Adding side-force generators for improved knife-edge flight
Today, almost-ready-to-fly (ARF) airplanes dominate most local flying fields, and so there’s little room for personal expression. RC modelers, however, can always customize their models to make them unique. This can include various cosmetic changes, from a simple color-scheme change to the addition of panel lines, rivets, and weathering effects to a scale model to improve realism. Some builders may also decide to make design modifications to alter their plane’s flight characteristics. The options are endless.
I recently completed a Hangar 9
Ultra Stick 30cc ARF. Although it is a great giant-scale sport model in its stock configuration, I wanted to add a counterbalanced rudder and sideforce generators (SFGs) to the wings. The addition of the counterbalance was mainly cosmetic, even though it does decrease the load experienced by the servo. For the wing mods, I eliminated the wingtips to allow a mounting surface for two extremely large SFGs. The result was astonishing, and the modified Ultra Stick is now capable of extremely slow knife-edge flight at high angles relative to the horizon. It can also perform extremely tight knife-edge loops with minimal pitch coupling.
TIME FOR CHANGE
The first step is to remove the covering from certain areas. I first examined the structures and where certain colors were located to minimize any additional covering seams. I then trimmed the covering along the top edge of the last wing rib and, on the underside, removed covering from the outermost rib bay. (While removing the covering, you need to be certain that you do not cut through the surface and weaken the wooden structure.) Occasionally,
the color for the covering may stick to the surface and the only portion of the covering you end up removing is the top clear. If this occurs to you, use a scrap piece of covering, iron it to the color you wish to remove, and peel back the scrap covering; the color should easily lift away. Repeat, as needed, until all the color patches are completely removed from the wood surface.
The stock model has a sheet wingtip as well as straightedge gussets that were part of the front and rear spar. Use a hobby knife, a razor saw, and then multiple sanding blocks to cleanly remove the wingtip structure. After that, the structure is fairly easy to modify for the attachment of the SFGs. I assembled the aircraft in my workshop with the main landing gear, wheels, and tailwheel installed to determine the clearance that existed between the centerline of the wingtip rib and the floor. It is not as simple as measuring down to the floor, however. You also have to consider the length of the grass at your flying field, if applicable. Also, if a landing is not perfect and your model banks when it touches the ground, you want to have some room for safety. I placed my Hangar 9 Angle Pro on the wing, banked the aircraft 10 degrees, and added a spacer under one of the main wheels to maintain this attitude. I then measured the distance from the centerline of the wingtip to the floor and subtracted a few inches to represent grass. This determined how tall the SFGs could be.
After removing the covering from the vertical stabilizer and rudder, I examined the structure.
You need to be aware of any structural changes and the weight added with respect to the model’s center of gravity (CG). Component locations may need to shift due to a CG change, or (as a last resort) weight might need to be added to the nose to restore the proper balance point.
NUTS AND BOLTS
The SFGs that I designed were built following
I ELIMINATED THE WINGTIPS TO ALLOW A MOUNTING SURFACE FOR TWO EXTREMELY LARGE SFGS. THE RESULT WAS ASTONISHING, AND THE MODIFIED ULTRA STICK IS NOW CAPABLE OF EXTREMELY SLOW KNIFE-EDGE FLIGHT AT HIGH ANGLES RELATIVE TO THE HORIZON. IT CAN ALSO PERFORM EXTREMELY TIGHT KNIFE-EDGE LOOPS WITH MINIMAL PITCH COUPLING.
the same overall layout of the other surfaces produced by Hangar 9. I wanted the SFGs to look as if they were designed specifically for the airplane. On a large piece of cardstock, I drew a centerline and marked the mounting points where the SFGs would be secured to the wingtips. Once I liked the outline of the SFG, I drew the internal structures and all the needed supports, modeled after the vertical stabilizer and rudder. All wood thicknesses and supports were determined in these plans to simplify the build process. To modify an aircraft, you don’t need to be an aerodynamicist, but a firm understanding of certain basics for building and flying is beneficial. It is important to keep your model as light as possible, so don’t overengineer it. On similarly sized models, I’ve fabricated SFGs out of 1/4-inch balsa and spruce stock. On this model, though, I made the SFGs the same thickness as the vertical and horizontal stabilizers, and to minimize the additional weight, I used contest-grade balsa wherever possible.
