Hi Paul-
I'm still looking for a prop to use when first testing/tuning the
engine. My thought was to get a prop -- any prop -- that would at least
prevent the engine from over-speeding, but not necessarily be one I
could fly.
Well, I've just tripped across a prop that might do both, and at a very
modest price (compared to the $2500 I was looking at with performance
props).
Anyway -- my question is whether this is a flight candidate. I know
there are a couple of folks on here with magic prop software, perhaps
they could run the numbers & tell me if this would work for me.
Attached is a photo of the prop data plate. Airplane is Mustang II.
VNe
is 235mph. Desired cruise is anything north of 200MPH. 13B at 230HP.
Tracy's rd-1c redrive. Typical prop on a M2 is 68" 2-blade. I
*really*
want good climb performance, since my field is at 6873' and summer
DA is
typically 9000' - 10000'. 6000' long runway.
Mark
I don't think that 60 inch prop is going to give you good take off
performance.
Should or could be stalled for a long time on the runway. I think
you need
a much longer prop. Perhaps 3 blade. Around 70 inches. Maybe 72 inches.
With less pitch.
That prop might be OK for Dave's turbo engine.
The prop in question is a 3 blade - not sure if that was obvious. In
any case, it would be interesting to know what the numbers for this one
work out to be. One of my desirements is to find a way to tame the
13b's fuel thirst at cruise. That may be unrealistic, I know. My fuel
tank is only 23 gallons usable - so I have an issue right off the bat if
I can't get consumption down somehow. Performance props was
recommending prop-limiting the RPM by having a really aggressive pitch
(although not quite so aggressive as this). They also thought 70 or 72
inches was the right number for a 3-blade. But .. I could get this one
instantly at 1/3 the price. So I'm trying to see if it would work as
something to get me flying for a while.
Mark Supinski
It might be worth buying. Since it is a Catto you can problay get what
you payed for it if it is not damaged.
A crude way of calculating pitch figure 80% slip at 200 MPH at cruise RPM.
In theory 70 inch pitch at 2700 RPM is 189,000 inches per minute
or 15,790 feet per minute or 262 feet per second or 178 MPH
at no slip. At 20% slip that is only 142 MPH. A lot depends on how
they measure pitch angle but that is true pitch.
Let's run their numbers... 60 inch pitch at 3137 RPM or 188,220... 15,680
261 feet per second 178 MPH at no slip. Interesting.
Obviously pitch is measured with a protractor on the bottom
of the blade airfoil. The real true pitch is higher than this.
I'll need to plot a typical prop airfoil to figure the difference.
Takeoff will be what ever you get.
Check my math :)
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"Pitch is the measured of the length of the cylinder of air thus moved by the prop in
one complete revolution, assuming 100% efficiency. It therefore gives a theoretical
speed for the airplane, for any rpm. But not all props have the same efficiency.
Theoretical speed = rpm x pitch x eff."
Pitch is usually measured at 75% of the radius.
Is in the ball park of 26 to 32 degrees in terms of blade angles.
--------------------------------------------------------------
This is interesting.
http://www.nclra.org/Programs/PropCalculator.html
You put in 200 MPH, 30 inch radius, 25 degrees, 2500 RPM and you get
98" pitch.
Lets see 35r times .75 is 26.25r for a 70" dia prop
lets try that radius.
We get 88 pitch. Now we are getting in the ball park.
The 3/4 radius is the key.
Still not making sense.
-----------------------------------------------
OK I think I have things nailed down. Prop makers are calculating
the pitch based on the angle from the lower flat part of the
blade. The real pitch is higher by about 10% to 15% ....my guess
so far..... until I can plot a blade airfoil.
What that means is this formula gives erronous results.
Theoretical speed = rpm x pitch x eff."
You must add 10% to 15% to the manufactures pitch numbers to get the
correct result.
"If the propeller were turning in solid material, as is the case with a screw in
wood, it could act without slipping, and so the distance traveled in one revolu-
tion would be equal to the theoretical travel of the propeller. Inasmuch as the
propeller is turning in the air, however, there is a certain amount of "slip"; there-
fore its actual travel distance falls short of its theoretical travel distance. In
the illustration (Fig. 128) the propeller at point A in one revolution should reach
point C if there were no slip but as the slip cannot be eliminated, the
actual travel of the propeller will be from point A to point B. In two revolu-
tions the propeller will travel the distance A-D. The slip in question is a theo-
retical slip, that is, one which occurs when the propeller is working at its best
-when the blades are striking the air at such a small angle that a maximum
thrust is obtained with a minimum drag (in this case called torque)."
Paul Lamar ...No rotor no motor.
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