Chapter 2 - OVERVIEW


You can observe a lot by just watching.
- Yogi Berra

2.1 Indoor vs Outdoor - return to top

Indoor models are flown in a controlled environment. Air currents are usually limited to drafts from heating or air conditioning vents or the occasional opening of a door, except for the more prestigious contests where even these are eliminated.

Indoor flying is like flying free inside of a box. Models will occasionally strike the ceiling or a wall. Event rules may permit "steering" during competition, using devices like a helium filled balloon on a string to push the model into a safer flight path.

Length of time aloft (duration) is extended by building models light. In accordance with the laws of physics, lightweight models suffer the least damage when they do strike an obstacle. A thirteen inch wingspan scale model built using conventional Japanese tissue covering and weighing about 0.8 ounce may achieve two minutes duration in the hands of a beginner, while 25.5 inch microfilm covered competition models weighing less than 0.1 ounce have achieved over one hour duration. Microfilm (nitrocellulose film) covering thickness is about .0000056 inches thick. These gossamer creations are an awe-inspiring study in slow motion.

Rapid advancements in electronic miniaturization and battery efficiency have placed indoor radio controlled models, powered by electric motors, within the capability of an experienced modeler. In the early days of radio controlled (R/C) models an airborne system's weight was measured in pounds. Currently, a complete radio system for a small R/C aircraft can be purchased weighing .85 ounces less batteries. That's less than the weight of three 25 cent pieces. See for links to light weight aircraft and related equipment.

Indoor modelers also fly hand-launched gliders and rubber powered models of every description. They are flown free flight or controlled; for fun (sport) or competition. They can fly year around, and they have an advantage when it comes to access to a flying site - any gym will do.

Outdoor modelers experience different challenges. High winds may ground all but the largest and heaviest models. At the other extreme, warm days produce updrafts (thermals) that can lift a model out of sight and carry it for miles. Free flight models are often equipped with "dethermalizers"; timed devices that change the position of flying surfaces to make the model slowly descend. This will happen unless the craft becomes "hung" (caught up) in a strong updraft. At this point, it is said to be in the hands of Hung, the nemesis of free flight modelers. It has been proven that the various incantations or attempts at paying tribute have little or no effect in this situation. Outdoor models are launched into the wind to achieve the advantage of the greater lift afforded by the airflow over the wings at the time of launch.

Outdoor models may be flown free, controlled by lines (U-Control), or radio controlled (R/C). They may be powered by twisted rubber, CO2 motor, gas engines, a ducted fan gas engine, pulse jet engine, turbojet engine, electric motor, or no engine at all (gliders). They may be flown for sport or in competition. They may be designed for endurance, aerobatics, speed, or scale. Scale models must adhere to the shape and color of a full sized prototype aircraft.

2.2 Free Flight vs Controlled Flight - return to top

Free flight models are designed and trimmed (adjusted) to fly in circles in order to restrict the distance they travel from the point of the launch, whether from the confines of a field or the walls of an indoor site.

Their power source is timed to achieve a desired height, then shut down to glide to the ground. In the case of an indoor free flight model the power is reduced gradually, ideally becoming exhausted at the moment of landing. This occurs when a rubber model winds down or an electric motor's battery stops producing power.

Outdoor free flight gliders may be hand launched, hand-pulled aloft by a towline, catapulted aloft using a line composed of a cord tied to a length of surgical rubber tubing (Hi-start), or by a powered winch that reels in the towline to pull the glider aloft like a kite. The glider flies off the line at the desired height and begins a circular flight path.

Free Flight modeling is represented by many special interest groups. To see the scope of this activity go to the Yahoo Free Flight Web Ring at: and type Free Flight Web Ring and click Free Flight Web Ring. Also see: for many related links applicable to all categories of model aviation, and for free flight introduction and related links

U-Controlled models are usually guided by lines extending from a hand held handle to a bellcrank (A lever having its fulcrum at the apex of the angle formed by its two arms) within the model. A link from the bellcrank extends to the moveable part of the horizontal tail affording an up or down movement that results in control of the altitude of the model's circular flight path.

