Help for "Sand Bag" Wing Load Testing

[ALMartins]

I'm looking for a good reference (book, report) on the
practical aspects of wing load testing of
light/experimental aircraft. I refer to the good old
"sand bag" method, where one spreads bags full of sand
on the lower surface of the wing supported "upside
down" by its structural fittings, to simulate the
spanwise and chordwise load distribution.

Does anybody have a good reference indication? Thanks!

 

[JohnboyFaye]

Aerodynamic excitation may be less forgiving than static tests. I used to use a forklift and scale and rules to evaluate racing sailboat parts but under real racing some failed after that. A finish that is less elastic than the wing might also help give you a stress map. I would LISTEN INTENTLY if it is a composite you are testing that is too near the limits.

 

[Sparweb]

A useful source of information on the distribution of lift on the wing would be NACA TM 948 [

http://naca.larc.nasa.gov

]. There are many more complex reports on calculations of load distributions out there, but this one is simple and conservative, and before the FAA was the FAA, the US aircraft regulators recommended it in their regulatory material. With this distribution known, you then multiply by the load factor you are using for your aircraft. Acrobatic aircraft use a factor of more than six (6), which is also multiplied by 1.5 (6x1.5=9) when testing to ultimate (failure).
Bear in mind that there's also a chordwise pressure distribution, and that you should have this properly represented, too. Wings don't just bend off, they also can twist off.
On an experimental aircraft, whose gross weight is 1000 pounds, you have to suspend it in a 15,000 pound crane or jack and get at least 6000 pounds of sand. That's a lot. I doubt you could get that much stacked on a wing before the pile tipped over. Using bags of lead shot, which are about 5 times denser, is more manageable.
Spread out the bags from root to tip, on each wing symmetrically + simultaneously (ie. get a few but not too many people to help you and make sure they pay attention to what they're doing). A suggestion I would make is to take measurements of how high the wingtips are off the floor when you start, and once you have 1000 pounds stacked on (in the correct 1g distribution), take another measurement. How far they went down should be a good predictor of how far they'll go down by the end of the test.

A brief structural analysis will tell you a ballpark number on how far the wing will bend during the load test. Knowing this before the test will tell you what to expect. If things are bending or twisting too much, stop, unload, think, check your analysis, think again, and try again.

There are a lot more things to think of, which are particular to a give wing's structure. I haven't looked, but maybe the old ANC-18 can tell you more. If this is your own basement design, test it for your own safety. If it's plans-built from scratch, do it anyway. If this is a kit project, trust the designer. They might have already done this.

P.S. If you break your wing, please don't blame me!


STF

 

[Wairau]

if you apply the airload only you will be subjecting the wing to higher loads than it experiences in flight. this is because of "Inertia releif' from wing weight which opposes the airload. ie If u are applying 6G of airload u should subtract 6* the wing weight. This is a complex issue as u need to determine the wing weight distbn both spanwise & chordwise.

all this means that airload only results will be conservative so if u experience a failure at a known bending moment, shear & torsion lods you can work that back to determine at what flight load factor that correspondes to for your aircraft.



Graham Murphy

 

[Sparweb]

You can see it done here:

http://www.aeris.ch/show.asp?catid=128&lang=1

STF