Can I use a different grade of aluminium for my microlight wing spar?

[TB74]

TL;DR

Is it ok to substitute one aluminium box section with two smaller box sections to make up the same overall profile in a Microlight aircraft wing spar?

I'm considering building a microlight aircraft from a set of plans. According to the plans the wing spar required is a 6m long rectangular box section 100mm x 50mm with wall thickness of 2mm, it is made of aluminium grade 6060 T5. I'm struggling to source a suitable piece of aluminium so was wondering if I could use two pieces of grade 6082 T6 aluminium, each 50mm x 50mm section joined together to make the 100mm x 50mm profile? I appreciate that this would result in a heavier spar but I'm unsure what the effect would be on the strength of the spar. I would be grateful for any advice.

I'm sure this is a very simple question to anyone familiar with the physics, sadly I'm not!

 

[berkeman]

Welcome to the PF.

Probably your best bet would be to contact the person/company who supplied the plans. They would be the most familiar with the design constraints, and they would be much more able to give you advice on variations from their published plans. Asking this on the Internet is probably not the smartest thing to do for this sort of thing.

 

[TB74]

Thanks for your reply, I have already contacted the designer and I'm waiting to hear back from him. I certainly wasn't intending to make any decision based solely on replies to an internet forum question but was interested in any views on the physics involved.

 

[Lnewqban]

... so was wondering if I could use two pieces of grade 6082 T6 aluminium, each 50mm x 50mm section joined together to make the 100mm x 50mm profile?...

Not to be taken as solid advice, just giving you a general idea that you could discuss with the designer:
The key problem in this case is the "joined together" part.
I would say that could become the weakest link of the made up spar.
Even if a solid connection is originally achieved, it could become weaker in physical strength with use and with time.

In real flight conditions, the spar will be subjected not only to pure vertical bending stress, but a combination of torsional and lateral bending stresses, as well as damaging vibration in the unfortunate event of the wing suffering fluttering.

The cyclic nature of loading and unloading the wings for each flight introduces the phenomena known as material fatigue, which will work in a different way for the (hopefully tested) design spar and for the made up spar.

Please, see:
https://en.wikipedia.org/wiki/Bending

https://en.wikipedia.org/wiki/Fatigue_(material)

 

[256bits]

Welcome to the PF.

Probably your best bet would be to contact the person/company who supplied the plans. They would be the most familiar with the design constraints, and they would be much more able to give you advice on variations from their published plans. Asking this on the Internet is probably not the smartest thing to do for this sort of thing.

I meant to push LIKE not the HA HA

 

[Baluncore]

An inspection certificate will probably be needed before the aircraft can be registered. It is in your interest to make sure the inspector is aware of and accepts your engineer's advice, and your implementation, before you invest in a costly and unacceptable solution.

Changing the specified alloy may reduce the price but it may also be more susceptible to fatigue fracture. The welding of air-frames is not recommended since heat treatment will be lost, and X-ray inspection will be required. Welding makes later adjustments and repairs more difficult. Instead, use aircraft grade bolts to fabricate the structure.