Introduction To Rotary-wing Flight

Introduction to Rotary-Wing Flight

"Helicopter pilots don't fly, they beat the air into submission!" This is a popular saying among pilots of rotary-wing aircraft, who consider themselves to be a unique breed, not like fixed-wing pilots. It is easy to see why—helicopters seemingly defy the laws of physics. The only thing that keeps them aloft is a constantly beating rotor, which acts as a rotating wing, producing lower pressure above the rotor and higher pressure below it and thus pushing the helicopter up. While fixed-wing aircraft progressed rapidly once the Wright Brothers proved that powered flight was feasible, rotary-wing aircraft advanced slowly. Rotary-wing engineers had to overcome several major hurdles before helicopters became possible, and they had to advance rotary-wing technology further before helicopters became practical. With early fixed-wing aircraft, the propulsion system (the engine and propeller) and the lift device (the wing) were relatively simple and the challenge was developing control systems to keep the aircraft in level flight and to change direction. But with helicopters, the propulsion system, the lift device, and the control systems were all part of the same mechanism. This presented several major challenges. The first challenge was the need to provide sufficient lift. With a fixed-wing aircraft, even a relatively low-powered engine could propel an aircraft fast enough for its wings to generate lift. But with a helicopter, more powerful, and lighter engines were needed and it was many years before relatively lightweight, powerful enough engines were developed that could turn a rotor fast enough to life an aircraft off the ground. It was not until the 1930s that such engines existed to run a helicopter's rotors. Another major problem was control. Once the helicopter was lifted off the ground, how could the thrust be aimed so that the aircraft moved in a specific direction? The solution to this—pitching the rotor blades so that they pushed the aircraft in a certain direction—was difficult to perfect, after all, the control devices themselves were constantly moving at high speed, not affixed to an unmoving wing. There were also a lot of other problems that designers had to overcome, such as the tendency for the rotors to flap. An additional problem was that the whirling rotors produced a twisting motion, or "torque" in the opposite direction. This motion had to be counteracted. Eventually, the most common way of accomplishing this was by placing a small vertical propeller on a tail so that it blew air in the direction opposite of the twisting force, thereby preventing the helicopter from spinning around and around. Finally, there was the ever-present headache of vibration. Early fixed-wing aircraft had a small propeller spinning at a high rate of speed close to the engine. The propeller rotated, but did not otherwise change pitch or angle. But a helicopter's rotor was large, spun very fast, and was connected to the engine by a relatively long drive shaft. In addition, the engine's power had to be redirected from a horizontal drive shaft to a vertical one and an additional drive shaft had to be run to the tail rotor. This created a lot of vibration, and early helicopters threatened to shake themselves apart. Even today, after over six decades of rotary-winged flight, many helicopters vibrate considerably more than aircraft. The first successful helicopter was built by French aviation pioneer Jacque Bréguet in the mid-1930s. Bréguet's Gyroplane-Laboratoire flew in 1935. Although it established a number of records, it was still a limited craft, with relatively short range and lifting capability. The next major advance in helicopter flight was made by Germans Heinrich Focke and Gerd Achgelis. Their Fa 61 aircraft used a conventional biplane fuselage, but instead of wings it had two large rotors mounted on outriggers on either side of the aircraft. The Fa 61 had its first flight in 1936 and was a far more capable aircraft than Bréguet's machine. It was the first truly practical helicopter. But it also had serious control problems in low speed turns—a flaw that was inherent to the layout of the rotors. During the late 1930s, another person was experimenting with helicopters. His name was Igor Sikorsky and he was a Russian ‚migr‚ to the United States. Sikorsky's VS-300 made its first free flight in 1940. Unlike the earlier French and German designs, Sikorsky's helicopter used a small tail-rotor to control torque. During World War II, although the Germans fielded small numbers of helicopters, Sikorsky built hundreds. They served in minor support roles during the last year of the war.

