Contest Tower and Omni-Pad
by George Gassaway
I have had a tower similar to this one before for 30mm. But refined it somewhat for the 40mm version.
The heart of the tower is the Base Cylinder, which holds the rails aligned. The Base cylinder has fillets that match the surface of the rails to hold them in alignment, so that the only thing necessary to attach the rails to the cylinder is to simply tape them using electrical tape or vinyl tape. It's not "elegant", but it works well.
The tricky part is fabricating the cylinder. Ideally it would be a tube that is the perfect diameter to hold the rails just far apart for a loose slide fit of the model without being too loose. I wasn't able to find such a perfect fit of tube. What I used was BT-60, with some tweaking. Now of course BT-60 is not stiff enough to hold up to such a task, and neither would the perfect tube for that matter. The tube needs reinforcement inside. For the BT-60 that was somewhat easy to do. First, I glued a piece of Totally Tubular (Jim Fackert) I used some Estes BT-20 to BT-60 centering rings to help place a BT-20 inside. One ring was slid about 1.5" inside, the other just barely inside so as to just align the BT-20. Thick CA was dribbled inside the end with the 1.5" recess to help to seal off that end so nothing would leak. Then, I got out some old Alumilite that was too old to cast good parts with, but still useful for filling bulk volume. I mixed up enough to pour in and fill the 1.5" recess. This is why the recess had to be sealed well, so Alumilite would not leak inside. After that cured rigidly, then the centering ring at the other end that held the BT-20 was removed. Then more Alumilite was mixed up to fill the rest of the void. The strength was overkill, if I was doing it again with fresher good Alumilite I'd add some filler to the middle mixes since only the last 1.5-2Ó or so at the ends really need to be sturdy.
So, that produced a good sturdy stiff cylinder, but the BT-60 was a bit too thick. I peeled off a lot of the outer layers of the BT-60, but the peeling did not produce enough of a reduction in diameter. So I ended up doing some sanding with a sanding block at three places 120 degrees apart to get the desired diameter. That was a crude way to do it, not ideal, but that's what I ended up with. If I did it again, I might try making a very thick-walled fiberglass tube on the aluminum mandrel, using some 1.5 oz. cloth or many layers of 3/4 oz. cloth, to be sure the tube would be large enough in diameter and relatively sturdy to be able to do the same thing as done with the BT-60 regarding filling it with resin.
With the cylinder of the right diameter, the last thing was to give it fillets to align the rails. That was done by using two special styrene discs with holes at the exact spacing needed for the desired diameter. I drew up a pattern in MacDraw for the center lines of the holes, and rubber cemented the pattern to some styrene so the holes could be made.
With the two discs at either end of the rails, with the cylinder inside, the rails were checked to be sure they were parallel to each other with no twist. Five Minute epoxy was dabbed along the cylinder and the rails to produce the beginnings of a fillet. I could only do two fillet sides at a time. After the first six fillets were done, then I followed up with some more epoxy to make the fillets larger. After allowing a very long cure time, then it was time to remove the rails from the cylinder. Well, that was a problem. I had used vinyl tape so the epoxy would pop off, but the epoxy stuck a bit better than I expected. I did finally get the rails off, but in the process broke parts of a couple of fillets. Fortunately the fillets still were fine for aligning the rails, so I just left them that way. If I did it again I'd apply a lot of mold release on the rails where the fillets were created.
Once that was done, a test assembled it by laying the rails in place and wrapping with tape. Just fine, nice solid assembly, sturdy, and fast (no bolts!).
That's not to say the tower was done, but that was the most important part.
The main rails are 40" of .505" fiberglass tubing. The upper rail extensions are 25" making for a total tower length of 65"
12" of .414" tubing are used for joiners. A little bit of mylar tape is used to help get a bit snugger fit.
The top ring is always a problem for towers. However, I already had one, an adjustable one I used for an older 30mm tower made similar to this one. The tower top ring dates back to a design I used for a top ring in 1978, made out of spruce and using some telescoping brass square tubing with a good friction fit for adjustment. The spruce top ring itself is 6-sided, three bolts and wingnuts hold the 3 pieces together, so the top ring can be disassembled for transport. Also the brass/music wire spacer rods are removable too. That's not to say I might not want to get hold of a better top ring for the WSMC, but this one worked fine.
The tower is attached to the Omni-Pad by bolting one of the rails to the pad. Due to the piston, some of the rail length is not used for guidance, so there is no problem by attaching bolts to the side. Besides, the tower is more rigidly attached that way than a bottom attachment. I did originally think of attaching via the side of the cylinder, but attaching via the side of a rail was simplest.
