Tandem rotors

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CH-46 Sea Knight USMC CH-46.jpg
CH-46 Sea Knight
Animation Tandem rotor.gif

Tandem rotor helicopters have two large horizontal rotor assemblies mounted one in front of the other. Currently this configuration is mainly used for large cargo helicopters. [1]

Helicopter Type of rotor craft in which lift and thrust are supplied by rotors

A helicopter, or chopper, is a type of rotorcraft in which lift and thrust are supplied by rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forward, backward, and laterally. These attributes allow helicopters to be used in congested or isolated areas where fixed-wing aircraft and many forms of VTOL aircraft cannot perform.

Helicopter rotor rotary wings and control system that generates the lift and thrust for a helicopter

A helicopter main rotor or rotor system is the combination of several rotary wings and a control system that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor, which connects through a combination of drive shaft(s) and gearboxes along the tail boom. The blade pitch is typically controlled by a swashplate connected to the helicopter flight controls. Helicopters are one example of rotary-wing aircraft (rotorcraft). The name is derived from the Greek words helix, helik-, meaning spiral; and pteron meaning wing.


Single rotor helicopters need a mechanism to neutralize the yawing movement produced by the single large rotor. This is commonly accomplished by a tail rotor, coaxial rotors, and the NOTAR systems. Tandem rotor helicopters, however, use counter-rotating rotors, with each cancelling out the other's torque. Therefore, all of the power from the engines can be used for lift, whereas a single rotor helicopter uses some of the engine power to counter the torque. [1] An alternative is to mount two rotors in a coaxial configuration. The first successful tandem rotor helicopter was built by Nicolas Florine in 1927.

Aircraft principal axes

An aircraft in flight is free to rotate in three dimensions: yaw, nose left or right about an axis running up and down; pitch, nose up or down about an axis running from wing to wing; and roll, rotation about an axis running from nose to tail. The axes are alternatively designated as vertical, transverse, and longitudinal respectively. These axes move with the vehicle and rotate relative to the Earth along with the craft. These definitions were analogously applied to spacecraft when the first manned spacecraft were designed in the late 1950s.

 These rotations are produced by torques about the principal axes. On an aircraft, these are intentionally produced by means of moving control surfaces, which vary the distribution of the net aerodynamic force about the vehicle's center of gravity. Elevators produce pitch, a rudder on the vertical tail produces yaw, and ailerons produce roll. On a spacecraft, the moments are usually produced by a reaction control system consisting of small rocket thrusters used to apply asymmetrical thrust on the vehicle.
Tail rotor small tail-mounted helicopter rotor

The tail rotor is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity allow it to develop thrust in the same direction as the main rotor's rotation, to counter the torque effect created by the main rotor. Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis.


NOTAR is a helicopter system which avoids the use of a tail rotor. It was developed by McDonnell Douglas Helicopter Systems. The system uses a fan inside the tail boom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom utilizing the Coandă effect. The boundary layer changes the direction of airflow around the tailboom, creating thrust opposite the motion imparted to the fuselage by the torque effect of the main rotor. Directional yaw control is gained through a vented, rotating drum at the end of the tailboom, called the direct jet thruster. Advocates of NOTAR believe the system offers quieter and safer operation over a traditional tail rotor.

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, [1] and the need for two large rotors.

The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set of scales or load cells. The center of gravity affects the stability of the aircraft. To ensure the aircraft is safe to fly, the center of gravity must fall within specified limits established by the aircraft manufacturer.

Transmission (mechanics) machine in a power transmission system for controlled application of the power;gearbox,uses gears/gear trains to provide speed,torque conversions from a rotating power source to another device;reduces the higher engine speed to the slower wheel speed

A transmission is a machine in a power transmission system, which provides controlled application of the power. Often the term transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.

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. [2]

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. [3]

Flight dynamics (fixed-wing aircraft) science of air vehicle orientation and control in three dimensions

Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of gravity (cg), known as pitch, roll and yaw.

Helicopter flight controls

A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way. To tilt forward and back (pitch) or sideways (roll) requires that the controls alter the angle of attack of the main rotor blades cyclically during rotation, creating differing amounts of lift (force) at different points in the cycle. To increase or decrease overall lift requires that the controls alter the angle of attack for all blades collectively by equal amounts at the same time, resulting in ascent, descent, acceleration and deceleration.

