In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform. Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction. Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem. Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling. [Illustration: A combination of several interesting features] [Illustration: A skilful adjustment of the front plane and skid built by Percy Pierce]