Hal, Oops! Sorry I attributed code to you that came from somewhere else. :-) I now seem to recall experimenting with your model at that time - actually replacing the older one in JSBSim cvs with yours, then adding the estimated propwash effects.
Hal wrote:
These were never in any of the code I worked with and were
removed before I started working on the FDM. My current Cmde function looks
like this:
<function name="aero/coefficient/Cmde">
<description> Pitch_moment_due_to_elevator </description>
<product>
<property> aero/qbar-psf </property>
<property> metrics/Sw-sqft </property>
<property> metrics/cbarw-ft </property>
<property> fcs/elevator-pos-rad </property>
<table>
<independentVar> velocities/mach </independentVar>
<tableData>
0.0000 -0.9
0.66 -0.6
0.74 -0.4
1.0000 -0.05
</tableData>
</table>
</product>
</function>
This is using the qbar-psf which is not influenced by prop
wash. The Cmde function Jon has above has a lookup table that goes from MACH
0 to MACH 2 in a linear fashion. This looks like something intended for a
supersonic aircraft and is not what I would expect from a subsonic aircraft.
The table I am using goes from MACH 0 to MACH 1 and has a strong inflection
at MACH 0.74 which is unlike the one in Jon's function since it is
non-linear.
Yes, the above mach effects table looks better. I can't remember where that
came from.
...
Jon thanks for the above code. I will look into integrating
this into the current P-51D. Also shouldn't the same sort of thing happen
with the rudder?
Yes - in fact, in the version I have in JSBSim cvs, the modified qbar is
used for: CLde, Cldr, Cmde, and Cndr. I haven't tested any of these for
validity, though.
And Jon do you have any ideas on how to go about writing a
function to implement downwash pitch moment affects?
Yes, that's another thing that could be done. I've thought about that
sometimes, too. When the wing is generating lift, the airflow is being
deflected downward behind the wing, and so the alpha that the tail sees is
affected, since the normal airflow has been given an additional component in
the body Z direction (downward from the pilot perspective). Also, if there
is an appreciable pitch rate, the alpha at the tail is affected. Finally, if
the propeller is producing thrust, then there is that affect, too. So, how
do we calculate the alpha at the horizontal tail?
I'm making this up right here as we go, but here are my thoughts. First,
calculate the wind velocity that the H. tail sees:
U_adjusted = U + U_prop
Next, the Z axis velocity that the H. tail sees:
W_adjusted = W + q*ht_arm - i_wing
Where W is the aircraft Z axis wind velocity at the CG, q is the pitch rate,
ht_arm is the distance from the CG to the horizontal tail, and i_wing is the
induced velocity produced by the wing. At zero lift i_wing would be zero,
and at lift=weight (such as at cruise) the i_wing value would be some value
- and it might not be trivial to calculate what that would be, though you
might be able to estimate it to produce a plausible qualitative effect.
There is a NASA paper that might be helpful in calculating this: "The
Calculation of the Induced Velocity Field of a Wing". It is the translation
of a technical paper by Klaus Gersten. It is NASA Technical Translation TT
F-12, 436, and you can download it at this URL:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690023298_1969023298.
pdf.
Anyhow, then, of course, the calculation is just this:
alpha_tail = atan2(W_adjusted, U_adjusted)
Maybe I've gone wrong somewhere here, but something similar might work.
Also, in situations like a flat spin or tail slide this probably falls
apart!
Jon
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