Flettner double rotor

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The Flettner double rotor ( intermeshing rotor ) is a construction principle for helicopters that uses two rotors coupled via a gearbox, whose axes of rotation are inclined at a small angle to each other and whose rotor heads are side by side at a relatively small distance in the direction of flight. With two-blade rotors, the blades of both rotors are at right angles to each other. The first helicopter to use this principle, named after its developer Anton Flettner , was the Flettner Fl 265 , built in six copies and tested by flight captain Richard Perlia from May 1939. The Flettner Fl 282 , with which Ludwig Hofmann first flew on October 31, 1941 , was only ready for operation.

Flettner Fl 282

The Flettner double rotor should not be confused with the Flettner rotor by the same inventor, a drive for ships that uses the Magnus effect .

construction

Flettner double rotor in detail. The rotor blades are not in the normal flight position, but have been swiveled into the "parking position" in the blade holders in order to save space in the hangar .

With this rotor configuration, the rotor heads of the two main rotors are not arranged one above the other as in the coaxial rotor , but side by side. Since the pylons of the rotor masts and thus the axes of the rotor shafts are in an upwardly open V-position to each other, in contrast to the two-rotor tandem configuration , the rotor surfaces overlap for the most part, the rotor planes are seen from the front in the V- Angle of the rotor masts to each other. To ensure that the rotor blades do not touch each other when they mesh with one another, the opposing rotors must be permanently synchronized by the main gearbox (with the usual two-blade rotors with a 90 ° phase offset to one another).

These structural features ensure that the rotor blades always move alternately just above the neighboring rotor mast. In order for the rotor blades to have enough ground clearance at the edge of the rotor circumference due to the outwardly inclined axis of rotation, the rotor heads must be arranged high enough.

The system is also known as intermeshing rotors .

Slow motion film of a Flettner double rotor (Kaman K-Max), the Flettner flaps are also clearly visible

control

It is controlled by cyclic and collective rotor blade adjustment, with the models currently in use usually with Flettner flaps , i.e. small control flaps attached to the end strips of the rotor blades at around 3/4 of the rotor radius, which are operated by push rods inside the blades and which, through their control deflection, bring the rotor blade, which is freely rotatably mounted in the longitudinal axis of the blade, into the required angle of attack position.

The control is carried out by a linkage similar to the swashplate in both rotor heads:

  • Ascent / descent is controlled by simultaneous and equally large changes in the angle of attack of all rotor blades (collective pitch ).
  • Nodding movements are caused by a common cyclic adjustment of the two rotors forwards and backwards;
  • Around the longitudinal axis ( rollers ), simultaneous cyclical blade adjustment on both rotors is used to control the desired rolling direction.
  • To yaw around the vertical axis, for example, with the Kaman K-Max, the pitch components of both rotors (collective blade adjustment) are adjusted differently in opposite directions by a certain amount in order to generate a desired, uncompensated torque through a higher resistance of a rotor; at the same time, to support the yaw movement, one of the rotors is steered forward and backward in the same direction with cyclical blade adjustment.

(Source: K-MAX Flight Manual)

Advantages and disadvantages

Kaman K-Max with Flettner double rotor. Recognizable on the rear edge of the rotor blades: the Flettner flaps for controlling the blade pitch.
Kaman K-1200 K-Max from the front. The outwardly inclined V-position of the two rotor shafts is clearly visible.

Advantages of the Flettner double rotor are:

  • No torque transmission to the airframe, so no tail rotor is necessary and no drift is generated (see tail rotor configuration ).
  • The power requirement for the tail rotor is omitted and is therefore available for lifting and propulsion.
  • The gearbox and the shaft for the tail rotor are omitted.
  • Compared to the tandem configuration , the advantage lies in the space-saving design with a central drive, compared to the coaxial rotor in the simpler mechanics.
  • Due to the mostly larger total rotor area compared to a single rotor system, the same load can be lifted with less power. For example, the Kaman K-Max has a similar performance to the Eurocopter EC 135 or Bell 429 , but has a significantly higher maximum take-off weight ( MTOW ). Similarly, types with a similar take-off weight such as Bell 412 or Kaman SH-2 have a considerably higher power requirement.

Disadvantages are:

  • The main gear is more complex than a single rotor.
  • The cruising speed is lower due to the lower power required - but also the considerably larger total rotor area.
  • The rotor plane has very little ground clearance in the lateral area transverse to the longitudinal axis due to the sideways inclined rotor masts, which requires great caution on the part of the pilot and ground staff (see the warning label "Warning - approach from front" or "Warning - only approach from the front" on the Rotor pylons of the two helicopters shown).

This results in advantageous areas of application in which high payloads are often flown in hovering flight or only at moderate speed (during assembly work as a flying crane, external load flights in forestry or construction) and the achievable flight speed and the distance to be covered are of secondary importance.

Models built according to the Flettner principle

Web links

Individual evidence

  1. ^ Steve Coates: German helicopters 1930–1945. Stuttgart 2004, p. 85.