January 30, 2012
Helicopter speeds have traditionally been limited by a phenomenon called “retreating blade stall,” which describes what happens to the main rotor at high speed. The relative wind on the retreating blade is reduced by the forward speed of the helicopter to a point where it no longer generates lift, and the helicopter rolls off to the side with reduced lift.
Tilt-rotor vehicles like the Bell Boeing Osprey turn the rotor blades into propellers. Now a less complex solution that dates back several decades is re-emerging: the compound helicopter, which uses a dedicated propulsor to produce forward speed. The Piasecki X-49A modified a Sikorsky UH-60 with a lifting surface made up of two wings, plus a ducted fan in the tail to push the aircraft to speeds of around 200 mph. The wings take up part of the lift load so the main rotor doesn’t have to provide as much at high speeds, and blade stall no longer matters as much.
The Eurocopter X3 is a modified EC155, itself a derivative of that company’s Dauphin (the U.S. Coast Guard operates one version). The X3 has achieved speed bursts as high as 267 mph, propelled by two tractor propellers driven by the main engines and mounted on stubby wings just beneath the main rotor. At high speed, the six-blade main rotor is slowed to reduce the drag of the advancing blade, while the wings provide necessary lift.
Sikorsky’s X-2 has advanced far enough to become the proof-of-concept for a prototype military aircraft now designated the S-97 Raider, a high-speed attack and scout helicopter. The X-2 proved the efficacy of a coaxial main rotor blade in which both sides of the helicopter have advancing blades opposite retreating blades, providing a symmetrical lift that requires no wing. Forward thrust is provided by a pusher propeller in the tail, an arrangement that took the experimental craft to a top speed of 290 mph, and, in a descent, to 300 mph. There is no tail rotor à la the traditional helicopter, and the X-2 maneuvers about the yaw axis by applying differential torque to the two main rotors. The little X-2 (it weighed less than 8,000 pounds on takeoff) was retired after only 22 hours of flying.
The X-2 is not Sikorsky’s first compound helicopter. Its S-72 employed a fixed wing and two turbofan engines to achieve a design speed of up to 345 mph. The wing generated enough lift to allow the craft to fly without a main rotor, which it actually did during testing. The X-Wing, another 1980s experimental craft, employed large, rigid main rotor blades that were intended to be stopped inflight to form an X-shape wing that supplemented lift from a conventional wing. Two turbofans provided thrust.
The closest the U.S. military came to buying and operating a compound helicopter was when it launched the Lockheed AH-56 Cheyenne with a contract in 1966 for 10 prototypes. Problems and delays during development led to cancellation of the Cheyenne, but the Army followed that with the AH-64 Apache, which is not a compound helicopter but is in operation today.
Here’s a demo of Eurocopter’s X-3 at last summer’s Paris Air Show:
August 10, 2011
The Army’s CH-47 Chinook helicopter has flown a stunning but standard maneuver—the aft-wheel pinnacle landing—since 1962. In Iraq and Afghanistan, the move has reached its peak.
This month as many as 5,000 pairs of boots will leave the ground, with a goal to extract 33,000 by next September. Many will exit the same way they were inserted, by the back door of a CH-47.
In the photo above, Australian special forces practiced for insertion into Iraq in 2003 using a CH-47D for Operation Falconer.
In Afghanistan (photo below, in Kandahar), where clear and flat land is even more the exception, there are few places to land a 52-foot fuselage. Add the Chinook’s rotors and its length stretches to 99 feet. The Army’s other workhorse, the UH-60 Blackhawk, can nose its way in. As for a CH-47, just give a pilot a patch big enough for its 12-foot width, plus a few feet for a ladder and a prayer, whether on a rooftop or a wind blasted summit.
A pinnacle landing is challenging even before you add enemy fire, darkness, or time pressure. Winds gather force as they sweep up the slope. At the same time the Chinook’s engine loses ability with high altitude and high temperature.
Pilots need to adjust the power level to sustain a hover by considering the current altimeter and pressure-altitude reading as well as the engine temperature, any of which may be unreliable whether it’s from a lack of field data for remote deployments or combat damage to the Chinook.
“Make room for error; don’t figure in a wind factor when determining the power required to hover,” says Randall Padfield in Learning to Fly Helicopters. “If no wind, you have the correct figure; if windy, which is very likely, the increased performance will be gravy. Don’t go in unless you have a huge power reserve and an extremely important reason for landing at the site.”
May 31, 2011
In 1966, Second Lieutenant Larry Liss was on the Czech-German border during a snowstorm, freezing his varlata off, when he saw something beautiful. It was a Bell UH-1 helicopter, still on the ground. The pilot—who was wearing short sleeves and drinking a cup of coffee—took one look at Liss and shook his head. “He said, ‘You’re such a jerk’—he used other words; I’m trying to keep it clean—” recalls Liss more than 40 years later. “I said ‘Why?’ He said, ‘Look at you. You’re freezing your ass off, it’s snowing, look at me, I’m warm. You should sign up for helicopter training.’ ” So Liss did.
Four days later he was stateside. Because of a pilot shortage, the U.S. Army had created an accelerated training program: Four months primary instruction at Fort Wolters, followed by advance training at Fort Rucker. The first five hours were spent learning how to hover. “My whole class,” says Liss, “was able, after a couple of hours, to take the helicopter up to a hover. I was already in the ninth hour and still couldn’t hover. And you have to solo in 10 hours. So I’m beginning to panic.” Out of 84 pilots, Liss was ranked 82.
