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Thread: Applied load factor

  1. #1
    Mike Switzer's Avatar
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    Applied load factor

    Question: I see many experimental designs advertised as being designed for 9 gs. Does this mean they are designing for the 6 g applied load factor required by the FAA for aerobatic flight x a 1.5 factor of safety, or are they using 9 g as the base load factor which x 1.5 means they are designing the airframe to handle 13.5 g ultimate loads?

    (it makes a big difference in airframe weight)

  2. #2

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    Mike, your first assumption (6 G design load, 9 G failure) is typical. Van's RV-3, 4, 6, 7, and 8 are that way.

  3. #3
    Mike Switzer's Avatar
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    Thanks. That's what I assumed, but you know what happens when you assume....

    While strength / weight is not really a 1:1 when using structural members, I'd hate to try to design something for 13.5 g

  4. #4

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    13.5 would be instant blackout city for most of us. 9 would almost certainly put me out too. When we can redesign humans for 12 g then we should design wings for 13.5. :-) Just my 2c.

  5. #5
    Mike Switzer's Avatar
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    I'm starting (preliminary) work on the fuselage geometry & I have to program the formulas into MathCad & Excel for the static analysis. (Starting with pencil scratchin's on paper, this stuff will change a lot before I am done)

    (I'm sure glad I'm not doing a composite or riveted aluminum structure, then I would feel like I had to do finite element analysis, and the last time I did FEA was 24 years ago (by hand, on paper), I don't ever want to do that again, even with software that i don't want to spend money on...)

  6. #6

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    I don't know what the kit builders are advertising, but if an aircraft has a 6g design load factor with a 9g ultimate, it should be advertised as 6g. The 1.5 safety factor is just that, a fudge factor for the engineering calculations, not a usable number a pilot should be concerned with.

    If you have a 6g design load factor, the plane only must not actually break at 9g... but it's allowed to bend past 6g, and stay bent.

  7. #7
    Matt Gonitzke's Avatar
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    Quote Originally Posted by Dana View Post
    ... the plane only must not actually break at 9g... but it's allowed to bend past 6g, and stay bent.
    It can break at 9g, but must withstand the load for 3 seconds first.

  8. #8

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    Quote Originally Posted by Matt Gonitzke View Post
    It can break at 9g, but must withstand the load for 3 seconds first.
    OK, yes... but it can break instantly at 9.001g...

  9. #9
    Matt Gonitzke's Avatar
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    Make fun of it all you want, but that's what it says in regs.

  10. #10

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    Of course. I was merely pointing out that the 9g number is not something that a pilot should ever plan on flying to; 6g (in this example) should be the maximum flight value.

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