There’s just no way to add 100 mph to the speed of a helicopter. Or is there?
- By James R. Chiles
- Air & Space magazine, September 2009
When it comes to hoisting cargo on a sling, dueling with insurgents barricaded in an apartment building, touching down on mountaintops, or slipping between looming obstacles, nothing can match helicopters. But in the overall ranks of aircraft, they remain a niche player. Whether you measure in passenger miles, number of registered aircraft, or number of pilots, airplanes have helicopters beat.
One reason is that helicopters have a woefully low speed limit. Today, we might call it Nelson’s Speed Limit. In May 1861, Mortimer Nelson, a Manhattan greeting-card printer, won the first U.S. patent for what we would now call a helicopter. About 33 feet long and weighing half a ton, Nelson’s Aerial Car—to be powered by supercharged whale oil—would have two sets of rotors: a big pair for lift and a little pair, flanking the rudder in the back, that acted as propellers. Nelson predicted that the latter set would drive the Aerial Car at speeds of up to 180 mph. His design never flew, but his prediction about the limits of helicopter velocity has proved remarkably accurate: Today, most production helicopters’ top practical speeds are under 180 mph.
For the Sikorsky Black Hawk, the limit at cruise speed is about 140 mph, and, if pushed to “dash speed,” a little over 200 mph, sustainable for only 10 minutes at a time, since longer periods exert too much wear on the mechanical systems, such as the gear box. For a Bell AH-1W Cobra without external ordnance, absolute top speed is 220 mph, according to Marine Lieutenant Colonel Wade Hasle, who spent two tours flying Cobras over Iraq. He himself never reached such a speed; “You only get going that fast in a dive,” he says. The official world helicopter speed record isn’t that much higher: In 1986, a Westland Lynx reached 249 mph.
Today, though, three companies—Piasecki Aircraft, Sikorsky Aircraft, and Boeing—are actively working on designs they hope will make Nelson’s speed limit as quaint a piece of helicopter history as whale-oil fuel.
Most of the Sluggishness of standard helicopters can be blamed on the rotor disk. Unlike an airplane’s propeller, a helicopter’s rotor disk has to move through the air sideways. On one side of the disk, the blades are rushing into the wind, and they experience a relative wind that is near (and sometimes above) the speed of sound. Slicing through wind that fast takes a lot of energy and is hard on the equipment. But just a few dozen feet away, on the other side of the disk, blades are rushing away from the direction of travel. If cruise speed goes too high, the retreating blades stall. Consequently, the aircraft is shaken by strong vibrations, and the nose is forced upward. At speeds beyond 100 mph or so, the drawbacks of the disk loom larger and larger until a point at which no extra engine power can shove the machine along any faster. (This became obvious when, beginning in the late 1950s, lightweight, high-power engines enabled helicopters to lift more weight, but didn’t fix the speed problem.)
For military helicopters, the speed limit represents a vulnerability that the enemy can take advantage of. For instance, if air support needs 15 minutes to reach a hot spot, the enemy can plan to stage an attack and then retreat before the defending helicopters ever show up.
Furthermore, says Colonel Greg Lengyel, who flew the Sikorsky MH-53J/M Pave Low for the Air Force and now commands the Florida-based First Special Operations Wing, a lot of military missions take place under cover of darkness. If helicopters are too slow, Lengyel explains, the plan must include finding a spot for the aircraft and crews to hide during daylight. “Any gain in helicopter speed would be helpful,” he says. “But if we’re talking about doubling it to 230 or 250 knots [about 265 to 290 mph], that’s what opens up the field.”
Over the years, every attempt to field a radically faster helicopter has resulted in some inadequacy (see sidebar, left). Added speed has meant poor payload capacity, instability, maintenance difficulties, high fuel consumption—or all of the above. Only three production vertical-takeoff-and-landing (VTOL) craft have reached high operational speeds, and all avoided the conventional-helicopter route entirely: one tiltrotor, the Bell-Boeing V-22 Osprey; and two designs that use engine exhaust for takeoff thrust: the McDonnell Douglas AV-8 Harrier and Yakovlev Yak-38 Forger. And the Forger is no longer in production.
One solution has been tried repeatedly: the compound helicopter. Compounds have hardware—fixed wings, propellers or jet engines—that enables them to take off like helicopters and then cruise as airplanes, flying at higher speeds. Creating a compound can be as straightforward as attaching wings and a propeller on a standard helicopter, or it can mean designing and making one from scratch in hopes of keeping tradeoffs, such as additional weight and mechanical complexity, to an absolute minimum.