[Illustration:  Fig. 134.—­Front skid and attachment to backbone.]

Bearings.—­We have still to deal with the propellers and their bearings.  The last, TN and TNl (Fig. 133), are simply two tiny pieces of tin about half a gramme in weight, bent round the propeller spar HS3 at B and B1.  Take a strip of thin tin 1/4 inch wide and of sufficient length to go completely round the spar (which is 1/4 by 1/8 inch) and overlap slightly.  Solder the ends together, using a minimum amount of solder.  Now bore two small holes through wood and tin from rear to front, being careful to go through the centre.  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.  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.