On Mon, 2008-12-15 at 22:15 -0700, John Denker wrote:
> 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.
Note that Tom is using the old engine model in 1.0.0 so his numbers will
be different, and horsepower is not available in that version.
> > 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.
The propeller has a huge impact on propeller performance. It is best to
consider them as a unit.
> 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.
Yes, this pressure drop is by design. The RPM has increased between the
first and second line by some 400 RPM, and the second line is 46 rpm
over the red-line. By design, the engine model's intake manifold flows
exactly enough air to satisfy the engine at maxrpm (2700 rpm in this
case). Exceeding the red-line begins to starve the engine for air and
the manifold pressure drops just as if the engine has been throttled.
> 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 change in RPM *IS* the explanation, as stated above, the RPM will
not affect full throttle manifold pressure _until_ the RPM exceeds 2700
RPM so you won't see a proportional change.
> -- 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.
No, the mixture does not choke the engine. Excessive rpm causes the
engine to draw too much air through the carburetor. The impedance path
is the carburetor.
1: Engine airflow = (RPM / 4) * Displacement * Volumetric Efficiency.
Our model defines maximum airflow for this engine as:
2: (2700 / 4) * 320 * 0.8 = 172800(in^3 min) or 100 cfm.
at 2743.1 rpm our engine is drawing 102 cfm, the intake system is
supplying 100 cfm, causing the manifold air density to drop, leading to
the indicated manifold pressure drop.
3: 100 cfm / 102 cfm = 0.98
4: 20.14 inHg / 20.49 inHg = 0.98
> Remarks:
> *) When testing this model, be sure to adjust the mixture.
Absolutely true. Also realize our engine is very robust and will
happily produce power at mixture levels that may destroy a real engine.
The cylinder head and exhaust temperature aren't perfect but they are
there.
> 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.)
Please don't blame the model for a cheap joystick. You do have this
mapped to a joystick axis right?
> *) 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.
As mentioned above, the MAP readings are not a "bug"
Thank,
Ron
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