Plans for the tandem Airbike call for the root and strut fittings to be made from aluminum bar 3/16 by 3/4 from 2024 T4.  I already have 3/16 by one inch wide 6061 T6511.  Would that be an acceptable substitute?

I'm not an engineer so I cant say if its acceptable, I can say it will be weaker. What will the correct material cost you? What will the wrong material cost you?

I wouldn't - I'm having trouble finding my spec book and internet searches aren't giving me exactly what I'm looking for but it appears the heat treat you have for the 2024 isn't near as strong as the 6061.  There are other properties as well that make the 2024 a better choice as well.  So, without knowing the loads and what the designer had in mind - I'd go with what he specified in a part this critical to my wife's happiness - she hates insurance companies.

I wouldn't substitute one alloy for another.  Characteristics vary widely between alloys.  The right stuff isn't going to cost that much.

Tom

6061T6 is about 3/4's as tough and strong as 2024T4. You can find the properties here: http://www.aerospacemetals.com/aluminum-distributor.html Would the 6061T6 work? Probably, giving up some safety factor.  The safety factor of these fittings might be 10 with the 2024 alloy and 7 with the 6061. Considering that these are critical structural elements, a safety factor of 10 or more might be justified.

In my opinion people don't worry enough about safety factors.  Safety factors aren't just a multiplier in case you put more stress on something than intended.  One reason for safety factors is that we often can't perfectly calculate the maximum stress that will be imposed on the structure so the factor helps with any inadequacies in the prediction of stress.  However there is another important function.  A properly worked out safety factor will be attempting to get the stress low enough in the part under the intended load so that the part either has passed the "fatigue limit", that is it can be stressed to the design level any number of times without failure, or in materials with no known fatigue limit you are trying to get the level of stress down to a level where the fatigue is so low that the item will have a useful life which is long enough so that it is extremely unlikely it will be in service long enough to fail.  Reduce the safety factor and even if you never exceed the designed stress the part may fail in a number of cycles that is unacceptable.

This is why when you see someone doing aerobatics beyond the intended loading of their aircraft using the rationalization that the the "g" loading they are imposing is less than the "safety factor" are mis-using the concept of safety factors.  I would never reduce a safety factor once it had been determined by the designer.

Tom

Materials that are cheaper will save you money on your build, but will tend to cost you more on funeral expenses.  

Tom's viewpoint is well taken.  I was merely offering information so dbaker could decide for himself.  He may have already had that info when suggesting a 1" wide 6061 bar might be substituted for a 3/4" wide bar of 2024.  I think if I were making the substitution, I'd also want extra edge distance for the bolt holes and would pay special attention to dressing out the parts. I doubt that Ison did a fatigue analysis when selecting the 2024.  More likely, the availability of the 2024 bar, its use in the design in sufficient quantity, and its slight weight saving justified its specification.  Fatigue is an issue for Boeing.  Let's also consider that the spar fittings connect to the strut and fuselage carry-through which are both 6061.

2024 is about 50% stronger than 6061 for tensile strength (60 ksi vs 42 ksi) and has about the same margin for fatigue resistance (140 vs 97).

A 3/4" bar of 2024 drilled with a 1/4" hole has 1/2" of material left.  A 1" bar of 6061 drilled has 3/4" of material left....a 50% increase.  Roughly the two materials are equal in strength in this application.

Quoted from FearTheH

Materials that are cheaper will save you money on your build, but will tend to cost you more on funeral expenses.  

Not true if the cheaper material is equal to the more expensive material in critical respects.  It's just so easy to put the "fear factor" into a quick, not well thought out response.

Quoted from radfordc

2024 is about 50% stronger than 6061 for tensile strength (60 ksi vs 42 ksi) and has about the same margin for fatigue resistance (140 vs 97).

A 3/4" bar of 2024 drilled with a 1/4" hole has 1/2" of material left.  A 1" bar of 6061 drilled has 3/4" of material left....a 50% increase.  Roughly the two materials are equal in strength in this application.

