CL Aerobatics
Bob Hunt
Congratulations to the 2008 national winners
BEFORE WE GET back to Project Hole Shot, I'll take the opportunity to congratulate David Fitzgerald for his excellent Gold Medal performance at the 2008 CL World Championships. He flew his new Thunder Gazer (David, how exactly do you "gaze" at thunder?) that was powered by a P.A. .75 engine.
Not only did David win the individual competition, but the US F2B (CL Precision Aerobatics, or Stunt) team that consisted of him, Orestes Hernandez, Paul Walker, and Paul Ferrell won the team Gold Medal.
Our new Junior F2B team member, Paul Ferrell, performed well. He is one to watch at this level in the future. I am very proud of them all.
While I'm on the subject of congratulations, I extend a big one to Orestes Hernandez for winning his second-in-a-row National Championship crown in CL Stunt at the 2008 Nats. He accomplished that feat just days before he left for the World Championships—now there's a great warm-up.
On a personal note, I give a couple of big "attaboys" to Igor Burger, who won the Silver Medal in F2B at the World Championships and to my buddy, Mike Palko, who placed eighth in Open CL Precision Aerobatics at the Nats. Both flew electric-powered models.
Foam-Wing Prep: In the last column I covered the history of foam wings in CL Stunt and went through the basics of how a foam core is cut. If you have your own foam-cutting equipment, you've probably already experimented enough to have figured out how to cut a straight, warp-free core. The only thing I stress here is that you cut a core with a fully rounded LE in foam.
As I wrote the last time, the practice of cutting the LE flat and then adding a balsa block, which then has to be carved and sanded to achieve the proper shape, is where many potentially good-flying models are ruined. It is far more accurate to cut the LE shape at the same time you cut the core and then mold a piece of balsa that will attach to that radius.
This will be a new and perhaps uncomfortable step for many, but that's why we are doing this project in the first place: to learn new techniques.
So you either cut your own cores or purchase a set from one of the foam cutters I mentioned in the last column. If you purchase a set and cover the cores yourself, make sure you have the cutter install the full-depth 1/8 Lite-Ply (poplar) spars. These will add a great deal of strength to the finished wing and double as an attachment point for the bellcrank.
When these full-depth spars are installed, it is important that they sit just below the surface of the core (approximately 1/16 inch down from the surface on each side of the core). If the plywood spar is installed to the surface, there will be a hard point at the high point of the wing. Then when the wing is sheeted with balsa, a stress riser will eventually result as the finish ages and shrinks. You will be able to see the point at which the soft balsa passes over the hard-plywood spar as a ridge.
After the spar is installed, fill in the area between the top and bottom faces of the spar and the top and bottom surfaces of the foam core with strips of balsa. These strips should be glued in place and should extend slightly above the core's surface.
When the glue holding these strips to the top of the spar has dried, sand them to conform to the core's surface. At this point you will have material of the same approximate density in contact, and there will never be a chance of a stress riser or ridge. It seems like a small thing, but it is important and it is another fine detail that will make your model stand out from others on the field.
It is important to vacuum the cores thoroughly after any surface sanding. Dust from sanding the core or the spar caps will prevent the glue from doing its job correctly when we apply the wing skins.
We need to discuss molding the LE caps. There is not much to this, but you will need a foam mold buck cut to the exact shape of the core’s LE radius. Your foam cutter should be able to supply this piece.
The foam LE mold bucks will be a bit flimsy and prone to bending while wrapping the balsa LE cap to them during the molding phase, unless you glue a 3/16–1/4 plywood backing piece onto each of them. This piece will stiffen the mold bucks and ensure a straight LE cap piece.
I use pieces of 3-inch-wide 1/16 balsa for the LE caps on the Hole Shot wing cores. These pieces, along with the rest of the wing-skinning stock (also 1/16 x 3-inch balsa), should be trimmed with a straightedge and a knife, and then edge-sanded to make sure they are straight.
I use a Perma-Grit 22-inch-long bar sander (item SB560) for the edge-sanding process. This brand of sanding bars is made from accurate aluminum extrusions with thin steel plates attached to the flat sides. The steel plates are covered with small pieces of tungsten that is welded on to form the sanding grit.
There are typically two grits on each Perma-Grit bar: one side is coarse on the other. I use the fine-grit side for the balsa edge-truing process.
