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propeller.md

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Propeller

Propeller {
  SFVec3f shaftAxis          1 0 0  # unit axis
  SFVec3f centerOfThrust     0 0 0  # any vector
  SFVec2f thrustConstants    1 0    # any vector
  SFVec2f torqueConstants    1 0    # any vector
  SFFloat fastHelixThreshold 75.4   # [0, inf)
  SFNode  device             NULL   # {RotationalMotor, PROTO}
  SFNode  fastHelix          NULL   # {Solid (or derived), PROTO}
  SFNode  slowHelix          NULL   # {Solid (or derived), PROTO}
}

Description

%figure "Propeller axis"

propeller.png

%end

The Propeller node can be used to model a marine or an aircraft propeller. When its device field is set with a RotationalMotor, the propeller turns the motor angular velocity into a thrust and a (resistant) torque. The resultant thrust is the product of a real number T by the unit length shaft axis vector defined in the shaftAxis field, with T given by the formula:

T = t1 * |omega| * omega - t2 * |omega| * V

Where t1 and t2 are the constants specified in the thrustConstants field, omega is the motor angular velocity and V is the component of the linear velocity of the center of thrust along the shaft axis.

  • t1 somehow represents the volume of fluid moved by the propeller: large helices will have a large t1 value.
  • t2 roughly represents the friction on the fluid opposing the motion of the propeller: aerodynamic robots evolving in a low viscosity fluid (like air) should have a low t2 value.

The thrust is applied at the point specified within the centerOfThrust field. The resultant torque is the product of a real number Q by the unit length shaft axis vector, with Q given by the formula:

Q = q1 * |omega| * omega - q2 * |omega| * V

Where q1 and q2 are the constants specified in the torqueConstants field. The meaning of q1 and q2 is pretty similar to the one of t1 and t2.

More details about the above formulae can be found in "Guidance and Control of Ocean Vehicles" from Thor I. Fossen (ISBN: 9780471941132) and "Helicopter Performance, Stability, and Control" from Raymond W. Prouty (ISBN: 9781575242095).

The [propeller.wbt]({{ url.github_tree }}/projects/samples/devices/worlds/propeller.wbt) example shows three different helicopters modeled with Propeller nodes.

Field Summary

  • shaftAxis: defines the axis along which the resultant thrust and torque will be exerted, see this figure.

  • centerOfThrust: defines the point where the generated thrust applies, see this figure.

  • thrustConstants and torqueConstants: coefficients used to define the resultant thrust and torque as functions of the motor angular velocity and the linear speed of adavance, see above formulae.

  • fastHelixThreshold: threshold in [rad/s] from which the helix representation is switched from slowHelix to fastHelix. The default value equals to 24π [rad/s].

  • device: this field has to be set with a RotationalMotor in order to control the propeller.

  • fastHelix and slowHelix: if not NULL, these fields must be set with Solid nodes. The corresponding Solid nodes define the graphical representation of the propeller according to its motor's angular velocity omega: if |omega| > fastHelixThreshold, only the Solid defined in fastHelix is visible, otherwise only the Solid defined in slowHelix is visible.