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.

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.

This sounds much more reasonable. While I'm certainly no expert
in these matters I'd be pretty surprised if these highly paid
engineers and designers made such a horrendous error as to result
in a one third higher than expected drag figure.
--

-Gord.