thinning down from 6061 to 2024

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Norm Langlois

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Bill you know I have fought this battle for years. Since back in 2007. I was reluctant then to join the EAA forums before, I had something to present more than a concept. Even then I had the Dana's and Victors.
Fortunately I had some actual help.
 

BBerson

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Yes, I know. But you and Boku and I have actually designed and built and flown something. So ignore most of the others.
 

Victor Bravo

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A nice, older, visually impaired lady was walking across the street looking for the local Wal-Mart. Someone ran out into the street and tried to help get her over to the crosswalk where the cars would be stopped.

The little old lady scowled and said "All I wanted was to find a Wal-Mart, and now you're trying to drag me in a whole other direction... if you don't know where Wal-Mart is then shut up and stop pissing on me".

Yes ma'am.
 

Dana

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Dana wants me to engineer first, then make something and test.
I am just making and testing ,concept. its far cheaper to buy a sheet of aluminum than to pay an engineer...If Dana could help he's not offering, He does not like triangles.
Believe it or not, education is even cheaper. Disclaimer, I haven't read the book, but have heard it recommended by those who have. And I am trying to help... help you from killing yourself by suggesting you do some learning about aircraft structural design, instead of saying that because you don't know how to do it, you don't need to do it. If that constitutes pissing on you, I give up.
 

pictsidhe

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Hi Norm,

You're definitely on to something with your trapezoid cross-section spar. It can have enough bending stiffness against both lift and thrust/drag, and enough torsional stiffness, that you need relatively little structure in the rest of your wing. It's a pretty elegant solution to a lot of the problems of ultralight construction.

However, I can tell that you are struggling conceptually to figure out how it reacts the stresses of flight, and I worry that you won't know when you're in over your head until it's too late.

The answers you seek are out there, and a little bit of math and some engineering would reveal it relatively easily. As a relapsed liberal arts dropout who learned beam theory in their 40s, I heartily endorse suggestions earlier in this thread to get some help to apply some actual engineering to the problems at hand.

As a start, I'd suggest going to have a good look at a Vans RV-series wing spar or something similar. What you'll see is a relatively light shear web in the middle (what you are calling a "diaphragm") with big fat bars of aluminum top and bottom. The overall effect is to replicate the sort of I-beam that engineers and ironworkers have known since the 17th century is the lightest way to create a beam that resists bending. Concentrating material at the top and bottom, and having as little as you can get away with in the middle, maximizes the strength and stiffness per unit weight.

As a counter-proposal to your proposed design revision, I'd suggest something like the attached. As you can see, it's the sort of trapezoid you're familiar with. But it also features strips of aluminum riveted on at the three vertexes. These bars would react the accumulated tensile and compressive stresses that result from bending.

View attachment 85063

As to how big those bars need to be, well, that'd take a bit of engineering, and I'm not qualified to do that without some help. I might take a whack at some estimates, but I can't promise anything.

As for your question about 2024 versus 6061 for the webs, the answer is not clear on the surface and is probably irrelevant. 2024 is stronger, of course, but it has the same stiffness as 6061, and stiffness is what is important for buckling resistance. And what you've been seeing so far is buckling failures. But 2024 is harder to bend and harder to work with, and switching to it might drive fabrication compromises that negate the greater strength.

--Bob K.
This is exactly how I thought that it should be changed, too.
Despite lacking BoKu's experience ;)

Seriously Norm, BoKu is one of the most knowledgeable people who posts here. When he tells someone else here (even me) that they are doing something wrong, he is almost always right... If you want to carry on with your empirical design, you can probably save a lot of work and broken spars by trying some of the suggestions from the more schooled or experienced here. Even if you aren't up to a full analysis of a thin walled beam, some more basic calculations would be better than nothing. Like looking at the maximum compressive stress on the apex of the spar.
Personally, I am in awe of your fabrication talent and would love to see it applied to a better design. I think your tri-spar is worth further development, but it does need further work. Yes, we are picking holes in your design, because we want you to build something better. Hopefully one of our suggestions will appeal to both you and the perverts here who actually enjoy calculating stuff.
These threads also help us with our own designs by seeing all sorts of ideas around that we haven't thought of ourselves.
 

