Compound Helicopters: Current and Proposed Concepts - Literature review Example

?Compound Helicopters 0 Introduction Among Leonardo da Vinci’s works in the 15th century can be found a sketch of a machine that, with its screw-type propeller, seemed meant to achieve vertical flight. It was Gustave de Ponton d’Amecourt of France, however, that coined the word ‘helicopter’ in 1861.1 At about the same time, Mortimer Nelson of the United States was granted patent by the US Patent Office for his design of a flying machine, which he called Aerial Car. The flying machine had a length of 33 feet and weighed half a ton and was meant to be used with a hot air balloon. It had a tapered body, a pear-shaped rudder at the stern, and two standing poles attached at the top of the fuselage with two sets of rotors meant to spin in opposite directions, acting as the lifting and propelling forces.2 Nelson calculated that the smaller set of rotors located at the rudder will propel the car to a speed of up to 180 mph. The Nelson Speed Limit holds true for helicopters even today. 3 Although helicopters have achieved what is considered the “Holy Grails of powered flight,”4 because of their ability to fly vertically, their limited capacity in speed and maneuverability have led aircraft builders to explore ways to augment performance metrics. One of the solutions offered by research is the compounding of the basic design of conventional helicopters with additional components to help achieve better speed and efficiency. Background: Compound Helicopters A compound helicopter is a conventional helicopter that has undergone modification with the addition of several components for the purpose of augmenting and enhancing basic performance metrics such as lift-to-drag ratio, propulsive efficiency and maneuverability.5 The most overt additions in a compound helicopter are fixed wings.6 Compound helicopters are often called hybrids because they are a combination of conventional helicopters, which are powered by rotors, and fixed wing aircrafts like planes, although some compound helicopters do not have wings. Compound helicopters have the important capability of conventional helicopters - VTOL (vertical take-off and landing).7 What additional features precisely make a helicopter compound has not been unanimously agreed however. John Watkinson, for example, described a compound helicopter as one in which the production of forward thrust in cruise is not up to the rotor but by some other device while Ray Prouty depicted them as having wings and a propulsion device, which could be a jet engine, ducted fan, or propellers for the purpose of alleviating the rotors from lifting or propelling tasks.8 Leishman simply defined it as one in which additional parts are appended for the purpose of enhancing and augmenting basic performance metrics.9 Figure 1 shows a diagram of a compound helicopter with a four-blade main rotor atop its body as is commonly found in conventional helicopters. However, fixed wings or flaperons can also be observed attached to its fuselage. Fixed wings function to off-load most, if not all, of the rotor’s duty to lift the helicopter at high speed. The presence of a ducted propeller at the rear can also be observed, which likewise serves the function of taking away from the main rotor the task of driving the helicopter forward. Some compound helicopters may or may not have either fixed wings or a propulsion mechanism other than the main rotor, but all additional appendages in a compound helicopter serve the purpose of driving it at speed not available to conventional helicopters.10 Fig 2 Diagram of a Compound Helicopter11 2.0 History of Compound Helicopters The first known compound helicopter was developed in Germany in the 1930s by Anton Flettner, but two other models were also developed in that part of the world. The Fl 184 had two airscrews, fitted with propellers at each end facing at opposite directions, and attached to either side of the helicopter fuselage. The airscrews functioned to counter the torque effect of the three-blade rotor and helped the aircraft propel forward. The Wn 342 was designed as observation platform during WWII and was equipped with three-bladed rotors that had jet tips for pumping fuel with air into the rotor blades to propel the rotor to spin. It was discovered later that its maximum endurance was only 15 minutes due to its high fuel consumption. On the other hand, the single-seater VFW H2 had a rotor propulsion mechanism driven by an air compressor, which forced cold air into and out of the blades causing them to spin. A later version, the VFW H3 had no tip-jets and forward flight was driven by two seven-bladed ducted