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Special Content from the Western Museum of Flight

YF-23 PAV2 Proof Load Test Fred Peitzman

Every USAF airplane type must undergo structural

testing to prove it is safe to fly before ever leaving

the ground for the first time. For production air-

craft this usually means building one or more extra

airframes and testing them to failure.

For a prototype program, like the YF-23, that

would add huge cost and time to the development

program so they allow one of the prototypes to be

used for a proof load test. This requires that one of

the prototype airplanes be successfully loaded up

to 110% of the maximum load the airplane will see

in flight before the planes are allowed to fly.

A major component of the Laboratory Test Organization at Northrop, which I headed during the

YF-23 program, was structural test. We were tasked with designing the test setup, instrumenting the

airplane, and conducting the proof load test. The test setup was basically a very large box, the edges

of which were 30-inch flange steel I-beams to take the large loads that would be imposed on the air-

plane. Unfortunately, no photographs exist since the program was classified. All files had to be

turned over at program conclusion and were apparently eventually destroyed. Sad loss for history.

This box was big enough to enclose the entire airplane, which is about 67 ft. long, with a 44 ft. wing-

span, and 14 ft. high. The sides of the box were open and the top had structural members to which

large hydraulic cylinders were attached. These were connected to members called whiffletrees (a

term left over from the gear used to hook teams of horses together!) that distributed the loads to the

various parts of the airplane. These were then attached to pads bonded to the surface of the airplane,

wing, fuselage, and tails. There were strain gauges applied to the critical parts of the airplane to as-

sure that the stresses felt by the structure were within acceptable limits.

The airplane that was proof tested was PAV-2 (Prototype Air Vehicle 2). This is the one we have at

the Western Museum of Flight.

The test was conducted in the same building where the aircraft was assembled. The aircraft was

rolled into place, all the structural load fixtures put in place, instrumentation checked out, and testing

begun. Loads were applied in small increments, with extensive checking of instrumentation readings

at each step. As the loads got closer to flight loads, the test drew more and more attention by pro-

gram personnel and management. After all, this was an airplane we wanted to fly - no one wanted it

to break! Once in a while you would hear a noise and everything would stop to make sure it wasn’t

something bad.

With all the sophisticated instrumentation, the

thing that everyone paid attention to was a simple

dial, like a clockface, that went from 0% to 110%

load. As we neared 100%, then passed it and con-

tinued to 110% there were lots of important peo-

ple holding their breath. When the needle hit

110% and the loads were released, there was a

cheer from the watching crowd. Success! The air-

plane could fly! Our Lab Test engineers and

technicians did a magnificent job.

Whenever I see our YF-23 on the pad at the east

end of the field, I think of that test -

sure glad we didn’t break it!

Photos, from top: • PAV-2 in flight over Mojave Desert. (Northrop Grumman) • YF-23 PAV-1 Roll out. (Northrop Grumman)

This article originally appeared in the August 2020 edition of the E-Supercruiser (Vol. 20.4).

The Western Museum of Flight honors and champions the Aerospace Industry Heritage of Southern

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