Engineers need to be allowed to do their jobs. When politicians get in the way, very often the results are disastrous.
As a young engineer, I worked at a small airframe manufacturing company. Among the individuals with whom I worked and learned from, was one man who spent many years of his engineering life working for Lockheed during the C-5 development program. Among the things that we discussed was the development of the wing on this-at the time-largest aircraft ever to be manufactured. These conversations go back well over 30 years, so my memory of them may have a few errors, but I will do the best I can.
Large military programs such as the C5 Galaxy invariably have politics mixed in with it. Congressmen, senators, as well as governors would very much like to have their districts or states contribute to these massive efforts by having design, testing or manufacturing-or all three if possible-performed by members of their districts. Spreading the cash out to various parts of the United States is also critical to getting these programs funded in the first place. Why should a senator from North Dakota vote in favor of appropriating money for any government-funded development effort if their state is not to share in the contracts? They are not going to do this simply out of patriotic fervor, altruistic reasons, etc. Their state needs a piece of the pie. Kind of helps when election time comes around.
When the efforts get extremely large- as in the development as complicated and expensive as the C-5 Galaxy-often international politics enter into the picture. This was the case with the C-5.
A large British aerospace firm was given the task of designing the wing for this massive aircraft. Having not been there during the design, analysis and construction phases, I cannot speak to specifics, but only to the generalities shared during our lunch time conversations. The fundamental problem with the C-5 wing was a very short fatigue life.
Aircraft are made largely of aluminum. Aluminum has some great qualities, being quite light in comparison to steel, also being easier to form in its soft state as well as the ability to be alloyed with other metals and to be heat-treated to bring its tensile yield strength well over that of many steels.
It’s major downfall is fatigue life. When studying the fatigue characteristics of steel, the cycle life will go to infinity at some level of stress. This value varies markedly depending on the type of steel. As compared to static yield stress, this could be 25% of the yield stress or up to 70% of the yield stress, depending on the alloy. The rule of thumb for garden-variety structural steels is to reduce the ultimate stress to one fourth of its value for its high-cycle fatigue strength.
Aluminum does not behave in this fashion. As the number of stress cycles increase, the tolerable stress levels continue to decrease, but do not reach a bottom value- there is no unlimited fatigue life for aluminum as there is with steel.
This is why what looks to be perfectly usable airframes are flown into scrapping centers-typically located in the desert somewhere-where usable parts are removed such as engines, landing gear, instruments etc. and the airframe is chopped up for recycling.
The C-5 wing design provided by the British had a known problem-a very low cycle life. Aircraft undergo some fairly severe stress cycles during landing and takeoff and are under constant load changes during flight. Pressurization stresses also takes a pretty severe toll, as was graphically seen some 25 years ago when the top of a Boeing 737 blew off of an Aloha Airways inter-island commercial flight. These aircraft typically fly several routes daily during which time they climb to well over 20,000 feet, cruise for a short time followed by descent and landing. The flights seldom last for more than 30 minutes.
A classic case of fatigue failure due to pressurization and flight loads was during the 1950s with an early turbojet design known as the DeHavilland Comet. Several of these aircraft self-destructed in midflight with the entire fleet subsequently grounded until the culprit was located. In this instance, small hairline cracks caused by punching-as opposed to drilling-rivet clearance holes caused the fuselage skins to eventually fail.
I don’t have any details of what limited the fatigue life on the C-5, just the end results, which was the need to redesign and construct a completely new wing for this aircraft. The design flaws of the original wing were well-known in advance by those whose business it was to know this, and entirely preventable.
Let the politicians stick to what they do best. Leave engineering to the engineers.
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org