propellers, attached at each side of the fuselage while the rotor autorotated.12 In the UK, compound helicopters were first developed in 1946 by the Fairey Aviation Company Limited, which built two models. The Fairey Gyrodyne had a three-blade rotor, a horizontal tailpin at the rear upon with vertical fins attached at either side in addition to a set of wings attached at mid-level fuselage. The Jet Gyrodyne built on that model and added rotor jet-tips to a much larger rotor and a set of wings fitted with propellers. The added weight and power made it difficult for the aircraft to sustain level flight and vertical landing became dangerous because of power shifting during descent. A later 1957 model, the Fairey Rotordyne added a jet-tipped rotor, but emitted noise that did not meet the 96 decibels requirement.13 In Russia, the first compound helicopter designed was the Kamov Ka-22 in 1954, which was impressive in size, speed and vertical lifting capability. Fig. 4 shows that the Ka-22 had four-bladed rotors attached to the aircraft’s wings, driven by a 5,900 hp Soloviev TV-2VK turboprop engine also found at the tips of the wings. The program was eventually scrapped after two separate incidents involving Ka-22 that occurred in 1962 and 1965.14 15 In the US, six aircraft manufacturing companies launched their own programs for compound helicopters: Gyrodyne Company of America (GCA); the McDonnell Aircraft Corporation; the Piasecki Aircraft Corporation; Bell Helicopter Company; Kaman Aircraft Corporation; Lockheed Aircraft Corporation, and; Sikorsky Aircraft. In 1949, the GCA-2A, a modification of the coaxial Bendix Model J, was tested. This model had outriggers on the left and right sides with a 100 hp Continental engine each to power the propellers. In 1954, McDonnell built the first compound helicopter that first saw flight in the US. The McDonnell XV-1 was an experimental aircraft and had a 550 hp Continental R-975-42 engine as propulsion driver while propellers were attached to the sides at the rear. The XV-1 and its subsequent variation had a cruise speed of 138 mph (top speed at 203 mph), with the wings providing 85% of the total lift and the remaining 15% by the rotor blades autorotating. 16 In 1962, the 16H-1 Pathfinder was tested by Piasecki Aircraft Corporation. The 16H-1 was remembered for its obtrusive Ring-Tail – a 5.5 ft three-bladed ducted propeller at the tail end of the aircraft that was responsible for forward propulsion. It had fixed wings, retractable main landing gear and a tail wheel at the rear portion. It achieved a 170 mph top speed, which fell short of the company’s aim 220 mph target. Pathfinder II was launched in 1965, with a more powerful engine with its 1,250 shp General Electric T58-GE- 8 shaft-turbine engine, larger rotor, longer fuselage, new drive system and propeller.17 Lockheed Aircraft Corporation, for its part, tested the XH-51A. A set of 16.9 feet long wings with spoilers were added and the engine was changed to a 2,500 lb static thrust Pratt & Whitney J60-P-2 turbojet. At this configuration, the aircraft achieved a maximum speed of 272 mph and reached 230 mph in 45 minutes from a hover. When additional bracing to the windshield was made to avert aerodynamic pressures caused by high speed, it achieved a top speed of 302.6 mph. The downside to this is high fuel consumption. In 1967, Lockheed developed the AH-56A Cheyenne, which was envisaged as escort and support for ground troops during the Vietnam War. It was powered by a 4350 shp GE T64-GE-16 gas turbine and had features such as wings, 10 ft pusher propeller, main and tail rotors and a sophisticated weapon system. 18 The aircraft was able to achieve a forward speed of 195 mph, sideways speed of 27.5 mph and rearward speed of 23 mph. However, it became unstable at a speed of more than 200 mph and was bugged by a technical problem called ‘1/2 P hop’. In 1969, this technicality caused a Cheyenne to crash when rotor oscillations caused the rotor to hit the fuselage and sliced it in half.19 Fig 5 Cheyenne20 Meanwhile, Sikorsky Aircraft was also conducting its own test for compound helicopters beginning in 1964. It tested the S-61F, S-69, S-72, S-72X, and finally the X-Wing. The S-61F, which modified the SH-3A conventional helicopter, had retractable landing gear, powered by two 3,000 lb static thrust Pratt & Whitney J60-P-2 turbojet engines, large horizontal stabilizer, 170sq ft wings and new six-bladed rotors. It had achieved a maximum 255 mph speed in its 113 test flights. In 1972, the company tested the Advancing Blade Concept, where two rotors were built to spin in opposite directions under a project called S-69 (or XH-59A). The model, which was without wings, eventually achieved a top speed of 303 mph. Nonetheless, the model had vibration issues and weight and drag problems. The S-72, on the other hand, was equipped with main and tail rotors, retractable landing gear, full-size wings with adjustable incidence, foot stabilizer, turbofan engines and ejector seats. Finally, the X-Wing system had a rotor system that was used only for lifting and landing and was stopped once the aircraft was aloft to serve as wings in the shape of an X. It was approximated that the rotor could be stopped at the speed of 196 mph and the aircraft can reach the top speed of 518 mph with the rotor stopped. 