Radio Control: Scale
Improving Old Models
Our ancient Douglas Mailplane, before its retirement into the new AMA museum, was flying better than at any time during its 13-year life. The important difference in flight handling came about when the old four-wire servos were replaced with new three-wire Royal Titans. We never regarded the older servos as being less reliable in operation, but proof of improvement is in the comparison between flights before and after the change. The most pronounced improvement was the new-found ability to fly with hands off the transmitter controls for extended periods. This is the result of more positive servo centering and greater torque output. Most noticeable change was the improvement in aileron response. Four very large ailerons could now be moved and centered with the 38 in.-oz. torque for a more positive return to neutral.
When the Douglas went to the museum, its servos were installed in our 12-year-old Emeraude to replace the old and heavy four-wire types that had been used since the model was new. Improvement in performance was a revelation. The model became much more active and responsive to control inputs. Once set up for straight-and-level flight, it would become a fly-away if something wasn't done to bring it back. An additional benefit was the decrease in model weight with the Titans. All of the servos are mounted a considerable distance behind the center of gravity; the change of servos actually moved the CG forward for a pronounced increase in stability. The model will perform tailspins and snap rolls on command, but the inadvertent snap roll at slow speed is now absent. The Emeraude may now be a bit nose-heavy, a preferred condition. It now requires a lot less attention while airborne and is downright relaxing to fly.
Improved curiosity in the Emeraude's flight aroused curiosity as to what would happen if newer servos were used in our 15-year-old Sopwith 1 1/2 Strutter. A large airplane, with 1,100 sq. in. wing area, it is quite light at 6 1/2 lb., and it was reasoned that higher-torque servos wouldn't be necessary. Bantam Midgets were installed. There was some doubt about the ability of an 18 in.-oz. servo to move four large ailerons. This proved to be no problem, as aileron response is positive at all speeds. With the change in servos, the lightweight Sopwith lost one-half pound! For the first time, the model's true potential as an aerobatic aircraft could be used to advantage. Most surprising, the amount of pilot effort to get scale-like flight was considerably reduced. The old four-wire servos may be serviceable for throttle and rudder; however, our preference for aileron and elevator applications is the more active and positive-centering three-wire types.
Keep old models flying. The models mentioned earlier have actually improved with age, though accidents have put them out of service temporarily. The Sopwith has strained through trees along the runway at Rhinebeck. The Emeraude encountered a tree at Toulouse, France. The Mailplane once scattered itself over a large area of the flying field at the Kitchener-Waterloo Scale Rally, survived, was restored to usefulness and went on to win contests. Restoration of a crashed airplane demands much less time than starting from scratch. Contrary to common belief, a modeler's top-quality scale model in particular may have a thousand hours construction time and shouldn't be scrapped willingly. A sport plane assembled in several weeks is not comparable. Investment of time in a scale model is too great to be written off hastily. There is also the investment in documentation and special parts made exclusively for the model that can't be used on another plane. Too much is lost if the model isn't rebuilt.
Preservation and Maintenance
With the large investment in time and money that is normal in scale modeling, it makes sense to expend effort in the direction of preservation. Our No. 1 admonition is to make certain of good battery life before each flying session, since batteries are the most common source of problems that lead to airplane damage. Cyclers have their place, but they aren't really insurance against failure. We are much more inclined to rely upon an expanded-scale voltmeter, with a resistance load, for a quick battery check. The first time that the voltmeter needle drops suddenly, or doesn't come up to the proper level, a good scale model will have been saved from possible destruction on the next flight.
Overworked receiver cells, carrying the load of a swarm of servos and subject to vibration, are the weak link in an RC system. Using larger-capacity cells isn't a solution, because a dead cell drops voltage below requirements. Larger cells only permit more flights and larger servos. Of our 18 scale airplanes, the nine that are flown regularly have the same charging jack for the sake of convenience. It is a Deans-type socket in the switch wiring harness. Plugging in the meter is a simple matter which leaves no excuse for omitting the check. The same meter can be used for checking transmitter batteries. For peace of mind alone, it's worth the $16 price (Tower Hobbies).
