Aviation Forum / General / Homebuilt / July 2009

So... how strong is your wing?

The only sure way to know is to TEST it.

How do we do that?  We roll the airframe over on it's nose, support
the main spar with a structure of some sort... Douglas Fir 2x4's works
okay... then we stack a known weight onto the wing.

Still confused?  No problem; just follow me through.  We already know
what the bird weighs., thanks to all those imaginary flights out to
Catalina and back.  Empty weight is 318 pounds..  Nine gallons of
gasoline is 54 pounds and one sorta-skinny pilot is 155 pounds, so
we've got 318 + 54 + 155 = 527 pounds... and we wanna see if the wing
will support 527 pounds.  (Yeah, I know... lemme work up to it.)

Start by removing the spinner and the prop, then bolt-on a wooden
bolster that weighs exactly the same as the spinner & prop.  Now we
whistle-up some help and we ever to gently roll the plane forward,
first onto the bolster and then onto the support structure that pokes
up into the cockpit and bears against the main spar about where my
legs would go.  The structure is kinda high because we dont want the
vertical stabilizer to touch the driveway.  But we finally get it
rolled over and supported on the structure we've made (now THERE was a
fun project).  Is it level?  We check it out.  Then we position a
couple of yard-sticks just off each wing tip. Now we cover the wing
with cardboard and start stacking on the weight.

What kinda weight?

Well... back in the Good Ol' Days, whenever that was, our local EAA
chapter would have a couple thousand pounds of lead weights all neatly
marked in matching pairs, and they'd deliver it and bust their backs
helping you do the Static Test (which doesn't have anything to do with
radio).  Nowadays your best bet is probably bags of Portland cement or
other building material ( ...such as Plaster Sand...  ) available in
bags, each marked with the weight of the contents.  ( Anywhere outside
of the USA it'll probably be marked in kilograms instead of pounds.
Not a probelm; just work it out. )

Now you lay the weights onto the protected surface of the upside-down
wing, starting in the middle and working your way out toward the
tips.  Five hundred and twenty-seven pounds is about six bags of
Portland cement so you'd have three bags per side.

Out at the wing tips, the marker aligned with your yard-sticks (meter
sticks across the pond) probably won't show any deflection at all.
Five hundred and sixty-four pounds ( that is, six times 94 ) equals
one g, which means you are cruizing along in level flight.

So what's your plane rated for?  If it's non-aerobatic it's probably
rated for Utility Class, which is about 3.3g,   Which means 3.3 times
517 or about 1706 pounds.  Which happens to be about 18 bags of
Portland cement, so that's what you stack on, starting at the middle
and working your way out toward the tips, keeping an eye on those yard
sticks.

Odds are, you won't have any problem at 3.3g's -- the wing probably
won't deflect at all.  Now all you've got to do is take a buncha
pictures and get everyone to sign the log.  Or you could keep piling
on the weight until something breaks.  That would indicate the
Ultimate Load for that particular structure but you gotta be careful
dealing with that amount of weight, especially if you're working in
somebody elses hangar, because when something fails it's liable to
flip those bags of cement around like a frisbee.
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A lot of times you aren't working with a finished airframe; lots of
times you'll only be working with a part of an airplane, such as it's
horizontal stabilizer, or perhaps an engine mount.,  That's when
things can get interesting, because you may be trying to achieve 8 or
9 g's.  That's when you'll want to have a couple of video cameras
running, because when the failure occurs it's liable to happen fast.

Sometimes you may be testing nothing more than a main spar, probably
bolted to a fixture you've designed to support it.  If the main spar
is made of wood there's a good chance that you've fabricated this
sample out of something less expensive than Sitka Spruce and aviation
plywood.

Or you could be testing the strength of a part with the wood at a
different orientation. (Which is what this message was all about to
begin with.)  :-)

Gravity and persistence can teach you a good deal about aeronautics
without ever leaving the ground.  You'll have to fabricate your
supporting structure and line-up a suitable supply of weight, but
having done so you may find there is more to aeronautics than you
realized...

I'll leave the next step up to you because when you get right down to
it, YOU are the mechanic-in-charge.

-R.S.Hoover