ver thirty years after the Wright Brothers absolutely right in terms of a so-called “pure” helicop- attained powered, heavier-than-air, fixed-wing ter. However, the quest for speed in rotary-wing flight Oflight in the United States, Germany astounded drove designers to consider another option: the com- the world in 1936 with demonstrations of the vertical pound helicopter. flight capabilities of the side-by-side rotor Focke Fw 61, The definition of a “compound helicopter” is open to which eclipsed all previous attempts at controlled verti- debate (see sidebar). Although many contend that aug- cal flight. However, even its overall performance was mented forward propulsion is all that is necessary to modest, particularly with regards to forward speed. Even place a helicopter in the “compound” category, others after Igor Sikorsky perfected the now-classic configura- insist that it need only possess some form of augment- tion of a large single main rotor and a smaller anti- ed lift, or that it must have both. Focusing on what torque tail rotor a few years later, speed was still limited could be called “propulsive compounds,” the following in comparison to that of the helicopter’s fixed-wing pages provide a broad overview of the different helicop- brethren. Although Sikorsky’s basic design withstood ters that have been flown over the years with some sort the test of time and became the dominant helicopter of auxiliary propulsion unit: one or more propellers or configuration worldwide (approximately 95% today), jet engines. This survey also gives a brief look at the all helicopters currently in service suffer from one pri- ways in which different manufacturers have chosen to mary limitation: the inability to achieve forward speeds approach the problem of increased forward speed while much greater than 200 kt (230 mph). Despite the retaining the helicopter’s unparalleled advantages in tremendous impact of the helicopter, Sikorsky realized vertical flight. the inherent speed limitations even at the outset, and While the last 70 years has seen more than two dozen predicted that the speed of the helicopter would never different combinations of rotors with propellers or jets, match that of the airplane. For the most part, he was the compound is by no means ancient history. Today 30 VERTIFLITE Summer 2006 we see numerous new concepts for compound helicop- ters, capable of both hover and high-speed forward What is a Compound? flight. Within the next few years, we should see the pro- Mike Hirschberg, Managing Editor peller-augmented Sikorsky X2, the ducted propeller rof. Barnes McCormick tells the story that when he first Piasecki X-49A, and the jet-powered Groen published his Aerodynamics of V/STOL Aircraft (Academ- Brothers/Adam Aircraft Heliplane all begin flight tests. P ic Press, 1967), he printed an artist’s concept of a tandem In addition, Bell completed successful ground tests of its rotor aircraft with stub wings on both the fore and aft rotor Propulsive Anti-Torque System (PATS), planned for the pylons as an example of a compound. Frank Piasecki called him up and said the example given was not a compound now-cancelled Unmanned Combat Armed Rotorcraft because it didn’t have any auxiliary propulsion. He sent (UCAR), and is considering it for other applications. McCormick a photo of the Piasecki Pathfinder, which is what Additionally, compounds continually emerge from is found in the later editions. high-speed rotorcraft studies, whether sponsored by the This vignette highlights a lack of agreement within the Army, NASA or internally funded. Naturally, there are rotorcraft community that continues to this day on exactly weight, drag and fuel consumption penalties from what defines a compound helicopter. Does a rotorcraft need adding wings, propellers or jets, so designers must care- wings, an auxiliary propeller or jet, or both to be classified as a compound? Two excerpts from rotorcraft design books fully consider the mission requirements to determine show the nuances in attempts to develop a cohesive defini- the optimal solution for a design. While no compound tion: helicopter has ever reached production, the future has • The Art of the Helicopter by John Watkinson (Elsevier Butter- never looked brighter. worth-Heinemann, 2004) offers: “The compound helicop- ter…is one in which the rotor does not produce any for- The Hybrids Emerge ward thrust in cruise. Instead the thrust is provided by other means.” an’s quest to fly has resulted in a multitude of • Ray Prouty, in Military Helicopter Design Technology (Krieger, 1989) states: “These aircraft may be considered to be aero- aircraft configurations over the years, each of planes which have an effective low-speed lifting device. Mwhich has had its own advantages and disad- They use a wing and some form of auxiliary propulsion vantages when compared with other designs in the such as propellers, ducted fans, or jet engines to relieve the