Applied load factor

[Mike Switzer]

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)

[Kyle Boatright]

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.

[Mike Switzer]

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

[spungey]

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.

[Mike Switzer]

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...)

[Dana]

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.

[Matt Gonitzke]

... 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.

[Dana]

It can break at 9g, but must withstand the load for 3 seconds first.

OK, yes... but it can break instantly at 9.001g...

[Matt Gonitzke]

Make fun of it all you want, but that's what it says in regs.

[Dana]

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.