Step two: The pitch determines airspeed only in combination with the airframe. A 20-inch-pitch propeller sounds fast. But if the airframe has a high level of aerodynamic drag, fixed landing gear, and straight wings, this drag prevents the airplane from ever reaching the propeller's theoretical top speed. The result is that the propeller cannot reach its maximum rpm because the extra airframe drag increases the propeller's air load.

For step three, the best propeller size for a given engine, in a 40- to 60-size, high-drag sport RC model, is that one compromise between the largest diameter and highest pitch that still allows the engine's maximum ground rpm to be roughly 1,000-1,500 rpm higher than its high-torque (maximum twisting power) rpm. (This figure is after the high-speed mixture has been adjusted to be 500 rpm less than absolute peak.)

Why this rpm? Once the airplane is flying, there is an average increase of 500 rpm. This happens because the aircraft's forward speed acts to decrease the propeller's AOA. Another way to picture this is that the air "flowing" into the propeller from the front is helping the engine turn it. It isn't, of course, but it is decreasing the engine's workload by reducing the effective AOA.  

An airplane stops flying faster when the propeller's AOA nears zero. But once the aircraft's nose is pointed skyward, more of its weight is placed on the propeller and therefore on the engine's turning ability. The airspeed decays, and those 500 "free" rpm disappear as propeller drag increases with the escalating AOA.

The stress of pulling the aircraft upward increases the power demands on the engine. It responds by turning more slowly, just as a car's engine does when going up a steep hill until extra energy, in the form of stepping on the gas, is applied. But the model engine is already at full power; there is no extra "gas" to give.

In fact, the engine's rpm will drop until it reaches its high-torque rpm. If the engine is the right size for the airplane and the climb angle is not steeper than what the engine/airframe combination was designed to maintain (usually at least 45¡), the rpm reduction stops here and the airplane maintains a constant climb rate.

Why not use a propeller that allows the engine to rotate at its peak horsepower rpm? The horsepower ratings for most .40-.60 two-stroke engines are usually at so high an rpm that they are nearly unusable for sport applications. Most reach peak horsepower well in excess of 13,000 rpm.

At this number, model pilots do not need to worry about their airplanes' performance because most clubs won't let them fly such loud models. Even if they can fly them, the propeller disc must be so small—7-9 inches—that little thrust can be applied to the air (step one). The result is an inefficient propeller, an airplane flying roughly 35 mph, and a screaming engine trying to tear itself apart.

As shown, the highest ground rpm does not translate into the fastest airspeed or the best climb rate. Under the demands of a climb, the smaller 10-inch propeller discs could not transfer the engine's power to the air as effectively as the 11-inch discs could.

An inch may not seem to be a big difference. However, the 11-inch propeller has an effective area of 95 square inches versus the 10-inch propeller's 79 square inches. The engine's "force area" is 20% larger using the 11-inch propeller.

The larger disc is the reason why the 11 x 6 propeller produced a 20% better climb rate than the 10 x 7, despite the slower climb speed. The extra power required to turn the 11 x 7 propeller when climbing proved more than the ol' engine possessed. Climb and top speed suffered, but landing speed was the slowest, probably because the idle speed was less than 2,100 rpm. The 11 x 7 might cause engine overheating in hot weather.

The 11 x 6 allowed the aircraft to leave the ground in the shortest time at the slowest airspeed, reducing airframe wear. It produced a climb rate up to 50% higher, and its top speed was only 6% less than the highest but up to 18% higher than the remaining propellers'. During your next visit to the flying field, check the propellers on most .45 two-stroke engines. Most will be various types of 11 x 6s.  

Choose the propeller-and-glow-plug combination that permits the engine to turn the largest-diameter propeller approximately 1,000-1,500 rpm higher than its peak torque speed on the ground. Start with the largest diameter and lowest pitch recommended for your engine. If the engine will not turn this propeller fast enough, drop to the next smaller diameter, again with the lowest pitch. Increase the pitch if the engine turns too fast. Continue until you find the right size combination.