Intermeshing rotors

The Flettner rotor Double (german intermeshing rotor ) is a design principle for helicopters that uses two rotors whose axes are inclined at a small angle to each other and their rotor heads in a relatively short distance in the direction of flight based side are adjacent. The first helicopter that used this named after its developer Anton Flettner principle, was built in just six copies Flettner Fl 265, which was tested by captain Richard Perlia from May 1939. Operation reached maturity but only the Flettner Fl 282, on October 31, 1941 first flew with the Ludwig Hofmann.

The Flettner rotor double is not to be confused with the Flettner rotor of the same inventor, a vessel propulsion system that exploits the Magnus effect.

Construction

In this configuration, the rotor rotor heads of the two main rotors are not above the other as in the coaxial rotor, but side by side. Since the pylons of the rotor poles and thus the axis of the rotor shafts are available in an open-ended V- position to each other, the rotor circuit surfaces overlap in contrast to the also zweirotorigen tandem configuration here for the most part, the rotor planes are here viewed from the front in the V- angle of the rotor poles to each other. In order to touch the rotor blades during the engagement combing not, the counter-rotating rotors must firmly synchronized by the main gear (in case of the usual two-bladed rotors with 90 ° phase shift to each other).

These design features ensure that the blades always move alternately just above the adjacent rotor mast away. Thus, the rotor blades through the outwardly inclined rotation axis have enough ground clearance outside circumference of the rotor, rotor heads must be arranged accordingly high.

The system is also called the intermeshing rotors.

Control

The control is effected by cyclic and collective rotor blade adjustment, wherein the currently used specimens usually with flettner flaps so small, attached to the trailing edges of the rotor blades at about 3/4 of the rotor radius of the control valves, which are actuated by push rods in the leaves and bring their tax by the rash in the blade longitudinal axis freely rotatably mounted rotor blade to the required angle position.

It is operated by one of the swashplate linkage similar in both rotor heads:

  • Climb / descent is controlled by the simultaneous and equal large change in the angle of attack of all the rotor blades ( collective pitch).
  • Pitching movements are effected by a common cyclic movement of the two rotors to the front and rear;
  • About the longitudinal axis (roll) is controlled by the simultaneous cyclic pitch on both rotors in the jeweiligs desired rolling direction.
  • To yaw around the vertical axis, the pitch components of the two rotors ( collective pitch ) differentiated by a certain amount in the opposite direction adjusted to produce by higher resistance of a rotor one wanted, uncompensated torque for example, when Kaman K- Max; simultaneously the yaw motion is controlled by cyclic pitch in the same direction each one of the rotors to the front or rear for support.

(Source: K- MAX Flight Manual )

Pros and Cons

Advantages of the Flettner rotor are double:

  • No torque generation, thus no tail rotor is necessary, and there will be no drift generated (see tail rotor configuration).
  • The power requirement for the tail rotor is eliminated and is thus for lifting and jacking available.
  • The gear and the shaft for the tail rotor omitted.
  • Compared to the tandem configuration is the advantage in space-saving design with a central drive, opposite the coaxial rotor in the simpler mechanics.
  • Because of the generally higher overall rotor surface against a Einrotorsystem the same load can be lifted at a lower power. For example, the Kaman K -Max has a similar performance as the Eurocopter EC 135 or Bell 429, but has a much higher maximum takeoff weight (MTOW ) on. Analog have types with same start weight such as Bell 412 or Kaman SH-2 has a considerably higher power requirements.

Disadvantages are:

  • The main transmission is more complex than with a single rotor.
  • The cruising speed is due to the lower power needed - but also the much larger total rotor area - lower.
  • The rotor plane is in the side area abeam to the longitudinal axis due to the sideways inclined rotor poles a very low ground clearance, which takes great care of pilots and ground staff (see the warning saying " Warning - approach from Front " or " Caution - to approach only from the front " on the rotor pylons of the two pictured helicopters).

An advantageous application areas thereby resulting areas is where flown with a heavy load often in hover or at a moderate speed ( for installation work as a flying crane, external load flights in forestry or construction area ) and the achievable flight speed and distance to be traveled have rather low priority.

Built models after the Flettner principle

  • Flettner Fl 265
  • Flettner Fl 282
  • Kaman K 225
  • Kellett XR -8 and 10
  • Kaman HTK and HOK
  • Kaman HH -43 Huskie
  • Kaman K -Max

The Kaman K -Max comes in the transport of external loads and to fight forest fires used.

Hangar Rotorhead Helicopter rotor Contra-rotating propellers Swashplate (aeronautics) Blade pitch
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