SOMETIMES WE become so involved with the various types of model engines, performance ratings, and displacements that we overlook something important. Just as a fully race-prepared car engine is useless until its power is transmitted to the ground, a model engine is useless until its power is transmitted to the air. Most often, model aircraft use propellers for this job. Our engines also need to burn fuel to produce power, and they need to ignite that fuel. These functions, which are directly connected to the engine, are this month's subject.

Propellers: A model's engine is only as good as its propeller. The propeller's size, shape, and composition determine how much of the engine's power is transmitted to the air and the manner in which the aircraft can best use that power. The best combination of propeller characteristics for a particular model is a compromise.

The pilot must choose a propeller that produces the best performance based on the aircraft's mission (training, racing, aerobatics, combat, etc.), the engine's power range, and the flying-field conditions.

A racing airplane would do best if its propeller were designed solely to produce high airspeeds while rotating at the same rpm at which the engine produces maximum horsepower. This is the right choice even if durability, climb, and acceleration rates are sacrificed.

Choosing the right propeller requires understanding and a few "prop tips," one of which is that a propeller's blade rigidity is important. A propeller is nothing more than a rotating wing. All propellers have airfoil shapes and direct their lift in a horizontal path, called thrust, instead of a vertical direction, as does the aircraft's main wing. Thrust pulls the aircraft forward.

Imagine how much of your aircraft's wing lift would be lost if the outer third of the wing were to flex enough that its incidence—its angle of attack (AOA) to the oncoming airstream—significantly decreased during every turn or climb. In the same way, a propeller in which the tips flex does "flatten out," reducing its incidence during acceleration and climb, thereby losing thrust when it is most needed.

Unlike a wing, which develops lift along almost its entire span, a rotating propeller produces the majority of its thrust centered around the 75% point of each blade's length. This makes the thrust lost caused by tip flexing even more critical.

Stand slightly behind and to the side of the spinning propeller and watch the tips. If they follow a wavy path, that signals excessive pitch loss (lower propeller AOA), which results in power lost transferring the engine's energy to the air.  

The first 20% of a propeller blade's length—its span—produces much drag but little thrust. This section is the area where the propeller's round center—the hub—tapers into the working "wing" of the blade, which does all the work. There is little "wing area" here.

This area also moves the slowest through the air since it is closest to the center of the "disc" formed by the rotating propeller. However, this inner section does rotate and therefore produces air drag. This is why spinners make propellers more efficient.

The next 50% of the blade's span is the area where the LE-to-TE width—the chord—increases to maximum and the airfoil becomes fully developed. Some thrust is lost until the blade is fully formed, and more is lost because the center-section rotates more slowly than the remaining outer blade area. Since a wing's total lift depends, in part, on its airspeed, the lift produced by different blade sections depends a great deal on their rotational speeds.

How different are these rotational speeds? The blade section 1 inch out from the hub of an 11-inch-diameter model propeller rotating at 11,000 rpm has an "airspeed" of just 96 feet per second (fps), or 60 mph. The middle of the blade is rotating through the air at 260 fps, or 180 mph, and the 75% point is moving at 396 fps, or 264 mph.

Even though the blade's area near the tip (90%) is much less than that near the middle, it is moving nearly twice as fast, at 475 fps, or 317 mph, and is therefore producing more thrust than the center-section is.

Please study that last rotational speed. The tip itself is moving at 530 fps, which is approximately the same speed as some .45-caliber bullets. If you want to know what happens if you are careless enough to put a hand into a spinning model propeller's arc, envision pointing a Colt .45 at your hand and pulling the trigger! Not an attractive image.  

Please be careful. Tune your engine while standing behind the propeller, never stand directly to the side of a spinning propeller, and keep children away from your engine at all times.

Since rigidity is important to propeller performance, a major factor to consider when choosing a propeller is its construction. Today they are usually made from one of four basic materials: fiberglass-filled nylon composite, fiberglass-reinforced nylon, wood, or carbon fiber (CF).

Pure nylon propellers were once manufactured, but for the most part they have been replaced by nylon composite construction. The fiberglass-filled nylon propellers are safer and stiffer than the old nylon-only variety, but they remain the most flexible kind.

Most fiberglass-filled nylon propellers have large blade areas to improve their performance. They produce excellent thrust for a given rpm but tend to rotate more slowly than same-size propellers of different construction. These propellers suffer the most thrust loss as the airplane climbs steeply since the outer blade areas flex the most under stress.

However, this flexibility is a major advantage for newer model pilots. The blades bend well on those poor landings—those that bend the nose wheel back nearly far enough to touch the fuselage bottom. Fiberglass-filled nylon propellers bend backward and usually do not break in those situations. They also last the longest when flying from paved runways.

This durability saves money and keeps newer pilots flying on those days when they would have exhausted their supply of more rigid propellers. Most RTF trainers are equipped with the fiberglass-filled nylon variety for exactly these reasons.

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