Fin Placement Template
Bonsoir, mes amis. I return now to fill in the gaps in your knowledge concerning the mounting, filleting, smoothing, and painting of model rocket fins. At this time you should have a set of fins ready to mount. If you do not, then please refer back to my posting of the twelfth of June of this year.
Above you will observe, if you please, a series of blue concentric circles with black lines drawn upon them. This is a template to help you mark the placement of fins on your rocket body tube. Print the diagram, and lay it flat upon the table. Stand your rocket body tube on this diagram, and use the circles to help you place the tube as close to the center as possible. Hold the tube very still, and use the lines and a pencil to make a small mark on the tube where the appropriate lines touch it. By appropriate, I mean to say that, if your rocket has four fins, use only the lines that form a cross. If there will be three fins, use only the top line and the two diagonal lines. The fins must all be separated by the same distances, the same angles; this should be easy to tell.
Now we must draw lines to indicate the locations for gluing the fins. For this, we need a V-shaped channel in which to lay the rocket body tube for marking. The edge of an open drawer, the underside of a shelf, a short piece of angled metal, or any of several other things you may find, or, if you have the needed skills, can build. Place the tube in the channel so that one of the straight edges touches one of the marks you have made, and, using a pencil, lightly draw a line to mark where the root edge of the fin will be anchored. Repeat this procedure for all fins. Do not draw the lines too far, just so, because you will otherwise later be trying to cover these marks with paint, and it may be very difficult to do it.
In the old days, we had only available to us white glue; none of this cyanoacrylate gel; but now, you may use the glue of your choosing. For white glue, or school glue, as you may know it, put a small amount along the root edge of the fin, and then smooth the glue with your fingertip until it becomes thin and “tacky.” For cyanoacrylate glue, you must omit the part about touching it, or you will be walking around all day with a fin stuck to your fingers. Quickly and carefully, touch the bottom corner of the root edge to the bottom of the line on the rocket body and then press the root edge quickly to the entire line. For the modern glue, you must hold it in place for ten to fifteen seconds, and then you are done. For white glue, you will hold it a bit longer. Also for white glue, you will need to support the fin for a longer time with masking tape, or some other type of tape, such as painter’s tape, that does not hold too firmly. The fin must point directly away from the rocket body. With white glue, you may want to wait approximately two hours between fins, but if you are careful, you may be able to install all the fins at one sitting. Do not proceed, however, until the glue is dry and the fins are firmly anchored.
Now that your fins are attached, we must apply something called “fillets.” No, mes amis, not filets. Will will concentrate on dinner at another time, perhaps. No, we will create a smooth transition between the surfaces of the fin along the root edge and the body of the rocket. Smoothing this area will both decrease the drag factor of the rocket and will greatly increase the strength where the fin and rocket body meet. We do this with the white glue. Lay the rocket down on its side; gently, now; the fins are not yet at their full strength; and place something under nose end to keep the body level. A book or two will work for this purpose. If your rocket has four fins, it will naturally lie in the optimal position, but if it has three fins, use books or tape to orient the rocket so that two fin root edges can be seen from above. It is these edges to which we will apply our first fillets.
Open your glue bottle, but not too far, only just enough so that you do not have to squeeze the bottle too firmly. Apply a line of glue to the root edge-body joint, and then do the same to the top-side joint of the other fin. Run your fingertip along each joint to smooth the glue, and wipe up any excess with a wet paper towel. Do not spread the glue too far onto either the fin or the paper rocket tube. When you have done this, it is time to shift your focus onto another task, as you must allow the fillets to dry. This will take several hours, after which time you must repeat the process in order to make the fillets thick enough. But a better way is to allow only a couple of hours, just long enough so that the glue can no longer run, and then you can rotate the tube to another position in order to apply fillets to the other fins. Do not make the fillets to thick on the first try, or you will have glue dripping everywhere, spoiling the model. But neither should you make the fillets in their final form too thin. Take your time. Do it well, and you will have a rocket model to be proud of.
Now that the fins are attached and the fillets are completed, it is time to seal the surfaces of the fins to prepare them for painting. This you will do with a substance called sanding sealer, which is a type of model dope. Dope is also the name for the type of paint you will typically use for your rocket. Apply a smooth, thin layer of sealer to all fins, and allow it to dry completely. This will take several hours, perhaps overnight. When you are certain they fins are dry, use fine sandpaper to smooth them, sanding in the direction of the grain. Repeat this process until the fins are very smooth, changing to finer grits of sandpaper as you go. It will take several iterations of this to obtain good results, but it will be well worth it in the end, mes amis. As you smooth, you should also try to refine the shapes of the rounded leading edges and the tapered trailing edges.
When all fins are filleted and filled, and the launch lug is attached and filleted (we did not speak about the launch lug, but do not worry about that for now), we are ready for painting. I like to begin with a white primer and do everything with a brush, as opposed to using spray paint, which is seldom able to produce a smooth finish. The smoother the finish, as I am sure you know by now, the less drag our model will have. When your primer is completely dry, and no excess pencil marks are showing, then apply your final color. Usually one coat of dope will be enough if you have used a primer, and too much paint, especially when you are using types of paint other than dope, can add weight, thus affecting the rocket’s performance.
That is all for now, friends. Enjoy your model building.
