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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)
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.
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
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.
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...)
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.
It can break at 9g, but must withstand the load for 3 seconds first.Quote:
Originally Posted by Dana
OK, yes... but it can break instantly at 9.001g...Quote:
Originally Posted by Matt Gonitzke
Make fun of it all you want, but that's what it says in regs.;)
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.
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.
...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.