CHAPTER 5 PROPELLERS

PROPELLER SIZE: Propeller size is designated by two numbers separated by a dash (6-3, 8-4, 10-6,12-5, 16-6, etc.). The first number indicates the exact diameter of the prop in inches. The second number is the "pitch" of the prop in inches - the theoretical distance the prop would travel forward during one complete revolution. As an example: a 10-6 prop measures 10 inches in diameter and would travel forward 6 inches during one complete revolution.

We say that the pitch measurement Is theoretical simply because a propeller is not 100% efficient. Air is not a perfect fluid, it compresses and distorts as the propeller passes through it. The prop's exact shape, blade airfoil, blade area, and finish also have an affect on the propeller's ultimate performance in flight. Therefore the pitch measurement should not be considered as the actual in-flight performance of the prop, but rather as a simplified designation of the blade angle for comparison purposes only.

PROPELLER SELECTION: Don't take the selection of a propeller for your model too lightly. It is just as important a part of your airplane as the engine or wing. If you use the wrong size prop your model will not fly right, or may not fly at all. You can also damage the engine by running it with the wrong size prop. For ultimate performance, the prop must be properly matched to the engine and model. Always use a propeller within the range specified by the engine manufacturer in the instruction sheet that came with your engine. The following prop charts are offered as a general guide.

These prop charts are intended to provide an R/C flyer with a safe, dependable starting propeller to use on a typical sport/trainer type model airplane. While the "STARTING PROPELLER" listed may not deliver optimum performance in every single case, it should get the model off the ground and flying nicely with the engine operating in a safe RPM range. This will provide a starting point from which other size props, either from the "ALTERNATE PROPELLERS" list or from the engine manufacturer's instructions, can be tried and compared. The model's size, weight, drag, wing loading; the type of engine being used (sport, pattern, racing, etc.) and its actual power curve; the type of fuel being used; and even the altitude at which you are flying are all factors in finally determining the optimum propeller for each different airplane. This can be done only by flying with different props and noticing any differences in the model's speed and climb.

In general terms, a higher pitch prop will pull the airplane faster in level flight. A lower pitch prop will cause the airplane to takeotf quicker and-climb faster. Some full-scale airplanes have adjustable pitch props so they can use the most efficient pitch in each situation. The pilot will select low pitch for the takeoff and climb to altitude, and then switch to a higher pitch for better level flight speed and fuel economy. It's exactly like shifting gears in a car! Low gear provides quick acceleration from a stop, while high gear is used for better fuel economy after the car is up to cruising speed. Even owners of simple full-scale airplanes with fixed pitch props, like a J-3 Cub, can choose between at least two different FAA-approved propellers - one called a "climb prop" (low pitch) and another called a "cruise prop" (higher pitch).

On a model airplane you should not only try different pitch props, but different diameters as well. For example, let's say you are running a .60 2-stroke engine and start out with an 11-7 prop. The model will very likely fly real nice. Next put on a 12-6 prop, readjust the needle valve, and fly again. Watch carefully! This time the model should be able to takeoff in a little shorter distance and you will be able to pull the nose up a little steeper on the climbout without stalling. However, the level flight speed will probably be slightly less than with the 11-7. So while the engine turns both propellers at approximately the same RPM, the flight characteristics of the airplane are slightly different with each prop. Don't be afraid to try another different size propeller and note any further changes. Usually the changes will be very small, and there will always be a tradeoff of some kind - what you gain in one aspect of performance you may lose in another. By trial and error you will eventually determine which size prop best suits your particular model and how you want it to perform.