The late Ed Heinemann was a designer at Douglas Aircraft. He was responsible for the A-4 Skyhawk, a single-seater with an empty weight of less than 10,000 pounds that served for a long time as the smallest carrier-based attack aircraft in the Navy's arsenal. It embodied the now-famous design philosophy emblazoned on Heinemann's office wall: "Simplicate and Add Lightness."
While the meaning of Heinemann's motto seems clear enough, there is a subtlety to it. When designing a device like an airplane whose dimensions are a function of its weight, one finds that changes reverberate throughout the design. They earn compound interest. Take, for example, the A-4's wing. With a span of only 27 feet it may at first have seemed too small to achieve the required approach speed for carrier landings. But making it small eliminated the need to fold it for hangar-deck storage. It also removed the weight of hinges and actuators and the complexity of controls, monitoring systems and backups; this in turn made it possible to pare a little material from the wing spars, landing gear and so on. In the end so much weight was saved that the target approach speed was possible after all.
That was a case of adding lightness by reducing size. Simplication works similarly: removing a system often allows you to remove other systems as well, because many services on an airplane require subsystems for verifying the behavior of the principal system, protecting against inadvertent operation and providing backup in case of failure.
At the same time, you have to guard against taking away too much or not quite enough, as I was reminded last week while installing the panel controls for the retractable landing gear in my homebuilt.
First, a quick flashback. In the mid-1970s I stopped in Newton, Kansas, to visit the factory where Jim Bede was getting ready to turn out BD-5 kits. I was in my old homebuilt, Melmoth, which I had first flown in 1973 and had been modifying and improving ever since. Somehow or other my nosewheel doors had gotten tangled up with the gear strut-I've forgotten how this happened-and in the course of removing the damaged doors I had had to pry them open by hand. My visit having ended, I climbed into the airplane, fired up the engine, and started to taxi, only to have the right main gear collapse. Bede very generously-especially considering that he knew that I was not among his most zealous admirers-let me use his shop and equipment to make temporary repairs. I returned to Los Angeles late that night without functioning flaps or a door on the right main.
What had happened? I'd shot myself in the foot. When I pried the nosewheel doors open, trapped hydraulic fluid in the gear circuit had to go somewhere, and it went to the right main actuating cylinder, unlocking the overcenter knuckle on the retracting link. As soon as I put an inward side load on the strut, it caved in. But where in the design was the fault? After all, manually pulling the nosewheel doors open was not something I had ever anticipated doing. The fault, I think, was in the very complexity of the system, which prevented my foreseeing the consequences of my action.
