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cheapracer

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That airplane design, like some people here, is at least 62 years old. Most markets change every half century or so.
Yeah, not really on the designer, but since 2024 T3 2" .058 wall tube seems hard to find, I wonder how easy it was 62 years ago.

Interesting point is that today's 6061 T6, is most likely stronger than 2024 T3 was 60 years ago.

The recipes are the same, but the quality and purity of the compounds has much improved over the last 100 years. There is a couple of scientific references to this on the Net.

For a rudder post, I would use 6061 T6 replacement without hesitation, it is not weaker than the 2024 as an initial structure, it just has an earlier yeild point when it gets bent 'X' distance, compared to 2024's further 'Y' distance, question is, why on earth would your rudder post ever get bent that far .....
 

Bigshu

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On the other hand if it is a torsional member or a vertical stab spar, the answer gets more complicated. We need a lot more info about what it is doing, how the part is processed, etc.

What is the airplane?

Billski
It's a Volksplane VP1. Lots of people are still buying plans, and presumably building them. I wonder what their source is for the rudder tube (vertical stab spar)? If it's as simple as a call to ACS to see if they have them that's easy enough.
 
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REVAN

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I'm stumped. I don't have the expertise to figure out an acceptable substitution for a rudder tube. The plans call for 2" X .059 wall, 2024 T3 aluminum tube. I'm not having luck finding that size in that wall thickness, so I wondered if 6061 T6 with a larger wall thickness would work instead. 6061 is a lot weaker according to the spec sheets, so how thick would I have to go to get comparable strength to the 2024 .059? I could go thicker wall in the 2024, but that gets real expensive, quickly. Why do designers spec out a difficult to find material in the first place?
I'm thinking that is supposed to be 0.058 wall thickness. 0.059 wall tube isn't a standard as far as I know. Maybe someone made a typo in the plans.
 

REVAN

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"I'm not having luck finding that size in that wall thickness, so I wondered if 6061 T6 with a larger wall thickness would work instead. 6061 is a lot weaker according to the spec sheets, so how thick would I have to go to get comparable strength to the 2024 .059? "
The area of inertia property for a square tube is Ixx=Iyy=1/12*(L_outer^4-L_inner^4).

2 inch 0.058 wall: 58_Ixx = 0.2879 inches^4

2 inch 0.065 wall: 65_Ixx = 0.3143 inches^4

If 65_Ixx/58_Ixx >= (the yield stress of 2024 T3)/(the yield stress of 6061 T6), then the thicker but weaker material should be as strong or better than the thinner but stronger material.

In other words, if the 6061 T6 is at least 92% the strength of the 2024 T6, the substitution should work (from a beam and spar strength perspective).

The nest step would be to look at the fasteners to make sure they are also okay. You may need different holes and rivets or bolts than what is called out. That analysis requires design details that you have not given here.
 
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Bigshu

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I'm thinking that is supposed to be 0.058 wall thickness. 0.059 wall tube isn't a standard as far as I know. Maybe someone made a typo in the plans.
No, I made a typo in my post! Thanks for the explanation on comparing different alloys to determine if substitution is acceptable. That makes me feel better about what to do if the search for the size from the plans continues to be fruitless. I think I probably should have gone straight to the VP builders group and asked what everyone is using for the part, but It seems like the majority of folks there built their plane a while back, so they might have had better luck than me finding the right spec tube. I came to the Metal aircraft group because it's a strength of metal question, and the VP folks are more focused on the wood construction aspect.
 

Dana

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Interesting point is that today's 6061 T6, is most likely stronger than 2024 T3 was 60 years ago.

The recipes are the same, but the quality and purity of the compounds has much improved over the last 100 years. There is a couple of scientific references to this on the Net.
I've never heard that; certainly there improved alloys today but the published values for standard alloys haven't changed as far as I know.

For a rudder post, I would use 6061 T6 replacement without hesitation, it is not weaker than the 2024 as an initial structure, it just has an earlier yeild point when it gets bent 'X' distance, compared to 2024's further 'Y' distance, question is, why on earth would your rudder post ever get bent that far .....
I would NOT make that substitution without a careful analysis. 6061 has both a lower yield and lower ultimate strength. The fact that the designer specified the more expensive 2024 indicates that he believed its higher strength was necessary.

The area of inertia property for a square tube is Ixx=Iyy=1/12*(L_outer^4-L_inner^4).
But we're talking about round tubes here.
 

Bigshu

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"But we're talking about round tubes here."

I wondered if that makes a difference. What is different about the analysis of different alloy round tubes? My issue with the 2024 tube isn't that it's more expensive, it's that I'm having trouble finding the specified wall thickness, and I'm not sure what effect a heavier tube will have on the design. It's at the rudder, so the arm makes for a big moment difference even if the weight increase is small, right?
 

WonderousMountain

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CheapRacer is right. The consistancy has improved.
In the early days you would get every sort of issue.
Tabulated values are the same, actual condition is a
fair amount better. SOP was to upsize every member,
trusting the figures just wasn't done, by responsible
configurators.

I am not at all worried about buckling a thicker tube,
which is the talked about failure mode. However, the
attachment & wear qualities do give pause. 2024 T-3
is substantially tougher in hardware fixture. A welded
flange should have extra strength in the thousands, but
A single through bolt, etc. could be an issue.

