Mike Patey spar testing.

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mcrae0104

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As much as I hate to pick on Mike, since he's accomplished so much, this test is not meaningful--at least not in the way it's being presented.

From this test, one could derive the maximum bending moment the spar is capable of (assuming certain restraints from buckling). However, the load application does not represent the way loads are actually applied to a wing (point load at the end of the cantilever). The case he's testing is arguably more conservative than the real-world case(s), but this is just playing around in the garage, not engineering. He holds himself out as an engineer; did he forget his statics courses?
 

BBerson

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I thought it was interesting but some of his words were sloppy. Such as "fatigue moment" or whatever it was he said.
Point load at the end is the same moment as double if distributed. So his 800 lb point load goal would be 1600 distributed for the cantilever portion.
 
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Pops

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He designed the spar and modeled it in solidworks.
The test replicated the computer results.
So the extrusions passed quality control.
You are correct, but at least it's better than most of the other videos.
 

TFF

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What he did not do is correlate how a 1500lb pull equates to, just for sake of argument, a 10g wing. He proved the spar has whatever strength that it does with a one point load, but thats more of a crash/ground loop loading, not full flight load. I think the wing setup was pretty good, he needed more of a whiffle tree deal, but of course he would also have to pull 10,000lb or whatever, and that starts getting dangerous unless in a real lab.
 

Tiger Tim

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I think for the layperson watching he did an excellent job of showing just how a one-piece metal spar fails: you strain it a bit, it springs back, you strain it a bit more, it springs back, you strain it more still, it doesn’t come back and never will.
 

BBerson

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Not a huge safety factor.
He said 4000 pound airplane. So subtract 200 pounds for wings and you have 3800 non-lifting load.
3800 x 6 = 22,800 pounds (4g limit and 6g ultimate).
Assuming each wing has 1/3 on the outboard cantilever and 2/3 is on the inboard. So both wings is 6/3 total to distribute.
22,800 divided by 6 is 3600 pounds distributed for the cantilever. Or 1800 pounds concentrated end load for the cantilever. Not the 800 pounds he said.
 
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Kyle Boatright

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Not a huge safety factor.
He said 4000 pound airplane. So subtract 200 pounds for wings and you have 3800 non-lifting load.
3800 x 6 = 22,800 pounds (4g limit and 6g ultimate).
Assuming each wing has 1/3 on the outboard cantilever and 2/3 is on the inboard. So both wings is 6/3 total to distribute.
22,800 divided by 6 is 3600 pounds distributed for the cantilever. Or 1800 pounds concentrated end load for the cantilever. Not the 800 pounds he said.
You're skipping enough steps I'm challenged to follow, but are you accounting for the fact that the lift will be distributed on the outboard portion of the wing, not concentrated at the tip?
 

BBerson

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You're skipping enough steps I'm challenged to follow, but are you accounting for the fact that the lift will be distributed on the outboard portion of the wing, not concentrated at the tip?
Yes, I did.
The correct loading is 3600 pounds distributed on each cantilever section. Or you can do a point end load of half.
It's easier to test with point load as he did. I test my ultralight wing with 800 pounds of distributed or 400 end load.
The end load method is roughly same as gross weight.
edit: removed 11" spar depth comment.
 
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rbarnes

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I have a feeling Mike intends to the sandbag the whole wing and this test was more about validating the extrusion construction method and solid work's predictions of said extrusion vs a traditional cap and web riveted together.
 

Orange4sky

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This test is not meaningful--at least not in the way it's being presented.
He said at the beginning "I want to emphasize... this is to test how strong a current cub spar is compared to what Scrappy's gonna be and a relative load increase I'm going to be able to do." Then he describes the FAA's required test with fully assembled wing, sandbags, etc. In other words, this test was simply to compare the stock spar with his new one... and to the solidworks data... in the context of this simplified test.

Did anyone else notice the Draco RC kit in the background at 9:08? Fanbois gonna be squealing with joy!
 

proppastie

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Or you can do a point end load of half.
while you are correct for a uniformed cross section beam ......I am pretty sure my last wing panel would not be able to survive 1/2 the wing load at the tip......my spar is designed at any point to only accept the load outboard of that point and is very weak at the last wing panel as there is only so much distributed load on the last panel......my limit load for the last wing panel is 30 lb....my limit load for the 1/2 wing is 700 lb and I know my last panel would not take 350 lb.
 
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BBerson

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while you are correct for a uniformed cross section beam ......I am pretty sure my last wing panel would not be able to survive 1/2 the wing load at the tip......my spar is designed at any point to only accept the load outboard of that point and is very weak at the last wing panel as there is only so much distributed load on the last panel......my limit load for the last wing panel is 30 lb....my limit load for the 1/2 wing is 700 lb and I know my last panel would not take 350 lb.
Yes, your spar is tapered and can't be tested with a simple wing tip load.
 

mcrae0104

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He said at the beginning "I want to emphasize... this is to test how strong a current cub spar is compared to what Scrappy's gonna be and a relative load increase I'm going to be able to do." Then he describes the FAA's required test with fully assembled wing, sandbags, etc. In other words, this test was simply to compare the stock spar with his new one... and to the solidworks data... in the context of this simplified test.
Yeah, I get it. But he didn't even describe a sandbag test correctly--he said the load would be evenly distributed. Maybe we shouldn't be too hard on him for misspeaking (even for referring to pounds of pressure), but the roundabout method he's using doesn't appear to conform to the way a wing spar actually is designed. I know, it's a YouTube video, not a college course...

His methodology seems to be: a Cub spar is good for X, and this one is good for 2.5X, therefore the new plane's gross weight can be 2.5x a Cub. I guess that's OK if everything is the same as a Cub--same span, same strut attach location, same airfoil, same tail loads at extreme angle of attack, etc. etc. I don't know that that's the case, but Mike should.

If it's a test to validate that a real-world generic test case matches the same Solidworks generic test case (the deflection graphs he shows at the end), then it's reasonable to assume that the actual fight load cases in Solidworks should be reliable. That's fine as far as it goes. If it's a test to validate the ultimate strength of the new spar, that's fine too--but under the test conditions (generous lateral support) I'd be surprised if this simple test came out much different than the flexure formula would predict. That's what differentiates this test from a conventional test of an entire wing. One validates the flexure formula, which we have known to be valid for hundreds of years, the other validates all of the other variables that go into the wing design and construction.
 
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