Engineers must be free to do their jobs. When politicians interfere, the results can be disastrous.
The C5 Galaxy
As a young engineer, I worked at a small airframe manufacturing company. One of my mentors had years of experience at Lockheed during the C-5 development program. We often discussed the wing design for this, at the time, largest aircraft ever made. These talks are over 30 years old, so I may not remember every detail, but I’ll share what I can.
Large military programs like the C-5 Galaxy often mix in politics. Congressmen, senators, and governors want their districts or states involved. They push for local design, testing, or manufacturing work. Spreading the funding across the U.S. is crucial for getting these programs approved. Why would a senator from North Dakota support a project if their state doesn’t get any contracts? They won’t do it out of pure patriotism. Their state needs a share of the benefits, especially during election time.
When a project gets as big and complex as the C-5, international politics can also come into play. This was true for the C-5.
A major British aerospace firm designed the wing for this aircraft. I didn’t take part in the design, analysis, or construction phases. So, I can only share general insights from our lunch talks. The main issue with the C-5 wing was its short fatigue life.
Aircraft are constructed mainly from aluminum. It is lightweight compared to steel and is easier to shape. It can also be alloyed and heat-treated to increase its strength. Yet, aluminum has a significant drawback: its fatigue life.
Steel fatigue characteristics allow for infinite cycle life at certain stress levels. This varies based on the type of steel. Compared to static yield stress, it might be 25% to 70% of the yield stress, depending on the alloy. A good rule is to lower the ultimate stress to one-fourth for high-cycle fatigue strength.
Not possible to remove the adverb. As stress cycles increase, the allowable stress levels decrease without reaching a minimum. There is no unlimited fatigue life for aluminum, like there is for steel.
This is why usable airframes often go to scrap centers in the desert. Usable parts, like engines and landing gear are removed, and the airframe is recycled.
The British design for the C-5 wing had a known issue: a very low cycle life. Aircraft undergo intense stress during takeoff and landing. They experience constant load changes while in flight. Pressurization also adds stress, as seen 25 years ago when the top of a Boeing 737 blew off during a flight.
A classic example of fatigue failure from pressurization is the DeHavilland Comet, a plane from the 1950s. Several of these jets self-destructed mid-flight. They grounded the fleet. They discovered the cause: tiny cracks. These cracks resulted from improperly punched rivet holes, which led to fuselage failures.
I don’t know the specifics that limited the C-5’s fatigue life. But, it required a complete redesign of the wing in the end. The original wing had known design flaws. Those responsible could have fixed them, and they were preventable.
Let 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