm on the outer edge of the left wing and 7 mm on the right wing. When the leading edge is now pinned to the flat building board, the correct wing twist is achieved. The upper sheeting is then glued on from the trailing edge to the leading edge spar; after drying, the wing can be removed and the remaining sheeting can be fitted freehand.

Once the center wing section is also sheeted, the excess rigid foam sheeting is trimmed all around. The end ribs are attached, and the cutout in the trailing edge is covered with 1 mm strips of the sheeting material. The leading edge is reinforced with an approximately 6 mm wide strip of paper..."

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