RE: Re: propellor and rubber of a F1D model
Kevin
I am not clear what your propeller block is like and how you are using it
but I think it is easier to ignore it for the moment. You mention that at a
radius of 50 mm you have a blade angle of 64 degrees. The sums are correct
but is the angle calculated from an assumed pitch of 650 mm or is it
measured?
In any case the blade angle so close to the hub is not a good reference
point. The significant factor in propeller performance is the blade angle
at the radius three quarters of the way out towards the tip. I think with a
modern F1D that you will find this angle is usually about 32 – 36 degrees.
That is the place to check your propeller and by measuring the angle, not by
some guessing at what the prop block may give.
It is a pity that pitch has such a hold in model stuff. (in propeller design
text books it is rare to see the term mentioned.) It is imprecise and
virtually impossible to measure and is usually assessed by a calculation
from the blade angle, so why not just stay with the blade angle.
Le Roy
Kevin and Bill Gowen have already explained very well the benefit of
specifying rubber as grams per metre but I would like to add a further
point. When using rubber width to define the motor it is common for people
to assume the thickness is standard at the manufacturers 0.04”. This is not
the case and difference of 0.002” are not uncommon. As an example of the
effect of thickness variation take the case of the 0.6g motor made from 1.3
g/m rubber that I gave in an earlier post.
The motor was 1.3 g/m it made up into a loop 9.1” long and took 1594 turns.
If the rubber had been 0.002” thicker then the g/m would have been higher
with the following result.
Motor 1.365 g/m would make a loop 8.7” long and would take 1482 turns.
A turns formula based on g/m takes care of these variations much more
easily.
John
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Received on Mon Mar 10 2008 - 14:29:30 CET
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