BLADE SECTIONS

The choice of aerofoil section for propellers is also conditioned by structural factors, especially near the. hub, where strength is very necessary and the profile must be thickened considerably. Fortunately, this part of the blade is least important from a thrust

point of view. Because of the high r. p.m. at which the model engines run, Reynolds number effects are less significant, on all but rubber-powered models, than for wings. The blade Re of a model propeller usually is comparable with that of a light aeroplane. Accordingly, aerofoils which are satisfactory on large propellers prove the same on models. Many of the boundary layer flow characteristics that plague model wings tend to disappear with propellers, again except for the rubber driven variety. In addition, the •boundary layer flow on a propeller is very much more complex than on a simple wing. The lowest layers, which are dragged along almost at the same velocity as the blade itself, are accordingly subject to strong centrifugal forces, which extend upwards to the rest of the boundary layer in proportion as the air travels round with the blade rather than staying with the general airflow. Within the boundary layer there are strong cross flows. Small vortices form, which almost certainly turbulate the air and probably prevent laminar flow altogether. Partly for this reason, not much effort has been put into designing very refined, low drag profiles for propellers on model aircraft With rubber driven and indoor types, although there has been much experiment and experience over the years, it cannot be claimed that any very startling improvements have appeared.