That is actually not true at all.
6061 is also weaker in shear by approximately the same ration. In this application with  a single 1/4" bolt the fitting will fail in shear by the bolt ripping through the 6061 and the extra material around the hole will not make up for any of the lost bearing strength.  
This pic illustrates the failure mode , I keep it specifically for people who don't understand this concept.

George is absolutely correct.  It is very dangerous to substitute materials unless one is really and truly very well trained in materials science and structural analysis.  In our business we used to keep what we called the "Little Shelf of Horrors" showing all sorts of things that people were sure were OK which had failed for one reason or another.  These are very simple aircraft but the structural analysis is just as critical as in more complex structures.

Tom

You're right, it's very easy to put the fear factor in a response when you're betting your life on an airframe, that if failure occurs would essentially turn into a 5,000 foot AGL free falling paper weight.  
Engineered prints are just that, engineered.  I just don't see the point in compromising structural integrity to save some money, especially in an aircraft...kinda hard to pull those over in the event of some sort of catastrophic failure.  

Quoted from 71

That is actually not true at all.
6061 is also weaker in sheer by approximately the same ration.

The shear modulus of 2024 is 290 MPa vs 210MPa for 6061.  So 2024 is about 30% stronger in shear.  If the edge distance of the 6061 fitting is appropriately greater than that of the 2024 fitting would it not be equal in strength.  The edge distance of the 3/4" fitting is 1/4"...the edge distance of the 1" fitting is 3/8"....about 50% greater.  What am I missing?

Quoted from FearTheH

You're right, it's very easy to put the fear factor in a response when you're betting your life on an airframe, that if failure occurs would essentially turn into a 5,000 foot AGL free falling paper weight.  
Engineered prints are just that, engineered.  I just don't see the point in compromising structural integrity to save some money, especially in an aircraft...kinda hard to pull those over in the event of some sort of catastrophic failure.  

I guess having flown so many hours in planes that had 1/8" 6061 fittings at the spar to wing joint I'm just not sold on the idea that a 3/16" 6061 fitting is substandard.

Quoted from radfordc

The shear modulus of 2024 is 290 MPa vs 210MPa for 6061.  So 2024 is about 30% stronger in shear.  If the edge distance of the 6061 fitting is appropriately greater than that of the 2024 fitting would it not be equal in strength.  The edge distance of the 3/4" fitting is 1/4"...the edge distance of the 1" fitting is 3/8"....about 50% greater.  What am I missing?

I corrected my spelling but I'm sure that was not the problem.
Apparently you are missing the meaning of "shear"
When you sheer material the force is always the same it doesn't matter how much of it lays outside of the sheer line. I cannot debate on this level.

George, it almost seems that you're saying the size of a piece of metal is irrelevant to it's strength?  Surely that's not what you mean.  Looking at the picture you attached, imagine the difference in trying to shear a piece of .016 sheet vs a .125 sheet.  It seems to me that edge distance is a critical factor in shear strength. http://www.machinedesign.com/fasteners/what-s-difference-between-bearing-shear-and-tear-out-stress
  

Hi All;
It appears that George is using a piece of flattened tubing for his example. Look close at the holes. It is also the edge of the tube that tore out and is curled up. How about it George.
Bob

You have your opinion and I have mine...that's really all it boils down to, so not really much reason to be rude about it.  I'm just a by the plans kinda guy, and as the OP stated "the plans call for..." no one said it was substandard, my point was that those specific plans call for the material to be used for a reason.  At the end of the day I'm sure it'd be fine, but everything fatigues and fails at some point regardless of what is used if not properly maintained and inspected.  That is all.

Yes, I'm sure there is a good reason that 2024 is called for.  For one, it's about 30% lighter than 6061 for the same strength.  That would be very important if you are trying to make legal UL weight.

