Multi-Bladed Props
TWO YEARS AGO I wrote an article on propellers that appeared in the October 1986 issue of Model Aviation. The purpose of that article was to give the modeler a way to approximate (under static conditions) the power absorbed by a propeller of any given pitch and diameter as a function of rpm. With this information, one could determine the power output of an engine or select the appropriate propeller for a particular engine. The article pertained to the more conventional two-bladed props used by most of us.
dB Facts
ANYONE WHO'S BEEN keeping up with the model press lately is aware of the noise issue and the various ideas about limiting and measuring the sound produced by our models. There can be no question about the importance of sound abatement. I'm sure that noise is far and away the most significant cause of the loss of flying sites today. In reading the reports and discussions on this subject you must have noticed the use of the term "dB," or "decibel." To understand the actual significance of some of the sound level standards, you have to know what "dB" means.
Vibration
THE VAST MAJORITY of our power models are equipped with single-cylinder internal combustion engines. Since there is no way that a simple single-cylinder reciprocating engine can be balanced, these engines are by their very nature vibration generators. Hence vibration is an unpleasant fact of life in the gas model airplane business. Not only is this vibration potentially damaging to the model and to the radio equipment in it, it also contributes substantially to the noise generated in flight. The purpose of this article is to explain why these engines produce vibration and why they cannot be balanced, how the model may react to vibration, and what methods may be employed in the aircraft to cope with the problem.
Centrifugal Force
OF THE SEVERAL FORCES that act on an aircraft in flight, centrifugal force is one of the most important. It will help us in both design and flight if we understand this force. If an object is traveling along a curved path, it will experience a force toward the outside of the curve. A childhood example is tying a rock to a string and whirling the string around your head. The force the rock experiences is called centrifugal force, and it is a very important factor in the behavior of airplanes.
Propellers
FOR MOST MODELERS the selection of a propeller is usually a matter of following the recommendation of the engine manufacturer or the airplane designer. Later on the modeler may experiment with different diameter and pitch combinations seeking the one which seems to provide the type of performance which is desired. With the advent of the larger chainsaw engines, big glow engines, and four-cycle engines, the modeler is faced with power plants having characteristics quite different from the glow engines that have been the standard of the hobby for more than 30 years. The larger engines may develop up to 5 hp at around 10,000 rpm.
