Rotor

The rotor element is used to associate the aerodynamic elements that model the blades of a helicopter rotor when some inflow related computations are required. By means of different inflow models, and by means of the aerodynamic load contributions supplied by the aerodynamic elements, the rotor element is able to compute the induced velocity at an arbitrary point on the rotor disk. This velocity term in turn is used by the aerodynamic elements to determine a better estimate of the boundary conditions. The syntax of the rotor element is:

    <normal_arglist> ::= <craft_node> ,
            [ hinge , (Mat3x3)<rotor_orientation> , ]
        <rotor_node> ,
        induced velocity , <induced_velocity_model>

The optional rotor_orientation is required when axis 3 of the craft_node is not aligned with the rotor axis; axis 3 of the rotor_node must be aligned with the rotor axis.

There are five models of induced velocity. The first is no induced velocity; the syntax is:

    <induced_velocity_model> ::= no

There is no argument list. This element doesn't compute any induced velocity, but still computes the rotor traction for output purposes, if output is required. The others have a fairly common syntax. The first three are uniform, glauert and mangler induced velocity models:

    <induced_velocity_model> ::= { uniform | glauert | mangler } , 
        <reference_omega> , <reference_radius> 
        [ , ground , <ground_node> ]
        [ , delay , (drive_caller) <memory_factor> ]
        [ , max iterations , <max_iterations> ]
        [ , tolerance , <tolerance> ]
        [ , eta , <eta> ]
        [ , correction , <hover_correction_factor> ,
            <ff_correction_factor> ]

• The reference_omega field is used to decide whether the induced velocity computation must be inhibited because the rotor speed is very low.

• The reference_radius field is used to make the rotor related parameters non-dimensional.

• The ground parameter is used to inform the rotor about the proximity to the ground; the z component of the distance between the rotor and the ground nodes, in the ground node reference frame (direction 3, positive), is used for an approximate correction of the axial inflow velocity [11].

• The memory_factor, the hover_correction_factor and the ff_correction_factor (forward flight) are used to correct the nominal induced velocity, according to the formula
  

  $$\left(\vphantom{ 1 - \mathtt{memory\_factor} }\right. \_factor \left.\vphantom{ 1 - \mathtt{memory\_factor} }\right)$$ U_effective =

  $$\_factor$$

  
  with
  U_nominal = $${\frac{{T}}{{2 \rho A V_{\text{tip}} \sqrt{ \cfrac{\lambda^2}{\m... ...orrection\_factor}^4} + \cfrac{\mu^2}{\mathtt{ff\_correction\_factor}^2} }}}}$$
  The delay parameter is used to linearly combine the current reference induced velocity with the induced velocity at the previous step; no delay means there is no memory of the previous value. The memory_factor parameter defaults to 0. The hover_correction_factor and ff_correction_factor parameters default to 1.

• The max_iterations, the tolerance and the eta parameters refer to the iteration cycle that is performed to compute the nominal induced velocity. After max_iterations, or when the absolute value of the difference between two iterations of the nominal induced velocity is less than tolerance, the cycle ends. Only a fraction eta of the difference between two iterations of the nominal induced velocity is actually used; eta defaults to 1. The default is to make only one iteration, which is backward-compatible with the original behavior.

  Note: the syntax of the correction factors input changed from MBDyn 1.1 to MBDyn 1.2, where two separate factors, according to conventional models (e.g. CAMRAD/JA), are now allowed instead of one. No backward compatibility has been implemented because this is a very specialistic parameter and it was introduced very recently.

The last induced velocity model uses a dynamic inflow model, based on [12], with 3 inflow states. The syntax is:

    <induced_velocity_model> ::= dynamic inflow , 
        <reference_omega> , 
        <reference_radius> 
        [ , ground , <ground_node> ]
        [ , initial value , <const_vel> ,
            <cosine_vel> ,
            <sine_vel> ]
        [ , max iterations , <max_iterations> ]
        [ , tolerance , <tolerance> ]
        [ , eta , <eta> ]
        [ , correction , <hover_correction_factor> ,
            <ff_correction_factor> ]

Most of the parameters are the same as for the previous models. The optional delay parameter is no longer allowed. The three states, corresponding to uniform, fore-aft and lateral inflow, can be explicitly initialized by means of the optional initial value parameter.

Output

The following output is available for all rotor elements:

1. element label
2. rotor force in x direction (longitudinal force)
3. rotor force in y direction (lateral force)
4. rotor force in z direction (thrust)
5. rotor moment about x direction (roll moment)
6. rotor moment about y direction (pitch moment)
7. rotor moment about z direction (torque)
8. mean inflow velocity, based on momentum theory
9. reference velocity at rotor center, sum of airstream and craft node velocity
10. rotor disk angle
11. advance parameter $\mu$
12. inflow parameter $\lambda$
13. advance/inflow angle $\chi$ = arctan$\left(\vphantom{ \mu/\lambda }\right.$$\mu$/$\lambda$$\left.\vphantom{ \mu/\lambda }\right)$
14. reference azimuthal direction $\psi_{0}^{}$, related to rotor yaw angle
15. boolean flag indicating convergence in reference induced velocity computation iteration
16. number of iterations required for convergence

The dynamic inflow model adds the columns

1. constant inflow state
2. sine inflow state (lateral)
3. cosine inflow state (longitudinal)

Private Data

The following data is available:

1. "Tx" rotor force in x direction (longitudinal force)
2. "Ty" rotor force in y direction (lateral force)
3. "Tz" rotor force in z direction (thrust)
4. "Mx" rotor moment about x direction (roll moment)
5. "My" rotor moment about y direction (pitch moment)
6. "Mz" rotor moment about z direction (torque)