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Greg et Al, ;-)
Being nothing more than a big fan a prop roughly
follows the fan laws.
Flow varies directly with RPM
Pressure varies with the square of RPM
Power varies with the cube of RPM
I say roughly because a prop is not a "perfect
fluid machine" and therefore these rules do not apply when the prop is
stalled or when the tips start to go transonic. The efficiency of the blades
also changes with varying Re Number so this affects things a bit.
This also only applies to fixed pitch
props.
Tune the engine on the dyno, the dyno operator
should know how. Develop your fuel map for that. Then select your prop to absorb
the desired hp at the desired rpm based on the dyno runs and your airframe
performance and flight level. Fine tune in flight.
Airplanes are different because of the vast range
of air density encountered in operation. Short of a air brake dyno in
an altitude chamber, you are going to have to live with this.
In an airplane with a fixed pitch prop, the
engine will never see full SL power, unless you use a climb prop and fly lower
than 1000 ft all the time towing banners at WOT. The prop will limit power
output for takeoff and climb. By the time the airplane is flying fast
enough to absorb full power in level flight, you will be at 75% of 75% SL
WOT power and a low air density. Typically operating at about a 50% or less
duty cycle compared to WOT SL performance. Boats at SL are much harder
on engines than airplanes.
If you have a turbo, and a CS prop, none of this
really applies and you are going to have a thermodynamic adventure.
YMMV
Monty
----- Original Message -----
Sent: Wednesday, October 28, 2009 11:56
AM
Subject: [FlyRotary] Re: Prop Loads
Greg;
I had the
same dilemma when
running my engine on the dyno.
The primary reason
I wanted to simulate a prop load so I could do tuning of the EC2 on the
dyno. But that never happened. I generated an approximate prop
load curve but could never follow it – IIRC, it was because you can’t set the
load, and there is more than one RPM/MAP combination for a
given load. Dyno work is basically about generating WOT HP and torque
curves. You set the mixture according to the A/F ratio. And you always
learn some other things along the way – flow rates, EGTs, fuel burn,
etc.
And
Gary, I don’t think
prop load varies as the square of the rpm – does it? I think some
aspects, like thrust, go as the square; but the drag goes more like the cube.
I generated one both ways, and neither is the real
world.
Al
G
-----Original
Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Greg Ward
Sent: Tuesday, October 27, 2009 6:43
PM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Prop Loads
Finally in
the dyno room building the mount plate to begin testing. Tracy says that
the engine should be tested with the prop on, and this is kind of hard in a
dyno room. We are mounting the engine without the PSRU, so that we don't
tear it up in testing, and instead hooking directly to the shaft which is
loaded by means of a water brake. We can put any load on it that we
want, problem is, how to calculate that prop load in foot-pounds, at different
settings. Talked to Craig Cato, and he is leaving for Europe, so doesn't
have time to run the calcs, and I am just a dumb high country
nail-banger. Any thoughts?
Lancair 20B
N178RG in progress
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