Speaking of the boat, if you build the dish will you please post Mangetout-style instructions on the proper construction of said foiliferous flower? I would very much appreciate it. Thanks!
I’m planning to. It won’t be updated on the site while it’s in progress because of issues too silly and subtle to properly explain, but when it’s all done, it will be fully documented.
The main issue that I see is the fact that 2/3rds of the parabola ends up basically being a flat sheet - that means a pretty poor focal point - maybe not an issue for a solar heater.
Also, I would be tempted to make the cutouts asymmetric - one side with an extra unit to under-lap and tape behind. Another better design (maybe) would not use straight sectors but curved ones (so the cutout looks like a submarine propellor) - the maths remains the same, but it may be easier to pull into shape, and if you make the cuts to near the center, get a bit more curvature into the center of the parabola. Using overlaps only works if the material you are working with is not too thick - 1 or 2 mm rigid plastic (pvc or similar) sheet would be ideal.
Si
Thanks - I have some ideas for the construction - it won’t be all that dissimilar to the stitch and tape method I used for the boat - it’s true that the centre of the dish will be flattish if the petals are cut so as to be naturally joined to a central disc, but I plan to cut the kerfs between the petals longer than the layout dictates, so there shouldn’t be a sharp corner where the flat part turns into the curve.
My plane shape is almost exactly a metre in diameter, and will have 16 petals - these will be narrower and thus easier to bend than 8 would be - but it also means any errors will be magnified more. I’m looking for a focal length of somewhere around 500mm - I don’t want it to be too deep, as that will limit the usefulness of the final object.
I’m planning to attach small blocks on the back of the petals at intervals along their radius - they will be drilled and a cord threaded through the holes - around the diameter of that sub-circle will be wound tight (Spanish windlass) so as to pull the dish into shape (then I’ll glue it).
You know there are easier ways to make a 1920’s Style Death Ray, right?
You could always do what physicist R.W. Wood did (and the Martia n named Number 774 in Raymond Z. Gallun’s “Old Faithful”) and fill a bowl with mercury and spin it. The surface will form a perfect parabola, as long as the rotation is smooth, and the bowl is pointing straight up.
(The idea has since been takemn up by lots of others:
http://www.sciencemag.org/cgi/content/summary/299/5613/1650
It has been used also to manufacture disposable contact lenses.)
That’s a great idea, but would limit the portability of my device - also, it would make it difficult to use for solar experiments - in its raw state, the device would only work as a solar concentrator at midday, when used near the equator. The angle of incoming light could be adjusted for by means of a large plane mirror, but it would be difficult to manipulate the focused rays (it could be done by making the focal length quite long, and diverting it with another plane mirror, but that would get in the way of the first one directing the incoming rays.
Also, juices from my solar-roasted meat would drip into the mercury - leaving me with the dilemma of whether or not to sop them up with my bread - on the one hand, it’s gravy - on the other, it’s toxic metal. Hey! you got your gravy in my toxic metal!
I sure wish Squinks link worked for me. All I get is “DNS lookup error”. My new internet service gives me all sorts of weird errors from time to time, but they usually resolve when I reload.
Well, the toxic mercury fumes are admittedly a deal-breaker for most people. You could use Wood’s metal or some low melting point eutectic, or gallium, I suppose. But you’re still limited in most cases to relatively long focal lengths and not a huge collection area.
Back in the 80’s one of my classmates built a parabolic reflector by spiral cutting his flat sheet of reflective material and then mounting it so that it was wound tighter thus pulling the outer edges up.
Something like this.
I don’t have time now to find a technical link for this but I’ll check back in later.
What you wind up with is something similar to a fresnel lens, just as a reflector rather than a lens.
That’s a seriously cool idea, but I think it’s probably beyond the scope of my materials (and I don’t want to buy too much for this - my substrate is a sheet of recycled ply).
One other idea I’m toying with though: Build a large shallow, sealed box, cut a big circular window in one of the flat faces, glaze it with mylar and apply a gentle vacuum to the inside of the box - it should stretch the mylar sheet into a bowl shape, although I don’t know how close to parabolic this would be.
I’m still struggling on the dimensions for my segmented paraboloid though. Si - are you there?
yep.
Here are the relevant elements of my spreadsheet - have a go at recreating it
Cell A2 - focal length (same units as C2)
Cell B2 - number of petals
Cells C2-C27 0 to 50 (in steps of 2) (these are X values, the radius of the Parabola - tweak these so that the R values are integers)
Cells D2-D27 =C2+(C2^3/(24*$A$2^2)) (these are the R values, the plane radii)
Cells E2-E27 =(PI()/$B$2)(C2^3/(24$A$2^2)) (these are the deltaW values, marked either side of the sector line on the R values)
hope this helps. PM If not, I’ll email you the spreadsheet.
Rather than making it too awkward, create a template, marking the curve using R and deltaW, cut it out, lay it on the sector lines and draw the line, then invert it and draw the other line.
Si
Excellent - I think I can do it now. Cheers.