davidjoyce(at)doctors.org
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 Posted: Wed Sep 05, 2007 3:17 am Post subject: Was Nothing new then now propeller twist |
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Bob & Ralf, There is the consideration that the central portion of most
props used on Europas will have a zero or negative angle of attack at speeds
over 100kts, which can't help the cooling!. Most props I have looked at have
a twist (taken as the difference between pitch at the tip and at 1/3
radius) of the order of 12 degrees. This is perfectly suited for a 1600mm
prop doing 2300rpm at about 35 kts, but at 115 kts at 2000 rpm you need
about 27 degrees twist to have every part of the propeller having the same
angle of attack in a uniform air flow through the propeller disc. It's not
likely that the tip will work too well at 27 degrees AOA, and if that is run
at + 5 degrees then the inner bit will be at - 10 degrees!
Props that I have looked at on Spitfires, Red Bull racers, etc &
fast turboprops various all have twists up in the 25 deg range. It is
interesting that the graph of twist needed for a given speed is not a
straight line, but a rounded top curve, rather like the course of a
projectile, so that for 1600mm prop, the figures I calculate are:
Knots: 36 61 100 130 184 246 307 at
2300rpm
Knots 31 54 88 113 164 214 267 at
2000rpm
Degrees 12 17 23 27 30 31 29
Twist needed
If you take the 2/3 radius point as having the ideal AOA then
with a12 deg twist the tip would have a pitch angle of 6 degrees less and
the 1/3 portion of 6 degrees more. That according to my calculations is what
you need at 36 kts, making our current props ideal for the initial
acceleration phase, but at130 kts, the twist is 15 degrees less than ideal.
That is to say that if the 2/3 point is ideally pitched the tip will be 7.5
degrees over ideal (and possibly stalled) and the 1/3 portion 7.5 degrees
under ideal pitch (and close to negative AOA).
On the other hand a prop with say 23 degrees of twist would be
ideal for 100kts but would be only about 5 degrees off perfect twist for 150
kts, and 6 degrees off perfect for60 kts.(with the AOA off-perfect for inner
and outer portions by only half these figures - probably still comfortably
in the working range for positive lift). The initial acceleration would of
course suffer. Going back to the original point we could hope that cooling
would be improved, as well as top speed and cruising fuel consumption.
I should say that these calculations are my own, based on the
angles needed to "screw" your way through a solid medium without slippage,
and are therefore fallible!! I would be interested to hear comments from
anyone with an insight in this area.
Regards, David Joyce, G-XSDJ
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