I’ve noticed that there was a Mod 152 which was a change of material of the wing spar. They went from aluminum 2024 to 7175.
Can anyone offer a view on what the likely reason might have been for this change?
Since Mod 151 was the TB “GT” model, c. year 2000, Mod 152 was probably after that.
Better static strength properties, worse fatigue properties. Is there any other mod about MTOW (or flight envelop) at the same time ?
Not sure if there is much difference on the corrosion front.
It could also be due to availability of plate thick enough in 2024 to machine the spars
No changes on MTOW or anything else; the GT POH is mostly dated from late 1980s because Mod 151 (the “GT” changes) was done so no POH changes were needed
See a comparison here
I’m not sure this is the right temper for the 7175 though
7075 is the typical “aircraft grade” aluminium. A comparison of 7075 and 7175 is here
Fairly equal these two, but the 7175 has slightly better fatigue strength. In the US, 6061 is common while in Europe 6082 is the common alloy. Lots of this is therefore simply “best practice”. Maybe it is the same for 7075 vs 7175.
Anyway, 7075/7175 vs 2024 is stronger, better fatigue strength and better corrosion properties. 2024 is much much more “formable”. It can be bent a lot without becoming brittle or develop cracks. It’s much easier to work with. If you don’t need to bend it, or deform it, then 7075/7175 is a better material.
The odd thing is not really why they changed, but why they used 2024 in the spar in the first place IMO. The only practical difference is probably that spars with 2024 will last for 10k h, while 7175 will last for 15k (or something like that)
I’ve not used 7175 but 7000 alloys are typically more expensive than 2024. I’d agree with @LeSving that this change may have had something to do with fatigue, perhaps in combination with a change in the cost structure. 2024 (used to be 24ST) is the long term workhorse for aircraft and is used for almost everything so it’s not surprising they used it initially.
6061 is ‘tooling material’, widely adaptable and inexpensive. Also easily weldable. As long as you don’t need high strength, which for many components is true, it’s a good material. It’s often used in place of 2024 in UL aircraft as a result of lower cost and the typically lower stresses of smaller structures.
If you want to understand the difference between e.g. 7075 and 2024 it’s useful to take a small piece and bend it 90 degrees in a vice. The 7075 snaps like a carrot whereas 2024-T3 elongates a lot before cracking.
It’s often used in place of 2024 in UL aircraft as a result of lower cost and the typically lower stresses of smaller structures.
It’s in the details For critical parts such as wing spars, fatigue is important. The fatigue limit of 2024-T3 and 6061-T6 are: (2024 on top):
2024 is 140 MPa and 6061 is 96 MPa. This means 6061 has only 67% of the fatigue strength of 2024. (I don’t know if that number is for 10^7 cycles or some other number of cycles, but it doesn’t matter)
This is not the whole truth. 6061 is highly non corrosive. It doesn’t corrode at all in normal atmospheric conditions. 2024 is the opposite, it’s one of the worst alloys there is. 2024 is therefore typically cladded with pure aluminium. The relative thickness depends on the thickness of the sheet, but let’s say 15% on thin sheet, 5% on thicker spar material. The clad material doesn’t contribute to strength, and therefore not fatigue either.
Then 2024 is an alloy with Cu as the main ingredient. Cu is heavy, so the density of 2024 is higher than 6061.
2024 is 3 g/cm^3 and 6061 is 2.7 g/cm^2. 6061 is only 90% of the weight (density) of 2024.
Then you have this section of spar that is 10 mm thick, made of 2024. You have to add 5% for the clad material, making it 10.5 mm. The weight is roughly 105% also. For the same fatigue strength, the 6061 spar would have to be 1/0.67 times the thickness. That is 49% thicker (than 10 mm), making it 14.9 mm. Than multiply by 0.9 to get the weight, and it is 134 %.
The total comparison will be 1.34/1.05 = 128%. What originally “looks like” an 49% increase in weight initially, is therefore only 28% in reality. But there is more. This is fatigue limit only. What about the actual strength?
The main thing is how strong it needs to be before yielding. You don’t want the spar to deform, and get a bent wing. 2024 is 320 MPa, 6061 is 270 MPa. 2024 is only 19% stronger than 6062. We have added 49% thickness with the 6061 for fatigue reasons, and this means the spar has become 25% stronger. For instance, instead of withstanding 6g before deforming, it will now withstand 7.5g before deforming.
When going from 2024 to 6061 on a spar, the spar will be 28% heavier due to fatigue considerations, but you have gained:
I’m not saying a spar made of 6061 is better than a spar made from 2024, or let’s say the “number one” alloy 7475. It’s just that it’s not all “bad” either. Even if you made the 2024 spar 28% heavier, it would only be 28% stronger vs 25% stronger for an equal weight 6061. The smaller and lighter it gets, then it is rigidity, not strength or fatigue that is important. At some point the aircraft just gets too flimsy, if strength and fatigue are the only factors, so there is very little gained in going for stronger materials. 6061 is more than good enough, and with the added benefit of zero corrosion problems. A spar is probably the one thing that still make some sense though, but not by much
However, Socata appear to have changed to 7175, not 6061. Are they the same?
Nothing like the same, at the opposite ends of the strength and cost spectra. Here is a bit of background. Most of what is described for 7075 applies equally to 7175. 7000 series alloys are quasi-exotic aircraft material.
I think 6061 was mentioned here because it’s inexpensive, well known, widely available and has been used recently in UL aircraft, without much weight penalty if the structure is mostly designed for stiffness or practical minimum skin thickness, not strength. That may be the case for e.g. boxy thin wall structures as opposed to wing spars. 7175 is conversely a high strength alloy, similar to 7075, expensive but good for highly loaded structure like wing spars. 2024 is in between, the workhorse alloy for aircraft. All of the above aligned with what @LeSving posted.
Yes, 7175 (or 7075, I don’t know what the practical difference is) is definitely “spar material”, more so than 2024.
The 6061 is just me wondering about the benefits of it compared with 2024, which are two very different materials.
Could it be that Socata had a load of 7175 lying around from their Airbus subcontract work?
If this change happened say 2000 then it would mean most GTs had this spar material. It is unlikely to have happened much later because by around 2001 the internal decision was made to drop TB production, and 2002/2003 aircraft were made from a pile manufactured mostly in 2000/2001.