On Tue, 30 Dec 2003 07:33:44 GMT, Chad Irby wrote:

In article ,
Mary Shafer wrote:

On Tue, 30 Dec 2003 02:21:26 GMT, Chad Irby wrote:

You don't need a few thousand hours on an airframe to figure out the
fuel usage. It's a fairly simple thing called "math."

Once you know what the drag is, sure. But predicting the drag is
fraught with error, as previous aircraft have shown. The usual
failure in prediction is trim angle of attack. It's wrong, which
means that the horizontal is set at the wrong angle, so the trim drag
is higher than predicted and the fuel usage is, too.

I'm trying to remember which airplane it was that was sweating out
something like 250 drag counts between predicted and as-flown a while
back. They were moving antennas, fidgeting with the cg to change the
trim angle, smoothing the skin--all kinds of stuff. It must have been
the F-16, I guess.

That difference is between the theory of a plane on the drawing board
and one in the air. They're *flying* F-35 airframes.

Well, big whoop. They're flying instrumented pre-production
airframes, not "real" F-35s, by the way. The production F-35 may
differ a fair amount from the pre-production models.

Have they done the performance points yet? They're not usually done
very early in a program. As I recall, the performance airframe is
often the fourth or fifth to go into test, because the early airframes
are dedicated to more urgent issues, like buffet and S&C and HQ.

As for the predictions, they're not from the drawing board. Rather,
they're from models, either wind tunnel or CFD. The predictions are
pretty good in general, but it is possible to miss now and then.
There are certain parameters that are disportionately sensitive to
small perturbations. Trim angle, for example.

Those drag changes don't make for a 33% change in performance. The
problems with the F-16 were over a 5% to 10% range, and that was between
the design and the flying airframe, not between the early flying
airframe and the production model.

I beg your pardon? Can you point to any place where I said that there
would be a 33% change or a difference between the pre-production model
(that's what we call "the early flying airframe") and the production
model?

The problem with 250 drag counts is that it's a hideously big amount
of drag. Really, really big. No, not enough to steal a third of the
range, but too much to meet the specs. And this was between the
predictions and the pre-production airframes, but the data from the
prototype YF-16 had been used too.

Mary

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Mary Shafer Retired aerospace research engineer