How can I calculate the speed of my machine from the size and pitch of my propeller? It is practically impossible to do so. If you multiply the pitch of a propeller by the number of turns at a given time, you will arrive theoretically at the speed. In practice, however, the slip of an aerial propeller is from twenty-five to forty per cent. It is very difficult to determine just what the slip is. Your calculation is likely to be a matter of thirty per cent off and is of little value. I am troubled with my machine trembling a great deal during flight. What can I do to make it steady? The framework of your model is probably too light for your motor. Strengthen your machine if it will stand it, or take off some strands of your motor, if it will stand it. The frame may be strengthened by using wire braces. This method so commonly used last year is being abandoned by the successful builders, as it makes it necessary to attach struts at right angles to the frame, which add to the weight and resistance. The wire in vibrating also offers an appreciable amount of resistance to the air. In a large passenger-carrying machine, these wire braces are absolutely necessary, but the model being so much smaller it is better to make the frame heavy enough to remain rigid. A great deal of extra work and annoyance is saved by doing away with wires. This trembling again may be caused by your propeller not being properly balanced. What is the lightest metal I can use? Aluminum has less specific gravity than any metal now available. Its cheapness also makes it practicable. Magnalium is a trifle heavier than aluminum but considerably stronger, and is preferred by many model builders. There are still others who prefer steel to either of these. Steel is three times as heavy as aluminum and about five times as strong. If you can get the metal in the proper size for model aeroplane building, steel is probably the lightest after all. Some model builders have great success with umbrella ribs, which is probably the most available form of steel for our purpose. As most of us have discovered, a steel bicycle spoke is by far the best axle for propellers. Should the propeller be in front or behind the machine? It will require some years of experimenting to answer this question definitely. On one hand it is argued that the forward propeller, or tractor as it is called, works in undisturbed air and is therefore more efficient. Most of the large passenger-carrying aeroplanes are driven by tractors. Others argue that the propeller churns up the air and that the planes would therefore balance themselves against the gusts thus set in motion, which makes their flight unstable besides requiring additional power. Practically all of the successful model aeroplanes this year are driven by propellers. What is the lightest practical model aeroplane?