Single line systems are sometimes used on speed models or additional lines may be used to control motor speed or other functions. In addition, electrical signals may be sent along the lines to provide added control functions such as wing flaps or aircraft carrier hook position for simulated carrier landings, etc.

Radio control of models enables the flier to guide his aircraft within restricted spaces. Doing so however, requires acquiring considerable piloting skills and experience. Ideally, the pilot must visualize himself/herself seated in the pilot's seat. When doing so and the model is flying away from you, signaling left turn makes the plane fly left. If the model is flying toward you, signaling left turn, the plane flies right as viewed from the ground. Unless the plane is flying upside down toward you, in which case, left would be left. This requires spatial visualization. Some are born with it, and others never acquire it. In addition, good depth perception is required to judge the point that engine power should be reduced to glide to a desired point on the landing strip.

It is possible, with a competent instructor, to start out with a docile trainer plane and learn to fly R/C. Perhaps the most "user-friendly" approach would be an "almost-ready-to-fly" (ARF) electric-powered R/C "Slow Flyer." However, a beginning modeler who starts with a simple free flight model, learning how to balance, trim and adjust the craft for flight, will be far more likely to reduce the learning curve for radio control. This means less expense when the inevitable crashes occur.

The ultimate RC challenge is to fly a small radio controlled plane in a gymnasium. With the availability of relatively low cost highly miniaturized equipment, this is being accomplished more than ever before.

2.3 Endurance vs Scale - return to top

Endurance models are designed for maximum time aloft. Without any design constraints the model would have unlimited power to climb to a desired altitude. It would be light and have a lot of wing area to permit an extended glide. But, you guessed it; modelers who fly in competition do have design constraints. For a given event upper or lower limits may be specified on: -wingspan or wing area -engine size or rubber weight for a rubber powered model -fuselage (body) cross section -construction materials permitted -propeller diameter or material -any other limits that may level the playing field

Despite many years of competition occurring in a given event, a canny competitor may find a loophole in the rules and enjoy a short-lived advantage. For example, in the early days the first gas models were flown in established rubber powered model contests. It was time for a rules change.

Scale models are built to resemble a full sized prototype as closely as possible. Since models do not behave in flight exactly as full-scale craft, experts in aerodynamics have had to develop a correction factor in order to use models in wind tunnels to study full sized aircraft. This correction factor, used to compensate for scale effect, is called the Reynolds Number. Often, the horizontal tail may have to be increased from exact scale for good model flight, for example. This is usually permitted in scale competition. In addition to true scale, proper colors, markings, and size limits, scale event rules also often include endurance requirements. The seasoned competitor, once again, will look for an advantage. The solution is to model a prototype that in full scale has the characteristics best suited for model performance. When maximum wingspan is specified, look for a full-scale plane with a wide wing (chord) to get the most wing area, giving the model the best lift performance. Eventually certain full-scale selections may be banned, again to level the playing field. In some contests all competitors are required to model the same prototype.

2.4 Competition vs Sport - return to top

Competition occurs in any of the categories covered so far, but it turns out that more modelers fly for personal enjoyment (sport) than those who compete. When flying a rubber powered model in the quiet air of dawn or dusk, the modeler will try to improve upon past performance. It could be said that one is actually competing with oneself.

A recent development in model aviation is the fun fly. Modelers attend these events to compare notes on design, and fly without the pressures of competition. These events have had a pronounced effect on the rapid acceptance of electric powered flight. But even in these low-key meetings, competitive fun events are flown. A mass launch of many attendees to see who lands last, or who can fly under the lowest horizontal bar (limbo), for example.

2.5 Other Types - return to top

Two examples of other challenging types of models are helicopters and ornithopters.

Model helicopters are gaining in popularity, powered by either gas engines or electric motors. It requires skill to fly these craft because like most RC models you must think ahead or the model will get ahead of your ability to control it. You can't just stop and think, soon I'll make a certain move, because with your hands on the controls every slight movement of hands creates an immediate response from the model.

Model ornithopters are propelled by the flapping of wings. Early experimenters studied the flight of birds and achieved some limited success with the wing flapping approach. A small but dedicated group of present day modelers enjoy the challenge of making ornithopters fly.

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Copyright 2002, Robert S. Munson. All Rights Reserved