On Sun, 2008-12-21 at 02:50 -0700, John Denker wrote:
> On 12/20/2008 10:39 PM, Ron Jensen wrote:
>
> > Here is a dynamometer test report of an engine intended for use in an
> > aircraft:
> > http://members.cox.net/alg3/Dynamometer%20test%20report.htm
>
> Ah, good, that's useful data.
>
> > Please note especially the RPM v. Manifold Pressure chart at the end:
> > http://members.cox.net/alg3/Dynamometer%20test%20report_files/image005.jpg
>
> > The shape of this curve is the reverse of the curves you've postulated
> > in your figure 3. It also remains above 28 inHg to nearly 5000 RPM.
> > Your proposed model appears unable to duplicate this feat, as your full
> > throttle line is below 0.94 (28 inHg / 29.92 inHg) MAP by 0.07 RPM.
>
> Then you may like these MAP (and power) curves better:
> http://www.av8n.com/fly/engine.htm
> The MAP curves stay higher longer ... and they are even concave
> down over part of the range, which makes them look more like
> the curves at that "alg3" site.
>
> The analysis runs parallel to yesterday's analysis, except
> that I used a smarter model of the throttle, namely a
> nonlinear "orifice plate" model. This is more plausible
> physics and generates nicer-looking curves IMHO.
At first glance that indeed does look better. There is still, however,
two big hurdles to putting this into FGPiston: K ("some constant") and a
("area of the orifice"). Given the data available in the configuration
file, how do we create values for K and a for all engines from the
smallest engines, like the Rotax582 in the Dragonfly to the Wasp R-5800s
in the DC6? And across the RPM spectrum as well, Rotax engines tend to
red-line in the 5800+ RPM range.
Also, in equation (5) a variable "O" appears without introduction.
> > While we're at it, please consider this dyno picture
> > http://aagearinc.com/supercharged_na.gif
> > Yes, its a motorcycle engine not an aircraft engine, but both function
> > according to the same principles and studying one will lead to
> > understanding of both.
> > The red line is a normally aspirated engine. You can clearly see the
> > power peak and fall off.
>
> OTOH please note the power output at the "alg3" site. The
> power is a monotone increasing function of revs ... indeed
> a strongly increasing function of revs ... something that
> the physics model has been predicting would happen under
> *some* conditions ... but heretofore has been, ummm,
> unappreciated, to put it politely.
>
>
> > The blue line is the same engine with boost. It produces linear power
> > to the top of the RPM run because it can breathe.
>
> Let's not pick-and-choose the data. You can't point to
> the alg3 data and say "the" MAP must remain high and
> then point to the motorcycle data and say "the" engine
> can't breathe.
>
> Feel free to model one *or* the other. Feel free to switch
> from one model to the other, if you switch cleanly. But
> it doesn't make sense to ride both horses at the same time.
There is no pick and choose. The inlet air pressure on the blue line is
boosted above ambient, and the intake orifice is probably larger, too.
If we drove the engine fast enough it too would start to fall off like
the red line, assuming it didn't fly apart first! These two lines
demonstrate power drop-off with throttling. The same engine develops
both curves, the only difference is air flow. I am suggesting we should
be able model both graphs by only changing theta(v)*a and A.
Thanks,
Ron
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