Special Content from the Western Museum of Flight

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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 aircraft 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 airplane. 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. wingspan, 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 assure 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 program 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 continued to 110% there were lots of important people holding their breath. When the needle hit 110% and the loads were released, there was a cheer from the watching crowd. Success! The airplane 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 California and inspires, motivates, and educates the dreamers and creators of today and tomorrow.

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