Maury Markowitz wrote:
Hello (all?),
Sorry, this is long...
I'm a private pilot, but I hven't been flying much lately due to a variety
of factor$, but I have been playing a lot of flight sims. One of these,
Fighter Squardron, is from an author who has an almost freakish attention to
detail. FS has a compete physics engine in it, planes are described as being
built up from parts, each part lists it's lift and drag curves, and the game
generates the flight characteristics on the fly. The result is one of the
most "live" feeling sims I've played.
It also has some gaping holes, a side effect of it being forced out the
door by their partent company. For instance, engine power is fixed and
constant at all altitudes. Right now it's in the process of being patched,
and as the resident armchair-physicist, I'm trying to help out on the damage
effects and engine performance.
I'm trying to come up with a system that the patch author can implement
that will give us something that's capable of simulating WWII aircraft
engines with a reasonable degree of accuracy - this is just a game, so 1st
order is good enough.
So I have some questions. I've been all over my various texts, but there's
a lot of questions they leave unanswered. Without further ado...
Assuming a normally aspirated engine for the moment...
with the throttle at 100% at sea level I get 1000hp (standing still)
with the throttle at 100% at 18kft, is it 500hp (standing still - a
mountain)
The question here is whether or not the power delivered by an engine is
directly related to the O2 content in the air, or if it's wildly non-linear.
If it's _fairly_ linear (a couple of percent) then I'll say it's linear.
same question, but throttle at 50%, sea level power is 500hp and at 18kft
250hp - or is that 500hp?
This is slightly more tricky. In the game you don't have an MP or
anything, just a big slider that goes from 0 to 100% (idle to full). So if
the engine can only generate 1/2 power, and I pull the power to 50%, should
I expect to get 25% power? I think the answer is "not in reality, but that's
the way we would like it to be!".
if I burn 10gph at 100% thottle at sea level, do I burn 5gph at 18kft?
- asked another way -
fuel consumption varies with power due to various ineffeciencies of the
engine, you burn a lot more fuel at 100% power than at 90% for instance. Is
this fuel/power curve linear with altitude - is it basically a constant
times the engine power being produced?
I've leaned out a lot of engines over the years, but I'm still not sure I
know the answer to this problem. AFAIK I'm leaning out because the engine
_can't_ burn the fuel so I'm simply removing it from the mix. Is there more
to it that that?
Ok, now I'd like to re-ask the same set of questions in the context of
various boosting systems.
if I add a supercharger with a crit alt of 18k, and set the thottle to
100%...
- am I using the same fuel flow as I would at sea level?
My current thinking is to simulate an engine as a device that has a max HP
and a fuel flow/power curve. The plane builder would then add on various
boost systems, each of which would list an HP boost for a given alitude (do
I need speed curves too, to simulate ram-pressure-recovery?). Will that get
me close enough for a game?
I guess the same questions are just as valid for jets and rockets (which
the game includes as little-teeny props!), but as I understand it these are
actually considerable easier. Fuel flow is directly proportional to thrust
for both, rockets don't have an altitude adjust, and jets do.
Maury
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