Sanding of fins

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vicmeister

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Hi - question regarding the sanding of the fins for mid and high power rockets. I obviously have plywood fins, 1/8 inch. I want to sand a somewhat airfoil shape to them, with a rounded leading edge and a long pointed shape to the trailing edge. I could do this by hand (tedious), or I could use the belt sander on a bench sander such as that put out by Powertec or Skill. Lowes has one for around $120. Any recommendations? Thanks
 
I use the bench belt sander for this and honestly use it a lot for rocketry. A good tool to have.

Greg
 
vicmeister said:
Any recommendations?
Click to expand...

Unless you have cleaned up the exterior of your rocket so that you cannot feel any airframe joints when you "scrape" your fingernail down the length of the rocket, and unless you have polished the external surfaces so that you can see yourself in the shine....

I would not bother with tweaking a knife-edge-sharp trailing edge on the fin. The aerodynamic drag increment for a bluff (square) fin T.E. is negligible, probably on the order of one percent of all the drag. If you were flying in a contest, I would say YES definitely go ahead, but for sport/utility flying you stand a good chance of sustaining fin damage at landing when you use sharp fin edges. A squared-off T.E. will stand up much better to day-to-day use. Round the leading edge to a nice smooth shape and you'll be good.

My two cents-
 
I agree with powderburner. Air foiling fins has very little effect on the performance of a hobby rocket. Personally, I round both the leading and trailing edges. I don't think there is a lot of extra strength to be gotten by leaving the trailing edge square, and round gets you a "little" bit of performance. Your choice.
 
For the 1/8 and even the 1/4 plywood fins I either round over all edges or knock off the sharp edges. I do this primarily so the sharp edges don't get damaged if the edge takes a hit in transport or my negligence moving the rockets around. The only time I shape the fins is when I am doing a scale rocket and the original had such fins.
 
Instead or telling you why you shouldn't do what you want to do, I'll make an attempt to answer your question.

Whenever I'm shaping, I'm always worried about going too far, so I offer this advice: if you go by hand with 80 or 120 grit or even a file to start then I doubt shaping 1/8 thick fins will take dreadfully long. And on the flip side, if using a power sander I'd start right out with 220, because power makes going too far too fast really really easy.

If I had the power sander I'd use it, but I don't think it's worth buying one. Just my $0.02.
 
Until my last build, I used a palm sander and 80 grit to cut airfoil shapes, followed by 120 and 220 to smooth out the scratches. It's a little tedious, but not terrible. On the last build, I used a Dremel tool with the sanding drum to do most of the work and then cleaned up with the palm sander. That went a lot faster, although you have to be cautious not to get the drum stuck in one place. I usually leave a bit of flat end where the fin will hit the ground, usually at the trailing edge tip. For everything else, I just leave a 1/32" or so flat along the edge to make it less fragile.

If you are using power tools, you have to stop and check sanding depth very regularly or you'll end up with knife edges.
 
I've always rounded leading edges (not a semi circle but more like a 3-1 blunt ogive nose) and put a "fake taper" in the trailing edge. What I mean by that is, I taper 1/2" - 1" down to the middle ply (in three-ply) or slightly into the next to middle ply (in 5-ply). The trailing edge is therefore not sharp, but at least 1/3rd of the thickness of the fin. It's a compromise between looking good, subsonic aerodynamics, and landing durability.
 
My new favorite tool for dealing with fin shaping up through small HPR size (1/8" fins like you're talking about) is the Dremel Multi-Max vibratory sander/saw. it is gentle compared to elliptical oscillating sanders but still gets the job done. They cost around $100 and have a zillion uses in rocketry in addition to shaping. The saw blades will slot tubes much easier than a Dremel rotary saw with less wander, and with finer sandpaper it also works great for sanding filler/primer on flat surfaces. Also great for removing paint from tubes/fins when refinishing. Smoothed out a Punisher nose cone pretty quickly too.
 
For thin fins like that, I'd do it by hand. Tedium is in the eye of the beholder, I suppose, but you don't have a lot of wiggle room, either. To get the profile you are after, I'd be afraid of any power tool taking too much off. You can always scrape away a little more... putting it back on can be a challenge, sometimes...