21 3.0 Advantages/Disadvantages of Compound Helicopters over Conventional Helicopters The problem with conventional helicopters, which compound helicopters seek to correct or amend are retreating blade stall and advancing blade compressibility. The airflow that goes through the rotor disc is not the same throughout but depends on the position of its blades. Fig. 4 shows a rotor disc. When in motion, the blade that moves parallel to the direction of the aircraft (right side in a forward motion) are in an advancing position and when moving parallel to the opposite direction (left side in a forward motion) area are said to be in a retreating position.22 The airflow at the right side is greater than at the left because of the forward direction of the helicopter. Thus, to generate the same amount of lift throughout the rotor disc, the advancing blade flaps up while the retreating blade flaps down. The result is a higher angle of attack on the retreating blade and an opposing lower angle of attack on the advancing blade. The problem, however, begins when the helicopter pushes forward at a higher speed because the difference of airflow between the advancing and retreating blades becomes greater. The high angle of attack in addition to a low blade speed causes the retreating blade to lose lift and stall. This impedes the helicopter’s speed and is usually manifested by low frequency vibration, upward nose pitch and rolling towards the retreating blade side.23 Fig 6 Retreating and Advancing Blade24 On the other hand, advancing blade compressibility occurs when the helicopter moves at a high speed causing additional airwave to the top of the advancing blade. This causes the formation of shockwaves on top of the advancing blade that separates airflow from the blade top further resulting in loss of lift, increase in drag and the erratic twisting of blade pitch angle. Advancing blade compressibility is manifested by airframe vibration, rolling tendency towards the side of the advancing blade, and an upward nose pitch.25 The compound helicopter seeks to solve the problems brought about by the structural mechanism of the simple helicopter by postponing the commencement of advancing blade compressibility or retreating blade stall. This is done by having the auxiliary propulsion mechanism take over the function of thrusting the helicopter forward and reducing the power supplied to the rotor resulting in slowing down the blade rotation.26 4.0 Configuration A compound helicopter, as earlier discussed, is a modified version of a conventional helicopter. Lockheed’s XH-51A Compound, for example, was built on the original XH-51A conventional helicopter and Sikorsky’s S-61F was a modification of the conventional SH-3A Sea King.27 These modifications come in the form of added wings, supplementary propulsion new rotor design, transmission and control. 4.1 Wing Design The addition of a set of wings in compound helicopters is significant in increasing the speed of the aircraft. It does this by delaying and reducing retreating blade stall. However, the presence of both rotor and wing in one aircraft may result in two negative effects: the inherent vorticity in the rotor may cause the uneven lift distribution of fixed wings, and; the position of the fixed wings in the fuselage vis-a-vis the rotor can exacerbate rotor problems. Some of the solutions to these problems include the use of variable incidence wing, flaps, spoiler and speed brakes, appropriate wing plan form geometry, choice of aerofoil section, proper circulation control, wing location and other enhancements.28 The use of variable incidence wing rather than a fixed incidence wing is useful to eliminate the download produced by the fixed incidence wing and 0° flap angle during lift and the stall produced by a 20° wing angle-of-attack during descent. A variable incidence wing allows the pilot to determine and adjust the angle-of-attack of the wings during lift and descent to prevent the download and stall they cause, respectively.29 The only downside to the use of this