Item by item discussion of analysis
Item 12: The “Ducted Prop” spreadsheet shown in part in
Table 8 and
9 began as a minor improvement to the existing spreadsheet discussed in Item 5 and evolved into a major revision of the calculation process. Mach number effects, Reynolds number effects, and a different process for adjusting 2-dimensional airfoil data into 3-dimensional wing (prop blade) data were incorporated. The foundation of the analysis is combined momentum theory and blade element theory presented in most classic helicopter rotor texts, modified slightly for use with propellers in forward flight.
Table 8
Table 9
The pitch distribution on the prop blades was revised by trial and error, now easy to do in the spreadsheet, to improve the propulsive efficiency. This somewhat higher efficiency will now be used in performance calculations. It is noteworthy, however, that a very similar companion spreadsheet for props not inside ducts seems to indicate that a ducted prop is not more efficient than a free prop at these relatively high flight speeds, which is consistent with “standard wisdom”.
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