How can I find the center of pressure of my model aeroplane? The simplest plan is to adjust the planes so that the model flies on a perfectly even keel, and then balance the machine. Since the center of pressure and the center of gravity must coincide to produce horizontal flight, this point of balance will be the center of pressure. As a rule, the center of pressure will be found to be near the front edge of the main plane, perhaps slightly back of this forward edge. What is the best position for the propeller? In the case of a model driven by a single motor, the propeller shaft should pass through the center of gravity and center of pressure of the machine. It is very important that the alignment should be perfect or you will have great difficulty in securing a horizontal flight. In case of a double propeller machine, the line midway between the two propeller shafts must pass between the center of gravity. My aeroplane starts off all right but often begins to rock from side to side like a cradle and then flutters to the ground. How can I prevent this? [Illustration: A beautiful model built by Stewart Easter] [Illustration: A successful model of 1910 built by E. G. Halpine. Note contrast in plane area] This "rocking" motion is probably caused by a too low center of gravity in your machine. If your planes are set at a dihedral angle, they should be brought nearer to the horizontal. Another plan is to place them lower down. A side gust of wind will start this rocking or vibration, and in order to right itself the machine loses momentum and falls. This difficulty is seldom experienced with horizontal wings, especially if their aspect ratio is high. I have no trouble in getting my model off the ground, but it seems to rise too fast and fall backward or uses up the power before it really gets under way. Is the trouble with the propeller or the planes, for my motor seems all right? If your model rises quickly, the planes are probably well designed and built, the motor is effective, and the propeller sufficient for all needs. The trouble is likely to be with the angle of the planes. Perhaps your front plane is elevated too high. The rear plane should be kept horizontal or very nearly so. An aileron at the rear of the main plane will help you to properly adjust same. As a rule, the angle of ascent should be a little more than that of the front plane. If it still acts badly, strengthen your motor and bring your large plane back a trifle. What is the best place to put the keel? The greatest stability is usually gained by fixing the keel at the center of gravity. There is great difference of opinion as to whether it should be above or below. Try both ways. What is stability for one aeroplane may mean disaster for another. Why should an aeroplane pitch and roll when rising or even traveling horizontally, but sail in a bee line when it begins to descend?