The hole must be just large enough to allow the
propeller axle to run freely, but not loosely, in it. Primitive though such
a bearing may seem, it answers admirably in practice. The wood drills out
or is soon worn more than the iron, and the axle runs quite freely. The
pull of the motor is thus directed through the thin curved spar at a point
where the resistance is greatest--a very important matter in model
aeroplane construction. 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.
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