But this self-described “really bad pilot,” along with pilots Tom Baca and Jack Swickard, and engineer Al Croteau, went on to save more than 100 South Vietnamese troops ambushed by the North Vietnamese Army—using an unarmed VIP Huey. Their heroic story is the subject of the Smithsonian Channel film “Helicopter Missions: Vietnam Firefight.”
It was May 14, 1967, and Baca had just 12 days to go in-country when he and Liss get the call about a group of soldiers that needed rescuing. “We were the only helicopter there,” says Baca, “and they needed our help. We were not going to say no.” When they got to the coordinates, they realized there was no landing zone; tall bamboo covered the area where they were meant to set down. Baca and Liss decided that the lack of a landing zone wouldn’t slow them down: they decided to use the Huey’s rotor blades to slice through the bamboo canopy. “We were a lawnmower, basically,” recalls Baca. Fully aware that damage to the underside of the rotor blade could cripple their helicopter, the two carved out a landing zone and set down. They picked up six casualties and headed back to camp. On the 15-minute ride, they got another desperate message: The remaining men were pinned down by a battalion of 600 men, and the entire company needed rescuing. It took Baca and Liss, along with a second helicopter piloted by Swickard and engineer Al Croteau, 11 hours to evacuate the men. On their final run, the defense perimeter was under the rotor blades.
In the film, the men return to the scene of their amazing rescue four decades later.
“Helicopter Missions: Vietnam Firefight” is available on demand from the Smithsonian Channel. Check your local listings to learn more.
May 13, 2011
Gamera, you’ll recall from Japanese horror movies, was a giant, fire-breathing, flying turtle that used to terrorize Tokyo (and battle Godzilla) back in the 1960s.
So what else would students at the University of Maryland—whose mascot is a terrapin—name their flying contraption, which yesterday appears to have become the first human-powered helicopter to fly. The flight didn’t last long, or get far off the ground. In fact, don’t blink (at about the 3:18 mark of this video) or you’ll miss it. But congratulations to the Maryland team, and to “pilot” Judy Wexler, whose furious four-limbed pedaling raised Gamera a few inches off the floor.
The team’s ultimate goal is winning the $250,000 Sikorsky Prize, which requires that they hover for 60 seconds, and reach a height of three meters. This worries me a little. How much faster can Judy pedal?
May 6, 2011
Plenty of buzz going around about the mysterious stealth chopper left behind by U.S. Navy SEALs after they shot and killed Osama bin Laden last Monday morning, local time, in Pakistan.
Having suffered technical problems and a hard landing, the helo apparently couldn’t fly back out of bin Laden’s compound, and became evidence to be destroyed by the SEALs as they left in the other helicopter(s). The obvious aim was to keep the stricken helo from falling into the hands of the Pakistanis, and thereby, their allies the Chinese. So the SEALs blew it up, probably with grenades.
In the fireball began a firestorm of speculation among flight buffs about what exactly the aircraft was.
The consensus is that the helo was a stealthed-out version of the H-60 Black Hawk, made by Sikorsky. The tail boom, which survived the explosion and fire relatively intact, didn’t look much like anything the rest of us have seen. Case in point: a “hubcap” over the tail rotor, presumably to muffle the noise and hide it from radar, and angular lines reminiscent of stealth airplanes. According to an anonymous, retired special operations aviator interviewed by the Army Times, “Certain parts of the fuselage, the nose and the tail had these various almost like snap-on parts to them that gave it the very unique appearance.” The rotor also appears to have more blades than a typical rotor, which could reduce noise levels or allow it to operate at a lower rpm, also reducing noise.
While the technology might be new, the idea of a “black” helo isn’t. Here’s a story about one in Vietnam.
More interesting, maybe, is that cell phone service and electricity in the vicinity of the compound went down just before the SEALs arrived, and came back up after they left, indicating that either the choppers or an aircraft loitering above employed advanced electronic countermeasures to further pull the wool over bin Laden’s eyes. It’s well known that electronic warfare airplanes such as the EA-18G Growler can jam wireless signals. But it is major news that they can jam electricity running through wires on the ground, if indeed that is what was happening.
Along with the Stealth Hawks, the new jamming abilities appear to be one more spoonful of secret potion now spilled to the world. And ain’t we just lappin’ it up?’
Air & Space contributor in the field Ed Darack, who spent time on the ground and in the air in Afghanistan in the V-22 Osprey and other rotorcraft (here’s his story on the Osprey in Afghanistan), knows a thing or two about helicopters. He emails from Ft. Collins, Colorado that the Black Hawk is one of the most conspicuous and mass produced helicopters in the history of military aviation, and “has a number of variants that the general public does know about, including those for special operations purposes, although these are seldom photographed.”
Darack sends a few images below that show a U.S. Air Force Special Operations MH-60G Pave Hawk. “The ‘M’ designates ‘modified,’ ” says Darack, “typically for special operations purposes. I was lucky to get these shots at the end of the expeditionary airstrip at the Marine Corps Mountain Warfare Training Center near Bridgeport, California, preparing to photograph a Marine Corps CH-46 Sea Knight “Phrog,” when out of nowhere this Pave Hawk showed up during high altitude training to refuel.
“The pilots didn’t seem to mind me photographing them and their aircraft,” he continues. “One of the distinguishing characteristics of the Pave Hawk is the refueling probe for extended duration flight. Was this model the progenitor to the ‘Stealth 60″ of the bin Laden raid? Maybe, but without a doubt, knowledge gleaned by pilots who fly these in training and actual combat went into crafting the mystery helicopter.”
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