Fin thickness & shreds

[Karl]

Does the thickness of your fins (depending on the motor) matter if your using large , well made fillets? Ive got some balsa which is like 1mm thick , or 0.5mm thick even , and im wondering wether to use it on my 2stage D to E rocket. And when that E kicks in , the rocket will be going faster so will it put too much stress on the fins to make them shred?
Thanks
Karl

 

[DynaSoar]

Originally posted by Karl
Does the thickness of your fins (depending on the motor) matter if your using large , well made fillets? Ive got some balsa which is like 1mm thick , or 0.5mm thick even , and im wondering wether to use it on my 2stage D to E rocket. And when that E kicks in , the rocket will be going faster so will it put too much stress on the fins to make them shred?
Thanks
Karl

It could well happen. Fillets keep them attached to the body. You could end up with ragged edges attached to the body. The thinner they are the more they flutter and that can rip them apart.

If you want to keep them that thin, consider laminating them with paper.

 

[Karl]

Would a layer of glass & a layer of paper do the trick? Or easier still , get some better fin stock ?
Karl

 

[powderburner]

Fins of just about any size at all, made from balsa stock that thin, will almost certainly shread during flight. If not on the first flight . . .

Why do you want to use such thin finstock? Are you after low weight or low drag? If it is low weight, then thicker fin material will weigh less than laminated thin fins. If it is low drag, you will be better off with a good airfoil sanded into the fins (not just rounded leading and trailing edges), which means that you have to use thick enough finstock to be able to sand an airfoil.

Thin sheet balsa such as you describe is normally used by model airplane builders to sheet the leading edges of wings, or to cover the sides of a fuselage.

 

[Karl]

Ahh , Ill pick up some 3mm stuff tomorrow.....What thickness would I need to airfoil them ? And what tools are required to do so? I've never airfoiled before but they look real nice when they are done! I was also wondering on this 2stage rocket....I wanted to use 'THOR' style fins , with the bottom set of fins on the booster and the upper fins on the sustainer. But would it be stable when the fins sperate from the '2fin design'?
I have rocksim but I'm no 'genius' at it im afraid
Karl

 

[Mad Rocketeer]

0.5 mm balsa? You could just about roll that into a tube without soaking it, I'm thinking. It would be bound to be pretty floppy. About like making the fins from a single thickness of unreinforced card stock, except floppier and less tough. There are probably good things to do with that balsa, but I don't think I'd use it alone as a fin. To do something fun with it, maybe you could use it for the skin of built-up fins.

I'd love to see a post on how to make good airfoils. Are there any jigs for that being sold? All I've seen are descriptions of doing it with a good eye, a steady hand, and lots of elbow grease. There ought'a be an easier way. Maybe a tool or a jig or something.

 

[DynaSoar]

Originally posted by Mad Rocketeer
I'd love to see a post on how to make good airfoils. Are there any jigs for that being sold? All I've seen are descriptions of doing it with a good eye, a steady hand, and lots of elbow grease. There ought'a be an easier way. Maybe a tool or a jig or something.

I reckon falling out of a window is easier than rolling downhill, but neither is very tough to do. We're talking balsa here. It takes no more elbow grease to airfoil a 3mm fin than it does to eat dinner.

4 to 8 push strokes of the fin across rough sandpaper laid on the table, for each side of each edge. For sharp airfoils, hold the fin at a steep angle to the sandpaper. For rounded, do it at a slightly different angle each time. For 4 big fins with 4 edges to round, such as a Big Bertha, it's a 15 minute job. This uses the same setup that you use to sand the fins all together at once to make sure all the edges are the same size. It's already there and ready to go.

If you want very consistent sharp angle airfoils, you can fold some cardboard into a wedge and use it to keep the fin at a constant angle to the paper.

Tip: always push the fin across the sandpaper with the grain. If you push them with the grain angled so as to dig into the sandpaper, it'll shred in tiny little fuzzies that'll have to be sanded off.

For very easy and quick airfoils on a small bird, I've even glued them on first, then took a small piece of rough sandpaper, pinched it between index finger and thumb, and run the fold down along the edges a few times. With each run, you pinch a little more, and it does different angles, producing a round edge.

 

[Mad Rocketeer]

Yeah, elbow grease was an incorrect term for what I meant. Mainly, I meant having to get the curve correct purely by hand. My tendency is to overshoot and take away too much balsa too fast.

Also, I was talking about a true airfoil, not just a rounded leading edge and a tapered trailing edge. A true airfoil has no straight edges but is one continuous curve. Tough (at least for me) to do strictly by eye and hand in something as soft as balsa and as small and fragile as a fin. I wind up like the amateur barber who ends up cutting too much here - trying to even it up - overshooting somewhere else - evening again - and finally ending up with a shaved head. Balsa's softness is a big part of the problem. My eye-hand co-ordination may be even more so.