Later!

--Coop
 
I know this thread is a few months old, but I've done this recently, with 1/8 inch plywood fins.

My sander is just a Great Planes hand sander. Little, meant for small jobs, not really ideal for larger rockets, but it was what I had, I tried it, and it did work.

I used a coarse grit - maybe 150, maybe 100. It was kind of tedious, but I enjoy the craft of rocket building.

I did about a fin per day, which cut down on the tedium.

As for people saying, "don't bother," I understand why they may say that, but every time I ask a how-to question on TRF and get a "don't bother" answer, I find it really frustrating. Because they don't know why I want to do it.

I airfoil almost all my fins, and I've heard the "don't bother" comment before. I suppose people think I'm trying to double my altitude or something, which airfoiling won't do. But it does help increase altitude somewhat, for one, and for another, I like doing it and I like the way it looks. Which is why I've asked these questions in the past - because I want the rocket to look good.

I don't blame anyone for not bothering to do certain things when building - filling tube spirals, filling fins, airfoils, etc. It's a pain. But I like the process, and the way it makes a rocket look, and "don't bother" is not a helpful response to beginner's questions.

As for how I did it, I just did what I do with balsa fins, but with more elbow grease. I usually do something like this:

[video]https://youtu.be/zYm_1R3vMo4[/video]
 
When I was younger I would get a haircut once a month. But now, I just don't bother. :facepalm:
 
Old thread, but I missed it the first time around...

Some of us shape fins to bring them closer to a scale appearance. It's important to me, even if most won't notice. I know from experience that some people do!

I bought a very cheap belt/disk sander from Harbor Freight with a coupon. It is one of my most used rocketry tools and I am very happy with it. It's probably not durable enough for some uses, but for sanding plywood fins to the proper shape, it's perfect for me.

I do a rough shape with the belt sander, and then finish it with a Dremel Multi-Max. That way I don't have to worry so much about removing too much material when I do a knife-edge!

Cheers,
Michael
 
Last edited:
Where do you find sanding sealer? i've looked at several places and NO ONE seems to know what i'm talking about. Why that it is BEYOND me, it's a pretty common product i figure. I thought just primer would seal balsa fin wood grain, and i quickly found out it doesn't.
 
Vigilante_A-20_DEMON said:
Where do you find sanding sealer? i've looked at several places and NO ONE seems to know what i'm talking about. Why that it is BEYOND me, it's a pretty common product i figure. I thought just primer would seal balsa fin wood grain, and i quickly found out it doesn't.
Click to expand...

I get mine from hobbylinc:
https://www.hobbylinc.com/htm/mid/mid71-4.htm

You should be able to find it at just about any local hobby shop.

I have seen others who use Minwax sanding sealer from Lowes with good results:

https://m.lowes.com/pd/Minwax-Base-32-fl-oz-Sanding-Sealer/3202081

Aerogloss just works too well for me to change. You can obtain a metal-like finish with some patience and a very fine sandpaper.

Cheers,
Michael
 
vicmeister said:
Hi - question regarding the sanding of the fins for mid and high power rockets. I obviously have plywood fins, 1/8 inch. I want to sand a somewhat airfoil shape to them, with a rounded leading edge and a long pointed shape to the trailing edge. I could do this by hand (tedious), or I could use the belt sander on a bench sander such as that put out by Powertec or Skill. Lowes has one for around $120. Any recommendations? Thanks
Click to expand...

I use 80-100 grit sand paper on a block, get into a good meditative state of mind, and git 'er done. I eyeball the lines in the ply layers as I go and move to 220 and 320 grit when I'm satisfied with the shape. A pleasant way to spend a rainy afternoon or evening. Now I must say I haven't done a real knife edge airfoil on plywood fins; for that I might go with some power tools. With all due respect to diversity and not judging but square leading/trailing edges just look wrong to me. :)



Vigilante_A-20_DEMON said:
Where do you find sanding sealer? i've looked at several places and NO ONE seems to know what i'm talking about. Why that it is BEYOND me, it's a pretty common product i figure. I thought just primer would seal balsa fin wood grain, and i quickly found out it doesn't.
Click to expand...