The edges of the Perma-Grit bars are square to the sanding surfaces, so you can place the tool on its side and then run the balsa against the sanding surface. I place the Perma-Grit bar at the end of a large piece of 3/4 flake board. Then I use a 12-inch-long Great Planes sanding bar fitted with 180-grit sandpaper to move the trimmed balsa sheets one by one against the edge of the Perma-Grit tool. The result is a smooth and square edge on each sheet.
Complete the preparation of the balsa sheets for molding by drawing a centerline lengthwise down one side of each sheet. Use a fine-point ball-point pen for this job.
To mold the LE caps, obtain a large, long, plastic storage box. You can find these at The Home Depot, Lowe’s, or Wal-Mart. Fill the bottom of the plastic box roughly an inch deep with very hot water.
I like to heat a large kitchen pot of water on the stove to just before boiling. I pour this into the plastic box and add a bit more very hot tap water. Be careful not to burn yourself when doing this.
Immerse the two balsa sheets in the hot water and let them soak for approximately 15 minutes. Remove one sheet from the “tub” and align the centerline, face down, on the LE mold buck. Line up the centerline on the sheet with the centerline on the end of the mold buck at each end. Once they are aligned, insert a straight pin through the balsa at either end of the mold buck to keep the sheet from moving laterally.
Fold the saturated balsa sheet around the mold buck, and wrap it snugly to the buck using an Ace Elastic Bandage. Secure the end of the Ace bandage with a piece of masking tape, and then let the balsa sheet dry thoroughly. Repeat this process for the other LE cap. The drying process usually takes at least 24 hours.
When the balsa is dry, unwrap the bandage and pull the pins out. You should have two LE caps that will perfectly fit the LE of the core.
To attach these LE pieces and to accurately sheet the remainder of the core, you will need a flat work surface that will not “bow” when you apply weight to it.
Place the lower cradle half from one core half onto the flat building surface. Lay a piece of 1/16 balsa in the rear half of the lower foam cradle. (The foam “cradles” are the upper and lower pieces that are left after the core has been cut.) This piece of balsa should span the entire length of the lower cradle. It will act as a shim to properly align the core.
Mix approximately a half ounce of Z-Poxy Finishing Resin (item PT40 from Super Glue Corporation). Use an acid brush to apply a thin but consistent film of it to the inside face of one of the molded LE cap pieces.
Place the balsa cap piece on the foam-core LE, and make sure the centerline on the cap piece is aligned with the centerlines on the ends of the core. Pin the cap piece to the core at each end, and then use short strips of masking tape to hold the cap piece to the surface of the core in three or four places along the span.
Align the core in the lower cradle half, and then place a piece of 1/16 balsa over the aft section of the core to act as an upper shim. These shims ensure that the core—with the LE cap installed—sits in the proper orientation in the cradle-core-cradle sandwich.
Line up everything perfectly and place roughly 20 pounds of weight (bricks, magazines, etc.) along the LE on top of the upper cradle piece. Let this assembly cure thoroughly.
I wrote about this sequence and posted it on both the Stuka Stunt forum and the Stunt Hangar forum awhile back. You can visit these sites and see the posts, which include sequential photos of the process.
For the Stuka Stunt version, go to the Web site and look up topic 295887. For the Stunt Hangar version, go to the Web site and then to the “Building Techniques” section of the forum. The LE caps how-to is locked permanently at the top.
Next time we will cover, well, covering.
If you have any interest in CL Stunt building/flying/competing, you should consider joining the Precision Aerobatics Model Pilots Association (PAMPA). This AMA SIG has a great 98-page bimonthly newsletter that is chock-full of all the information you need to be successful in this section of the hobby/sport.
PAMPA membership includes a membership directory that will help you find one-on-one help in your immediate vicinity. For information about how to join, write to the organization.
Sources:
Perma-Grit +44 (0) 1529 455 034 www.permagrit.com
Great Planes sanding tools (800) 637-7660 www.greatplanes.com/accys/sanders.html
Super Glue Corporation (800) 538-3091 http://zap.supergluecorp.com
Stuka Stunt www.clstunt.com/htdocs/dc/dcboard.php
Stunt Hangar www.stunthangar.com
PAMPA 15107 SE 145th Pl. Renton WA 98059 www.control-line.org
Transcribed from original scans by AI. Minor OCR errors may remain.
