Author: B.

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Author: D. Wischer
Edition: Model Aviation - 1984/11
Page Numbers: 46, 47, 135, 138
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Radio Control: Scale

Bob & Dolly Wischer

Scale Electrics

An Electric Spitfire Mk Ia, seen at Toledo, served notice that we can expect more Electric Scale to appear in our competitions. The 61‑in. wingspan Spitfire, by Keith Shaw, featured retracts, an AFI Cobalt 25 geared motor, and Jomar SC‑2 speed control. The distinctive difference between this model and all others at Toledo was its weight: a mere 25 ounces, without batteries! Total weight, ready to fly, is 80 ounces. At present, Electric Scale airplanes are judged along with all others, but this may be changing.

Electric Scale made its first appearance at the 1973 Nats, when Bob Boucher entered a Fournier and introduced us to the spectacle of Scale silent flight. Under ideal conditions, the Fournier could have been quite competitive, even then. Unfortunately, Scale flying days at the 1973 Oshkosh Nats were plagued with strong, gusty winds which prolonged the time needed for Bob's climb to each maneuver position. This meant curtailment of the time needed to complete a full flight schedule. Improvements in motors, drives, and control accessories will make Electrics more attractive as competition subjects. The Fournier was chosen as an ideal subject for Electric Scale.

Harry Apoian on Electric Scale

Harry Apoian, writing in the Electric Aeromodeling Association newsletter, has this to say about Electric Scale:

"Scale models are unique in that they look and fly like full-size airplanes. The challenge lies in the degree of difficulty to achieve these goals and to return a sense of accomplishment. With the use of electric power, this 'Scaleness' horizon may be expanded to include model types which, up to now, were considered impractical with reciprocating engines. For example, pushers, multi-engine planes, shaft-driven props, lighter-than-air, geared props, self-starting and air restarts are all possible and are practical.

"To promote Electric Scale we, in the Electric Aeromodeling Association, have to emphasize the positive aspects, and exploit those applications where electric power would be to our advantage. What is the best course to take? Should we compete with non-Electric planes at regular AMA contests? As a newly-formed special interest group within the AMA, we are in a position to dictate requirements that are to our advantage. Let's generate Scale rules for Electric power. We need new ideas for Scale competition. We need new categories for AMA records, for Scale RC, Scale Free Flight, Scale Indoor. AMA–EAA will have the opportunity to start fresh, dynamic enough to advance the state of the art. Please let us know your thoughts and ideas about the new organization. Let's improve capabilities and enhance enjoyment of our great hobby."

Please let us hear from Harry Apoian, 27704 Saddle Road, Rolling Hills, CA 90274.

Scaling Principles and Examples

Information on designing Scale models for electric power comes from Roland Boucher, Leisure Electronics. Roland has this to say about design: "A properly designed and constructed Scale model will perform in a manner similar to the full-scale airplane. It will take off and land in the same number of airplane lengths the real plane will; climb at the same angle; will snap and spin maneuvers in a scale-like manner. The plane will also fly at scale speeds allowing the same types of vertical maneuvers the real plane can accomplish."

This can be accomplished by following a few simple rules. The first rule is to keep the density (weight per cubic foot) of the airplane constant. The air the model is flying in is the same air the real plane is flying in, so it makes sense to keep the plane at the same density. If we reduce the model in size by a scale factor "S," then the wingspan will reduce to 1/S of the real plane, and with density constant weight will reduce to 1/S^3.

This has a remarkable effect on wing loading—it reduces directly as the scale factor. For example, a 1/4‑scale ship would have a scale factor of 4. It would have 1/4 the wing loading of the real plane. The scale speed the model will fly at is proportional to the square root of its wing loading. Our 1/4‑scale example will stall and glide at 1/2 the speed of the full-size plane. The power required to fly the model is directly proportional to its speed and weight.

The scale power required would then be reduced from the real plane's power by 1/S^3 (the weight factor) and 1/√S (the speed factor). Using our 1/4‑scale, 65 hp Piper Cub as an example, the power required would be (1/4) × (1/2) × 65 = 0.5 hp. The 65 hp Continental engine turned a 72‑in. diameter prop at 2,400 rpm. If an 18‑in. scale-size prop is used, it must turn faster by the square root of the scale factor, or 2,400 × √4 = 4,800 rpm. The example of a 1/4‑scale plane was given to keep the math easy. It results, however, in a 108‑in. span plane weighing 12 lbs.—not exactly a comfortable flier!

