Why are aircrafts made of aluminum? 6

[snappish]

Hellow all,

I am always confused, why not steel? Steel has almost the same specific strength... Please advise. Thanks.

 

[rb1957]

many smaller planes (and older ones too, history plays a part) are built to minimum gauge (thickness), so steel would almost certainly weigh considerably more. for example the pressure cabin of a typical small jet is less than 0.06" thick (in Al) ... the equivalent strength in steel would be 0.02", which is probably impractical (to handle).

another issue would be rivet counter-sinks. Al skins, being thicker, can accept bigger flush rivets than could the equivalent steel skin.

all that being said, there are many places where the equivalent strength steel would be acceptable (heavy gauge wing skins, for example), but still we stick with Al ... why ?, good question !

 

[GBor]

Probably worth asking this question in the metallurgy forum, but remember that at altitude, it gets cold even in summer. I realize the Titanic hit an iceburg, but there are some speculation that rivits in that really cold environment may have had some trouble.

We also have to think about corrosion. Even stainless steel isn't as protected galvanically than Aluminum.

Then, there is thermal expansion, machinability, strain to failure, fatigue, the list goes on...

I would think the bigger question would be Titanium. I know cost is a factor today, but increase the demand for titanium, and eventually production would increase to drop the cost. Its specific strength is greater than either steel or aluminum and we do use titanium in certain places already. Of course, carbon fiber composites have their place in consideration, too.

Materials are constantly improving. Particularly in the last 10 years, there has been a tremendous amount of research in composites, aluminum alloys, and metal processing. I'm curious to see what we are using 20 years from now...

 

[RPstress]

Specific strength is just one measure of a material's usefulness. Its crack growth characteristics and fracture toughness per unit weight are also very important.

There are also other design behaviours: if a skin is subject to compression then its buckling is important. This is more or less proportional to the product of its modulus and thickness cubed (this assumes that the load per unit width is a constant, and you are varying the thickness to meet the requirements; this is simplistic but not a bad initial approach). For a less dense material the thickness is much greater for a given weight, giving a greater benefit for local buckling. On the other hand overall deflection of a wingbox is slightly worse for a less dense material, as the greater skin thicknesses reduce the overall section properties a bit. Similar arguments apply to local deflection under aero pressure and overall: less dense is quite a bit better for local deflection and a bit worse for overall. Is the design dominated by surface quality for low drag or flutter?

A similar consideration applies to tubes subject to buckling: local (cylinder) buckling is more or less proportional to modulus and wall thickness cubed. However, overall (Euler) buckling is proportional to modulus and second moment of area. For a given weight the denser material is squashed into the thinner wall; local buckling suffers, but the overall EI is improved.

There are numerous other inceasingly complicated measures of behaviour and the efficiency of different material choices based on various practical constraints.

NB: the above applies to choices of metals; almost all structural metals have the same specific modulus. The main practical exception is aluminum-lithium alloys, which are about 10% better. (Beryllium and its alloys are VERY much better for specific stiffness, but are hard to handle.) Carbon composites can also be quite a bit better. Some of these materials can give improvements in both local buckling and deflection and overall as well.

With sufficiently clever design based on hierarchical materials and structures many of the disadvantages of the denser materials can be overcome, unless they have a fundamental disadvantage, such as copper alloys with their low modulus for their density. See

http://silver.neep.wisc.edu/~lakes/Hierarch.html

. Practically speaking, we haven't got very far with this sort of thing yet.

 

[KENAT]

The simple, incomplete, answer is its lower density, however the fact you say specific strength indicates you know this already.

The others above know more about it than I and have given some very good input.

Many materials, in many combinations, have been used on aircraft through history.

Materials that have been/are used for aircraft structures include wood, canvas, steel, stainless steel, composites, titanium in addition to aluminium and its alloys. This list is just off the top of my head without thinking about it, there are lots more.

KENAT, probably the least qualified checker you'll ever meet...

