Vance Jaqua wrote:
OK I am interested in a rough approximation so I
will accept a
5% error due to the disk loading effects.
Now getting back to the constant speed prop verses
the the fixed
pitch prop verses the infinitely variable gear box
driven prop.
Lets look at the constant speed verses the fixed
pitch prop.
Let us say the fixed pitch prop operates at about
2200 RPM and 120 HP on average
while the constant speed prop engine runs at 2700
RPM and 150 HP and
while the O-320 Lancair accelerates from zero to a
rotation speed of 100 feet
per second.
It took 27 seconds for the fixed pitch prop to reach
100 feet per second and
only 9 seconds for the constant speed prop to reach
100 feet per second.
First of all that chart is advertising hype and
probably bears very little relationship to real data
VJ
> Let us also say the gross weight of this airplane
was 1600 pounds in
both cases.
The average acceleration with the fixed pitch was
about 3.7 feet per second per
second or .11 G. While the constant speed prop did
it at 11 feet per second per
second or .34 G.
F = W * G's so in the fixed pitch case we had an
average thrust of 176 pounds
and 544 pounds average in the constant speed case.
At 50 fps we have available out of the 120 HP 2200
RPM engine 1320 pounds of thrust
or a prop eff of 13%. (thrust in/thrust out. HP
in/HP out). Out of the 150
HP 2700 RPM constant speed engine we have 1650
pounds of thrust or a prop eff. of
32%.
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
I think there is something wrong as you show the
eff. for the constant
speed prop at 30 MPH (44 fps) being only two points
difference than the
fixed pitch prop.
Your number for the fixed pitch prop is way too high
IMHO.
I don't think the difference in disk loading can
make up for such
a wide discrepancy.
It is 2 percent lower because the disc loading is
higher the fixed pitch prop is getting 3.6 lbs per hp
and the constant speed prop is getting only 3.3 lbs
per horsepower because of higher disc loading 6 hp
per sq ft versus 4 hp per sq ft - why is this hard to
believe? Vance
Why is 393 pounds of thrust out of the fixed pitch prop hard to believe? :-)
Because it is taking the O-320 Lancair 27 seconds instead of 9 seconds
to get to 100 feet per second with a fixed pitch prop. That is why 393 pound
of thrust out of the fixed pitch prop is hard to believe regardless of the
minor change in disk loading. These are real measured numbers ostensible.
Where did I go wrong explaining this? :-) Is my math wrong someplace?
Paul Lamar
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org
