Rotor definition

@ROTOR_DEFINITION {
@ROTOR_NAME {RotorName} {
@NUMBER_OF_BLADES {Nbld}
@ROTOR_SPEED {Ω}
@ROTOR_RADIUS {R}
@SHAFT_FRAME_NAME {shaftFrameName}
@LIFTING_LINE_LIST {LfnName1, LfnName2,... LfnNameN}
@INFLOW_NAME {InflowName}
@SENSOR_NAME {sensorName}
@NUMBER_OF_HARMONICS {Nharm}
@REFERENCE_LENGTH {Lref}
@REFERENCE_CHORD_LENGTH {cref}
@COMMENTS {CommentText}
}
}

NOTES

  1. A rotor is a component of an aerodynamic model and is defined as a collection of rotating lifting lines.
  2. The rotor is characterized by a number of blades, Nbld, an angular velocity, Ω, and a radius, R.
  3. The definition of the axis systems associated with the rotor relies on the definition of the shaft frame, shaftFrameName, which is used to construct the rotor frame.
  4. The rotor is associated with a collection of lifting lines, LfnName1, LfnName2, ..., LfnNameN, specifies all the lifting lines associated with this rotor.
  5. The induced flow generated by the unsteady circulation associated with the lifting lines of this rotor can be evaluated using an inflow model, InflowName.
  6. If the response of the rotor is periodic, it is often convenient to expand rotor specific quantities in Fourier series. In that case, the basic frequency of the Fourier transform is the rotor speed and a total of Nharm will be used in the transform.
  7. Lref and cref are required when AirloadsScheme = OVERFLOW. They are used for non-dimensionalization of airloads.
  8. It is possible to attach comments to the definition of the object; these comments have no effect on its definition.

Sensors

Sensors can be defined to extract information about rotors. The following SensorType and associated FrameName specifications are allowed for rotors: TOTAL_AIRLOADS. (Default value: TOTAL_AIRLOADS).

No u value or v value are accepted for rotors.

Axis systems

Figure 1 shows the axis systems used to represent typical rotorcraft problems. Although the conventions might be slightly different from one code to the other, the following axis systems are typically present.

  1. The Inertial frame is defined as FI = [O, I = (i1, i2, i3)], where point O is an inertial point and I a fixed orthonormal basis. This is the fundamental inertial frame used in the dynamic analysis.
  2. The Fuselage frame is defined as FF = [C, BF = (f1, f2, f3)], where point C is the center of mass of the rotorcraft and BF a fuselage attached orthonormal basis, BF. f1 is pointing to the front of the fuselage, f2 is pointing to the right of the fuselage, and f3 is pointing down. This system defines the directions forwards (or backwards), right (or left), and downwards (or upwards), on the fuselage, even in the case of a maneuver involving large angle rotations for the rotorcraft.
  3. The Shaft frame is defined as FS = [H, BS = (s1, s2, s3)], where point H is the hub point and BS a body attached orthonormal basis. This system is rotating with the shaft and, for clarity, is not depicted in fig. 1. s3 is pointing down the rotor shaft.
  4. The Rotor frame is defined as FR = [H, BR = (r1, r2, r3)], where point H is the hub and BR an orthonormal basis. This system is non-rotating. r1 is pointing right, and r3 is pointing down the rotor shaft.
  5. The Hub frame is defined as FH = [H, BH = (h1, h2, h3)], where point H is the hub and BH a body attached orthonormal basis. This system is rotating. h1 is pointing along the blade in its reference, un-deformed configuration, h2 towards the leading edge and h3 is pointing along the shaft.
  6. The Airstation frame is defined as FA =[A, BA = (a1, a2, a3)]), where point A is at the airstation location on the blade and BA a blade attached orthonormal basis. This system is moving with the blade. a1 is pointing towards the blade tip, a2 towards the leading edge and a3 is pointing upwards. One airstation frame is defined for each airstation. These frames are indirectly defined together with the lifting line: the frame has its origin at the location of the airstation. The orientation of the frame is determined by the triad defined at the airstation. Finally, the motion of the airstation is dictated by that of the associated beams, see section airstation motion.
Figure 1. Axis systems for a rotor.

These frames are defined in the following manner.

  1. The fuselage frame, FF. This frame is a fixed frame if the rotorcraft fuselage center of mass is an inertial point, or a moving frame if the fuselage center of mass is moving, as would be the case for maneuvering flight. The fuselage frame is defined in the aerodynamic interface definition.
  2. The shaft frame, FS. This frame is a fixed frame if the tip of the shaft is an inertial point, or a moving frame if the shaft is moving. The frame triad defines the direction s3 down the shaft. The other two directions are in the plane normal to s3, but otherwise arbitrary. The shaft frame is defined in rotor definition section.
  3. One hub frame, FH, per blade. These are moving frames rotating with the blades. A hub frame must be defined for each blade and is attached to the rotating shaft, at the hub. The hub frames are defined in section lifting line.

Based on the definitions of these frames, the rotor frame is then constructed. The rotor frame has its origin at the hub point, the origin of the shaft frame. The orientation of the rotor frame is defined according to the following relationships.

  • The third axis is parallel to the third axis of the shaft frame, s3,     r3 = s3.
  • The first axis is in the direction of the second axis of the fuselage frame, f2, i.e. to the right of the rotorcraft r1 = f2 - (r3T f2) r3. The second term in this equation orthogonalizes r1 to r3 by implying that r3Tr1 = 0.
  • Finally, the second axis is such that r2 = r3 x r1.

Clearly, this frame translates with the hub, changes its orientation according to the tilting of the shaft tip, but does not rotate with the shaft. It correctly defined the plane of the rotor even when the rotorcraft performs a large angle maneuver. For a tilt-rotor aircraft, the tilting of the nacelle takes place about the f2, and hence, the above definitions yield a proper rotor frame in helicopter mode, forward flight, and transition phase.