Scripts for making propellers for use in rubber powered model airplanes.
These OpenSCAD scripts generates a form for molding a propeller suitable for use on lightweight rubber band powered indoor model airplanes. The form is used to warp flat sheets of balsa into a curved and twisted shape needed to produce efficient propeller blades. The cover photo shows the MKI version for a 7" x 22" prop form and template for a Limited Pennyplane class model.
The usual procedure involves carving a wooden block for forming. This script automates that part of the process by generation such a mold from design parameters.
Details on design and construction of propellers for various endurance classes of rubber band powered model airplanes can be found on several websites, such as https://indoornewsandviews.files.wordpress.com/2012/10/inav_121_press.pdf Note, some competitions require that you use commercially manufactured propellers, so these scripts might not be useful there.
The MKII version is an attempt to improve on these by first rotating the form so the blade lies flat.
As an option, the blade shape can be cutout to an approximation of the design given here:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790015732.pdf
The latest versions of OpenSCAD include an optimizer panel that allows you to change parameters without modifying code. ![Using the Customizer] using_the_customizer
The development snapshots, such as OpenSCAD-2019.01.10 are available by scrolling down to the "Development Snapshots" area at http://www.openscad.org/downloads.html
The basic design parameters for a propeller are its diameter and pitch, or theoretical distance traveled in one revolution. The block width is also needed as this indirectly sets the blade's chord.
Additional options include whether the blades are flat plates or curved cross sections. Currently the script does not support other airfoil shapes, but that could be added. Finally, a parameter specifies the number of sections along the length of the blade. I suggest not increasing this too much (at least while adjusting the other parameters) as the script may become unresponsive. Values of about 50 seems like a good number here.
Of course, other blade shapes can be used by printing a recatngular block and using it to form any shaped blanks. One common variation is to placed the blade centerline ahead of the form centerline slightly. This is sometime done so the thin blade material flexes slightly under high torque at the beginning of the motor run and possibly yields something like a variable pitch propeller.
Describe the LarrabeePlanform.scad script.
Motivation: to provide additional features for the native OpenSCAD linear_transform() operation whith specific requirements for how verticies are mapped between endpoint.