Microsft Excel (NACA 4412)
Why design and build a propeller? .... ..... Why not. Actually the intended purpose of this propeller is for an airboat, like the ones found in the Everglades. Click on images to enlarge
This propeller was completely designed and handcrafted by Mark Grabwolski and Michael Duffy. From start to finish it took us about 3 months working off and on.
First, Mark wrote a propeller analysis code using C ++ and Blade Element Theory (BET). The purpose of the code was to determine the cord and angle of attack of each section. He optimized the propeller for a 10 ft/s inflow velocity, which is about as fast as we estimate the airboat will travel.
Next, I took the data from Mark's (BET) analysis and began to model the propeller in CATIA V5. It took several iterations to get it right. One of the nice features of CATIA is that it has a built in stress analysis package. I applied loads to the tip to synthesize a worst case bending stress. Mainly this was to ensure that there was no stress concentrations at the hub. Once the analysis confirmed the strength of our propeller,r we began the process of fabrication
I submitted a bunch of images and instructions to instructables.com and a printable pdf file with instructions is linked here (printable pdf instructions)
First step in fabrication was drawing the templates. To get the proper airfoil shape and blade twist, the CATIA model is sliced into sections and then printed out in a 1:1 scale.
Each station along the length of the blade is measured and a profile slice is made. The drawing to the left depicts the overall dimensions and shape of each airfoil section. I created the drawing using CATIA's drafting workbench.
Just for fun I created some slices through the propeller's longitudinal axis. The dark regions depict the exposed profile of the wood if you took a saw and cut slices along the top and bottom as if it were cheese at the deli.
Next, I measured the thickness at each station so that we could cut the wood into sections.
Then, Mark cut out the boards and glued them together. Note that the thickness varies from the center to the tip. This is to conserve wood, and save time cutting away the unwanted wood (hogging).
Next we print out the 1:1 scale templates and glue them to aluminum sheets so that we can cut them out and use them as profile templates.
Here I am filing down one of the aluminum templates.
Here the profile is cut out and we are ready to begin the fun part. Hand tools.
Mark starts hogging out the hub with hammer and wood chisel
I was nervous at first using the Pneumatic air chisel. But eventually I get the hang of it. And let me just say that it is lots of fun.
Mark gets fed up with slow progress and starts hogging out large chunks wood. I'm nervous in this picture.
After a long day of work, it starts to look like a propeller. But lots of work still to go.
Here I am holding the unfinished propeller.
Mark holding the unfinished propeller.
The next couple of weeks we meet at marks apartment to finish the delicate work of carving with the spoke shaves and draw knives.
Here is a look at the shape un-sanded and still unfinished. It took us about 4 meetings of about 3-4 hours to actually bring the propeller down to shape
Here is the almost finished propeller, still needs a sanding and few coats of varnish.
Propeller Duplicator Machine