Hi All
I agree with staying with the plans as far as material. I know a guy that is building a Nieuport 11. He has been searching for flying wire cable for about a year now. He can't find a hardware store that carries cable to suite him. I have told him Aircraft Spruce and many others carry the proper cable AND the fittings. Too cheap to buy the right stuff. Each of the 4 flying cables on my plane will hold 1600 pounds of tensile load. That is a total of 6400 pounds. What will he wind up with with hardware store cable? FearThe H is right. Funeral cost is rising as we speak.
Always LOWER the risks by using the proper material, AN bolts and hardware. I have seen stuff done to aircraft that they would not do to their cars because they don't want to get stuck on the side of the road. How can they think that way???
Enjoy the build, use the proper material and enjoy the flying. I have never heard of one of these plans crashing because of a wing coming off.
Bob

My 1100R plans specify the same 2024T4 bar for the spar fittings.  With a gross weight 62% of the tandem airbike.  Speaking of Aircraft Spruce, they don't appear to sell rectangular bars in 6061T6.  Hmmm...

My 2024 3/16" bar fittings attach to a 6061 1/8" wall tube (plans specified) in my struts.  The tubing end will tear out long before the bolt tears through the 2024 bars. AS does sell the 2024 tubing at $2 more a foot.

http://www.makeitfrom.com/compare/2024-T4-Aluminum/6061-T6-Aluminum

Fatigue strength of the 2024 is significantly higher, which is more than likely why these plans were engineered with that particular material.  My whole point is that I don't want to see another AOPA video or NTSB report about ultralight/LSA failure.  We all know how something so small can be catastrophic in aircraft configuration, such as using the incorrect bolt, or even a hairline crack somewhere.  I've seen videos of a guy building ultralights out of materials collected solely from home depot...even if a piece of white pine had over 6 rings per inch, I surely wouldn't use it if it weren't specifically spec'ed and engineered for my intended purposes.  

Quoted from FearTheH

http://www.makeitfrom.com/compare/2024-T4-Aluminum/6061-T6-Aluminum

Fatigue strength of the 2024 is significantly higher, which is more than likely why these plans were engineered with that particular material.  My whole point is that I don't want to see another AOPA video or NTSB report about ultralight/LSA failure.  

I'm beginning to think we should be using 4130 steel for these fittings!  Can't be too safe.

Quoted from FearTheH

I've seen videos of a guy building ultralights out of materials collected solely from home depot...  

Yes...here it is

https://www.youtube.com/watch?v=1Q-8TilAluQ

That's him...would you fly around on that thing?

Quoted from FearTheH

http://www.makeitfrom.com/compare/2024-T4-Aluminum/6061-T6-Aluminum

Fatigue strength of the 2024 is significantly higher, which is more than likely why these plans were engineered with that particular material...  

I think not.  The fatigue strength of 6061 from your source is 96Mpa.  This is the amount of stress that has to be cycled ten million times to produce a failure.  96Mpa is 13.9Kpsi.  If the stressed area of our bar is .09 in² and the maximum stress is 1000 lbs at 4.4g. Then the stress is 11Kpsi or 76.6Mpa.  So you could do ten million loops with a 4g pull-out and be safe.

Correct, but the fatigue strength of the 2024 was rated at 140MPa which is 44 MPa more than the 6061, which would still rate the 2024 as being higher, correct?  

The same engineering principles apply to steel of course.  Everything has to be designed specifically to meet all the requirements of the given situation.  The usual reason to use aluminum is normally that you can save some weight over steel.  It is all a matter of really studying the materials science and structural analysis.  Change the material and everything in that load path will have to be checked for the right level of resistance to all the stresses and the fatigue.  Design a steel part to the same strength as the aluminum and you may get a section thin enough so that there won't be enough bearing surface in way of a pin, so you have to change the pin, etc.

Tom

Are the tandem airbike struts steel or aluminum? If they're steel then the argument for 2024 is stronger.  If they're aluminum, what is the specified alloy?  Bolting 2024 to 6061 makes the fatigue argument moot.