Balance and Weight
Second in importance is the model's balance point. Tail-heavy models can be flown, with incidence angles of wing and stabilizer properly set, but the burden of controlling such an unstable beast is too great, even for experienced fliers. A nose-heavy model may be non-aerobatic, but its stability will keep it out of trouble from the dreaded inadvertent stall-and-snap-roll. It's true that adding weight to the nose merely increases total model weight, thereby raising wing loading and stall speed, but without it, the model may never fly properly or safely.
After rebuilding a crashed airplane, we find that almost invariably the balance point has shifted aft due to an extra few coats of paint or a bit of added plywood reinforcement. It happened most recently in the restoration of our crashed Ryan SCW, which now has 12 oz. of lead in the cowl, fastened as far forward as possible. Performance hasn't suffered too badly, and landing speed isn't perceptibly higher. It may need even more weight, because the aircraft has a long tail and short nose. In other words, resist the temptation to lavish extra finishing coats on a long tail.
Engine Access and Cooling
Another important consideration for a long-lived model is to provide a convenient means of adjusting the engine's needle valve. A needle that can't be adjusted leads to the temptation to fly when it isn't set at the optimum point for reliable fuel feed in all flight attitudes. A heavy scale model that needs full engine output in a climb may suffer an early return to earth when the power sags due to a lean setting. Fully cowled engines are most vulnerable. We don't like to poke a hole through the cowl in a conspicuous location to make the needle accessible. As a consequence, the engine runs hot because the needle can't be moved to a richer setting, and the closed cowl may contribute to heat build-up because of poor air circulation. This unhappy combination could lead to a short life for both engine and airplane. A fuel tank too low in the fuselage can also contribute to overheating—usually late in the flight when the level of fuel drops, causing the mixture to become lean. Muffler pressure to the tank or a Robart Pumper may be the answer.
Rebuilding After a Crash
After an accident, we have made a practice of retrieving every broken piece of the crashed airplane, one of the most disagreeable tasks in our modeling experience. It helps to have an assistant, someone who will resist the immediate inclination to burn the remains. Quite often the parts can be reassembled like a giant jigsaw puzzle. Each glue joint is stronger than the surrounding balsa. Wing spars and longerons may need thin plywood gussets for reinforcement. External cracks can be filled with spackling compound or Hobbypoxy Stuff, and they become invisible after sanding and refinishing. Some weight is added in the process, at least several ounces for an average-size plane. While balsa may be pieced together, plywood portions such as bulkheads need to be completely replaced.
Rebuilding a wreck may actually be an ideal opportunity to make improvements. Even though an old plane has already been thoroughly de-bugged, it may become evident that certain balsa parts were overstressed and should be replaced with plywood. A rough rule of thumb says that plywood having one-fourth the thickness of a balsa part will have equal weight and twice the strength. For example, a 1/4-in. balsa piece could be replaced by 1/16-in. ply at no sacrifice in weight and a gain in strength.
Materials and Landing Gear
Landing gears are certainly subject to abuse and damage, even in a minor accident. After the first landing mishap, the time has come to increase strength. For example, the brass-tube struts can be changed to steel. Brass may be quickly accessible in hardware stores and hobby shops, but steel has a longer life in high-stress applications. Aircraft steel tubing, on the other hand, doesn't telescope as do successive sizes of brass tube and is a bit harder to find at airports or aircraft supply dealers. It is worthwhile to search for the better material. Aluminum tubing can be used for single-strut-type landing gears, provided it is an alloy and not pure aluminum. The soft aluminum tubing sold in hardware stores is even less satisfactory than brass, unless very heavy wall thicknesses are used.
Preferred aluminum tube is:
- 2024-T3 alloy (first choice)
- 6061-T6 (second choice)
With proper choice of wall thicknesses, alloy aluminum tubes will telescope. Local steel supply houses carry these alloy aluminum tubes.
First Flights After Repair
First flights after a major repair are very similar to the very first of a new model. There are mixed feelings of anticipation and apprehension. If all goes well, as is the usual case, elation follows. Resurrection of a model that had been an unsightly mess provides satisfaction equal to completion of a new aircraft.
Bob and Dolly Wischer S-221 Lapham Peak Rd., Delafield, WI 53018.
Transcribed from original scans by AI. Minor OCR errors may remain.