methods by which it defeats the pull of gravity and rotor of all, or nearly all, or its lifting and propulsive duties overcomes the forces of aerodynamics. This menagerie at high speeds.” of aircraft sometimes makes it difficult to place them in Other references are more generic in stating that a com- a specific category. A good example of this is the tiltro- pound is a helicopter that has a wing or an auxiliary propul- sor or both. Perhaps the best way to understand what people tor, which has obvious features of both a conventional mean when they say “compound” would be to differentiate fixed-wing airplane and a helicopter. The inability to between the two types. According to Principles of Helicopter easily categorize aircraft as either an “airplane” or a Aerodynamics by Prof. J. Gordon Leishman (Cambridge Uni- “helicopter” will likely become increasingly prominent versity Press, 2003): as new technologies emerge and aircraft continue to A compound helicopter involves a lifting wing in addition evolve. As indicated in the sidebar, a compound heli- to the main rotor (lift compounding) or the addition of a sepa- copter may or may not include some form of augment- rate source of thrust for propulsion (thrust compounding)…. The ed lift, such as a fixed wing, depending on what the idea is to enhance the basic performance metrics of the helicop- ter, such as lift-to-drag ratio, propulsive efficiency, and maneu- designer hopes to achieve. When fitted, a wing is verability. The general benefit can be an expansion of the flight designed to offload much, if not all, of the rotor’s lifting envelope compared to a conventional helicopter. duties at high speed. Likewise, forms of augmented In other words, a “compound” may involve both lifting propulsion serve to relieve the main rotor of the major- and/or propulsive compounding. The goal is to “off-load” the ity, if not all, of its duties in propelling the aircraft for- rotor from its normal lifting and propulsive duties. This can be ward. Whether or not it includes a wing, the compound done by using a wing and/or auxiliary propulsion. Thus, the compounds described in this article would best helicopter is designed for one primary purpose: to per- be described as “thrust compounding” rotorcraft. Although mit forward speeds higher than those that are possible adding a wing may allow higher speeds, the real “quest for with conventional rotorcraft. speed” has required an auxiliary propulsion device to acceler- In a pure helicopter, two specific factors limit forward ate the rotorcraft beyond conventional helicopter capabilities. speed. One is retreating blade stall and the other is As can be seen in the article, unloading the rotor to the max- advancing blade compressibility. A compound helicop- imum amount possible, such that the rotor autorotates, ter is able to reduce or delay the onset of the negative allows an autogyro-like maximum speed while still preserving a hover and vertical flight capability. In fact, one could argue factors associated with both of these problems by limit- that there are only shades of gray – or maybe only semantics ing or even reducing the speed of rotation in the main – between a “compound helicopter” and an autogyro that is rotor as the aircraft gains forward speed by way of its powered for hover, take-off and landing. Vehicles like the XV- auxiliary propulsion. This is achieved by reducing the 1 and Rotodyne spent (figuratively) 95% of their mission fly- power supplied to the main rotor, the degree to which ing as an autogyro and only 5% as a helicopter. All other con- is determined by the amount of forward thrust pro- cepts discussed in this article approached this extreme in duced by the auxiliary propulsion. At the same time, varying degrees by unloading the rotor in flight. Vol. 52, No. 2 31 vertical flight capabilities are retained by diverting sion of a compound helicopter. This aircraft, known as power back to the main rotor as the aircraft slows down the Fl 185, took to the air for its maiden flight in 1936. or enters a hover. While allowing increased forward (A preceding design, the Fl 184 may also qualify as a speeds, inherent design features generally make a compound, but it is unclear if it was capable of hover- winged compound helicopter less efficient in both ing or was merely a powered autogyro, as limited infor- flight regimes when compared to its respective fixed- mation on it has survived.) wing and rotary-wing counterparts. Unlike contemporary autogyro designs equipped Despite the penalties inherent in the compound hel- with propellers, this was a true helicopter, capable of icopter, such as increased weight and complexity, vertical takeoff, landing, and hovering. The most designers have returned to this hybrid configuration unusual feature of the Fl 185 was the method by which numerous times in attempts to meet the demanding torque from the 39.3 ft three-bladed rotor was coun- requirements for speed.