Spar sheer test!

[Mr B....]

A few weeks ago at a contest my Marauder sailplane landed in a tree off field (well you could say crashed). I went to go see if I could find it! Well I did 50' up in a Pine tree! It took some doing to get it down and added some more damage to it. It was time to replace it anyway.

Last year I started to build the main spar with full width sheer webs 1/2" wide. The spars are 3/16X1/2 spruce top and bottom with the sheer web in the middle. I also added two strains of carbon 12K tow to the bottom of the bottom spar. Been doing this for a few years.

Well that's how I got to this point. I cut out the spar from one of the main panels, the other one is still in the tree. I have one of those really cool real jobs that I get to Mfg things from carbon fiber and other really neat stuff. We have a test lab at work with a machine that can test shear load on parts. I asked if they could test my spar from my plane. I got OH YES! We don't get to break things very often. I helped set it up for the test at a span of 24". Setting it up across the machine at a angle to get max span. It came out to be 18" on the inside of the support blocks.

The photos below show the set up, the break and the data.


Safe Flying!

Merrill Brady, Chief Designer*
AMA 276833 LSF 7247

M M Glider Tech
Po Box 39098
Downey, CA 90239

[jrerickson]

Merrill,

It's so fun to break things. Reminds me of my Engineering Physics class where we built bridges out of balsa. Simple trusses always won. Interesting to see the spar failed in compression (top break).

158 lbs for an 18" span...not bad! I think our big planes pull around 250 lbs on tow, or at least that's when the line rated at 250 lbs breaks. Can your company do tension tests and see if our winch line is up to snuff?

John

[MarkusN]

Can you give total height of the spar, so we get a chance to calculate the tension at which the spar broke? It's not often that we get a chance to gather real-world data.

John: AFAIK Spruce usually fails on the compression side.

[Mr B....]

It's on the data sheet as Depth, d 1.125"


Merrill

[BigTilly]

Nicely done. As a guy who reviews mechanical test data for a living in the design of full scale aircraft, just a couple of comments:

1. Your compression failure is strange in appearance. I would have expected an upper cap buckle to accompany the failure. Did you have carbon on the upper cap?

2. John, be very cautious in intepreting the load in the test directly as a structural load carrying capability of the spar. This was a point load with a simple support, and is not completely analogous to a flight load distributed over a wing.

3. Not to nit-pick, but you didn't perform a shear test. Technically, this is a beam bending test, which has an induced shear load. Its too bad I couldn't have seen your setup before you started, it would have been much more valuable if you could have done a 4-point bend.

4. The most encouraging portion of the test was the load-deflection curve. You may not realize it, but your spar is very damage tolerant. Even after max load, you still had considerable load bearing capability, and continued to strain considearbly. This is likely because the lower spar cap hadn't failed.

Great work, glad to see that someone has interest in the engineering side of the house. The real question from here is: Do you know what to do with this data?

[jrerickson]

Tilly,

I'm not projecting out data for what the total bending moment would be, I'm just observing his lab condition with a point load. The line question was something different and I should have stated that. I question the rating on some of the winch line we use. Everyone has a story about a "bad batch"; the breaking point seems a lot less than the rated value. This also can be due to "compromised" conditions of the winch line (fraying, thinning, etc.).

Like you, I'm a fan of real world data. If you've seen one of Merrill's planes, they are stout and can take a full pedal zoom. I know the spar is strong!

John

[ChuckA]

Thirty years ago, I saw a lot more wing failures. I used to examine the remains in trash cans after a contest and never saw a compression failure in the top spar or a tension failure in the bottom spar. The failure was either caused by bad grain or a shear web failure caused by bad glue joint or poor shear web fit. The top spar always failed in buckling and I don't recall a pure r a tension failure in the bottom spar.

[Andy W]

.. delamination, whether it be wood or composite, is what I see most..
.. and it's "shear" (although, as pointed out, this test isn't).. ;)
..a

[Mr B....]

Here are a few more pic's!

The spar bent just over 1" before it failed. With out the carbon tow it would have failed sooner. I have made a spar with carbon on bottom of the top spar and carbon on bottom of the bottom spar. I have not had a chance to test that one yet and hope I won't have to. It's still flying!!!!!! OH Canada!

Hope to see all flying in Visalia!


Merrill

[flying_flip]

It's still flying!!!!!! OH Canada!

Merrill
I hope you never get the chance Merrill :) I love my Marauder :D

Phil

[joesoar]

A slightly less modern way of testing...

Did this a few years ago on an AVA center section that had a separated/damaged D-tube.

Weight was (and still is) 140 lbs. ;) I had to jump on it several times to have it let go...

Joe