Old and new all over again, questions on a V-2"ish" build

Alan Roberts · Feb 10, 2011

38 years since I've flown anything, all the rockets and launcher long gone, and a friend starts flying with his daughter. He went for a visit to Semroc, and I couldn't resist ... Now I have a V-2 kit to build.

So the Semroc instructions convinced me to use the semi-scale fins ... Stability without having to drill the nose and put in more weight seems good for a first build on my second round.

Since I'm not going to be scale, I'm not feeling any need to add drag with the servo pods, turbine exhausts, and pull-out plugs in the original Estes instructions.

That brings me to fin and fillet questions. The original scale plans show fins tapering to thin edges, but memory is telling me that this mattered for airfoils that went supersonic. For the model, wouldn't rounded-leading-edge, tapered trailing be best?

The instructions also show build-up of a fillet that gets very wide at the rear of the boat tail. Memory is that a modest fillet helped with strength and drag, but I don't remember making thick ones. Of course I think all my models back then had straight body tubes!

Looking at V-2 pictures on the internet, I can see the widening fillet, but I'm speculating that was to lessen drag from the hardware which moved the in-exhaust steering vanes. Given no steering vanes and that I've already departed from scale, is there aerodynamic benefit from building up the glue fillet to the width shown in the instructions, or am I just adding weight and drag?

Advice (or pointers to some archive thread with everything I wanted to know about fin shaping and fillets) appreciated!

gdiscenza · Feb 10, 2011

I'm not a materials engineer, but in my experience, if the fins are not TTW, a thicker fillet at the aft of the mount will help offset the stress of a hard surface landing.

The forward half of the fin mount, in a hard landing, is being pushed into the BT/tailcone, so a thinner fillet is all that is needed, but the rear section is being pulled away from the mount point, so a thicker fillet will keep the fin fully attached.

Of course, after a certain point, the fin will simply snap off, since if the wood glue is used properly, the joint is stronger than the material itself.

G.D.

GregGleason · Feb 10, 2011

Alan Roberts said:
38 years since I've flown anything, all the rockets and launcher long gone, and a friend starts flying with his daughter. He went for a visit to Semroc, and I couldn't resist ... Now I have a V-2 kit to build.

So the Semroc instructions convinced me to use the semi-scale fins ... Stability without having to drill the nose and put in more weight seems good for a first build on my second round.

Since I'm not going to be scale, I'm not feeling any need to add drag with the servo pods, turbine exhausts, and pull-out plugs in the original Estes instructions.

That brings me to fin and fillet questions. The original scale plans show fins tapering to thin edges, but memory is telling me that this mattered for airfoils that went supersonic. For the model, wouldn't rounded-leading-edge, tapered trailing be best?

The instructions also show build-up of a fillet that gets very wide at the rear of the boat tail. Memory is that a modest fillet helped with strength and drag, but I don't remember making thick ones. Of course I think all my models back then had straight body tubes!

Looking at V-2 pictures on the internet, I can see the widening fillet, but I'm speculating that was to lessen drag from the hardware which moved the in-exhaust steering vanes. Given no steering vanes and that I've already departed from scale, is there aerodynamic benefit from building up the glue fillet to the width shown in the instructions, or am I just adding weight and drag?

Advice (or pointers to some archive thread with everything I wanted to know about fin shaping and fillets) appreciated!

First of all, welcome and welcome back to the hobby!

I have to take these one at a time, so bear with me.

Q1. For the model, wouldn't rounded-leading-edge, tapered trailing be best?

A1. Depends on your goals:
For ease of assembly: flat leading/trailing edges would be best.
For ground-handling: regardless of the leading edges, flat trailing edges are best
For aerodynamic purposes: airfoiled is best, followed by rounded, followed by flat.

Q2: s there aerodynamic benefit from building up the glue fillet to the width shown in the instructions, or am I just adding weight and drag?

A2: Generally speaking, there is a benefit to fillets.
First, depending how they are made, there are structural gains regarding the fin's ability to stay connected to the airframe
Second, there is an aerodynamic benefit, up to a point. There is an optimal fillet profile, but my guess is that it will vary with airspeed, airframe diameter, and fin span. IIRC, there was a rule-of-thumb presented in the Handbook of Model Rocketry, 7th Edition (if you don't have that book, I would recommend that as one of the first hobby-related books that you should add to your library), but I cannot recall what that is.
Third, there is a weight penalty for fillets, so the goal is to make them as light as possible. For this reason, I am not a fan of epoxy clay because it adds so much weight to aft end, which is typically the worst place to add extra weight.

Greg

Stymye · Feb 10, 2011

annother option would be curved cardstock fillets ..if you want that scale V2 look without alot of weight.

the figure quoted in the handbook is the optimum radius at 4-8% of the fin root chord..any larger starts to be overcome by drag.
on small models it's not that critical

Rocket Al · Feb 17, 2011

To the original poster, the purpose of the knife edge fins and the tapered filets was to adhere more to scale. Since you're not building exact scale, but more of just the recognizable shape without all the finicky bits, I wouldn't worry about it too much....

Sandy H. · Feb 18, 2011

OT, but if you don't mind driving, the rocket club in Charlotte is flying this weekend. Check out the website (rocketrycarolina.org) and come by. The weekend should be great flying and you might see some things you haven't seen so far. Check out the picture gallery and youtube link on the ROCC site as well for some visuals.

Good luck on your build.

Sandy.

Alan Roberts · Feb 18, 2011

Thanks everyone. Found an (older) copy of Handbook of Model Rocketry in the library and saw the 4-8% number. I sometimes worry about my house sinking into the red clay from the weight of books

, but I guess the latest edition will have to be part of the next Amazon purchase ... Doesn't seem like a good book choice for the Kindle app on the smart phone!

Since I'm not pursuing scale I'll skip the sharp tapers on the fin edges in favor of less work and more strength, and not worry about matching the big fillet build-up at the rear of the boat tail.

Now that I've dealt with a few minor infrastructure purchases (the house bottle of TiteBond had turned into a solid block of orange gel, and all the sandpaper was used on the last interior painting project), perhaps I can make some progress this weekend!

Sandy H. I can't make Charlotte this weekend, but I do trek to Gastonia to see my Mom. I'll work on my timing to get to a future date, thanks!

Old and new all over again, questions on a V-2"ish" build

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