Tuesday, February 23, 2010

Sudden Mach fins

Now that I've figured out what to call it, and I've got the general design down, I'm trying to figure out what to use for fins, because that's pretty much the only work on it I can do till I aquire the nose cone, kevlar shock cord, rail buttons, and av-bay parts.

They'll be pretty standard swept trapezoidal fins. I haven't decided on the exact dimensions yet, but it'll have 3 fins, roughly 7" long and 2.5" wide, shaped like this:

The question is the material. I will likely use 1/8" plywood. It's light and strong, and I have plenty available. The problem is that I'm not quite sure if it is strong enough for speeds approaching Mach 1.8 (~ 600 mps, or ~1340 mph).

Balsa is right out because it would shred nearly instantly. Basswood is a possibility, but it's prolly very similar to my plywood in properties. Carbon fiber, used in the Wildman Blackhawk, is superstrong but heavy, which would mean giant draggy fins just like on the Blackhawk. Plus, carbon fiber is ridiculously expensive.

So, if I do need stronger fins, I have three options.

First, I could buy really nice 5/32" or 3/16" plywood, which is nice and strong, and better quality than my plywood. The problem is that that creates a lot of weight aft, which requires bigger fins or nose weight. Weight is not as much an issue with the Sudden Mach as with other, smaller Machbusters, but I need to keep the loaded weight below 12 ounces to directly compete with the Blackhawk, and below 10 to really out-accelerate it.

Second, I could buy custom-cut G10 fins. There's several small companies that do such work. I could have them perfectly cut and shaped, edges rounded, and very professionally smooth for a not-ridiculous amount. There's only two problems with that. First, Fiberglass is three times the density of birch ply (1.85 grams per cubic centimeter, versus 0.63). That means bigger fins, or nose weight. They're also against my self-imposed rules of no phenolic, Blue Tube, fiberglass, carbon, or metal. I intend to beat the Blackhawk with nothing but cardboard tubing, plastic, and wood.

Third, I could get aluminium fins. Incredibly thin, incredibly strong, and absolutely unshreddable. But again, they're not conventional fin materials, they're heavy, and getting them fabricated could cost me a pretty penny.


Murdock said...

Paper your fins, checkout out Apogee's posts on the method. I'm building a 29MM MD Aspire to go mach and it uses balsa fins with a layer of copier paper over them, they'll survive Mach :)

The EGE said...

Oooh, excellent idea. Thanks much.

DTH Rocket said...

It's true balsa is a dangerous choice, but I wouldn't say it would necessarily shred right away. I used balsa on mine and it worked just fine. You just have to build it right. Ditto what Murdock said. Paper skins on the fins.