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After years of carrying all its fuel in the fuselage, the B-52 transitioned to a wet wing (where the wing structure is used as one or more fuel tanks) with the B-52G.

This caused severe problems with metal fatigue in the wing structure (the wet wing increased structural stresses by approximately 60% compared to the A-F's dry wing, according to Wikipedia), which was already an issue due to the increased aerodynamic stresses associated with a recent switch from a high-altitude to a low-altitude nuclear mission profile (the high-altitude mission profile having become unsurvivable due to advances in Soviet SAM technology).

However, storing fuel in the wings of an aircraft normally decreases the stresses on the wing structure (by putting as much of the aircraft's weight as possible in the part of the aircraft that generates the lion's share of its lift), alleviating fatigue and increasing the aircraft's fatigue life. What made the B-52 different in this regard?

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    $\begingroup$ You are making two false assumptions regarding the B-52 change to a wet wing. 1) Early B-52s did not carry ALL their fuel in the fuselage. 2) It was not the wet wing design that resulted in fatigue cracks. It was the change to a lighter aluminum alloy wing design that caused problems. $\endgroup$
    – Mike Sowsun
    Commented Nov 14, 2019 at 15:30
  • $\begingroup$ In general, flight loads bend a wing upwards and ground loads bend a wing downward. When you add fuel to the wing it resists the upward loads in flight better, but increases the downward bending on the ground a lot more. Fatigue damage is cause by the cycling of load, so a wing skin that is in heavy compression on the ground and moderate tension in flight can have more fatigue than one that is in light compression on the ground and a heavy tension in flight. @MikeSowsun's answer is create with regard to the B-52, though. $\endgroup$
    – OSUZorba
    Commented Apr 10, 2021 at 1:47

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Earlier B-52s did not carry all their fuel in the fuselage. They also carried their fuel in the wings using rubber bladder-type wing tanks.

The new B52G “wet wing” was a weight savings measure which also increased the fuel capacity over the bladder-type in-wing tanks. The metal fatigue of the new wing was due to a poor structural design which was supposed to save weight.

http://www.joebaugher.com/usaf_bombers/b52_15.html

The B-52G had a lighter structure than previous versions in order to save weight, but it carried more fuel. This meant that fatigue problems resulting from the structural flexing generated by the stresses of low-level flying and midair refuelling manifested themselves earlier in the life cycle of the airframe. This was especially the case in the region of the wing structure where most of the weight savings had been achieved by using an aluminum alloy. Fatigue cracks got so bad that stringent flying restriction had to be imposed, pending modifications. In May of 1961, a program was approved in which the wings were modified and strengthened as part of the regular IRAN schedules for the aircraft in the B-52 fleet. The project was finally completed in September of 1964.

Early B-52B,C,D fuel chart showing multiple wing tanks. enter image description here enter image description here enter image description here

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The decrease in fatigue is not a property that's automatically conferred to the plane by the introduction of wet wing - it's a product of smart design. Usually the decision to use a wet wing is made during the initial design of a plane, and the support structure of the wing is built accordingly, so that it can take the dynamic strains that occur upon refueling (in case of military aircraft - possibly in-air refueling) and upon spending fuel from the tanks. The initial design of the (initially dry-wing) B-52, obviously, did not account for putting such loads on wing structure (the additional internal wing tanks carried 17 tons of fuel!).

During the modification to wet wing, the wing should've been reinforced to counteract the additional load. Instead, the range of the bomber was prioritized: G and H models were lighter than previous models, mostly due to extensive usage of aluminium and titanium parts, particularly - in wing structure; at the same time they carried much more fuel. Alloys used were less fatigue-resistant than ones used in previous models. The testing done in 1960 by Boeing and Air Force confirmed that combined with additional stresses induced by change of usage (i.e. low altitude missions and wing tanks operation) this significantly shortened the operational life of the aircraft. This result was made painfully clear by B-52G that crashed near Gainsborough in January 1961. That led to a new modification program being endorsed by the Air Staff in May 1961, which was finished in 1964 - it involved reinforcing the wings and other areas subject to increased fatigue ("The wing structural improvement program, carried out as ECP 1050, replaced the wing box beam with a modified wing box that used thicker aluminum. It also installed stronger steel taper lock fasteners in lieu of the existing titanium fasteners; it added brackets and clamps to the wing skins, added wing panel stiffeners, and made at least a dozen other changes. Finally, a new protective coating was applied to the interior structure of the wing integral fuel tanks." - Encyclopedia of US Air Force Aircraft and Missile Systems: Post-World War II bombers, 1945-1973, Volume 2).

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    $\begingroup$ But the question is, why is there additional load? If the fuel is in the fuselage, the weight has to be transferred to the wings through the main spar, creating a bending moment on it. When the fuel is moved into the wings, it is at the place where lift is created, so less force to transfer and less bending moment. So… there must be something else… $\endgroup$
    – Jan Hudec
    Commented Nov 11, 2019 at 22:19
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    $\begingroup$ @JanHudec I modified the answer with more info on the wing structure modification. Aside from that - the weight of fuel still affects the internal wing structure, even if the bending moment is being compensated by lift forces; the wing is not a single-piece solid body, and forces acting on it from inside and from outside induce stress on internal structure even if they are largely compensate each other. $\endgroup$
    – Danila Smirnov
    Commented Nov 12, 2019 at 4:37
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    $\begingroup$ Also, stress is not caused by load itself, but by load cycles, isn't it? So it they made it a bit lighter as the magnitude of stress was lower, they could have easily made it more susceptible to fatigue by mistake. $\endgroup$
    – Jan Hudec
    Commented Nov 12, 2019 at 5:09
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    $\begingroup$ The B-52G wing was not a modded "dry" wing. It was a totally new wing design that along with other changes, went from bladder fuel tanks to wet wing tanks. $\endgroup$
    – Mike Sowsun
    Commented Nov 14, 2019 at 18:10

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