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Thread: Applied load factor

  1. #11
    DBurr's Avatar
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    Aug 2011
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    17
    This is a big gripe of mine. Most numbers in aviation are meaningless without more detail, and let's face it, most of the numbers plastered in advertising have more to do with marketing than engineering. When a plane is advertised at '180kts cruise', is that TAS? CAS? IAS? At sea level or 10000 ft? What engine? 65% or 80% power? And so on. Same is true for loading. Is '6g' limit or ultimate? The specs should state that plainly. Naturally, they never do. Without clarifying details, I'd always presume the worst case. For example, if I read '6g', I'd think ultimate loading, since certified normal category loading is 3.8g limit, and with a 1.5 safety factor yields (there's a pun) around 6g ultimate. Designing for 6g limit loads is equivalent to the certified aerobatic load limit, and that means extra cost and weight. Most of the experimental planes out there are not designed for real aerobatic loads, even though you see folks looping and rolling them all the time. Done competently, those maneuvers are well within the normal category spec limits. Done poorly, the plane could get all bent out of shape really quick.

    Honestly, I expect this sort of spec chicanery in the advertising, but it does bug me when even the builder's documents for a kitplane don't cough up the proper numbers. Yes, build quality has an impact, but there are certain aspects of any design that are set by and known primarily by the designer, unless the builder wants to reverse-engineer the entire structure. Numbers like limit and ultimate loads, design dive speed, and the like. Publishing a V-n diagram for the airplane loaded and built as originally intended by the designer would answer most of the really important questions (Vso, Va, Vf, etc.)

    As far as I know, Vans doesn't release a V-n diagram either, though I wouldn't doubt that some builder has made one by now. I do know that Van's does a good job of telling users how to fly it's planes. They state right in their 'how-to-fly' document that most of the RV's are spec'd to 6g limit / 9g ultimate when flown below gross weight. (Something like 200 lbs below or so I think). The RV-9 and RV-10 are spec'd at the certified utility load limit of 4.4g and should not be flown as 'intensely' as the other RVs. If only other kit manufacturers would be as detailed and forthcoming with their numbers.

  2. #12
    David J. Gall's Avatar
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    Oct 2011
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    Sacramento, CA
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    11
    ...and the 1.5 safety factor only applies to metals and wood; composites need to go to 2X, various parts of the airplane even more.... See Neal Willford's article on stress analysis and safety factors in the back issues of Sport Aviation.

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