Horsepower, Torque, and Other RPM Numbers
Horsepower (Hp) is the power output of an engine or a motor. Most model engine manufacturers give the maximum, or peak, horsepower rating of their engines as one of their selling points. Horsepower is a calculated number. This number is calculated from the engine's torque and the RPM (revolutions per minute—also note that RPM is already a plural. RPM's is incorrect) at that torque. Torque is the actual twisting force on a shaft, and can be measured with a dynamometer, or "dyno".
When an engine is put into a dyno for testing, a calibrated load is put on the engine. This is usually some kind of weight. The engine is run at full throttle, and the RPM is measured. Different loads are put upon the engine, and the RPM is measured each time. With enough measurements, a torque versus RPM curve can be plotted.
At each point, the horsepower can be calculated. The formula is:
Hp = (Torque x RPM) / 5252
This is when torque is measured in foot-pounds. If the torque is measured in inch-pounds, then the formula is:
Hp = (Torque x RPM) / 63,025
(Fractions and some rounding are ignored in the above formulas.)
A curve with horsepower plotted against RPM will then show where the peak horsepower happens. Unfortunately, this can be misleading. The only time peak horsepower is needed, or desired, is in a racing installation, where every bit of power you can squeeze out of the engine is needed. For most applications, peak torque is needed. After all, it's torque that is the force that turns the propeller. Peak torque almost always occurs at a much lower RPM than the peak horsepower.
For example, a typical .46-size engine will develop its peak horsepower at about 16,000 RPM, but it will develop its peak torque around 12,500-13,000 RPM. For almost all uses, then, the engine should be set up so that it will turn 12,500-13,000 RPM at full throttle.
Another set of engine RPM is what some manufacturers term, "practical RPM". This is the RPM range in which the manufacturer has run the engine. One .46-size engine has that range listed as 2,000-17,000 RPM. Unfortunately, you will never be able to run the engine through the full range with one setup.
The low end of the range is the slowest speed the manufacturer could get the engine to run with one particular setup. The high end of the range was found with another setup, and was as high as was dared before the engine started to have a very significantly-shortened life. Much faster, and the engine will not hold together. Consider the top end of the range the "redline" speed of the engine.
If you can get the engine to run very well at low speeds, the top speed will be sacrificed, and the same is with the top end. You won't get a particularly low idle if you set your engine up for maximum RPM.
The actual operating range is somewhat towards the middle. The same .46-size engine with the 2,000-17,000 RPM practical range really winds up with an RPM range of about 2,700-13,000, when set up in most models. This RPM range will give an excellent throttle response along with a very good idle and long engine life (assuming proper operation and care). For a .46-size engine, this means a 10x7, 10x8, 11x6, or 11x7 propeller.
Nowadays, many engines have larger propellers listed than have typically been used in the past. This aren't necessarily the best propellers, but are suggested because the engine manufacturer is trying to help with noise reduction. Many places around the world have very stringent noise restrictions, so the propeller suggestion is to help reduce noise. A larger propeller will turn at lower RPM than a smaller one, so engine and propeller noise are reduced. Most engines won't develop their best torque or power at those RPM levels.
In the U.S., most noise restrictions aren't as severe as in Europe or some places in Asia, so the engine can be run at more efficient settings, allowing more power to be developed.