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As any cyclist knows, steel has many advantages over aluminium:

  • it's stronger (for the same size)
  • it can handle greater forces without needing to be stiffer
  • it has better fatigue characteristics
  • it's easier to work with
  • and most importantly of all, it offers a more responsive and comfortable ride.

On the other hand aluminium is much lighter (for the same strength), which obviously is pretty important in an aircraft.

Even so, steel-framed bicycles aren't always heavier, or significantly heavier, than aluminium-framed bikes of similar strength, and in engineering the strength of a construction is not only determined by its materials, but also its design.

When did aluminium become the default material for the vast majority of an airliner's construction, and is its strength-for-weight advantage the principal reason?

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    $\begingroup$ I'm not sure the steel-vs-aluminum bike is a good analogy. You may be comparing apples and oranges, for example if an aluminum bike weighs 10lbs and the steel one weighs 12lbs, that isn't a significant weight, but look at percentages, its 20%. Take an aircraft weighing 20,000lbs, 20% is another 4,000 lbs... $\endgroup$ – Ron Beyer Sep 11 '16 at 2:02
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    $\begingroup$ I disagree on the fatigue point: Aluminium is outright nasty. Steel is much more forgiving. $\endgroup$ – Peter Kämpf Sep 11 '16 at 7:39
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    $\begingroup$ @PeterKämpf Which fatigue point? $\endgroup$ – Daniele Procida Sep 11 '16 at 8:47
  • $\begingroup$ I think the expression intended is fatigue limit. Aluminium does not have a fatigue limit, that is why it's so nasty. $\endgroup$ – MSalters Oct 10 '16 at 14:32
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You are right, on first sight it is actually surprising that aluminium prevailed. It comes with some disadvantages:

  • Crack growth must come first in this list. With only a small cyclic load, cracks in aluminium will relentlessly grow. This requires frequent checks and has caused many accidents. Fortunately for aluminium, this fact was only discovered after WW II when the lifetime of aluminium airframes shot up from dozens to thousands of hours, so aluminium was already firmly established as the premier structural material.
  • To connect two aluminium parts, only riveting or bolting are available. Recently, bonding has become viable, too. Steel offers a third choice in welding, which gives it the advantage in vehicle construction. Again, history gave aluminium a head start, because when the first metal airframes were designed, welding was still relatively little developed, so the restriction to riveting was not felt as the disadvantage it is.
  • Corrosion: Here it is generally better than steel, but far worse than stainless steel which would be the prime candidate (next to maraging steels) for airframes were aluminium not available. During the Berlin Airlift, salt or baking powder could only be transported by seaplanes because it would had permanently damaged the landplanes. Seaplanes used special, corrosion resistant alloys which were needed in the marine environment.

In the end, however, this is still outweighed by the principal advantage of aluminium: Its low density. This allows to use thicker gages for the same weight, so the buckling strength of aluminium skins is far better than that of equally heavy steel skins. In aviation, the demands of aerodynamics make the use of crimps or reinforcing seams impossible, and only the labor-intensive addition of closely spaced stiffeners could remedy this. Such a stiffened steel skin would still be inferior when it comes to hail damage or handling loads, though, so aluminium was the obvious first choice.

In order to keep material cost down, Junkers built his first metal aircraft from steel but had to switch to corrugated aluminium in order to make them light enough. Note that the structural integrity of his designs was ensured by a steel truss, and only the covering was from corrugated aluminium. Later stressed-skin designs did away with the steel truss.

To compare the strength and stiffness of different materials independent of their density, you can use their breaking length and specific modulus, which can be expressed as their strain length. Surprisingly, most structural metals have a comparable value and only composites can set themselves apart.

Despite its nasty failure characteristics, aluminium has defended its position as the dominant structural material: By the time composites were developed, the engineers and certification authorities had learned how to handle aluminium and had created a barrier to entry in the form of a host of tests and approved construction techniques which severely harmed composites. If all the materials we know today had been available a century ago, aluminium aircraft would had become an oddity, not the norm.