kind of wing is that it adds to the weight of the aircraft and therefore, limits the installation of other important components like the undercarriage and fuel tank.30 Another solution to the download and stall during climb and descent, respectively, is the use of flaps as an enhancement to plain fixed wings. Since the addition of a variable incidence wing exacerbates complexity and weight, flaps can be used instead to control wing lift by minimizing wing drag. On the other hand, spoilers and brakes act to destroy wing lift in cases where the rotor needs to initiate autorotation as when the engine falters. Autorotation becomes difficult with wings because they significantly cover the rotor from the up-flow, which can initiate autorotation. Another important consideration is the design of the shape, size and other geometric concerns relative to the wings structure and position vis-a-vis the rotor. In planning the wing geometry, the main concerns are maneuverability at certain speeds and the prevention of retreating blade stall. The size, shape and the position of the wings in the fuselage relative to the position of the rotor determine these concerns. Thus, a relatively smaller set of wings allow more maneuverability and hovering efficiency, but a bigger set of wings can make the aircraft go longer distances. Bigger wings, however, take up more area and bigger load, which can both affect hover and low-speed flight, in addition of depriving the aircraft of other add-ons that can add more weight. The use of circulation control in tandem with relatively smaller wings can solve the lift coefficient issue of smaller wings. 31 Wing location is a very important consideration in the making of compound helicopters. Aside from the earlier discussed wing-rotor and rotor-wing interferences, wing location can also affect ease in cabin entry and egress. Moreover, the engines can also be affected by flow disturbances in the case of wings attached too high in the fuselage. Other enhancements are also useful in solving the problems brought about by the wing-rotor combination such as leading edge umbrellas and slats, which can lessen drag coefficients. Another device is the installation of prism on the surface of the upper wing, which has proven to reduce download by 30%. 32 4.2 Supplementary Propulsion A supplementary propulsion system enhances the performance and efficient productivity of compound helicopters. This system can take the form of a turbojet, turbofan, propeller and ducted fan. The use of turbojets in the past proved to be largely due to convenience, but studies have shown that their use present a lot of disadvantages such as noise and inefficiency. They use up a lot of fuel, which add to overall cost of flying as well a weight, which can reduce the aircraft’s payload capability. Moreover, it makes the aircraft exposed to heat-seeking missiles. Turbofans, on the other hand, offer more practicality in terms of efficiency because it is less noisy and use up less fuel. The downside, however, is that they still add to costs and are redundant during hover flight. Propellers are another form of supplementary propulsion and are considered better than both turbofans and turbojets in terms of efficiency, but they also have disadvantages. Propellers are heavy and noisy and can affect the rotor when the latter’s blades pass through their pressure fields. Moreover, their usual exposed state can pose danger to ground personnel. Finally, ducted fans (see Fig. ) are not only efficient supplementary propulsion but are safer because of controlled airflow that secures less interference. The duct also prevents debris from scattering out when the fan fails and makes work of the ground crew safer. Ducted fans are also less noisy. 33 5.3. Transmission A compound helicopter poses transmission problems because of the several components present in its system that simultaneously need power. Possible solutions to this problem include the use of additional systems that are either mechanical or pneumatic, variable-cycle engines and electric drives. Other than their weight, mechanical drives have proven to be efficient and safe. Recent advances in material technology could also mean that future mechanical drives would be less heavy and will no longer add to the payload penalties of a compound helicopter. Pneumatic systems have been installed in some compound helicopter models to drive the rotor disc from the blade tips. This system provides three advantages: it eliminates the torque that is transmitted from the rotor to the fuselage; it can turn on and off autorotation, and; it controls rotor circulation. On the other hand, the use of the variable cycle engine, a