To be sure that the tower rail will not rock side to side when bolted to the pad, I made up an interface piece out of balsa. Some 1" wide 3/8" thick balsa that has two 1/2" pieces of triangular balsa glued side by side, the triangles oriented inwards so that there is a 90 degree "V-notch" as a result.
To help keep the rails from bending apart near the middle of the tower, one of the two styrene discs used for the fillet process is also used. The disc is located a few inches below the top of the piston head. The ring has a 1/2" hole in it, which works fine with 11mm pistons.
Speaking of pistons, the tower was designed from the start with piston use in mind. That is why the tower is so long, since the piston takes up so much length.
The reason for the BT-20 cast inside of the base cylinder was to give a good universal means for attaching a piston to the tower. Any sort of attachment can be made up and simply plugged inside the BT-20.
The way the tower sits relatively close to the ground, there is enough room to be able to put the model into the tower without tilting it or using a chair. For someone shorter, that could be a problem. The first line of defense would be to mount the tower even closer to the ground, and indeed if I needed to I could have made it sit almost touching the ground.
Every tower needs some way to hold it in place and adjust the launch angle. Some like the Apogee/Medalist tower have their own legs. Most others use a separate device to hold thew tower, such as a Tripod or pad.
Too often the camera tripods are not sturdy enough. Often they wobble in the wind with a tower attached. In enough wind, the solution has been to add a sting to act as a guy wire to prevent the tower from blowing around too much. That's an on the field band-aid, not a really good solution. You can't easily change the angle to counteract weathercocking since the guy wire isn't easy to adjust, and if there are three towers all using the guy wire method in the same pad area, eventually you know what's going to happen.
So, here's some info on the pad I use. I call it "Omni-Pad" since it is used to holding all kinds of launch devices. C-rails, R/C RBG towers, Pistons, and Contest type Towers. I won't get into extreme detail, however.
Basic design criteria was that it be able to break down easily for transport, and not require any tools to assemble. The no-tools aspect is covered by use of wing-nuts or thumb-nuts, and the occasional thumb-screw. Fingers for assembly, no tools. 6/32 size used for commonality.
Original 1 x 2 Wood Omni-Pad
My first Omni-Pads broke down for transport OK, but they were made out of 1 x 2 wood, making them pretty heavy and requiring a 48" long shipping box. For 1996, version I made up a new lighter version breaks down into parts no longer than 30" and I can fit one inside a suitcase. It is made up of 3/4" square Aluminum tubing and 5/8" wood dowels. The Aluminum tubing is used for the main parts. The 5/8" dowel is used for extending the pad out for a larger footprint, for the front leg and outer legs. The dowels simply slide inside of the aluminum tubing. A threaded insert in the dowel and a hole in the aluminum tubing allows a thumb-screw to be used to secure each dowel in place.
The pad needs two diagonal struts for support. The lateral (side) strut keeps the pad from wobbling about the yaw axis. I use some "U" cross section aluminum extrusion for that, but it could be anything suitable.
The most important diagonal strut is the one for elevation. The lower end of it attaches to one of several 6/32 threaded rod at specific places along the forward 5/8" dowel (The 6/32 rods are really just 6/32 screws with the heads cut off, and CA'ed into the dowel). They are located in places that result in 90, 85, 80, 70, 60, and 45 degree elevations. Of course since the Omni-Pad was mostly meant for R/C use, the exact angle did not matter much. But it has been a bit of trouble when used for ballistic rockets.
For the flyoffs I changed that strut to one using a turnbuckle so the elevation angle could be adjusted several degrees. The turnbuckle uses 3/8" threaded rod. It was grafted to some fiberglass tubing to act as a joiner between the threaded rod and dowel with a hole at each end. You can see in the photo below what it looks like.
The angle is adjustable backwards too. When there's a lot of wind, it is useful to be able to tilt the tower AWAY from the wind so that as the model weathercocks it will weathercock vertical, and hopefully not weathercock much into the wind. Depends on some events as to how important that might be.
To help keep the pad from blowing over, the front leg is staked into the ground. A very large screw eye is attached to the front end of it, so a large 3/8" nail can be used to stake it down. Since I face the front end into the wind, it does not have to be staked elsewhere. If the wind changes, the pad normally is just pivoted around the staked front end, though in a tight space if need be the front end gets re-staked.
Now, there's all kind of ways to make pads and towers and so forth. I don't expect to see exact clones of the Omni-Pad or this tower. But there are some parts that folks might want to consider doing with their pads and towers.