Originally Posted by
JohnDixson
The Army had, as a result of Vietnam experience, created a crashworthy design requirement document which they cited for the UTTAS Request for Proposal. Both Sikorsky and Boeing designed to these very new requirements for their UTTAS competing aircraft. About the same time both of us were ready for first flight, they published these criteria as Mil-STD-1290. You can look it up and get an idea of the difference between designing for these combat related design crash survivability requirements, and where the various “ other “ helicopter structural design requirements are. One that stuck out was that as a fallout of the drop requirements in para 4.2 and the table 5.1 and following is that we had to drop the fuel tank, full of fuel and enclosed in the same structure enclosing it in the real aircraft, from 65 feet and not spill anything.
DS re the 42 ft/sec. Not sure I recall that number from the spec data ( and that could be age interference, I admit ). But we did have an occurrence during the test program wherein we overdid it a bit and smacked the tail down with a tail wheel vertical velocity of 42 ft/sec. You may have seen the video, as marketing put it out all over the place.The thing about it was that it occurred at 1530hrs local in CT. It was 652, ship 3. Sent the asst crewchief inside the tail three times looking for structural issues-there were none. The tail wheel tire was flat and the rim needed replacement. And, the rear 1-2 inches of the stabilator trailing edge was scrapped and bent up slightly. We did not have one and so the stabilator trailing edge was straightened with a pair of vice grip pliers and 652 flew at 0630 in the morning. There is a story about the asst crewchief going back into the tail the second and third times, but it has to do with human nature and not the technical facts. DS, this might be the source of that 42 ft/sec number?
John...
I recall seeing stills of the landing, but not sure I ever saw the video. Thanks for, as usual, adding plenty of meat to the bones of that story. I'm sure every development program has had many events like that, and of course the UTTAS was no exception.
I'm not a crashworthiness guy, and didn't remember the actual specs. A Google search quickly found USAARL Report No.93-15, "Basic Principles of Helicopter Crashworthiness by Dennis F. Shanahan." It's not a very clean reference, but it does refer to the "42 ft/sec design pulse." and that number rang a bell with me. But it seems questionable that the inadvertent demonstration would precisely meet the spec requirement.
If my maths is correct, a 65 foot drop corresponds to a 65 ft/sec impact velocity. The energy in the test, which goes as the square of the velocity, would be over twice that of a 65 foot drop (96/65)^2 = 2.16.
It would be interesting to know if the test that is the subject of this thread closely duplicates an actual accident, or is being used to gather data and perhaps validate a mathematical crash model. We'll likely never know.
DS