On 12/15/2008 04:31 PM, Tom Betka wrote a long note making
a number of good points.
Thanks!
Let me join the discussion here:
> HP = [(2pi)*(Torque)*(RPM)] / 33000 [1]
Right. That's the formula I've been using when quoting
shaft horsepower numbers.
> ...one can easily see that the only factor we can do anything about once
> the aircraft is in the air, is the engine RPM. Thus it should be
> apparent that increasing the RPM by any significant amount will indeed
> increase the horsepower. And while there isn't a horsepower indication
> that I could find in the property tree for the C172P,
Not HP directly, but there are torque and engine-speed ("rpm")
numbers in the property tree, so equation [1] is easy to apply.
> there is an
> indication of the thrust.
You could get a power number from thrust * tas, but that
would not be a direct reflection of /engine/ power; it
would have propeller efficiency factored in. Let's take
it one step at a time, i.e. let's debug the engine before
fussing with the propeller.
Here is another hint as to where to look for o-320 bugs.
Consider the following two lines of data: Using the
c172p from the rc2 package, under standard conditions:
model eng/rpm torque power % p_alt d_alt ias tas
RoC/fpm mixture thrttl map static ff/pph bsfc
c172p 2365.6 -198.6 89.5 55.9 10019.4 10048.6 56.8 57.2
160.4 0.933 1.000 20.56 20.56 71.1878 0.7957
c172p 2746.1 -282.4 147.7 92.3 10116.3 10145.9 59.9 60.4
376.4 0.773 1.000 20.14 20.49 66.2622 0.4488
The main thing I intentionally changed from one line to
the other is the mixture. (Additionally there are small
changes in altitude and airspeed, but they are small and
unintentional, and based on tons of evidence not shown
here, they are do not affect the points I am about to
make.)
The first line has the mixture fairly rich. We now compare
the second line to the first line. The mixture has been
pulled back slightly, and we observe
++ the fuel flow goes down, as expected
++ the rpm and % power go up, as expected
++ the bsfc goes up, as expected.
However,
-- The idea that this engine could produce 92% of its rated
power at 10,000 feet under any conditions is quite unrealistic.
But never mind about that at the moment.
-- I call particular attention to the difference between the
ambient static pressure and the manifold absolute pressure
(MAP). The difference is zero on the first line, and more
than a third of an inhg on the second line.
Analysis: As far as I can tell this defies the laws of physics.
I don't see any way that a slight change in the mixture could
have this kind of effect on the static-map difference.
-- The change in RPM does not explain it. The difference is
zero on the first line, and if it changes in proportion to
RPM it should still be zero or near-zero on the second line.
-- The mixture control is not a choke. There is no mechanism
whereby pulling the mixture control reduces the airflow.
The first line tells us there is a zero-impedance path from
the ambient static pressure to the MAP, and leaning the
mixture is not going to create an impedance.
Remarks:
*) When testing this model, be sure to adjust the mixture.
This is a pain, because the adjustment is very fussy.
(This is in itself unrealistic, because the real-life
adjustment is not so fussy; there is a rather broad
peak ... but let's not worry about this at the moment.)
*) There is a chance that fixing the aforementioned MAP bug
would make the engine overall _less_ realistic, by giving it
more MAP and more power at altitude. If so, this means
there is an even larger bug living somewhere else in this
engine.
On 12/14/2008 05:37 PM, Ron Jensen wrote in part
>> The engine model (eng_io320) currently uses a BSFC of 0.37. A value of
>> 0.42 might perform more accurately. Lower values for BSFC increase the
>> engine's power per fuel unit.
Interesting. I see from the file that 0.37 is the "design"
number:
<bsfc> 0.37 </bsfc>
On the other hand, in flight, the 0.44+ number tabulated above is
about the best I have ever been able to do, at any altitude or
rotation rate. I'm computing my bsfc from what the property tree
reports for torque, rotation rate, and fuel flow.
There's a 20% discrepancy here. I'm not sure how understand it.
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