BBerson

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Simple quick cantilever calculation is this:
Assume 500 pound gross weight, 26 feet span, 12" deep spar at fuselage, 30 pounds each wing.

500-60 wing weight = 440 load at 1g (Inertia relief neglected)
440 X 6 g (ultimate) = 2640 pounds total or 1320 each wing.
So one wing is 13 feet and let's neglect the last foot at tip because of tip loss call it 12 feet.
Put the concentrated 1320 pounds at midwing = 6 feet (conservative, distributed load is less stress)
The 6 foot moment lever arm and 1 foot thick spar results in increasing the 1320 load by 6 times.
So 6x1320=7920 pounds force compression in upper cap and 7920 pounds tension in lower cap.

For 6061-T4 (14,000 p.s.i. Compressive yield) we need: 7920 divided by 14,000 = .56 sq.inches of metal
For 6061-T6 (35,000 p.s.i. Compressive yield) we need: 7920 divided by 35,000= .22 sq. inches of metal
For 2024-T3 (38,000 p.s.i. Compressive yield) we need: 7920 divided by 38,000 = .20 sq. inches of metal

That assumes thick bar stock or angle that won't cripple in compression. (1"X1x.125" angle 6061-T6 = .25 sq. inches)
If the spar is 6" deep then twice as much cap metal needed.
6g ultimate is for a 4g limit load not to be exceeded.
As always, proof load is required.
 

mcrae0104

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Nearly. Once you have the moment at the root, use Fb=My/I. You have Fb (allowable stress), M (moment), and y (distance from the neutral axis to the extreme top or bottom). Now you can figure out I (area moment of inertia) that is required. I won't go on since this is in any textbook on the subject or YouTube videos or probably even in Billski's thread on beam theory.

I
is simple for rectangular beams and nearly as simple for I-beams, but you can find it for other shapes like circles or even thin-wall triangular tubes. (Find I for the "outside" triangle, then subtract I of the triangular void in the middle. Also keep in mind that the neutral axis is not in the middle for a triangle, which will change y in the Fb=My/I equation.)
 
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Norm Langlois

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Bill if I understand you . Had I installed the 1X1 X1/16 sq tube all the way along the top of the spar it would have held up. On MY BUILD
since I only went out 30 inches. I got the same result but at a greater load.
If inverted with the pair on top ,that relates to more than .25 inches with my new proposed spar . The riveted flanges create angles of 1X1X 1/8 X2
With the single in tension adding an inner strip should make it work.
 

Norm Langlois

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The books wont help my old brain . If I handed you a book to learn Chinese ,written in Chinese. and you have no translator will you get any where with the book?

FOR ALL OF YOU I AM THROUGH FLYING, I am just exploring what I know how to do.
No one is saving me from anything with negative opinions or safety concerns.
If I choose to waste money and time on experiments .All I have is time on my hands ,I'm retired.
 

BJC

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Builder asks engineering question on Homebuilt forum.
Forum reply: "Get a book"
Not fair, BB. The question was answered in the forth post in this thread.

Would also note that it is difficult to answer a technical question when the question clearly indicated that a basic understanding of the subject is deficient.

Norm: Congratulations on having designed, built and flown your own aircraft. To minimize risk of such undertakings requires either a willingness to copy proven designs and / or to develop the technical skills required.


BJC
 

Victor Bravo

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Builder asks engineering question on Homebuilt forum.
Forum reply: "Get a book"
Forum reply: "Wait a minute, you may be in over your head here, we're happy to..."
OP reply: "Quiet, don't tell me that I need anything other than what I thought I needed. My airplane hasn't crashed yet, so I must have done something right."

What exactly is the difference between someone who is sincerely concerned for another person's safety, and a negative naysayer?
 

Dana

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The books wont help my old brain . If I handed you a book to learn Chinese ,written in Chinese. and you have no translator will you get any where with the book?

