"Mike Marron" wrote in message
...
Scott Ferrin wrote:
Ed Rasimus wrote:

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.

Neat stuff, indeed. In fact, coinciding with the 100th anniversary of
the world's first powered, heavier-than-air flight, an exciting new
chapter in flight testing standards is being written as we speak.

Given the amazing success of Unmanned Combat Air Vehicles
(UCAV's) such as Predator and Global Hawk, the aviation industry
is currently working on flight testing standards for these unmanned
aircraft that are very similiar to those Ed described above for
manned aircraft.

The FAA has acknowledged the stunning growth of ultralight aircraft
in recent years and has found those of us involved in ultralights to
be a valuable resource for flight testing standards of pilotless
military and civilian aircraft. The goal is to allow unmanned military
and civilian airplanes to safely fly into the National Airspace
System in the near future.

As I mentioned above, flight testing an ultralight is not unlike
flight testing a sophisticated military jet. Once the design is
finalized, it has to conform to a given standard. For an ultralight,
this means it has to be loadtested to 6-G positive and 3-G
negative. Flight tests are also performed and documented.
Some of the flight tests include longitudinal stability tests, takeoff
performance, gradient of climb, dynamic stability, descent rate
and landing distance. All are done minimum/maximum weights
and at the different center of gravity positions available.

Several load tests must be performed in both positive and negative.
The limit load test must show that the wing is able to be held off the
ground indefinitely without suffering any permanent deformation (the
positive limit test on one wing I flew showed a measured load of
3,671 pounds). For example, here are the calculations for the 6-G
positive load test on the example I flew:

Aircraft, pilot, and passenger weight (less wing) =882 pounds
Multiplied by 6 = 5,292 pounds
Add wing at test weight = 105 pounds
Load to be applied = 5,397 pounds

The load has to be applied to a specific distribution which is laid
out by the controlling authority (e.g: the FAA). An aeronautical
engineer must witness the testing as is the signatory on the
documents.

A grid reference is drawn onto the wing. In this way the load can be
applied exactly according to the standard. Bags loaded with steel ball
bearings are used for the load. The bags have Velcro sewn in as does
the sail on the wing in order to prevent the bags from sliding off as
the wing is lifted.

For the positive load test, the wing is placed on the ground upside
down or undersurface up. A forklift is used to lift the wing from the
point where it normally attaches to the fuselage. A calibrated load
cell is used to measure the load (the measured load usually varies
slightly from the applied load).

In the case of the wing I tested in the example above, the final
measured load for the ultimate positive test was a whopping 5,345
pounds (remember, the wing only weighed 105 pounds so you can
imagine just how strong modern ultralight wings are!) The wing must
be lifted completely off the ground and held for more than three
seconds remaining intact.

I've seen photos of other wings that have literally exploded like the
Boeing 747 or 777 wing you mentioned above so if you fly a certified
ultralight these days you can fly with confidence knowing that the
wing has been thoroughly tested just like the wings the military and
the "big boys" fly.

Interesting process Mike.
Reminds me of a while back when Bellanca was demonstrating their Super
Viking at some of the shows we were flying. Bellanca has always been known
(at least until they had a major AD on the Decathlon's spar anyway :-) for
fantastic strength in their airframes. They had apparently hired a bunch of
professional Sumo wrestlers. They had these people standing on the wings
from the root to the tail as the airplane stood on the ramp behind the guy
telling everyone how strong the Viking was
.. Very effective sales technique....although I don't remember anyone buying
the exact airplane they were using for the demo :-)
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/ CFI Retired
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