Aspect Ratio

The blade’s chord along with its length determines the aspect ratio. The aspect ratio is defined as the ratio of prop radius to prop chord (R/C). A high aspect ratio prop blade – one with a narrow chord – is generally more efficient than a low aspect ratio or wide chord blade. A high aspect ratio blade shares the same characteristics with a high aspect ratio wing; the strength of the trailing edge and tip vortices is reduced. Therefore, the induced drag is also reduced, which increases the prop’s lift/drag ratio and hence, efficiency. But there is a limit to the blade’s high aspect ratio; problems may occur with blade stall and the strength of the blades due to the various forces acting on them. [See Prop Stress].

A prop with a low aspect ratio blade is known as a ‘paddle blade’ It has greater solidity and a higher activity factor, and will absorb more engine power than a high aspect ratio blade. But its efficiency may be reduced due to the blade wake affecting the thrust produced by the flowing blades. A wide chord blade also places more stress on the pitch change mechanism