Fin Design

[LSA Solutions]

I have a loc precision, Iris kit. The fins are flat on the top leading edge. Im curious if anyone sands down there fins to make them any more aerodynamic and to save weight?

 

[Art Upton]

Saving weight is not an issue, it would be negligible.

You can make them aerodynamic, then they will be more prone to dings in the air foil.
Most people either just round them a little, or leave them square.

Mine are generally square but sometimes round.

 

[cls]

leave the tip and the root square. round the front and back edges.

this is discussed in Handbook of Model Rocketry.

 

[bjphoenix]

You can make them aerodynamic, then they will be more prone to dings in the air foil.
Most people either just round them a little, or leave them square.

This has been my experience too. When I started building rockets it seemed that all the literature said to sand airfoils so I did. I've noticed that those old rockets have picked up a lot of nicks. So now when I build rockets I round the fin edges. I think this is the most durable edge and the easiest edge to make consistent.

 

[waltr]

I almost always put a taper on leading edge and maybe trailing edge.

I also have the LOC IRIS and only put a taper on the leading edge.
To do this I use a tapered router bit in a Dermal router table. Makes perfect tapers every time.
This makes the fins a bit more Scale looking.

This is NOT for weight saving since the amount of wood removed is way less the amount of filler, primer & paint applied.

 

[SolarYellow]

The simplest "airfoil" would be a rounded LE and tapered TE. Anything to reduce the area of the squared-off surface on the back and reduce base drag.

I'm surprised it hadn't occurred to me until this thread that it might be the case that a squared-off TE is better than a rounded TE, because the rounded TE has the same problems as a "round" object such as a cylinder or sphere. Could probably read on it pretty quickly. Need to do that, I guess.

 

[lr64]

How fast is a LOC precision? If close to Mach 1, other shapes may be better.

Many model airplane guys will reinforce the leading and trailing edges instead of leaving them blunt. For instance, one could slot the leading edge and glue in a thin piece of fiberglass or, in the case of balsa fins, birch plywood. Or, just glue on a thin layer of thin ply, 1/64" or 1/32" on the l.e. before shaping. On balsa trailing edges, I've used light glass or glued on a strip of spruce before shaping. Plywood fins that need to be significantly stronger could use heavier glass or denser wood, even thin metal strips (if that's legal). Come to think of it, one could just use an extra strip of glass cloth on the outside of the leading edge.

 

[teepot]

I airfoil almost every fin. 90% of my fins are 1/4" plywood.

 

[Bruiser]

I always round or taper leading edges on my sport rockets. Scale rocket models get whatever tapers the actual rocket had.

-Bob

 

[DaHabes]

For my bigger rockets I've gone to tapering the leading and trailing edges using a jig I found on John Coker's website. PDF attached. You need a disk sander but it works really well. I generally leave 1/8" of the edge flat to avoid splintering (if it gets too thin) and then harden the tapered edges with CA followed by a light sanding.

For LPR/MPR, I generally leave them square. Balsa or basswood fins break easily enough without tapering them.

 

[lr64]

If they are light enough, they don't need to be so strong. I saw a couple of rockets have a streamer or parachute failure today and come down sideways with no damage.

 

[Art Upton]

If they are light enough, they don't need to be so strong. I saw a couple of rockets have a streamer or parachute failure today and come down sideways with no damage.

Balancing the rocket so it comes down sideways is what competition folks do for duration models. It Slows the rocket way down on the , well, way down. I even used thin Kevlar that is fastened with a Mylar wrap right at the CG to help the balance.

Most sport rockets will not come down that way, but I have had a few do it it just out of the blue.

Many of my sport models with parachute failure had a chip taken out of a fin, or a crease on the body tube; unless the field was really really soft.

 

[LSA Solutions]

Thanks guys, I think im going to go the tapered route then.