How Much Load for a Load Test?

Neal Fulco wrote:
On Jul 4, 3:27 pm, Brian Whatcott wrote:
wrote:

...





Glad someone raised this topic. Too lazy to do it myself. Assuming
a LSA gross wt. aircraft (1320 lbs.) with a wing that weighs 200 lbs.
and you want to test the wing for 4 g's. 4 possible
scenarios.....The first three I have seen in "reputable"
publications...you know...the ones you have to PAY for. The 4th one
is the only other one I could think of. I think we've decided #1 is
wrong. Turning the plane upside down and supporting the frame on the
fuselage. How many bags of manure do you pile on the wing to test.
Assumptions: each bag of manure weighs 1 lb.
Scenario #1 1320 x 4 ( 5280 )
Scenario #2 1320 - 200 x 4 ( 4480 )
Scenario #3 1320 - 200 x 4 + 200 ( 4680 )
Scenario #4 1320 x 4 - 200 ( 5080 )
Neal F.
What a well-phrased question! :-)

It would be most reassuring, if one used the heaviest load, no doubt.
But you have specified a test only of the wing.
This wing must carry not only its own weight multiplied by the test
acceleration, but that of the rest of the airplane's gross weight
multiplied by the test factor.
As mounted in the test rig, it is already loaded with one wing weight,
so we should add another 3 wing weights and four weights representing
the aircraft gross less wing weight. What does that come out to, I wonder?

The load mentioned above represents
3 X 200 +
4 X (1320 - 200) =
5080 lbs.
As you can see, this is just another way of expressing your Scenario #4
This is not quite the heaviest option.....

But wait: that 3 times wing weight does not have to be reacted through
the wing attach at all.
That's one reason why fuel tanks in wings are favored - the wing lift
can react the fuel load locally, and that's a less stressful job than
holding the tank up on the end of a (more or less) long lever arm.
So we COULD allow ourselves the benefit of discounting the accelerated
load due to the wing itself, though the wing could still collapse say by
crushing or buckling, lets allow a 50% reduction.
Than we would load the wing with
3 X 200 X 50% +
4 X ( 1320 - 200 - (200 X 50%)) =
(thats an allowance for the self-support of the OTHER wing too)
total 300 + 4080 = 4380 lbs
Well, would you look at that: the smallest load of all! :-)

And that's when we start considering the merits of Barnaby Wainfan's
flying wing, made with flat panels, to boot. The WHOLE weight is locally
reacted by the wing.
How strong is THAT!

After considering the merits, you are going with the biggest number -
it's your life, after all.

Brian W- Hide quoted text -

- Show quoted text -

Check out this website for Irv Culver's design notes for his Windrose
sailplane. Looks like he's using Scenario #2. Laws of physics being
what they are...there's only one correct answer ( Scenario ) Trick is
finding which one is right.

www.continuo.com/windrose/culver.htm

Neal F

Hmmm....I can only offer the trite old aphorism:
Circumstances alter cases."


Best

Brian W