Tandem helicopter describes a helicopter configuration in which two main rotors of the same size are used as a rule , whose counter torques ( yaw moment ) are balanced by the opposite direction of rotation. There is no need for a tail rotor and the full engine power is available for lift. In principle, this also applies to helicopters with transverse rotors (side by side, mostly on booms) and the Flettner double rotor , which, however, have different flight characteristics.
The rotors are arranged one behind the other in the direction of flight, the rear of the two rotor levels always being higher than the front. The coupling of both rotors via their drive shafts ensures that they run synchronously in all operating states. The rotors are still set in such a way that they mesh with one another when viewed from above, i.e. two blades are never on top of one another and therefore cannot touch each other when they flap.
The design requires a minimum center distance of the two rotors of more than one blade length. For fluidic reasons, however, they are arranged so far from one another that the rotor planes usually only slightly overlap. This inevitably results in a great length of the helicopter.
In the case of helicopters with a total mass of less than nine tons, the tail rotor concept is seen as having a clear advantage, up to a take-off weight of around thirty tons the two systems are seen as roughly equivalent, while the tandem design has clear advantages for heavier helicopters. The following advantages and disadvantages of the tandem design compared to a conventional main or tail rotor system come into play:
- Since the tail rotor is omitted, which would otherwise reduce the power by up to 25%, the entire drive power is available here to generate thrust. There is also no lateral drift.
- The design effort for the tail rotor is eliminated, while the two main rotors - apart from the different directions of rotation - can be constructed largely identically.
- With the same blade tip speed, the smaller diameter tandem rotors can rotate faster, with lower torque and thus lower weight of the gear unit .
- The great difficulty of maintaining the narrow limits of the center of gravity with the single rotor because of the loss of control travel that is otherwise associated with this, is eliminated with the tandem arrangement. Shifts in the center of gravity can be controlled through differentiated collective adjustment between the front and rear rotor.
- Such helicopters tend not to hold the fuselage in the direction of flight on its own, but rather to move with sliding angles until it is completely transverse. With tail rotor helicopters of the long tail boom acts because of the large distance from the center of gravity due to the wind vane effect at least at higher forward speeds as rudder and causes a certain orientation and greed -Stabilization the cell toward the Fahrtwindanströmung. It is different with tandem arrangements, where the long fuselage has large lateral surfaces both in front of the center of gravity and behind it, which tend to favor transverse positioning in relation to the direction of flight rather than have a stabilizing effect in the direction of flight. Holding straight requires constant control inputs from the pilot and still remains “spongy”. In some situations the lack of longitudinal stability is advantageous because the tandem helicopter is not very sensitive to where the wind is coming from.
- The masses, which are arranged far from the center of gravity, result in a higher moment of inertia around the vertical axis and thus less maneuverability.
- Provided the helicopter is not hovering, the two rotors arranged directly one behind the other have a significantly smaller unimpaired air mass available than with a single rotor or two rotors arranged next to one another. With these, each rotor or the individual rotor captures its own stationary air mass in order to accelerate it downwards to generate lift, while with the tandem, despite the higher-seated rear rotor, a large part of the air that has already been accelerated downwards by the front rotor blades as an effect of lift generation is still there comes into the area of the rear rotor. This downwash component generated by the front rotor impairs the generation of lift in the rear rotor circular area and, overall, a greater drive power is required for the same lifting force. The disadvantage can be avoided, for example, if a tandem helicopter makes its climb with the fuselage almost transverse, at least until the increase in fuselage resistance requires the normal forward direction to be taken.
- Due to the assembly of the engines in the stern area near the rear main gear, which is common with current tandem helicopter types, a relatively long connecting shaft from the rear to the front rotor gear is required. This is structurally strong and fail-safe. In particular, this shaft has to ensure a reliable synchronization of the rotors at all times, also, for example, in the event that an autorotation landing is necessary due to engine malfunctions .
- The gearbox and rotor head are among the most complex and expensive components of a helicopter, which significantly increases the costs of a tandem arrangement, both in terms of acquisition and maintenance.
- The rigidity of the long fuselage is a major challenge in the construction: it is subject to heavy loads on bending in the pitch axis and vertical axis as well as on torsion in the longitudinal axis.
As with single rotors, it is controlled by swash plates in the two rotor heads:
- Ascent and descent are controlled by changing the setting angles of all rotor blades simultaneously and of the same size (collective pitch ).
- Nodding movements are caused by a differential collective adjustment and collective cyclical adjustment (lengthways) of the two rotors, which is superimposed on the regular overall pitch input.
- The longitudinal axis ( rollers ) is controlled by collective cyclic blade adjustment (laterally) on both rotors.
- To rotate ( yaw ) around the vertical axis, the two rotor planes are cyclically adjusted differentially (sideways).
The most successful manufacturer of helicopters with tandem rotors is to this day Boeing-Vertol with two models that were built in civil and military versions:
Other common tandem helicopters were:
Tandem model helicopter
Tandem helicopters are also used as model helicopters. As a rule, these basically correspond in construction to the man-carrying specimens; In practice, however, instead of shaft drives, the more readily available toothed belts are used to connect the rotor gears , which also ensure synchronization of the rotors. For reasons of center of gravity, the drive motor is usually arranged centrally between the main rotors. The recently used indoor ready-made models with tandem coaxial rotors deviate from the design principles in that the speed control (each individual rotor is driven by its own electric motor, i.e. four motors in total) with a fixed pitch angle are required . Here, the distance between the main rotors is often chosen so large that they do not have to be synchronized due to the lack of overlap. Different control modes (e.g. pitch control by cyclical forward control instead of pitch differentiation) are also possible here. What is technically remarkable about these light models is that they combine basic design principles (coaxial rotors in tandem), which have not been implemented in this combination in man-carrying aviation up to now.
- Raymond W. Prouty: Helicopter Aerodynamics. Ray Prouty's Rotor and Wing Columns 1979-2002. Helobooks, Mojave CA 2004, ISBN 0-9726368-6-2 .