On Tue, 30 Dec 2003 17:56:07 GMT, Scott Ferrin
wrote:

What's to argue about, anyway? If you know the drag, you know the
thrust you need. If you know the required thrust and you know the
engine, you know the fuel consumption. If you know that, you know the
range. What you do in flight test is find the drag and refine the
engine model. Then you hope the difference isn't so great between
what was predicted, which was enough to meet the specs, and what you
got that you can't come close enough to get a waiver. It's pretty
simple, really.

Do they EVER get a *pleasant* surprise? As in "wow this sucker has
300 miles more range than we thought"? It always seems like it's
heavier than they thought or less range or lower speed or weird
aerodynamics. Seems like nothing positive ever pops up.

Yes, they occasionally do. Not usually on weight or range, though.
Aircraft always end up heavier and draggier than designed. More lift,
better HQ, more thrust is more likely, but very uncommon. Maybe
better structural dynamics, like flutter, too.

The reason it seems that only bad stuff shows up in flight test has to
do with the purchasing process. The military gives the contractor the
contract based on the contractor's promise to build an airplane that
can do at least as well as specified in the contract. There's rarely
a reward for doing better.

So, things that don't meet the specs are a problem and have to be
fixed. They get the publicity. Things that exceed the specs are not
a problem in terms of the contract, so no one hears about them. (They
could be a problem in terms of design, that the contractor made
something too good, wasting money, since the idea on the contractor's
side is to produce an airplane that's just good enough.)

Mary

--
Mary Shafer Retired aerospace research engineer