modified gas turbine engine that allows control of power delivery, can eliminate redundancy and ease up payload penalty. Variable cycle engines can either be the use of separate turbines for each component that needs power, a single source of power that is connected to the rotor and pitch fan and a variable-area final nozzle. The advantage in the first is total control of each propulsion system, but the drawback is a possible compromised efficiency system. In the second system, where the pitch determines the amount of power to be delivered to the rotor, the advantage is decreasing mechanical complexity and maximizing geometric simplicity. Finally, the third type of pneumatic system allows the nozzle to be adjusted to conform to the amount of power needed by the jet thrust. This is the least complicated systems of the three. Figure __ shows the schematics of variable cycle propulsion using a single source of power with controlling valves that determine the amount of power to be delivered to the rotor and the supplementary propulsion system. 34 Fig. Variable Cycle Propulsion35 5.0 Conclusion: Current and Proposed Concepts At present, some aircraft manufacturing companies are conducting continuing research on compound helicopters. NASA likewise had conducted research for the best design of a compound helicopter with explicit weight and speed specifications. Other countries are also into compound helicopter research. Bell Helicopter, for example, is researching on PATS or Propulsive Anti-Torque System. The PATS system seeks the installation of a propulsion system in the tailcone of a helicopter with anti-torque capability and forward propulsion without the need of wings and tail rotor. The system is supposed to eliminate the weight problem associated with compounding. 36 On the other hand, Piasecki is developing the Vector Thrust Ducted Propeller or VTDP, which is similar to the Pathfinder II Ring Tail, but added advances in aerodynamics and thrust propulsion movement. Meanwhile, Sikorsky announced its newest project: the X2. The X2 promises more speed and maneuverability with the use of a coaxial ABC rotor system and pusher propeller. In addition, Carter Aviation Technologies and Groen Brother Aviation are also into more research on compound helicopter technology with the CAT Heliplane and GBA Heliplane. 37 At NASA, research was conducted in 2006 for the make and design of a compound helicopter that would weigh 100,000 lb with just one rotor and can fly at the top speed of 250 knots (287 mph) at 4000 ft/95°. Using CAMRAD II as rotorcraft analysis system, the research concluded that with respect to design parameters, a lower wing loading and higher design blade loading will result in an augmented aircraft lift-to-drag ratio, while the best sharing ratio between rotor and wing should be, rotor = 8-9% gross weight and wing = 91-92% gross weight.38 On the other hand, Indonesian and Malaysian researchers joined hands in exploring new design concepts for a compound helicopter that is a efficient, stable and fast but cost-effective at the same time. Specifically, these researchers investigated the possibility of building a compound helicopter with fixed wings, one propeller in the nose and the basic components of a conventional helicopter. The program is called the LCH Compound Helicopter.39 References: 2009, Retreating Blade Stall copters.com http://www.cantrell.biz/copters/aero/retreating-blade-stall/. 2010, UH-60/VTDP Compound Helicopter Search. http://www.personal.psu.edu/~users/j/f/jfh19/brg_webpage/compound_helicopter.html. Allen, P. (1996) The Helicopter. http://www.aviastar.org/helicopters_eng/lok_cheyenne.php. Edi, P & Yusoff, N & Yazid, A. (2009) New design approach of compound helicopter, WSEAS Transactions on Applied and Theoretical Mechanics Vol. 9(3):799-808. Chiles, J. (2009) Hot-rod helicopters: there’s just no way to add 100 mph to the speed of a helicopter. Or is there? Air & Space Magazine. http://www.airspacemag.com/flight-today/Hot-Rod-Helicopters.html?c=y&page=3. Federal Aviation Administration. 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(1953) ‘Flying Windmills’ The History of Helicopters. http://www.aviastar.org/history/index.html. Stepniewski, W. (1979) Rotary-Wing Aerodynamics. Courier Dover Publications. Van Sickle, N. & Welch, J. & Bjork, J. & Bjork, L. (1999) Van Sickle's Modern Airmanship. 8th Edn, McGraw-Hill Professional. Winchester, J. (2005) ‘Fairey Rotordyne’ The World’s Worst Aircrafts. http://aviastar.org/helicopters_eng/fairey_rotodyne.php Yeo, H & Johnson, W. (2006) Optimum Design of a Compound Helicopter. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070017929_2007016665.pdf. Read More