ELECTRIC FLIGHT U.K. - British Electric Flight Association

angle is reduced so as to reduce the pitch, the blade acquires a lot of wash out.
Neither of these extreme conditions is desirable in a propeller for the same reason
that they are not desirable in an aircraft wing. It is hard to see how such a
propeller can be set “exactly to the desired pitch”.
I believe that this is the way that most variable pitch propellers work, including
the full size ones, and is the result of twisting the entire blade to increase the
pitch. To maintain a constant pitch it is necessary to reduce the twist as the blade
tip is approached, which seems quite a difficult thing to do. To minimise the wash
in when increasing pitch the designer could introduce a little wash out when set at
fine pitch and also keep the increase of blade angle to a minimum.
There is another way of varying the pitch of a propeller blade, and that is to
increase its diameter. We will do this in table 3 adding 10mm to each radial point
on our 250 diameter 150 pitch propeller, and recalculate the pitch. The pitch has
increased as expected but we have a blade with some washout. If we reduce the
blade diameter by 10mm we get in table 4
r new r α p new r α p
25 35 43.7 210 15 43.7 90
50 60 25.5 180 40 25.5 120
75 85 17.7 170 65 17.7 130
100 110 13.4 165 90 13.4 135
125 135 10.8 162 115 10.8 138
Table 3 Table 4
The pitch has reduced but we have strong wash in. Combining the 2 methods by
increasing the radius by 10 mm and rotating the blade by 2.5° gives us table 5.
And increasing the radius by 20 mm and the blade angle by 5° gives us table 6:
new r new α p new r new α p
35 46.2 229 45 48.7 322
60 28.0 200 70 33.0 286
85 20.2 196 95 25.2 281
110 15.9 197 120 20.9 288
135 13.3 200 145 18.3 301
Table 5 Table 6
If we decrease the radius by 10 mm and reduce the blade angle by 2.5° we get:
new r new α p
15 41.2 82
40 23.0 107
65 15.2 111
90 10.9 109
115 8.3
Table 7
105
38 E.F.-U.K.