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  • $\begingroup$ Wikipedia has a page on specific modulus, which appears to be the same thing you are calling "strain length" (= Young's modulus per density). It never mentions the "strain length" term there. $\endgroup$ – Jan Hudec Sep 12 '16 at 19:04
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    $\begingroup$ @JanHudec Strain length is a theoretical term: The length of a vertical rod hanging under its own weight where the top doubles in length from the strain. This gives a very descriptive figure which is totally unrealistic. You are right, the page lists what I mean. I edited the answer accordingly. $\endgroup$ – Peter Kämpf Sep 12 '16 at 19:21
  • $\begingroup$ I may be missing something, but isn't aluminum repair easier than composite repair, or repair on steel, due to lower hardness in the latter case ... or is that too broad of a statement to classify as a pro or a con? The design criteria of "maintainability' has entered the field (it was certainly a criterion for the F-18) but maybe not so much for other models? $\endgroup$ – KorvinStarmast May 18 '17 at 12:53
  • $\begingroup$ @KorvinStarmast: That is a hot topic. I have done several composite repairs on gliders and find them much easier to repair than a metal structure. The toughness of stainless steel is a drawback, granted, but then weldability is a huge advantage. With metal, if you do it right, you better replace a part completely. $\endgroup$ – Peter Kämpf May 18 '17 at 20:29
  • $\begingroup$ OK, I have a few friends working composite repair for UAV's, but that's another topic. $\endgroup$ – KorvinStarmast May 18 '17 at 22:06
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Good strength to weight ratio, manufacutrability, resistance to corrosion, have been the driving factors behind the use of aluminum alloys. However steels are used in high strength applications such as the landing gear. Stainless steels has been used in a few airframes such as the MiG-25 for their higher yield strengths when heated.

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It really comes down to cost/benefit.

Let's consider two aircraft that were built out of stainless steel, and performed very well: the XB-70 and X15. Stainless steel was used to resist the heat of Mach 3+ flight, which would weaken aluminum to a failure point. At the time the XB70 was originally designed, titanium wasn't available in sufficient quantity to consider a large (100+ airframe) production run.

The X15 did achieve a speed of Mach 6.7, where skin temperature would be quite high. However, it was a pure research rocket powered aircraft, so it doesn't make as appropriate a contrast, and that speed was maintained only briefly.

By the early 1960's, titanium was available in enough quantity (albeit by using shell corporations to purchase it from the Soviets) to build the smaller and less produced A12/SR71. Like the XB-70, it was designed to sustain Mach 3+ for over an hour, and the subsequent high skin temperatures that preclude the use of aluminum.

The key issue to understand here is sustained Mach 3 flight: for over an hour. Aluminum interceptors such as the MIG25 have a brief Mach 3 dash capability, five or ten minutes, where the skin temperature won't rise as much. After ten minutes at Mach 3, the MIG25 is very low on fuel and it's engines are ruined.

To keep weight low, the XB70 used a complex honeycomb design that is expensive to manufacture, and the resulting aircraft still was heavier than an aluminum aircraft of similar size. The XB70 made up for this by using compression lift - literally riding it's own supersonic shock wave - for very efficient Mach 3 flight. The A12/SR71 used the shock wave in it's engines to achieve a similar level of efficiency.

Note that the B-58 also used a honeycomb construction to keep weight low, but was made from aluminum, as it's design speed was Mach 2. The Concorde was made from aluminum, but it cruised at Mach 2.2. Beyond that speed, the heat generated from skin friction rises dramatically, to a level where aluminum would be critically (and fatally) weakened.

Presumably, a commercial aircraft made from stainless steel would also require the complex honeycomb design to achieve a reasonably efficient weight. It would still not be as efficient as an aluminum aircraft. As the primary reason not to use aluminum was to resist sustained Mach 3 heat, a speed that only the SR71 and XB70 can maintain for any period of time, the extra expensive of honeycomb steel (or titanium) and the greater weight isn't really justified for a subsonic airliner.

The XB-70 was canceled as it was thought to be vulnerable to the SAM2 missile the Soviets had developed. Ironically, the SR71 proved this not to be the case: being shot at over 1000 times (including many SAM2's) and never hit.

Yes, you can build an aircraft out of stainless steel. But, the honeycomb construction is expensive, and the result less efficient than an aluminum aircraft. The only reason steel was used in those two aircraft was to sustain very high skin temperatures. You could build one out of titanium, and it would be even more expensive.

While there have been airframe failures from aluminum fatigue, countermeasures far less expensive and lighter than moving to stainless steel have been developed.

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Aluminum has a worse fatigue behavior than steel, but it has two important advantages: strength to weight ratio and corrosion resistance. In the beginning of aviation there were some aircraft made of steel. Some of the engines in that era also had cast iron blocks. But the resulting weight was terrible. As for corrosion resistance, see for yourself: you still see 45-year old Boeing 727's at airports with their airframes shining like new!

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  • $\begingroup$ The Wright Flyer used aluminum in the engines and that's why it was able to fly contrary to all engines of her era. Sources please for the cast iron blocks? $\endgroup$ – ymb1 Sep 13 '16 at 23:30

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