Helicopters, Guitars And Other Inventions

Charles Kaman passed away on Monday at the age of 91 - the last of a generation of engineers who, in the 1940s, transformed the helicopter from a dangerous circus trick into a practical machine, each with his own trademark design.

It is said that when young Charlie Kaman started promoting his helicopter concept at United Aircraft, where he was working in the early 1940s, the response was:  "We've already got one inventor around here, and his name is Igor Sikorsky". Kaman headed out on his own, starting with a mobile ground-based rig, and by 1947 had his first "synchropter" airborne.

The design hasn't changed much between then and the K-Max, which today is the basis of Lockheed Martin's unmanned cargo aircraft program.

One of its unique features is obvious: the side-by-side, canted, intermeshing rotors. Like all counter-rotating helos, the Kaman design doesn't throw power away with an anti-torque rotor, and does not unduly mind hovering in a crosswind, a useful attribute in tight corners. It does not need the long body of a tandem, or the complex controls and mast of a Kamov/Gyrodyne coaxial.

The Kaman craft were never fast, but had a high useful load fraction. The most widely produced version was the HH-43, which for many years was used by the USAF as a flying fire truck, carrying a firefighting rig on a cable. 

The synchropter was also the basis for first turbine helicopter to fly - in 1951, powered by a Boeing engine (really), and (in a neat piece of foreshadowing) the first remotely piloted helicopter, tested in 1954.

The other unique feature of the synchropter - which was carried over to the company's most successful helicopter, the single-rotor SeaSprite - was the servo-flap control system. Each rotor blade, at the three-quarter-span point, carried a small outrigger-supported flap, operated by control rods that passed through the blade. Shazam! The blade's flexibility in torsion, a nuisance in most helicopters, becomes part of the control system, using much lower forces than trying to graunch the whole thing around by hydraulics at the root.

I recall then Navy secretary John Lehmann landing after a demo flight in the SeaSprite, grinning from ear to ear. "It makes me look good", he said.

Kaman was a prolific inventor of details: better bearings and couplings. I was looking at a helicopter at an HAI show in the early 1980s and noticed a wonderful, elegant flex coupling in a tail rotor drive. I asked about it. "That? It's a KAflex - we get it from Kaman."

Kaman's company was also a US leader in fiberglass rotor blades and produced them not only for its own helicopters, but for other manufacturers such as Bell. Combined with Charlie Kaman's musical talents, and the acoustic and vibration challenges inherent in blade design, that work led to a unique diversification effort: the Ovation guitar, with a one-piece composite bowl replacing the normal wooden back and sides. I have played Ovations - whether they make me sound good, I'll leave to the audience.