lllazar wrote:How can we cut specific size props out of balsa - wouldn't we have to bend it. I'm very confused on this topic....
I know of three basic approaches to making props/rotors out of balsa. Not sure which are better/worse from a time or effectiveness point of view, for that we'll need a couple years of data from competitions! OK, the basic approaches:
1. Build a balsa framework in a helical shape and cover. Not sure if there's an explanation on the web, so I'll try a quick one here until I can get all the pictures of the copter I'm building now finished and load them up.
a. You need two 38.9 cm spars, a bunch (8 or so) of ribs and covering. A building board and parts to make a jig.
b. Pin the first spar to your building board, mark its center.
c. Take the second spar, locate it say 1.5 inches above the first (you ARE going to need a jig for this) and rotate (ccw for an upper rotor, cw for a lower) it about the center so the tips are spread say 4 inches or so looking down from the top.
d. Mark both spars every 5 cm in from the end.
e. Cut ribs to fit well between the spars starting at the tips and moving in at each 5 cm radial position. Glue in position. You may actually want to start at the first 5 cm position from the center and work out!
f. Remove rotor frame from board and jig. Cover from first rib at 5 cm radius out to tip.
g. If you made a lower rotor, glue it nice and square to motor stick about ¾ inchs down from upper bearing.
h. If you made an upper rotor, add another spar at the center and your prop wire to hook the motor too. Assemble to prop hanger.
2. Take thin balsa, soak it in water and mold it to helical shape on a form. Attach blades made this way onto a center spar.
a. See: http://www.indoorduration.com/INAVHobbyShopper.htm
This is a multi-page article see especially the last section. Note, this article is about building a CRAZY light balsa plane including a prop only a little smaller than needed for SO. Use it as inspiration and procedure. Details of course need to be adapted to SO rules. Thicker wood, larger, etc. For SO I'd think in terms of 3/32 to 1/8 inch spars and 1/32 to 1/16 inch thick blades. But the process is right and explains it FAR better than I can.
b. In addition to using a form like in this article, you can also form the props on a curved surface and get a close approximation of a helical prop. Haven't found an online explanation, but form the blade about 15 degrees off axis on a 6 inch cylinder should be about right.
3. Carve from a raw block of balsa.
a. Here's an article that shows how to lay out a block for helical pitch props: http://www.indoorduration.com/ftp/GrantPropINAV94.pdf
b. Kind of assumes you know a lot already, hmm this is pretty detailed, see also: http://www.gryffinaero.com/models/ffpag ... carve.html
c. Another approach which takes more time prepping, but less carving: http://www.hbrf.org.nz/joomla/index.php ... &Itemid=59
A simplified approach to 2. would be to simply attach flat blades to the spar at appropriate angles. Works, if not very efficient. Probably fasted way to get a copter built.
Another approach to 2. would be to carve the blade, see: http://www.gregorie.org/freeflight/shee ... index.html
and then attach to spar.
Yet another approach to 2 would be to build a balsa frame on that surface you molded a solid prop blade on and cover. See: http://www.indoorduration.com/F1DPropConstruction.htm
FOR PROCEDURE ONLY. Again, this is about building crazy light stuff, but the process is right. Scale up for SO. Again, probably 1/16 inch square outline and ribs.
Approach 1. is the conventional approach for indoor copters. I suspect approach 2. with the built up blades will give a little better performance because it should be possible to reduce drag. Approach 3. will be a challenge to make rotors light enough.
Hope that helps, feel free to ask followup questions.