Stupid question about flight testing and "the envelope"

On Sat, 20 Dec 2003 15:41:47 GMT, Ed Rasimus
wrote:

On Sat, 20 Dec 2003 09:31:05 GMT, Scott Ferrin
wrote:


In this month's "Combat Aircraft" there is an article on the F-22 and
in it one of the people invloved in flight testing says "Flying at the
edge of the envelope pushes this aircraft harder than any operational
F/A-22 will ever be pushed." Is that hyperbole? There are instances
in the past where aircraft have gone FAR beyond the brochure numbers.
So do they go *beyond* the brochure numbers or are they saying they go
to the typical +9 -3 g's and the flight system prohibits anything
more? Obviously they aren't going to test it to failure in *flight*
but every componant has a design spec and a safety factor. Do they go
*beyond* spec but not beyond the safety factor? Just curious.

The "envelope" is a lot more than simple structural limits. Flight
test is an extended process of gradually expanding the allowable
operating limitations of the system. First flights, for example, often
are done without cycling the landing gear, IOW, a takeoff, low speed
climb to altitude and basic exploration of approach and landing
airspeed.

As testing proceeds, airspeed, bank angles, altitudes, AOA ranges and
accelerations are expanded. Pushing the envelope involves moving those
lines on the performance charts gradually outward to expand the
capabilities of the airframe.

As for structural limits, those are typically explored to failure in
static ground tests. They used to have a big hangar down at Eglin
(probably at Edwards as well) that they would load up an airframe and
keep increasing the load until flex limits or destruction. Fascinating
stuff.

Yeah, on one of those Discovery shows they showed either a 747 or 777
wing being bent to failure. It was quite a bang. It didn't bend, it
pretty much blew apart. Then I remember reading that the machine that
was testing the that box-like structure the wings attach to on the
Tomcat busted before the Tomcat did. Neat stuff.