 

[berkshire]

Snappish,
Check out the old Piper Cub. The fuselage is about 75% SAE 1020 mild steel tube, the rest is chrome moly SAE 4130 steel tube. You cannot see the tubes because they are covered in fabric.
B.E.

 

[KENAT]

A quick google turned up:

http://exp-aircraft.com/library/heintz/material.html

http://adg.stanford.edu/aa241/structures/structuraldesign.html

Aluminium wasn't widespread in A/C structures until just before WWII. Even then other materials continued in largescale use for some time after, especially for lower performance A/C. [An obvious exception to this would be the Mosquito, primarily wood (ply & balsa) it had very high performance.]

KENAT, probably the least qualified checker you'll ever meet...

 

[GregLocock]

You'd probably find that the tooling cost for aluminium is lower than for sheet steel. The actual material cost may well be a tiny part of the cost of the component. For example, if you have a low volume machined component it makes very little difference whether you use titanium, steel or aluminium, the cost of the part will only vary by 20% (so we used titanium, as we needed strength for weight). Similarly for a typical simple suspension link (track bar) the material cost is perhaps 2 bucks out of a part cost of 15 bucks.

One big difference between cars and planes is that a/c seem to be designed for fatigue life, whereas cars are pretty much designed for a given stiffness these days - that is, once the design is stiff eniough then it is also strong enough. There are a few parts fot he car where this si not true and so you can use highs trength steels, with a 350-450 N mm-2 yield. These are actualy cheaper than normal steel in terms of load carried per dollar.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tbuelna]

As GregLocock pointed out, it's all about economics. Aircraft are made primarily of aluminum for the same reason cars are still made from steel. It gives you the most bang for the buck.

However, "conventional" aircraft are not just made entirely of aluminum alloys. There are also components made of steel alloys like bearings, gears, linkages, landing gear and fasteners(4340, 9310, MP35, AerMet 100, etc), turbine blades and fasteners made from nickel alloys (Inconel 718, 15-5PH, A286, etc), structures, compressor blades and hi-loks made from titanium alloys (6Al-4V), wiring and bushings made from copper and copper alloys (BeCu 172). Each material is carefully selected for a given application, but in the end it is always a compromise between various conflicting requirements, such as weight, strength, raw material cost, fabrication cost, material availability, etc.

The weight-to-cost implementation trade-off for a production commercial aircraft like a 737, used to probably be somewhere around $300/pound. With current high fuel costs, that number is probably much higher. And that's why the new generation of commercial aircraft like the 787 are using lots of (very expensive)carbon composites and titanium. Even older aluminum structured aircraft, like the 737, used titanium for all of its permanent fasteners (ie. hi-lok's), because even at $40 or $50 per pound, titanium was still attractive for its weight savings.

A hard lesson that Boeing has just learned with their 787 is that even though you can engineer and prototype an aircraft in rare and exotic materials like carbon composite and titanium, you must also ensure that you will have a reliable, consistent and stably-priced supply of those materials for the duration of production. If you doubt me, just check with your local supplier for delivery lead-times of titanium fasteners or carbon pre-preg.

 

[Kwan]

I think RP has hit the nail on the head. Aluminum gives more inertia with less weight as compared to steel. (with a simpler shape). Aircraft are critical in buckling.

 

[amorrison]

In designing a homebrew flying machine I have found that 90% of the frame design "problems" are from compression member buckling failure since 100% reverse loading capability is required for all structural members.

As Kwan states "aluminum gives more inertia with less weight as compared to steel".

The above also applies to wood designs.

 

[swall]

When I was in the aircraft wheel and brake business, the only serious alternative to aluminum wheels was titanium. Some were made,tested and performed well. A plus was the corrosion resistance compared to aluminum. Titanium could be used bare vs aluminum which has to be anodized and painted. The factor that killed titanium was the cost--none of the airlines would have been able to afford titanium wheels.