Let's see what happens if a more practical scale factor of 1/8 in. = 1 ft. for the J‑3 Cub is chosen. This choice of scale results in a 61‑in. span model weighing 37 oz., and powered by a .08 hp motor turning a 10.5‑in. diameter prop at 6,285 rpm. Today, modern '.05' geared electric flight systems will fit this requirement quite well. Table I gives the scaling factors for this model.

Surprisingly, the '.05' fits almost exactly and its weight is nearly in proportion to the real power plant. The 65 hp Continental engine weighs about 200 lbs. and the 15 gallons of fuel aboard weighs another 90 lbs., for a total of 290 lbs., or 38% of the Cub's flying weight. The '.05' flight system weighs about 17 ounces, or 42% of the gross model weight. Not bad for an '.05' electric. For those of you who don't have a calculator with square roots, Table II gives the scaling factors for the more popular scales.

Tables

Table I — J‑3 Cub Scale 1/8" = 1 ft. (S = 6.86)

  • Wingspan (full-size): 35.3 ft. → Linear factor 1/S = 1/6.86 → Model: 61 in.
  • Area (full-size): 354 sq. ft. → Area factor 1/S^2 = 1/46.9 → Model: 63 sq. in.
  • Weight (full-size): 850 lb. → Volume/weight factor 1/S^3 = 1/321 → Model: 37 oz.
  • Wing loading (full-size): 24.05 lb./sq.ft. → Factor 1/S = 1/6.86 → Model: 2.50 oz./sq.in.
  • Speed (full-size): 70 mph → Speed factor 1/√S = 1/2.62 → Model: 27 mph
  • Power (full-size): 65 hp → Power factor 1/S^(2.5) = 1/116 → Model: 0.56 hp
  • Prop (full-size): 72 in. → Linear factor 1/S = 1/6.86 → Model: 10.5 in.
  • RPM (full-size): 2,400 → Speed factor 1/√S = 1/2.62 → Model: 6,285

Table II — Scaling Factors Commonly Used

  • Scale 1/4" = 1 ft.
  • Fractional scale: 1/48
  • Scale factor (S): 48
  • Area (1/S^2): 1/2,304
  • Volume/Weight (1/S^3): 1/110,592
  • Linear (1/S): 1/48
  • Speed (1/√S): 1/6.93
  • Scale 1/8" = 1 ft.
  • Fractional scale: 1/96
  • Scale factor (S): 96
  • Area (1/S^2): 1/9,216
  • Volume/Weight (1/S^3): 1/884,736
  • Linear (1/S): 1/96
  • Speed (1/√S): 1/9.80
  • Scale 3/16" = 1 ft.
  • Fractional scale: 1/64
  • Scale factor (S): 64
  • Area (1/S^2): 1/4,096
  • Volume/Weight (1/S^3): 1/262,144
  • Linear (1/S): 1/64
  • Speed (1/√S): 1/8.00
  • Scale 1/6" = 1 ft.
  • Fractional scale: 1/72
  • Scale factor (S): 72
  • Area (1/S^2): 1/5,184
  • Volume/Weight (1/S^3): 1/373,248
  • Linear (1/S): 1/72
  • Speed (1/√S): 1/8.49
  • Scale 1/12" = 1 ft.
  • Fractional scale: 1/144
  • Scale factor (S): 144
  • Area (1/S^2): 1/20,736
  • Volume/Weight (1/S^3): 1/2,985,984
  • Linear (1/S): 1/144
  • Speed (1/√S): 1/12.00

Practical Examples

Examples of Scale models suitable for Leisure LT‑50 geared motors:

  • Leisure LT‑50 6033 (2.5:1 geared, 7 cell)
  • Heath Parasol — 63 in. span
  • Taylorcraft giant scale — 90 in. wingspan (catalog info: 4‑cycle 75–90)

(Manufacturer and catalog notes omitted here for brevity.)

Roland Boucher has come up with some handy numbers to lead us into the proper scaling procedure, and to avoid the headaches that come from building too large or too small, and also to keep the model weight and power within the ballpark. With Electric Scale now flourishing, this is the type of information needed. In fact, it also applies to all Scale models. Glow‑ and gas‑powered models only permit us the luxury of building heavier aircraft and getting away with it. We wonder who will be first to successfully build and fly a quarter‑scale electric? Maybe it's already been done.