FOR ALL OF YOU I AM THROUGH FLYING, I am just exploring what I know how to do.
No one is saving me from anything with negative opinions or safety concerns.
If I choose to waste money and time on experiments .All I have is time on my hands ,I'm retired.
Norm, you don't give yourself enough credit. The fact that you've successfully designed, built, and flown an airplane proves you have the ability to learn new things and solve problems. Yes, there are a few questionable aspects of your design and you did some design/build iterations that might have been avoided with some research, but overall it's a good design and looks well built. You don't have to learn Chinese or the aeronautical equivalent; we're talking introductory texts here, written for people who don't yet know the language.
 

BBerson

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Bill if I understand you . Had I installed the 1X1 X1/16 sq tube all the way along the top of the spar it would have held up. On MY BUILD
since I only went out 30 inches. I got the same result but at a greater load.
If inverted with the pair on top ,that relates to more than .25 inches with my new proposed spar . The riveted flanges create angles of 1X1X 1/8 X2
With the single in tension adding an inner strip should make it work.
It might have worked. I don't know what you mean "by only went out 30".
Generally you want all sort of extra strong metal (not weak 6061-T4) at the root, perhaps four times what is at the half way out and almost nothing at the tip. Calculating all that is more work. The bent flange alone is probably enough the last four feet. Look at Ercoupe and Cherokee wings for hints. My Cherokee spar was just bent channel outer spar but extremely thick extrusion at root.
The numbers I suggested are possible bare minimum. For fatigue safety you might double or triple the amount of cap metal.
And correct for holes drilled, etc.
 
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BBerson

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Not fair, BB. The question was answered in the forth post in this thread.

Would also note that it is difficult to answer a technical question when the question clearly indicated that a basic understanding of the subject is deficient.

Norm: Congratulations on having designed, built and flown your own aircraft. To minimize risk of such undertakings requires either a willingness to copy proven designs and / or to develop the technical skills required.


BJC
I don't agree with the post 4 answer. Alloy does matter (in my view) in this case.
 

BBerson

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Forum reply: "Wait a minute, you may be in over your head here, we're happy to..."
OP reply: "Quiet, don't tell me that I need anything other than what I thought I needed. My airplane hasn't crashed yet, so I must have done something right."

What exactly is the difference between someone who is sincerely concerned for another person's safety, and a negative naysayer?
I think your comments on this thread to Norm were tactless.
I didn't want to comment. My view is the demeanor is lacking lately and this is concerning me. This pfarber character replied to me in an uncalled for manner. I didn't respond. I don't know what else to do other than limit my forum time somehow.
 

Victor Bravo

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Never intended to insult anyone in the beginning. There's an old joke about what is the difference between ignorance and apathy. The punch line is "I don't know and I don't care". Not knowing is something this forum can help with. Not caring is a bigger problem. If several of us were trying to politely say his spar was funky, and he lashed out in anger, sorry but the gloves come off at that point. But I will apologize to all parties if anyone thinks that my comments were rude but Norm's comments were acceptable.
 

pictsidhe

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I think your comments on this thread to Norm were tactless.
I didn't want to comment. My view is the demeanor is lacking lately and this is concerning me. This pfarber character replied to me in an uncalled for manner. I didn't respond. I don't know what else to do other than limit my forum time somehow.
My ignore list has grown a lot recently.
Has pfarber managed to say anything polite to anyone yet?
 

VP1

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In general I like your spar and I think It can work, but it’s upside down. The widest spot needs to be on the compression side (the top) of the spar. Apologies if this has been mentioned.

Edit- Norm- love your ambition and experimental attitude.
 

BBerson

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My ignore list has grown a lot recently.
Has pfarber managed to say anything polite to anyone yet?
pfarber was an extreme example. I was concerned about the driving off of Briggs experimenter Kevin Armstrong and the crude comments about Choppergirl's trailer needing a 5000 pound hitch to haul a 250 pound ultralight....
and now Norm. Why drive away experimenters?
 
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