Andy Ross tried again!
> Vivian Meazza wrote:
> > As does this (2):
> > cruise-speed="308" cruise-rpm="2850"
> >
> > This does not (3):
> > cruise-speed="308" cruise-rpm="1360"
>
> Again, these are *wildly* different propoellers you are
> specifying. The second one is going to end up with four (!)
> times the force coefficient. In general, multiplying any
> number in the configuration file by a factor of two and
> expecting the aircraft to perform similarly just isn't going
> to work. Is there another typo? What are you trying to accomplish?
>
> Andy
>
The engine I'm trying to specify developed 1140 HP at engine revolutions of
2850 rpm at a boost pressure of 9 psi. It was fitted with 1:0.477 reduction
gearing, which I think means that the propeller turned at 1360 rpm. Thus, if
I have understood all of the various emails correctly, leads to a
specification file:
eng-power="1140" engine power output = 1140 HP
eng-rpm="2850" @ 2850 rpm (supercharged)
turbo-mul="2" Turbo multiplication factor = 2
wastegate-mp="48" Boost Control Valve = 48 in Hg
absolute
cruise-alt="17500" Cruise altitude = 17500 ft
cruise-speed="308" cruise speed at cruise altitude = 308 kts
cruise-power="1140" Power absorbed by propeller at cruise = 1140
HP
cruise-rpm="1360" Propeller cruise rpm 2850 * 0.477 =
1360 rpm
takeoff-power="900" take off numbers
takeoff-rpm="1200"
manual-pitch="true"
gear-ratio = "0.477" reduction gear of 1:0.477
I'm reasonably confident that the numbers are in accordance with the
published data for the engine. This results in a YASIM output;
Iterations: 1
Drag Coefficient: 1000.000000
Lift Ratio: 1.000000
Cruise AoA: 0.000000
Tail Incidence: -0.00000
Approach Elevator: 0.000000
CG: -2.528, 0.000, -0.270
FGFS locks up attempting to run with these settings, not unexpectedly.
I made an alternative assumption as an experiment: that cruise-rpm was the
engine rpm at cruise - 2850. They are also about the lowest values for which
YASim converges. With these settings, YASim converges with these results:
Iterations: 2320
Drag Coefficient: 6.279826
Lift Ratio: 360.380524
Cruise AoA: 0.770977
Tail Incidence: -0.8144328
Approach Elevator: 0.939014
CG: -2.523, 0.000, -0.276
FGFS runs with this input, but when throttle = 1 engine rpm = 6928, which is
as expected for a propeller rpm of 2850 and a gear ratio of 0.477. I had
hoped to see the engine rpm stay constant, and the propeller rpm to drop,
but, as I say, I was just experimenting. Apart from the engine rpm the
model performs well with these settings. I think we have to assume that
either the published engine parameters are outside YASim's calculations in
some way, or that I still have some fundamental misunderstanding of what
goes where in the file.
Sorry to be a nuisance with all these queries, and thank you for your
patience and help.
Regards
Vivian
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