Vance Jaqua wrote:
Paul - The programs that I gave you, and that you
cleaned up for quick basic will do all those things
for you. The PRPSIZ will suggest a fixed pitch prop
based on input of BHP, RPM, and estimated top speed of
your bird for thst power - it suggests maximum
diameters based on tip speed and allows you to select
that diameter. it will then give you pitch, blade
widts at
.9,.7,.5,and .3 radius points, angles at those points
for true helical pitch and a computed efficiency at
that point and 70 percent cruise. This program and the
other one are based on matching results of blade strip
element (using lift and drag coefficients for actual
airfoil data at typical prop aspect ratios) and disc
momentum thrust. The other program PRPLRIT you
describe the selected prop and it will iterate
through selected speed and rpm ranges and provide you
with thrust, shaft HP, and efficiency for each of
those points, it will also seperate out pumping eff
from total eff and and estimate installed thrust and
efficiency with the airframe added drag from the prop
slipstream. If you select the long print out it even
provides the working angle of attack for each of the
blde radius points. This program has limited
"anchoring", and of course it has to assume that the
prop shape remains reasonably constant in operation
(which can be a major factor with some wodd props and
easpicially the so called "almost constant speed"
props with scimitar shape). I think it is a pretty
good tool but of course I am prejudiced since I wrote
it. It has been widely circulated in various stages,
and some of the other programs available might be
based or patterned , or independently invented like it
- It is certanly not perfect, and you are free to not
believe the output - Vance
Too soon to say whether I believe it or not. I need to get
out there and take some actual take off data. So far I am still in bed.
The data I do have you dismiss as advertising hype.
Based on the chart of runs you made and uploaded showing such a small advantage
to a constant speed prop over a fixed pitch prop raised my doubts.
This goes against my considerable flying experience with constant speed
props verses fixed pitch props.
The biggest problem I have is with your efficiency numbers and the lack
of RPM column and pitch column for the constant speed prop. Constant speed implies
max RPM with max HP for the constant speed prop. I know from experience
that constant speed props rev higher on takeoff and therefore the engine
is generating way more HP.
I also have the same problems with the var gear columns.
Here again is your chart.
fixed pitch 70X78 const spd var gear
mph rpm bhp f lbs eff f lbs eff f lbs eff
0 1940 108 407 0 603 0 525 0
30 1950 109 393 28 530 26 505 25
60 1960 110 353 51 480 50 460 47
90 2080 118 327 66 425 63 420 62
120 2400 130 307 76 366 73 365 73
160 2500 149 283 81 299 80 299 80
190 2720 160 268 83 268 83 268 83
Paul Lamar
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