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National Advisory Committee for Aeronautics, Technical Notes - Propeller Tests to Determine the Effect of Number of Blades at Two Typical Solidities

Propellers with equal total blade area, but with dif—
ferent numbers of blades, were tested at Stanford Univer—
sity.

The tests show generally that, for equal total blade
area. propellers with the larger number of blades absorb
the greater power and, provided hubs have equal drag, de»
velop the higher efficiency.

It is shown that the differences found are in agree-
ment, qualitatively, with what might be predicted from
simple blade—element theory.

The simple blade—element theory as developed by
Drzewiecki shows that between two propellers with simi—
lar blade plan forms and blade section profiles and with
equal total blade area, but with different numbers of
blades, the power absorbed and the efficiency developed
by the propeller with the larger number of blades should
be the greater. The larger power absorption would be ex-
pected from the increased lift coefficients for blade 61‘
ements of higher aspect ratio. A gain in efficiency
should arise from increased L/D of blade elements.

In the practical case, unless the aerodynamic superi-
ority of the many-blade propeller is considerable, the
propeller with fewer and wider blades might be chosen,
since, particularly for the controllable—pitch propeller,
the mechanical features will be less complicated and the
original cost no doubt smaller.

At the request and with the financial assistance of
the National Advisory Committee for Aeronautics, the sub—
sequently described experimental study was undertaken.

The purpose was to determine by test the quantitative dif—
ferences in aerodynamic characteristics between two— and
three—blade propellers having'equal total blade areas, and
between three~ and four~blade propellers, again having
equal total blade areas but, in this case, 3Z~l/3 percent
more area than for the two-blade —— three—blade comparison.