implicitly correcting for that yet. The plane now makes it to 11000ft on
autopilot *slowly*. I think I'll have to look at the prop parameters
next - altering the diameter and blade angle just slightly can have
quite an effect. If you want to play then try changing the values
prop_diameter and blade_angle in FGNewEngine::init. Keep blade
angle between 20 and 25 (since those are the only values for which
I've entered data and I interpolate between them) and ignore
FGProp1_Blade_Angle since that's an old variable that isn't used. I
really ought to read all the engine and prop parameters from file to
avoid recompilation when tweaking !
the actual air pressure and temperature from the LaRCSim model
instead of assuming that it is at sea level as before. This has
reduced the ceiling from over 60000 ft to about 9000 ft. This is a bit
low (should be around 13 - 14000 ft I think) but I still have some
stuff to do with the engine power correlation and its ignoring the
temperature at the moment so I'm not panicking yet :-)
I've also changed the mixture-power correlation to one from a
published paper since the curve from the IO360 manual seemed to
be a load of rubbish, and didn't have any numbers on the mixture
axis anyway.
I've also knocked the full rich mixture down a touch in line with
Riley Rainey's recommendation, and cleaned up the code a bit.
Added fuel-flow and total fuel to the LaRCSim model. Its still a bit
rough for now but it works, except the engine dosn't stop when fuel runs
out at the moment since there's no refuelling capability in the sim just
now. It takes about 4 gallons use before you see the fuel guages begin
to drop since there's 28 gal per tank but the guages go to 26.
Basically I've rewritten the prop model along similar lines to how
Jon has done his - using published efficiency and coefficient of
power data. It works *much* better - try pulling the throttle back
to idle and putting the plane into a dive before and after updating
and you'll see what I mean. It doesn't require a fudge factor either
:-)
bring EGT down to a more reasonable range. EGT is now returned in
deg Fahrenheit (yuk!!) by the accessor function since that is what
the guage is calibrated in, and the absolute max value that can be
output (max power mixture at max power) is about 750 deg F. Dave, I
suggest that you set the guage to run from 450 - 750 deg F between
the four big marker ticks. What do the offset and scale actually
refer to in the .xml config file BTW?
Fuel flow, better handling of manifold pressure wrt engine speed, and
proper consideration of altitude effects next, hopefully.
temperature. The cylinder head is assumed to be at uniform
temperature. Obviously this is incorrect, but it simplifies things a
lot, and we're just looking for the behaviour of CHT to be correct.
Energy transfer to the cylinder head is assumed to be one third of the
energy released by combustion at all conditions. This is a reasonable
estimate, although obviously in real life it varies with different
conditions and possibly with CHT itself. I've split energy transfer
from the cylinder head into 2 terms - free convection - ie convection
to stationary air, and forced convection, ie convection into flowing
air. The basic free convection equation is: dqdt = -hAdT Since we
don't know A and are going to set h quite arbitarily anyway I've
knocked A out and just wrapped it up in h - the only real significance
is that the units of h will be different but that dosn't really matter
to us anyway. In addition, we have the problem that the prop model
I'm currently using dosn't model the backwash from the prop which will
add to the velocity of the cooling air when the prop is turning, so
I've added an extra term to try and cope with this.
In real life, forced convection equations are genarally empirically
derived, and are quite complicated and generally contain such things
as the Reynolds and Nusselt numbers to various powers. The best
course of action would probably to find an empirical correlation from
the literature for a similar situation and try and get it to fit well.
However, for now I am using my own made up very simple correlation
for the energy transfer from the cylinder head:
dqdt = -(h1.dT) -(h2.m_dot.dT) -(h3.rpm.dT)
where dT is the temperature different between the cylinder head and
the surrounding air, m_dot is the mass flow rate of cooling air
through an arbitary volume, rpm is the engine speed in rpm (this is
the backwash term), and h1, h2, h3 are co-efficients which we can play
with to attempt to get the CHT behaviour to match real life.
In order to change the values of CHT that the engine settles down at
at various conditions, have a play with h1, h2 and h3. In order to
change the rate of heating/cooling without affecting equilibrium
values alter the cylinder head mass, which is really quite arbitary.
Bear in mind that altering h1, h2 and h3 will also alter the rate of
heating or cooling as well as equilibrium values, but altering the
cylinder head mass will only alter the rate. It would I suppose be
better to read the values from file to avoid the necessity for
re-compilation every time I change them.
and prop inertia and passed the timestep from LaRCsim in order to have
the engine rpm behaving according to the applied torque and the laws of
physics.
