Re: [Jsbsim-devel] C-172 Performance

[David M. <dav...@gm...>]

On Wed, 13 Oct 2004 09:11:32 -0400, Bill Galbraith <bi...@ao...> wrote:

> 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).

I think that would be pretty pointless anyway -- we'd be measuring
with more precision than the performance difference among individual
planes in the fleet.  There can be drastic differences between planes
of the same make and model, depending on how new the paint job is, how
worn the tires are, how recently the plane was rerigged, how worn the
cylinders are, etc. etc.  This is especially true for FBO rental
workhorses like the Cessna 172 -- it's not unusual to see a 5-10 knot
difference between two 172p's.

> 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?

A lot longer takeoff run because of the drag from the nose-high
attitude.  Ditto for flap usage -- the POH says to apply flaps before
the start of the takeoff roll, but many pilots apply them just before
rotation so that they don't have the extra drag during the initial
acceleration.

> 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.

And if it does, you're probably bound by a non-disclosure agreement anyway.

> 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.

Not for most makes and models, but we do have a lot of pilots on the
list and people can and do contribute in-flight data for the simpler
planes.  I remember that Alex Perry did some tests a few years back
for the 172, and I did some for the PA-28-161.

> 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 so -- for phugoid, dutch roll, yaw-roll coupling, roll rate, etc.
we have to rely on general impressions -- does it feel about right to
pilots who have actually flown the plane (difficult to measure, since
computer controls move so differently), and does its handling match
published pilot narratives (i.e. "tends to spin", "docile in stall",
"needs a lot of rudder in a turn", etc.).

For some models, we do have information available from published
sources.  For example, Roskam publishes linear coefficients for quite
a few aircraft, though they come from a computer model rather than
actual flight tests.  NASA also publishes some data.


All the best,


David

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http://www.megginson.com/