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. [4] [5] Tandem rotor helicopters tend to have a lower disk loading than single rotor helicopters. [6]

Disk loading

In fluid dynamics, disk loading or disc loading is the average pressure change across an actuator disk, such as an airscrew. Airscrews with a relatively low disk loading are typically called rotors, including helicopter main rotors and tail rotors; propellers typically have a higher disk loading. The V-22 Osprey tiltrotor aircraft has a high disk loading relative to a helicopter in the hover mode, but a relatively low disk loading in fixed-wing mode compared to a turboprop aircraft.

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. [7]

List of some tandem rotor helicopters

Columbia Helicopters Boeing Vertol 107-II and Boeing 234 PH-10171a.jpg
Columbia Helicopters Boeing Vertol 107-II and Boeing 234
A Piasecki H-21B at Elmendorf AFB in the early 1960s Piasecki H-21B USAF.jpg
A Piasecki H-21B at Elmendorf AFB in the early 1960s
A Yakovlev Yak-24 at Central Air Force Museum Yakovlev Yak-24 at Central Air Force museum (3).jpg
A Yakovlev Yak-24 at Central Air Force Museum

See also

Related Research Articles

Tiltrotor powered lift convertiplane where transition is accomplished by tilting the rotors between horizontal and vertical positions

A tiltrotor is an aircraft which generates lift and propulsion by way of one or more powered rotors mounted on rotating engine pods or nacelles usually at the ends of a fixed wing or an engine mounted in the fuselage with drive shafts transferring power to rotor assemblies mounted on the wingtips. It combines the vertical lift capability of a helicopter with the speed and range of a conventional fixed-wing aircraft. For vertical flight, the rotors are angled so the plane of rotation is horizontal, lifting the way a helicopter rotor does. As the aircraft gains speed, the rotors are progressively tilted forward, with the plane of rotation eventually becoming vertical. In this mode the wing provides the lift, and the rotor provides thrust as a propeller. Since the rotors can be configured to be more efficient for propulsion and it avoids a helicopter's issues of retreating blade stall, the tiltrotor can achieve higher speeds than helicopters.

Piasecki Helicopter Corporation was a designer and manufacturer of helicopters located in Philadelphia and nearby Morton, Pennsylvania, in the late 1940s and the 1950s. Its founder, Frank Piasecki, was ousted from the company in 1956 and started a new company, Piasecki Aircraft. Piasecki Helicopter was renamed Vertol Corporation in early 1956. Vertol was acquired by Boeing in 1960 and renamed Boeing Vertol.


Contra-rotating, also referred to as coaxial contra-rotating, is a technique whereby parts of a mechanism rotate in opposite directions about a common axis, usually to minimise the effect of torque. Examples include some aircraft propellers, resulting in the maximum power of a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are also common in some marine transmission systems, in particular for large speed boats with planing hulls. Two propellers are arranged one behind the other, and power is transferred from the engine via planetary gear transmission. The configuration can also be used in helicopter designs termed coaxial rotors, where similar issues and principles of torque apply.

Piasecki X-49 experimental aircraft by Piasecki Aircraft

The Piasecki X-49 "SpeedHawk" is an American four-bladed, twin-engined experimental high-speed compound helicopter developed by Piasecki Aircraft. The X-49A is based on the airframe of a Sikorsky YSH-60F Seahawk, but utilizes Piasecki's proprietary vectored thrust ducted propeller (VTDP) design and includes the addition of lifting wings. The concept of the experimental program was to apply the VTDP technology to a production military helicopter to determine any benefit gained through increases in performance or useful load.

Boeing X-50 Dragonfly experimental drone by Boeing

The Boeing X-50A Dragonfly, formerly known as the Canard Rotor/Wing Demonstrator, was a VTOL rotor wing experimental unmanned aerial vehicle that was developed by Boeing and DARPA to demonstrate the principle that a helicopter's rotor could be stopped in flight and act as a fixed wing, enabling it to transition between fixed-wing and rotary-wing flight.

Coaxial rotors

Coaxial rotors or "coax rotors" are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (contra-rotating). This tiltrotor configuration is a feature of helicopters produced by the Russia Kamov helicopter design bureau.

