Friday, March 27, 2015

Mid Power - Building the Quest Big Dog (Part 4)

Click here for Part 1

The fins have been sanded into an airfoil shape.

This will improve flight performance; the rocket should go much higher with its fins sanded into a teardrop shape, rather than left square. As I've written on this blog before, rather than just going for it, I decided to cut some extra fins from 1/8 inch balsa wood. With these, I'd practice a new airfoiling technique, and also see if I couldn't find a better way to fill the wood grain so the fins would be smooth on the finished rocket.

Six practice fins, airfoiled, sitting on top of the four fins that came with the rocket. The kit fins are still square.
A lot of people don't bother shaping their fins, or at least, they stick with simply rounding the leading and trailing edges. Unless you're doing competition rocketry, you may not notice a difference in performance. But a rounded or airfoiled fin can give you a boost in altitude - sometimes by well over 100 feet.

Airfoils are tricky, but they look cool, so I really wanted to get a good shape on these, and they came out rather well.

I'm actually really proud of these. This was not as hard as I had thought it would be, and I plan to write an upcoming post about how to do it. The basic idea is to mark guide lines parallel to the leading and trailing edges of the fins, and sand evenly from that line down to a pointed edge. The trailing edge has a long taper, while the leading edge should be rounded off. This is much easier to show than to describe, so look for an upcoming post with lots of pictures, and probably a video. It takes some practice, but I think it's doable if you take your time.

Airfoiling the kit fins took a lot longer than the balsa practice fins I made. The kit says the fins are made of balsa, but I'm not sure I believe that. Balsa comes in many different grades, but it's usually a soft wood. These were really tough. If you flicked them with a fingernail, they sounded solid. Perhaps they're a high grade of balsa; perhaps another wood. I keep hearing about "tung" wood, which is sometimes used as a balsa substitute, but I don't know what tung is supposed to be like.

I certainly can't find any sheets of tung for hobby use online. The balsa sheet I get at hobby shops is so flimsy, so soft, I'm not crazy about it. Most kits have balsa fins which are a little tougher, with tighter wood grain. I couldn't even make a slight pencil mark on the balsa sheet without leaving a deep mark in the wood.

The Big Dog kit fins, however, they're something else.

While the balsa practice fins took me perhaps five to ten minutes to airfoil, the kit fins took much longer. I could do several balsa fins at once, but I needed a break from sanding after each Big Dog fin. And my hands started hurting! And I dropped a fin several times, with no damage at all.

Compare that with the damage I did to one of my first rocket fins, the Cosmic Explorer, from mere sanding:

A misshapen gouge taken out with some gentle sanding with fine, 400 grit sandpaper. I was able to repair this later.

All this is to say, these are going to be some tough fins.

I'll save fin filling for next time. We'll also talk about papering fins - a faster, less messy way to get a smooth surface on your fins, and which I'm really bad at doing - but I've at least got some pictures.

Click here for Part 5.

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