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A tandem-rotor aircraft is an aircraft with two large helicopter rotor assemblies mounted one in front of the other in the horizontal plane. This configuration is mainly used for large cargo helicopters. Such aircraft are often informally referred to as "Chinooks," after the CH-47 Chinook, one of the first widely adopted heavy-lift helicopters with a tandem-rotor configuration.
Advantages of the tandem rotor system are a larger center of gravity range and good longitudinal stability. Disadvantages of the tandem rotor system are a complex transmission and the need for two large rotors.
The two rotors are linked by a transmission that ensures the rotors are synchronized and do not hit each other, even during an engine failure.Art of the helicopter by John Watkinson, p. 13
Tandem-rotor designs achieve yaw by applying opposite left and right cyclic to each rotor, effectively pulling both ends of the helicopter in opposite directions. To achieve pitch, opposite collective is applied to each rotor; decreasing the lift produced at one end, while increasing lift at the opposite end, effectively tilting the helicopter forward or back.
Tandem-rotor helicopters have the advantage of being able to hold more weight with shorter blades, since there are two sets. However, the rear rotor works in the aerodynamic shadow of the front rotor, which reduces its efficiency. This loss can be minimized by increasing the distance between the two rotor hubs, and by elevating one hub over the other.Principles of helicopter aerodynamics by J. Gordon Leishman p. 73.Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 197. Tandem-rotor helicopters tend to have a lower disk loading than single-rotor helicopters.Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 185
Tandem-rotor helicopters typically require less power to hover and achieve low-speed flight as compared to single-rotor helicopters. Both configurations typically require the same power to achieve high-speed flight.Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 200.
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