I built the external framework using 1/2-inchsquare balsa and a combination of 1/8 x 1/2-inch balsa, 1/2 x 1/2-inch balsa triangle stock, and
1/4 x 1/4-inch balsa triangle stock for internal bracing and supports. For added strength, I used 1/4 x 1/2-inch spruce at the center of the SFG, where the mounting bolts would secure the SFG to the wing. Two mounting points are required, at a minimum, for each SFG. To save hardware weight, I used 4-40 socket-head bolts, neoprenebonded steel washers, and 4-40 blind T-nuts. Occasionally, a spacer may be required for the T-nut installations. Always measure the distance from the end of the T-nut to the flange where the prongs are located to make certain the T-nut won’t protrude from the surface. The Ultra Stick is unique in construction as a front and rear spar that interlocks with the wing ribs. Since the spar locations are strong, having the mounting holes near these locations is perfect.
Make sure to use straight, unwrapped balsa parts. To cut the ends of the larger 1/2-inchsquare stock to a perfect 90 degrees, I used a Midwest Products hobby miter box as well as a belt sander. Once all parts are pinned to the building bench, check the alignment of the parts with a ruler for straightness. I used medium CA from Zap throughout the build.
Since 4-40 bolts and wood secure the SFG to the wing and the outermost wing rib was lightened, I fabricated two light-ply-and-balsasheet plates for each wingtip, which I secured to each spar as well as the rib section. Each plate is oversized, so it could be glued in place on the inner surface of the rib so that it could not pull out in flight. I then added an 1/8-inch balsa block to fill the area between the plate and the outer face of the rib. This provided area for the covering to be secured to.
RUDDER WORK
With this airplane, two horizontal edge pieces exist on the vertical stabilizer, running from the base and the top cross-support tip. Using a saw, I cut above the second horizontal support and doubled this brace to prevent any warping once the vertical stabilizer was covered. When using thin balsa supports, the new covering can often warp a surface if pulled too tightly.
I then laid the rudder and the top of the vertical fin down on a flat bench and glued the two pieces together. Having double-beveled edges, I added triangular stock to the bevels to produce flat, mating edge surfaces. I then added a horizontal support that would become the lower base edge of the counterbalance. For additional strength, I added triangular stock to where the trailing edge of the vertical stabilizer and the upper horizontal support meet at the counterbalance. Since the added counterbalance eliminated a stock hinge location, I installed another hinge in the vertical stabilizer to keep the hinge count the same as the original.
Whenever building model parts for the board, make sure your building surface is perfectly straight and is made out of a material (I prefer laminated balsa) that allows the balsa items to be held in place using Du-Bro T-pins or similar product. To prevent gluing any items to your plans, use wax paper or a clear plastic plan protector. After all the parts were sanded smooth, I used an UltraCote that matched the original color scheme to re-cover the new parts. Lastly, give your aircraft a complete assembly check for proper fit and function. Having the SFGs removable makes any future repairs easier and also improves transportation tasks.
FINAL THOUGHTS
Taking a little extra time to customize your aircraft can be extremely beneficial. The art of building has decreased to some extent over the years due to the large variety of ARF models. But maintaining basic building practices is still in need. If you require help during a build or have general construction questions, seek the advice of a fellow giant-scale enthusiast. And remember to enjoy every part of this great hobby while seeking out new learning experiences. Until next time, fly hard!
RC MODELERS CAN ALWAYS CUSTOMIZE THEIR MODELS TO MAKE THEM UNIQUE. THIS CAN INCLUDE VARIOUS COSMETIC CHANGES, FROM A SIMPLE COLOR-SCHEME CHANGE TO THE ADDITION OF PANEL LINES, RIVETS, AND WEATHERING EFFECTS TO A SCALE MODEL TO IMPROVE REALISM. SOME BUILDERS MAY ALSO DECIDE TO MAKE DESIGN MODIFICATIONS TO ALTER THEIR PLANE’S FLIGHT CHARACTERISTICS. THE OPTIONS ARE ENDLESS.