William Mills wrote:
> Graham et al,
> I think you will find, if you do the vector diagrams at intervals along
the
> blade, that the faster the forward speed of the A/C, the more twist you
need
> in the blades for them to provide lift along the whole length of the
blade.
William, To an unfulfilled mathematician (one who discovered nurses at a
sensitive stage in his emotional development and got diverted into
medicine!) this was too tempting to ignore. For a 62" diameter prop doing
2300rpm, tip speed 369 kts, I got the following figures for tip angleof
incidence, angle at one third radius out from centre, twist (the 1/3 angle
less the tip angle) and speed of travel. These figures relate simply to the
angles needed to keep up with the airflow. In practice of course an angle of
attack has to be added to produce lift.
TIP ANGLE 1/3 ANGLE TWIST SPEED (Knots)
5.7 16.7 11.0
36.6
9.5 26.6 17.1
61
14.9 38.6 23.7
98
18.4 45.0 26.6
123
21.8 50.0 28.2
148
26.6 56.3 29.7
184
That much nicely confirms your statement, although interestingly enough if
you continue the table the trend for increasing twist goes into reverse,
thus:
33.7 63.4 29.7
246
39.8 68.2 28.4
307
45.0 71.6 26.6
368
This extended bit of table is pretty irrelevant to any Europa, although it
does imply that if we optimised the prop for130 or 140 kts it would be
pretty good for all speeds above. The snag is that the optimal twist for
say 123 kts is 15 more than you need at 50kts and 10 more than at 60kts. So
a 120 kt prop is going to be bad at getting you off the ground, when at
least half of the blade is likely to either in reverse thrust or stalled
mode - and no doubt this accounts for manufacturers generally optimising
their props on figures below 100kts.
This situation is also probably highly relevant to the blade cross
section. I don't have any figures for critical angle of attack of different
prop blade sections, but I could imagine that the relatively wide Woodcomp
section might have a range of perhaps 12 degrees from zero lift to stall
whereas the thin, sharp nosed section of the Arplast and the Warp Drive may
only be half that. If so then the Woodcomp type of section would allow you
to produce some lift (if not maximum) over a significantly greater
proportion of the blade when in 'Too slow for the Twist' mode.
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