> Thanks,
> My question could be a question to Jon, Dave, and any JSB specialist.
> I just wonder, about, the opportunity to get profit of your work for
> developments on the JSB branch. The properties should be the same on
> the global FG level.
> Both FDM ->YASim ->JSBSim with a different "philosophic" approach should
> give the same end results.
> >
> --
> Gerard
See the FGPiston.h file in JSBSim. See the constructor in FGPiston.cpp for
exact specification in the new XML format (documentation pending). Here is some
data on what can be specified:
Models Dave Luff's Turbo/Supercharged Piston engine model.
Additional elements are required for a supercharged engine. These can be
left off a non-supercharged engine, ie. the changes are all backward
compatible at present.
- NUMBOOSTSPEEDS - zero (or not present) for a naturally-aspirated engine,
either 1, 2 or 3 for a boosted engine. This corresponds to the number of
supercharger speeds. Merlin XII had 1 speed, Merlin 61 had 2, a late
Griffon engine apparently had 3. No known engine more than 3, although
some German engines apparently had a continuously variable-speed
supercharger.
- BOOSTOVERRIDE - whether the boost pressure control system (either a boost
control valve for superchargers or wastegate for turbochargers) can be
overriden by the pilot. During wartime this was commonly possible, and
known as "War Emergency Power" by the Brits. 1 or 0 in the config file.
This isn't implemented in the model yet though, there would need to be
some way of getting the boost control cutout lever position (on or off)
from FlightGear first.
- The next items are all appended with either 1, 2 or 3 depending on which
boost speed they refer to, eg RATEDBOOST1. The rated values seems to have
been a common convention at the time to express the maximum continuously
available power, and the conditions to attain that power.
- RATEDBOOST[123] - the absolute rated boost above sea level ambient for a
given boost speed, in psi. Eg the Merlin XII had a rated boost of 9psi,
giving approximately 42inHg manifold pressure up to the rated altitude.
- RATEDALTITUDE[123] - The altitude up to which rated boost can be
maintained. Up to this altitude the boost is maintained constant for a
given throttle position by the BCV or wastegate. Beyond this altitude the
manifold pressure must drop, since the supercharger is now at maximum
unregulated output. The actual pressure multiplier of the supercharger
system is calculated at initialisation from this value.
- RATEDPOWER[123] - The power developed at rated boost at rated altitude at
rated rpm.
- RATEDRPM[123] - The rpm at which rated power is developed.
- TAKEOFFBOOST - Takeoff boost in psi above ambient. Many aircraft had an
extra boost setting beyond rated boost, but not totally uncontrolled as in
the already mentioned boost-control-cutout, typically attained by pushing
the throttle past a mechanical 'gate' preventing its inadvertant use. This
was typically used for takeoff, and emergency situations, generally for
not more than five minutes. This is a change in the boost control
setting, not the actual supercharger speed, and so would only give extra
power below the rated altitude. When TAKEOFFBOOST is specified in the
config file (and is above RATEDBOOST1), then the throttle position is
interpreted as:
- 0 to 0.95 : idle manifold pressure to rated boost (where attainable)
- 0.96, 0.97, 0.98 : rated boost (where attainable).
- 0.99, 1.0 : takeoff boost (where attainable).
A typical takeoff boost for an earlyish Merlin was about 12psi, compared
with a rated boost of 9psi.
It is quite possible that other boost control settings could have been used
on some aircraft, or that takeoff/extra boost could have activated by other
means than pushing the throttle full forward through a gate, but this will
suffice for now.
Note that MAXMP is still the non-boosted max manifold pressure even for
boosted engines - effectively this is simply a measure of the pressure drop
through the fully open throttle.
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