[sorry to the OP for perpetuating the hijack]

I get this at Home Depot:

Deft sanding sealer.jpg

Not your classic hobby shop sanding sealer but works for me.
 
Yeah, some elbow grease and 200 grit sand paper is the trick. That, or a belt sander. I have a combo disc/belt sand and use it for all kinds of things. You can find them modestly priced at any hardware store. Mine is relatively small which I can move around the shop from table to table if I want. It's pretty convenient and a great tool to have. A small band saw is a great complement. And a metal lathe. I'll stop there :).
 
I use a block plane. It is super sharp and works well on plywood. I might have to adjust the angle a bit for the cross grain layers.

To answer the op's question: Yes and it will be handy to do a lot more with it as well.

I know that this is an old thread...
 
How about ELMER'S Wood Filler? I tried it today on a scrap piece of balsa, and it worked quite nicely. It's a thin putty like paste, comes in a squeeze tube. I spread it on the balsa with a plastic bondo spreader waited 15 minutes and sanded it smooth. I will look for the other stuff suggested here, this wood filler was kind of expensive, $5.41 for a 93 g tube.
 
Last edited:
lcorinth said:
I know this thread is a few months old, but I've done this recently, with 1/8 inch plywood fins.

My sander is just a Great Planes hand sander. Little, meant for small jobs, not really ideal for larger rockets, but it was what I had, I tried it, and it did work.

I used a coarse grit - maybe 150, maybe 100. It was kind of tedious, but I enjoy the craft of rocket building.

I did about a fin per day, which cut down on the tedium.

As for people saying, "don't bother," I understand why they may say that, but every time I ask a how-to question on TRF and get a "don't bother" answer, I find it really frustrating. Because they don't know why I want to do it.

I airfoil almost all my fins, and I've heard the "don't bother" comment before. I suppose people think I'm trying to double my altitude or something, which airfoiling won't do. But it does help increase altitude somewhat, for one, and for another, I like doing it and I like the way it looks. Which is why I've asked these questions in the past - because I want the rocket to look good.

I don't blame anyone for not bothering to do certain things when building - filling tube spirals, filling fins, airfoils, etc. It's a pain. But I like the process, and the way it makes a rocket look, and "don't bother" is not a helpful response to beginner's questions.

As for how I did it, I just did what I do with balsa fins, but with more elbow grease. I usually do something like this:

[video]https://youtu.be/zYm_1R3vMo4[/video]
Click to expand...

What an awesome post. I enjoy this hobby, being a second-time BAR. Part of the joy is improving my hobby skills and when someone with greater skills assists me on here when I ask a question I truly appreciate it.

I just bought an Aura since it will be my first non-Aerotech MPR and my first plywood fin rocket. I'm glad this question was asked! Well done with your reply lcorinth!

Mike
 
mkadams001 said:
That may have wax in it. You would want to use Bullseye SealCoat which has no wax.
Click to expand...

Oh geez, there's so much that can screw you up. Thanks for this heads up, i had no idea THIS sealer had WAX IN IT! It does not list it on the jar. I tested on a piece of scrap balsa. Here is the result of black paint over the sealer. It left white pin pricks which seams to be the result of the wax in the sealer. I'll find some wax free of this sealer which is sold also,DSCF0352.jpg or i'll try to find the exact brands in this thread.
 
I am old school on prepping balsa for rockets. I use Aerogloss sanding sealer or balsa filler. For me it is the simplest method. A few coats with sanding in-between does not take long and it's ready for the finishing process.
 
Regarding shaping of 1/8" ply fins,
I use the Great Planes Easy Touch Hand Sander sanding block.
Use a block, it keeps your rounded or tapered edges consistent.
Knock off the corners with 150 grit then move to 220 then 400 grit.

If you use belt or power sanders, practice on some scrap 1/8" ply before
committing to the kit fins. Power sanders can get away from you.

I avoid airfoils for the most part, I don't fly for altitude. Leading and trailing edges are usually rounded.
Airfoiled Alpha styled fins (trailing edges behind the body tube end) are more prone to breaking on a hard landing.
 

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