Always on the prowl for good ideas, I spent some time recently thinking about aero. That brought be to airfoiltools.com where you can see the coefficient of lift and drag and various other properties for many different airfoils profiles. Some googling brought me back to old GRM posts about how profile 'e423' could be a good one for lower speed motorsports since it produces relatively high coefficient of lift (or downforce) at low reynolds numbers (low speed and small wing area in air). Here is what the profile looks like:
You would make it upside down for a downforce application.
Unfortunately, that profile is pretty complicated and would be really time consuming to make out of common or cheap materials. I thought about making a template out of metal and then using a router to cutout about a hundred pieces and then gluing them all together to make a giant plywood wing, but holy moly not sure if I have that patience. I also worried that a bit of tear out at the pointy end could really affect the performance of the wing, and would be hard to avoid.
Then I learned that some folks are making airfoils for home-built wind turbines and such from sections of PVC pipe. Basically you cut the tube length-wise so you end up with a curved section like a slat from some window blinds, and then round the front edge and sharpen the back edge. airfoiltools also has some of those profiles:
and using the comparison feature, the section of pipe wing actually outperforms the e423 wing at low reynolds numbers (the pipe wing is blue). More lift at angle of attack 10-17 degrees:
AND less drag too!
So, a section of pipe wing actually seems much more feasible to create than hundreds of carefully profiled pieces, mostly because the thickness of the wing doesn't vary much. A piece of flat stock (i.e. plywood) could be used.
Here's where things get interesting:
The pipe-section profile I showed above specifies a thickness of 5% of the radius of the curve, and a chord length of radius/0.963. Let's say that I picked up about 16 pieces of 3/4 inch ply that are 16 inches wide and 8 ft long out of the trash, because I did (I think they were shelves in someone's basement and they got wet, so out they came and into the trash). If thickness is 3/4inch, radius is 20x that, or 15 inches. and then the chord length is 15.6 inches. I since the chord length doesn't take the curve into account, I'll need slightly more than that to get there. Someone else I'm sure can do the math on this, but 16 inches will be quite close.
If I use a router to round-over the front edge of the plywood, and then use a table saw to cut the back edge sharp (like maybe a steep angle like 60 degrees or something), then I just need to bend the plywood into a 15 inch radius. Enter an idea from another recent thread on here, and cut slots with the table saw:
Say the outside edge is where you want your 15 inch radius measured from (the airfoiltools profile say the middle, but this is for quick and dirty). So a full circle would have a circumference of 94.2 inches. A full circle with a 14.25 inch radius would have an 89.5 inch circumference. about 5% less. so the inside needs to have 5% less width than the outside. 16 inches minus 5% is 15.2 inches, or about 15 and 3/16 (or 1/4). So if the table saw blade is 1/8th inch thick, there would need to be 6 slots cut.
Now finally, we need something to bend this around. Tires from suv/trucks etc generally have almost 30 inch diameter. So, cut the slots, fill them with wood glue, ratchet strap the plywood to the outside of a few tires, wait for the whole thing to dry, and BOOM! you have a nearly-free wing that should actually perform pretty well.
What am I missing here?
Also, don't worry. I very much plan to do this and I intend to test the results as well and keep this thread updated.