Introduction

"Powered" model airplanes, that is ones powered by internal combustion engines, use propellers that have graduated from crude hand-carved wooden props to modern day carbon fiber marvels. The cost is up, too. Carbon fiber props cost more by weight than gold.

The operating range has been extended to increasingly high aircraft and engine speeds. Propeller tips in F1C routinely exceed Mach 1. Centripetal acceleration is in the tens of thousands of "G's".

This article is dedicated to taking another look at power model props with emphasis on control line model aircraft. The approach is mildly technical and attempts to show what the major factors are to performance as well as to do a better job of describing common terms such as "pitch" and " efficiency"

The next section reviews the fundamentals and geometry of how the propeller moves through space with some typical parameter values for competition C/L racing events illustrated.

The idea of inflow is introduced after that. The common knowledge that the propeller draws air in to the blade face while running on the ground is examined for flight conditions. The inflow may be as high as 10% to 20% of the forward speed and has a profound influence on efficiency in some events.

The general idea of prop efficiency is discussed. Defining it (it is NOT the amount of "slip", treating the "goesintas",and providing an example of estimated blade element efficiency for an F2C propeller.

Mach number effects, which are significant for several competition classes are treated with wind tunnel data and more from the F2C propeller example.

One theme is that the propellers are so small that high precision reproduction of airfoils is not possible, however, the principal factors affecting control line model propeller performance are the pitch distribution and major airfoil characteristics, thickens and chord. With modern materials the very thin airfoils that should be used are possible to construct. One-bladed propellers permit larger chords, much more likely to have good aerodynamic performance and usually well worth the loss in induced efficiency that a low number of blades incurs.