Definition of Section Offsets in
A HydroComp Technical Report
Product link: PropCad
expands on the coordinate specifications used to define propeller
blade section offsets in PropCad. Procedures to convert data from
other formats are presented.
adheres to the coordinate system used by the majority of commercial
stock manufacturers and designers. This system is based on developing
each section in the expanded view, as shown in the graphic below.
of each section is also known as the pitch line. By viewing all
pitch lines from root to tip, a pitch plane is developed.
feature of this definition is that the base-line is located at
the lowest point of the pressure side (i.e., the face) of the
propeller. This definition has been adopted because most commercial
stock propellers are based on a propeller type that has a significant
portion of its geometry as a flat face which will conveniently
lie on the pitch line. (For example, the Gawn, B-series, Kaplan,
AU and SK propellers all share this feature.) All blade offsets
will thus be positive values above this base-line.
by the Nose-Tail Line
format is to define the propeller section with a base-line passing
through the leading edge "nose" and the trailing edge
"tail". A section defined in this way will show the
offsets as distances above or below this "nose-tail line".
of this format is not recommended for a number of reasons:
- The numerical performance
models used in software such as PropExpert rely on pitch being
defined in the standard way. Often, the nose-tail line is not
parallel to the pitch line (as you can see from the B-series
section above), leading to an incompatible pitch value between
the performance model and the geometric definition.
- The flat face provides
a logical base-line for documentation or measurement, and for
repair with pitch blocks.
- Local changes to the
section may alter the nose or tail location without changing
the principal geometry of the section. For example, one cure
for root cavitation control is to cut back on the leading edge
for a more rounded nose. Another example is with cup, where
the majority of the blade remains unchanged, but the tail is
data in this nose-tail format to the standard format is easy -
simply find the maximum pressure side offset and lower the base-line
(or raise the offsets) by this amount. Keep in mind that the new
base-line will be parallel to the nose-tail line. You may need
to revise the offsets if you wish to locate the base-line along
the pressure face.
One additional refinement
can be performed if you wish to locate the sections in their exact
prior axial location in space. You can apply values for the rake
distribution equal to the base-line shift. (A negative value will
shift the section aft from the generating line). In other words,
you can exactly match the prior geometry by relocating the base-line,
then shifting the rake distribution by the same amount.