What I'd like to see is something one could lay a piece of balsa in and sand across the top of with a block to get the curve, then flip the fin over and lay it down (in a curved base this time) to sand the other side. Sort of a tray with sides curved into an airfoil shape. One could then use a sanding block, just make sure the block overlaps both curved sides of the tray, and sand away anything that sticks up.

 

[Elapid]

is the most accurate way to hand-sand airfoils...get two pieces of piano wire, one thin, one thick and align the thick with the leading edge, the thin with the trailing edge, and with a sanding block, you can get a really nice taper. flip them over, do the otehr side, then round the leading edge for a pretty decent airfoil, consistent between fins.

i've never tried this technique, though it's mentioned in all the texts.

good luck!

 

[Stymye]

also try sanding one long piece into an airfoil ,nice and evenly
than cut the fins from that, it's much easier than trying to match 3 or 4 seperate pieces.
once cut they are now easier to sand the taper along the semi span,if you want it.

 

[DynaSoar]

Originally posted by Mad Rocketeer
What I'd like to see is something one could lay a piece of balsa in and sand across the top of with a block to get the curve, then flip the fin over and lay it down (in a curved base this time) to sand the other side. Sort of a tray with sides curved into an airfoil shape. One could then use a sanding block, just make sure the block overlaps both curved sides of the tray, and sand away anything that sticks up.

I'm thinking that if you did taper the entire airfoil from a max at say 1/4 chord all the way back, the last 1/4 of it would be so thin it's tend to flutter and break off. I doubt the work it would take would result in any measureable control benefit over a double knife edge or a round leading/tapered trailing. Any flutter would negate any possible improvement in control characteristics.

 

[wwattles]

I tend to agree that a full-fin foil shape would result in very weak trailing edges that are prone to breakage and flutter. That being said...

Both leading and trailing edge shaping can be done with a shaped router bit mounted either on a router or a drill press. There are a wide variety of curves available for the leading edge, as well as straight bevels for the trailing edge.

If you don't have a router or drill press, you can use a dremel or similar rotary tool by hand. Measure off how far you want to run your bevel/rounding back into the fin surface, mark it with tape, draw a line with a fine tip marker down the center of your fin's leading/trailing edge, and slowly shape it down using a drum sander. It's a pain in the tail, but it'll do the job.

WW

 

[Mad Rocketeer]

Eapid: I've seen the wire technique, and I like it, but it results in a wedge rather than an airfoil. In an airfoil, the leading edge isn't the widest point along the chord. As stated by DynaSoar and wwattles, this wedge shape could get too thin too quickly, resulting in a weak trailing edge. I may try it sometime with plywood though. Thanks for the reminder. A true airfoil is a bit thicker for a bit longer as it extends to the trailing edge.

Stymye: The problem with sanding/shaping a long piece and then cutting it into fins the way you say is that the grain in long pieces tends to run the length of the sheet. That means that the leading edges of the fins will be parallel to the length of the sheet. Shaping the whole sheet at once will make the airfoiled cross-sections run perpindicular to the length of the sheet. But we want them to run parallel to the root edges of our fins. The only way to get that with this technique is to make rectangular fins whose leading edges stick straight out from the body tube. This would be great for that kind of fin, but it will introduce inaccuracies in swept fins. Still probably a lot closer than just rounding and tapering though. Cal me picky.

wwattles: Good ideas. I don't have any of these tools yet. I have a band saw, a circular saw, and a couple of hand drills.

I guess rounding and tapering are plenty good enough for my current purposes, but I'd like to see an easy way to guide the sanding of true foils. A true foil just looks so cool.

 

[sandman]

Going back about forty years I remember making a tiny power tool to sand the trailing edge on airfoils.

I found an old electric shaver in the garage, the CORDED kind!

The head was kinda rusty so I figured my dad was done with it and it was never going to be a shaver again.

I removed everything from the head and I found that all it was was a pair of coils and a plate that vibrated, ( at 60 hertz but I didn't know that then!).

The plate ended in a two prong finger about an inch apart.

I sharpened the tips of the two fingers and added little barbs with a hack saw.

I jabbed that into a block of really hard balsa wood about 1/4" thick and about 1.5" square.

Then glued some sandpaper onto the block.

I had to replace the wooden head and the sandpaper after every 5 or 6 models.

A tiny power sander!

Airfoils got real easy then!

 

[wwattles]

And so begins the legend of Sandboy... stay tuned for the next episode, Sandteen and the Mighty Oak!

WW

 

[DynaSoar]

Originally posted by wwattles
And so begins the legend of Sandboy... stay tuned for the next episode, Sandteen and the Mighty Oak!

WW

Foolish mortal...

We are Sandmen of Borg. Squareness is useless. You will be abraded.