RE: [Jsbsim-devel] C-172 Performance

[Bill G. <bi...@ao...>]

> Do you have any suggestions on an approach for initial=20
> validation of our C-172 model? Any comments at all? I haven't=20
> thought about this much, but I might give some thought to=20
> formal approach at this, which really hasn't been addressed=20
> yet. My background has included some validation of military=20
> and space vehicle simulations, so this does interest me, and=20
> I'd be interested in more discussion.
>=20
> Jon

This is of particular interest to me. Detailed testing of the C-172 =
would
require detailed flight test results. In general, you would set up a =
script
to EXACTLY mimic the flight test procedure, such as weight and blance =
(crew,
equipment, ballast for CG control), power and control application (need =
time
histories of control inputs), and atmospheric test conditions (temp, =
wind,
pressure).

Let me digress for a moment. I have a lot of experience in flight test =
data
reduction and application to simulation validation. For standard tests,
there are standard test procedures that the pilot is SUPPOSED to follow,
although they don't always follow. For an in-flight test, a particular
dynamic test might require the pilot to trim the aircraft to a =
particular
condition before performing a dynamic maneuver. The pilot might be =
sloppy,
winds may affect ease of trim, or the test point might be particularly
difficult to trim to. If you have time history information available to =
see
if he was trimmed, and you trim your simulator to the exact same =
conditions
(a slight rate of decent if that's what the pilot flew, etc), then apply =
the
same input that the pilot did. If your model is accurate and you have
faithfully replicated the pilot actions, you should get the same =
response.

Think about this: If you applied full elevator deflection from the very
start of a takeoff run, versus applying it at 45 knots, you will get
different results, right?

So, back to the C172 issue. Unless your shirt says "Cessna" on it, the
chances that you have this flight test data available are pretty slim.

So, what is the solution here?  Well, I've know guys that will do "bar =
stool
and yard stick" flight tests (yeah, really using a bar stool and a yard
stick for their instrumentation). You spend 10-20 hours in the air, with
some simple measuring tools (stop watch, hand-held force guage) and a
clipboard and record data this way. Probably not going to happen here.=20

Therefore, you are stuck digging through POHs, Jane's All The World's
Aircraft, and anything else that mentions the words "Cessna" and =
"C-172".
You have to decide if the information provided is valid for your =
aircraft,
and if there is enough information to be useable (set-up, conditions,
procedure, etc). You compare your results with generalize curves in the =
POH,
keeping in mind that you might perform better than the POH predicts.
Phugoid, short period, steady heading sideslip, flap response... Yeah, =
good
luck. You won't find any useful data.



Just as an aside (since the fingers are typing fast this morning), =
buying a
flight test package doesn't mean that the data is good. I worked on a
simulator for an H-60 helicopter. We were fortunate enough to get the =
raw
flight test data digitally, so we could look at ALL the channels of data
collected (128 channels or more, if I remember correctly). When the =
aircraft
was supposed to be trimmed, some channels indicated that it wasn't. We
traced it down to accelerometers that didn't read zero when sitting on =
the
ground, so we had to subtract out these biases from the flight test =
data. If
this wasn't done, you'd get some very different results. The pilot might =
be
asked to input a 1 inch step input and hold it, but flight test data =
showed
that he might overshoot to 1.2 inches, then release back to 1 inch, but =
the
stick would continue to drift back to 0.5 inches. If you input a perfect =
1
inch step rather than what the pilot input, you'd get different results. =
By
mimicing the exact setup and inputs as best as we could, the resulting
simulator turned out to be an excellent simulator, and the flight model =
has
been used on subsequent H-60 simulators without change.

Bill