Lesson 2: Installing Fins

In this lesson, we're going to install the fins and finish the motor mount. See lesson 1 on building and installing the motor mount itself.

Shaping Fins

My Tomahawk kit came with wooden fins, but if you are building a PML kit, it will come with G10 (fiberglass) fins. Either way, you will most likely want to dress the visible edges of the fins in some way. My instructions said to do this after the fins are mounted, but I find it easier to do when the fins are still loose.

If you are concerned with performance, you will want to get as much of an airfoil shape on the fins as possible. This generally involves a rounded leading edge and tapered trailing edge. For this kit, I chose to bevel (taper) all three exposed edges of each fin. Be sure not to round or bevel the root edge (or the tab ends)!

Safety note: Sanding G10 (or any fiberglass) produces very dangerous fine particles. Use a good respirator and sand outside (or in a well-ventilated area) to reduce your exposure as much as possible.

Clean Up the Fin Slots

Now that you have the MMT installed, test fit your fins into the slots. Clean up the slots as necessary and make sure they are long enough. For my Tomahawk kit, I had to even out the fin slots and make them slightly longer to accommodate the fins.

It's a good idea to check that the fin slots are parallel to the axis of the airframe as well. Lay your aluminum angle along the length of the body tube next to each fin slot and verify that they are straight and in line. If they are not, you can adjust them a bit by widening where necessary.

The straighter your fins are, the better your flight will look and the more altitude you will get. Very crooked fins can even result in a shred, although this will only happen with the more powerful motors.

Now is also a good time to sand the outside of the body tube around the fin slots. This will help the external fillets bond properly. We will mount the fins in the next step, but it's easier to sand the slots now than after the fins are mounted.

Install the Fins

In order to help align the fins, draw a good mark at the aft end of the airframe at the center of each fin slot. If your rocket has three fins, draw another mark exactly half way between each pair of fins.

Remove the aft CR and make sure the MMT and inside of the fan can are clean and well sanded. Place the body tube in a cradle so that it is securely supported in a horizontal position. (Robart or similar model airplane stands work well for this purpose.) Turn the tube so that one fin slot is at the very top and position the stand so that the aft end of the rocket is facing out toward you.

Take your first fin and put a nice bead of epoxy along the root edge and the forward edge of the fin tab (where it will meet the center CR). If you are using an epoxy system, it is a good idea to paint the bottom of wooden fins with some unthickened epoxy first. In the picture on the left, you can see one of my Tomahawk fins with unthickened epoxy along the edges and a thickened epoxy bead along the root.

Slide the fin into the fin slot and make sure it seats on the MMT all along its length. Visually sight along the rocket from the aft end and verify that the fin is straight up (straight out from the tube).

To make a good joint to the MMT, use a stick to put more epoxy along the root edge and make as good a fillet as you can along both sides of the root edge. This is the most important epoxy bond in your rocket so be sure you make it a good one.

Clean up the aft end of the body tube and MMT with alcohol and re-install the aft CR while the first fin bonds. (This will keep the body tube and MMT aligned while the fin bond cures.)

To make sure your fin is properly aligned, lay a long ruler along the aft end of the rocket, lining up the fin with the marks on the aft end of your body tube. Use a long piece of tape to hold the fin in position while it cures.

Once the fin has cured (or at least reached the gel stage), remove the aft CR and rotate the rocket so another fin slot is at the top. Do all your fins, but don't rush this part or you may get crooked fins. Clean up the aft end of the rocket with alcohol and replace the aft CR after each bond so that your MMT stays in proper alignment with the body tube.

Once the first fin is pointing down, you can also make the fillet inside the body tube on each side of the fin. When you are done, each fin will have a good fillet at the MMT and another one at the inside of the body tube. Make sure that epoxy doesn't run down the fin through any gaps between the slots and the fins. Use tape if necessary to seal the gaps.

Install the Aft CR

Once all fins are bonded and all internal fillets are done, you can bond on the aft CR. Mix up some epoxy and paint the aft ends of the fin tabs, the outside of the MMT and the inside of the body tube. Slide on the aft CR until it touches the aft end of the fins. Wipe off any excess epoxy with alcohol and let the fin unit cure fully. Now your rocket is really taking shape!