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1. 04:20 PM - Cowling: Percent power. How to calculate? (teamgrumman@aol.com)
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| Subject: | Cowling: Percent power. How to calculate? |
Please read and make comments and recommendations if you have any
additional information.
OK, so, the FAA has been working on the paperwork submitted by myself
and the DER for the last month or so. There seems to be a hitch in the
giddy-up: I used the POH to compute percent power for a 30F OAT, 5000
feet, and 10.8 gph for 75% power. (Note: altimeter setting was 29.84
at 120 MSL with an OAT on the ground of 52F)
Problem 1: The POH I used is not an FAA approved document.
Apparently, the FAA wants the equivalent computation
based on the charts in the Lycoming Engine handbook. They could not
tell me if the handbook was or was not FAA approved. It isn't marked
as such.
Problem 2: The Lycoming Engine handbook itself.
The problem is, based on feedback from LyCon Engine
Rebuilder's engine/performance specialist, that the charts in the
Lycoming handbook are built from full throttle operation with a
constant speed propeller used to limit RPM at a given manifold
pressure.
I've researched articles from the Lycoming website. Specifically,
"Lycoming Flyer." Case in point: From Lycoming Flyer, General
Operation, page 22-23 (Note: material in "quotes" is quoted from the
Lycoming Flyer article.)
"As an example, the standard fixed pitch propeller supplied with an
aircraft may allow the engine to dev
elop 180 horsepower at 2700 RPM at
full throttle, in flight at sea level, with a standard temperature. The
Lycoming O-360-A Series normally aspirated engine illustrates this
example."
For the test plane used (65 inch pitch), we could easily exceed 2700
rpm in level flight at 5000 feet. Therefore, we had to reduce manifold
pressure (throttle) just to maintain engine operation below the 2700
rpm redline. (Note: the 65 inch pitch propeller is the maximum pitch
certified for this engine/plane combination)
"Next, let us assume that this same engine/propeller combination is
operated at 75% power with a best economy fuel/air mixture setting.
Again, assume sea level and standard temperature to simplify and
standardize the discussion.
75% power will require about 2450 RPM with a brake-specific fuel
consumption of .435 pounds per brake horsepower hour. Also, 75% of the
180 rated horsepower is equal to 135 horsepower. Fuel usage at this
power and mixture setting will be 58.7 pounds per hour or 9.8 gallons
per hour."
Again, this is based on sea level operation. At 5000 feet, more
throttle is required, i.e., fuel flow, to obtain 75% power. The only
tool available to the owner/operator of the plane is the POH. So, now
what?
"With this information as background, it is easy to see that setting a
desired power with a fixed-pitch propeller can only be accomplished if
the pilot has a chart tha
t applies to the specific
aircraft/engine/propeller combination. Although the power chart for a
new aircraft may come from data obtained by test flying with a
calibrated torque meter, a fairly accurate chart can be derived for any
fixed-pitch propeller and engine combination. Briefly, this is done by
finding the maximum available RPM at any particular altitude and
applying data from the propeller load curve.
To conclude, the purpose of this article is to make readers more aware
of some operational aspects of the fixed-pitch propeller. Usually, it
is only necessary to accept the material provided by the airframe
manufacturer and to use the engine/propeller as directed."
As quoted by Lycoming in their own reference, "the airframe
manufactures data should be used. "
=======================
Gary
PS, any feedback is welcome.
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