|
|
How do propellers work? |
|
|
From the desktop of |
The twist (pitch) and inclination of a blade are defined by the angles between a datum plane normal to the axis of rotation and a number of datum lines fixed relative to the blade. A set of pitch datum lines are taken at a series of constant radii from the axis of rotation, and the angle between the datum plane and a pitch datum line is named the pitch angle.
|
|
|
The operation of a screw
can be illustrated by considering an annular element of one of the blades.
The figure below shows the forces acting on an annular blade element of
width dr
at a radius r from the
axis of rotation. A torque Q is
applied to the screw by driving the shaft, and the propeller and shaft
rotate at a rate of rotation . Owing to the reaction of the fluid in which
it operates, the blade element experiences a resultant force d
R which has two components, a tangential torque force dQ/rB, where B is the
number of blades, acting opposite to the direction of rotation, and a
thrust force dT/B acting ahead,
parallel to the axis of rotation.
|
|
|
|
The sum of the moments of the tangential torque forces on all the blade elements is balanced by the torque Q applied to the propeller, and the sum of the thrust forces on all the blade elements is equal to the thrust force T which propels the screw in a forward direction at a speed of advance.
|
If you have any suggestions or questions regarding these technical issues, or if you know someone who's willing to receive these technical letters, or if you wish to be removed from our mailing list, please send an e-mail to: technicalletter@ricepropulsion.com |