After World War II ended in 1945, the United States continued to lead the world in helicopter development. A number of new helicopter companies were created. Bell developed its highly successful Model 47. Piasecki soon developed a tandem rotor helicopter called the "Flying Banana," which had a large cabin capable of carrying a number of passengers or cargo. This was the forerunner to several later highly-successful tandem rotor designs. By the 1950s, Bell began development of the first mass-produced helicopter powered by a jet turbine. Designated the HU-1 by the U.S. Army, it quickly became known as the Huey and by the early 1960s Hueys were being procured in large numbers, with over 15,000 eventually entering service. (By that time they had been re-designated the UH-1.) The Huey was the most successful helicopter ever built and its distinctive "whomp-whomp" sound could be heard miles away. The Soviet Union did not begin developing practical helicopters until the late 1940s. Its first massproduced helicopters entered service in the 1950s. The largest and most powerful helicopter in the world was built by the Soviets. It was known as the Mi-6 Hook and entered service in 1957. It was soon followed by the even larger and more powerful Mi-6 Harke. By the early 1960s, the Mil design bureau developed the Mi-8 Hip, the most prolific non-western helicopter to enter service, with over 10,000 produced by the 1990s. The United States used helicopters in many new and innovative roles. Beginning in the 1950s, the U.S. Navy equipped helicopters with sonar that could be lowered into the water to listen for submarines and torpedoes to kill the subs that they found. The SH-3 Sea King entered service in this role and served for over three decades as a submarine hunter. The United States was the first country to arm its operational helicopters, conducting experiments in 1956. By the early 1960s, the U.S. Army was deploying armed helicopters to Vietnam. By 1967 it was operating the HueyCobra, a dedicated gunship intended to provide support for troops on the ground. Vietnam proved to be a major testing ground for the helicopter and a new form of warfare known as air mobility. No longer would armies fight each other along long fronts on the ground. Now they could be lifted deep into enemy territory, striking suddenly and powerfully. The Bell Huey and dedicated troopcarrying and heavy-lift helicopters like the CH-46 Sea Knight and the CH-47 Chinook were used to ferry troops in large helicopter assaults. Although the United States lost the Vietnam War, it emerged with a new way of fighting battles and other countries followed its example. By the 1970s, the United States began development of a new generation of helicopters based upon its experience in Vietnam. Both the UH-60 Black Hawk troop carrying helicopter and the AH-64 Apache attack helicopter were better protected against ground fire. The Soviets developed the Mi-24 Hind gunship. Although it was as powerful as it was ugly, it proved too large for its intended mission and eventually the Russians decided to develop smaller, more maneuverable gunships. Although helicopters were first and foremost military aircraft, they soon entered civilian service as well. Early civilian helicopters were used for various roles, including lifting cargo to inaccessible areas and the evacuation of critically injured patients. They also entered service with law enforcement, being used by various police departments to search for fleeing suspects and to monitor traffic. They are also used for drug interdiction and border patrol. But their cost and complexity limited their commercial service for many years. The Huey was marketed to commercial firms as a cargo-carrying helicopter beginning in the early 1960s. Some other helicopters were also used to ferry passengers, particularly between urban "heliports" and large airports. But they never proved very successful in this role. One area that helicopters proved successful was carrying crews to offshore oil rigs. By the 1970s and later, helicopters were in service in a large number of commercial roles, from crop dusting to searching for schools of tuna to lifting lumber out of forests to carrying rich business executives to meetings. Despite all of these uses, the helicopter has remained a limited aircraft and has never entered service in the same numbers as conventional aircraft. They are expensive, difficult to maintain, and have a short range, slow speed, and limited carrying capability. Attempts to develop hybrid aircraft that have the speed and range of aircraft and the vertical liftoff capabilities of helicopters have not been very

successful. The best example of such an aircraft is the Bell-Boeing V-22 Osprey tilt-rotor. But the search for an aircraft that can operate like a helicopter—but lacks its limitations—continues. —Dwayne A. Day