Gyrodyne rotorcraft

A gyrodyne is a type of VTOL aircraft with a helicopter rotor-like system that is driven by its engine for takeoff and landing and also includes one or more conventional propellers to provide forward thrust during cruising flight. Lift during forward flight is provided by a combination of the rotor, like an autogyro, and conventional wings. Due to a number of issues, there is some confusion over the term "gyrodyne", and the terms compound helicopter and compound gyroplane are frequently used to describe the same design. The gyrodyne is one of a number of similar concepts which attempt to provide helicopter-like low-speed performance and conventional fixed-wing high-speeds, including tiltrotors and tiltwings.

Rotorcraft Heavier-than-air aircraft which generates lift over rotating wings

A rotorcraft or rotary-wing aircraft is a heavier-than-air flying machine that uses lift generated by wings, called rotary wings or rotor blades, that revolve around a mast. Several rotor blades mounted on a single mast are referred to as a rotor. The International Civil Aviation Organization (ICAO) defines a rotorcraft as "supported in flight by the reactions of the air on one or more rotors". Rotorcraft generally include those aircraft where one or more rotors are required to provide lift throughout the entire flight, such as helicopters, autogyros, and gyrodynes. Compound rotorcraft may also include additional thrust engines or propellers and static lifting surfaces.

Powered lift aircraft capable of powered vertical takeoff and landing but which operates as a fixed-wing aircraft during horizontal flight

Powered lift or powered-lift refers to a type of aircraft that can take off and land vertically and functions differently from a rotorcraft in horizontal flight.

Fairey FB-1 Gyrodyne

The Fairey FB-1 Gyrodyne was an experimental British rotorcraft that used single lifting rotor and a tractor propeller mounted on the tip of the starboard stub wing to provide both propulsion and anti-torque reaction.

Piasecki PA-97 experimental helicopter/airship by Piasecki Aircraft

The Piasecki PA-97 Helistat was an American experimental heavy-lift aircraft, built by Piasecki by fastening four H-34J helicopters to a framework beneath a helium-inflated blimp envelope. It crashed during a test flight, killing one of the four pilots.

CJC-3 was an experimental tandem rotor helicopter developed in Taiwan during the 1950s under the director of Major General C.J. Chu (朱家仁).

Future Vertical Lift (FVL) is a plan to develop a family of military helicopters for the United States Armed Forces. Five different sizes of aircraft are to be developed, sharing common hardware such as sensors, avionics, engines, and countermeasures. The U.S. Army has been considering the program since 2004. FVL is meant to develop replacements for the Army's UH-60 Black Hawk, AH-64 Apache, CH-47 Chinook, and OH-58 Kiowa helicopters. The precursor for FVL is the Joint Multi-Role (JMR) helicopter program.

Berliner Helicopter

The Berliner Helicopters were a series of experimental helicopters built by Henry Berliner between 1922 and 1925. The helicopters had only limited controllability but were the most significant step forward in helicopter design in the USA until the production of the Vought-Sikorsky VS-300 helicopter in 1940. The 1922 flights of the Berliner and the de Bothezat H1 were the first by manned helicopters.

Multirotor rotorcraft with more than two rotors

A multirotor or multicopter is a rotorcraft with more than two rotors. An advantage of multirotor aircraft is the simpler rotor mechanics required for flight control. Unlike single- and double-rotor helicopters which use complex variable pitch rotors whose pitch varies as the blade rotates for flight stability and control, multirotors often use fixed-pitch blades; control of vehicle motion is achieved by varying the relative speed of each rotor to change the thrust and torque produced by each.


  1. 1 2 3 "Archived copy". Archived from the original on 2014-02-18. Retrieved 2017-12-03.CS1 maint: archived copy as title (link)
  2. Art of the helicopter by John Watkinson, p. 13
  3. "US Army FM 3-04.203 Fundamentals of Flight; page 1-33, para 1-81, 1-82" (PDF).
  4. Principles of helicopter aerodynamics by J. Gordon Leishman p. 73.
  5. Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 197.
  6. Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 185
  7. Rotary Wing Aerodynamics, W.Z. Stepniewski, p. 200.