My expectation is 65mil will compensate, however, that's
A long way from an assertion. More 6061 ultralights are
flying than 2024, strong doesn't compensate for dumb.

Bigshu,

The amount of weight you're fretting over is 1/1800th
of VP-1 empty weight, you'll get a stiffness upgrade &
Holding upgrade which may improve rudder authority.
 

Bigshu

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Ok, I think I've gotten good advice on how to proceed. I'll call to see if ACS has the .058 tube, I'll check their cross references for strength comparisons if they don't, I'll dig into Part 23 and run some numbers.....then I'll buy the heavier tube and not worry about the tiny weight increase! Thanks to everyone for weighing in, I learned some things, which is why I asked in the first place.
 

Bigshu

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Have you tried OnlineMetals.com, If it exists, they will have it. You may want to try to confirm that the wall thickness is in fact correct. As already mentioned, most homebuilts are 6061-T6.
I did try them in particular. And the other online metal dealers, and Airparts inc, I didn't try my local metal by the foot...The spec is straight from the plans, but the prevalence of 6061 is what prompted the question about substitution in the first place.
 

Bill-Higdon

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On the other hand if it is a torsional member or a vertical stab spar, the answer gets more complicated. We need a lot more info about what it is doing, how the part is processed, etc.

What is the airplane?

Billski

It's a Volksplane VP1. Lots of people are still buying plans, and presumably building them. I wonder what their source is for the rudder tube (vertical stab spar)? If it's as simple as a call to ACS to see if they have them that's easy enough.
Ask Fritz what he thinks
 

BBerson

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Any weight increase in the tail can be a CG issue.
I would probably use 2" .049"x4' tube with another inside lower bonded sleeve of 1 7/8" .058 x 12" =1.84 lbs.
(2" .058" x 4' = 1.69 lbs.)
Most of the bending load is in the lower 12".
.035" tube with a sleeve would be even lighter if .035" is available.
 

wsimpso1

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It's a Volksplane VP1. Lots of people are still buying plans, and presumably building them. I wonder what their source is for the rudder tube (vertical stab spar)? If it's as simple as a call to ACS to see if they have them that's easy enough.
OK, the main consequence of bumping the tail post to the next thicker wall is a slight increase in weight and a slightly aft shift of CG. You can calculate the added moment due to the weight, then calculate how much the engine must be shifted forward to maintain the same CG. It won't be much...

Does the VP1 really use an aluminum tailpost? How is it attached and how is the rudder attached onto it? Bolts? Rivets? Heat treated aluminum alloys go dead soft when welded, and then may grow some temper with age.
 

Yellowhammer

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First and foremost, unless you are an qualified to design and analyze the components involved, we usually prefer that you stick to the plans, even if something is hard to come by. The reasoning is simple, that way you will get what has already been designed, built, and proven out. The exception to sticking to the plans is when there is a known issue and a known good fix for the issue, then we usually advise sticking to the plans for the known good fix.

Getting past that, a lot depends on what a "rudder tube" is. Please tell us what this tube does and how it carries load and how it is attached to the associated pieces. Is it part of the rudder, or is it the rudder post, which is actually the aftmost part of the fuselage - a spar in the vertical stabilizer, with the rudder hinges mounted to it?

If it is a control push-pull tube, buckling under compression usually sets sizing, in which case 2024 is no better than 6061 of the same diameter and wall thickness. Got a copy of Evans' Lightplane Designer's Handbook? Good book and includes some old school compressive strength curves for standard aluminum and steel tubes. We can help some more with more information.

On the other hand if it is a torsional member or a vertical stab spar, the answer gets more complicated. We need a lot more info about what it is doing, how the part is processed, etc.

What is the airplane?

Billski
I bought the book as soon as I read your post Sir. Thank you. Quite a painless process. Paid $24.98 and opted for the PDF file download version.

Very intuitive manuscript.
-Yellowhammer
 

Yellowhammer

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OK, the main consequence of bumping the tail post to the next thicker wall is a slight increase in weight and a slightly aft shift of CG. You can calculate the added moment due to the weight, then calculate how much the engine must be shifted forward to maintain the same CG. It won't be much...

Does the VP1 really use an aluminum tailpost? How is it attached and how is the rudder attached onto it? Bolts? Rivets? Heat treated aluminum alloys go dead soft when welded, and then may grow some temper with age.

Do you think there would be that much difference in weight to have to move the engine forward a skosh?
 

Yellowhammer

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OK, the main consequence of bumping the tail post to the next thicker wall is a slight increase in weight and a slightly aft shift of CG. You can calculate the added moment due to the weight, then calculate how much the engine must be shifted forward to maintain the same CG. It won't be much...

Does the VP1 really use an aluminum tailpost? How is it attached and how is the rudder attached onto it? Bolts? Rivets? Heat treated aluminum alloys go dead soft when welded, and then may grow some temper with age.

CALL MR EVANS PERSONALLY. HIS DAD BUD, DESIGNED THE PLANE.
 

BoKu

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...Does the VP1 really use an aluminum tailpost?...
I believe that the VP airplanes have an aluminum spar for the all-flying vertical tail surface. I seem to recall that the tube spar pivots in bearings mounted to the aft fuselage.

Here is a page for someone building it. He offers a compelling argument for substituting 6061-T6: Rudder Construction
 
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