# Propeller and Slipstream Effects

PROPELLER NORMAL FORCE

The following method of estimating the propeller normal force is due to Ribner (1944). The normal force is expressed in terms of the derivative dCN/dap (see Sec. 3.4), which is given by

dCNp/dap = fCY;li{)

The factor / is the same for all propellers, and is given in Fig. B.7,1 as a function of Tc = TlpV2d2. The value of Су>ф0 varies with the propeller and its operating condition. The values for a particular propeller family are given in Fig. B.7,2. Extrapolation to

SBF IBF sb h |

Figure B.6,2 Effect of a fuselage on Cma. (From Royal Aeronautical Society Data Sheet Aircraft 08.01.07.) other propellers can be made by means of Fig. B.7,3, on the basis of the “side-force – factor,” SFF. This is a geometrical propeller parameter, given approximately by

SFF = 525(b/D)03 + (b/D)06] + 270(b/D)09

where (b/D) is the ratio of blade width to propeller diameter, and the subscript is the relative radius at which this ratio is measured.

Also given in Ribner (1944) are some curves which are useful for estimating the upwash or downwash at the propeller plane. These are reproduced in Fig. B.7,4.

Tc

Figure B.7,1 Variation of / with Tc. (From NACA Wartime Rept. L-25, 1944, by H. S. Ribner.)

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