RC Scale

Bob & Dolly Wischer

Unnecessary Scale Accidents

One month before our Scale team was to depart for the World Championships at Paris, Team Manager Dolly Wischer received the shocking news that one of our models was lost in an accident! Cliff Tacie was practicing with his Spezio Tuholer. Another flier, on an aborted takeoff, ran his model into the pit area, where its propeller chewed into Cliff's leg. The leg repair required 40+ surgical stitches. Unfortunately, Cliff's Spezio was in the air when this occurred, and control was lost. The airplane, with several thousand hours of labor in its construction and refinement, was totaled. Its performance had been much improved after changing the .60 engine to a new four‑stroke .90. The airplane that did the damage was of the type that can be assembled in a few weeks by an average builder.

There is a point to this story—and that is to fly a valuable Scale model defensively. Even when parked in the pit area, it's vulnerable, and we have had personal experience on at least three occasions in which damage occurred in the pits. Be ready to protect a valuable investment in time, effort and money. Don't be goaded into flying with other airplanes, just to prove fearlessness, when someone taunts with the expression "chicken." When this is heard, tell the person to build one of equal value, and put it into the sky in a game of combat.

Of course, we don't live in fear, but a strong sense of apprehension about mixing a valuable airplane with a swarm of quickies will help to preserve a model. We have heard our Scale airplanes called "closet queens," because it was assumed that they were seldom flown due to fear of damage from pilot error or other reasons. The name-callers have long since gone back to golf (or whatever else they are doing), and the models have been preserved and continue to fly.

The true Scale modeler has a long-term commitment to our sport and hobby. He (or she) isn't a transient, looking for instant gratification, who moves on to other endeavors as fancy dictates, or when the thrill of controlling a missile is diminished. We have stopped counting near-misses in the air. One midair collision in 28 years of RC has been more than enough.

Defensive Flying

Defensive flying to promote preservation of the model isn't entirely a matter of protecting against the other guy. Some accidents are the result of performance beyond the model's capability. We fly our trainers through maneuvers and then expect to do the same with a Scale model.

Tailspins near the ground claim some airplanes that could have survived with a bit more air underneath at the time. Some spin recoveries are delayed intentionally, and others need decisive action to affect recovery, particularly with WWII aircraft. Avoiding an inadvertent spin is just a matter of maintaining flying speed all the way down to a landing.

There are not too many uses for down‑elevator control in Scale flying, but spin recovery is the right time to apply it. If it happens near the ground, use down‑elevator to gather speed, then follow with gentle up‑elevator and add power to prevent re‑entry into a secondary spin. We have used the technique to save a model. Of course, it was possible only because there was sufficient altitude initially, but the model appeared to be well on its way to destruction.

Steep turns near the ground are also an invitation to disaster, because too much of the wing's lift is being used to change direction. This leaves too little lift for supporting the model's weight, which is now increased by centrifugal force added to gravity. A snap‑roll into the ground is actually a one‑half‑turn spin.

Paint Coverage

Paint seems to have less covering power today, as compared with what was normal in our earlier days of modeling. Dope is one of the materials that seems to have changed. After several coats, framework remains visible in silhouette through the covering when held up to bright light. Experts tell us that this is the result of lead being banished from paint manufacture. Apparently, the term "lead sled" was an apt description for those earlier, heavily finished Scale models.

An opaque finish is still possible, without the penalty of excessive weight build-up, by using a coat of aluminum paint just beneath the final color coats. For example, on a doped finish, mix aluminum powder into the last coat of clear dope. This has the additional advantage of leveling out color differences in the substrate. An amazingly small amount of color is then needed to cover the aluminum for a uniform coating. One heavy spray coat—or two lighter applications—will suffice.

We prefer to run the receiver antenna through the fuselage interior. There have been stories that this was hazardous on a fuselage sprayed with aluminum paint, but our experience indicates no problems with signal loss, even when many aluminum coats were used to simulate a metal fuselage.

Bob and Dolly Wischer S‑221 Lapham Peak Rd., Delafield, WI 53018.

Transcribed from original scans by AI. Minor OCR errors may remain.