tion. To strengthen this spar further against torsional
forces, run gut ties from B and Bl down to the bottom of the rear vertical
skid post; and from B to B1 also pass a piece of very thin piano wire,
soldered to the tin strips over a little wooden bridge, Q, like a violin
bridge, on the top of the central rod, to keep it quite taut.

[Illustration: FIG. 135--"Centrale" wooden propeller.]

Propellers.--To turn now to the propellers. Unless the reader has already
had fair experience in making model propellers, he should purchase a
couple, one right-handed and one left-handed, as they have to revolve in
opposite directions. It would be quite impossible to give in the compass of
this article such directions as would enable a novice to make a really
efficient propeller, and it must be efficient for even a decent flight with
a self-launching model. The diameter of the two propellers should be about
11-1/2 to 11-3/4 inches, with a pitch angle at the extremities of about 25
to 30 degrees as a limit. The "centrale" type (Fig. 135) is to be
preferred. Such propellers can be procured at Messrs. A. W. Gamage, Ltd.,
Holborn, E.C.; Messrs. T. W. K. Clarke and Co., Kingston-on-Thames; and
elsewhere.

For the particular machine which we are considering, the total weight of
the two propellers, including axle and hook for holding the rubber, should
not exceed 3/4 oz. This means considerable labour in cutting and
sandpapering away part of the boss, which is always made much too large in
propellers of this size. It is wonderful what can be done by care and
patience. The writer has in more than one case reduced the weight of a
propeller by more than one-half by such means, and has yet left sufficient
strength.

The combined axle and hook should be made as follows:--Take a piece of thin
steel wire, sharpen one end, and bend it as shown at C (Fig. 136). Pass the
end B through a tight-fitting hole in the centre of the small boss of the
propeller, and drive C into the wood. Solder a tiny piece of 1/8-inch brass
tubing to the wire axle at A, close up to the rubber hook side of the
propeller, and file quite smooth. The only things now left to do are to
bend the wire into the form of a hook (as shown by the dotted line), and to
cover this hook, as already advised, with a piece of valve tubing to
prevent fraying the rubber skeins.

[Illustration: FIG. 136.--Axle and hook for propeller.]

Weight.--The weight of a model with a T-shaped central