Fiberglass-reinforced propellers are stiffer and sometimes feature undercambered (concave-bottom) airfoils. They have tips with a small area but quickly widen to large chords just short of the tip. The tiny tip area helps the engine stay quiet and increases the propeller's efficiency. One of the most efficient wings ever designed employs elliptical wingtips that reduce drag by reducing wingtip vortices; just ask any Spitfire pilot.

Fiberglass-reinforced propellers have wingtip designs that most closely resemble the elliptical wing shape. The reduced tip drag allows the propeller to accelerate quickly and to reach a higher top speed. That combined with the more rigid blade make fiberglass propellers famous for excellent climb performance. The middle areas of many reinforced blades are usually the largest in their respective size classes. This helps increase overall thrust, again adding to the aircraft's climbing ability.

However, these stiffer fiberglass-reinforced blades still flex a bit under load and are easy to break during hard landings. Paved runways are rough on them since the tip area is small and may be destroyed with one contact, even if the propeller is not rotating.

Wood propellers are more rigid than fiberglass-filled and fiberglass-reinforced nylon types. Some wood propellers have special tip designs to produce increased thrust and rpm. Most have roughly the same blade area as fiberglass-filled propellers that are the same size. However, wood propellers must be carved—not molded—and therefore do not usually feature the more exotic blade designs that are so common in some molded model propellers. Depending on their design, wood propellers produce excellent top speeds and quick acceleration because they are light and stiff.

Wood propellers break easily with any ground contact, and prolonged use on grass runways results in excessive blade wear. They also require the most balancing effort because density and water content may vary in a single propeller. 

You know that all propellers must be balanced, right? Unbalanced propellers cause excessive vibration, resulting in three major problems. First, the engine's bearings wear quickly. Second, the onboard radio components, especially servos, suffer excessive wear and can fail early. Whenever a servo quits in flight, much of the fun of flying RC models is diminished.

Third, an unbalanced propeller will cause a 3%-4% rpm loss. An engine that would have turned a balanced propeller at 11,000 rpm turns an unbalanced propeller at only 10,600 rpm. Climb rate and aerobatic performance are reduced.

Many different propeller balancers are available. I will cover these in the last edition of this segment of the series.

CF propellers are the ultimate in rigidity; they have almost no detectable flex. They can assume any airfoil shape and blade area as they are molded. Some are solid and others are hollow. CF propellers are light, allowing for the fastest engine acceleration possible, and hollow ones accelerate even more quickly. The solid and hollow kind feature excellent performance across the entire aerobatic spectrum. You can even purchase them prebalanced.

However, despite their superior performance, few modelers use CF propellers. There are two good reasons for this, the first of which is cost. CF propellers vary from nearly $30 to $120 for the larger sizes. A modeler can buy an abundance of wood, fiberglass-filled, or fiberglass-reinforced propellers for $30. 

Second, trainers and many lower-performance sport models are unable to take full advantage of the performance increase that such a propeller provides. From level flight, a 40-size trainer may be able to perform a 100-foot vertical climb. If a CF propeller provides a 15% climb increase, that trainer will perform a 115-foot vertical climb. It's not that noticeable of a difference for the money.

But install that propeller on a 40-size Pattern airplane, and its normal 250-foot vertical climb stretches to nearly 300 feet with enough remaining airspeed to provide excellent control.

After construction, the next important factor in picking the right propeller is size. Two numbers label their dimensions, and the first is diameter in inches. The second is pitch, which represents the distance in inches the propeller would travel forward in one revolution if there were no friction, drag, or other limiting factors. This is the propeller's AOA, or incidence.

The numbers are separated by the usual "by" designation: "x." An 11-inch-diameter propeller with a 6-inch pitch is called an "11 x 6."

Understanding both numbers' performance implications is critical. They interact in a complicated dance of airflow, engine performance, thrust, and geometry. Fortunately the dance becomes easy to understand once you know the few simple steps. Well, step one, which follows, is not all that simple but is easy to understand with a little geometry.

The propeller's efficiency for a given task is determined by the amount of air it moves per revolution and its speed. On a sport airplane, if a propeller can move a huge amount of air, but only at a slower speed, that is better than moving small amounts of air at high speeds. 

The diameter of the disc that the rotating propeller produces has more effect on the power transmitted than a speed increase does because the disc area increases by the square of the radius. Therefore, an increase in diameter (radius) moves additional amounts of air by the square of the radius. This is a large force multiplier.

Without tripping over the math, airstream speed increase has an even less than linear effect increasing the amount of power applied to the air. This is a small force multiplier. The idea is that as long as you have enough pitch to fly at the speed you need, diameter is king, offering faster acceleration, better climb, and shorter takeoff runs.