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Seems like most everyone has a question about props. Some like what the manufacturer has provided or recommended but, many wish to increase the performance of their aircraft by replacing the prop with something better.First off, it is best to stick with the designer or manufacturer's recommendation as changes may not enhance the performance of the aircraft and may even become dangerous. Secondly, the propeller material may be a factor in the aircraft's performance. All materials exhibit a natural frequency of resonance such that at certain rpm values the material of the prop may resonate to the point of damage or destruction. It's best to follow recommendations as to the type of prop to use based on the material of its manufacture. Wood propellers exhibit a change in pitch as airspeed is increased making for a better cruise speed over all. Other materials, however, do not exhibit this tendency and are, therefore, not as efficient. Wood props handle foreign object strikes fairly well and seldom result in catastrophic failures. Carbon fiber blades will shatter whenever a foreign object strikes causing an sudden unbalance situation requiring emergency action. Metal props have a definite frequency of resonance and certain ranges of RPM cannot be sustained over lengthy periods of time. Most propeller blades utilize the "Clark Y" airfoil section. However, in recent years other airfoil sections have been employed to provide some enhancement in performance. Length of the prop, that is the diameter, must be kept at optimum values to maintain prop efficiency as high as possible. This results in better climb capability, such as is seen on the Helio Currier. This was a factor on some of the military aircraft used for artillery spotting as these aircraft operated from short un-improved strips. A two bladed propeller is more efficient than a three bladed prop since the two blade prop is of a much larger diameter than the three bladed prop. Tip speed is of much concern regarding the efficiency of propellers. Metal bladed propellers must not exceed 950 ft/sec. (feet per second) at the tip while in rotation. Wooden propellers have tip speed a bit lower and should be below 850 ft./sec. For a propeller of known diameter the tip speed can be determined from the formula tip speed = 3.14159*(rpm/60)*diameter(ft.) for instance a propeller of 5 feet in diameter turning at the rate of 2150 rpm will have a tip speed of 3.14159*(2150/60)*5 tip speed = 563 ft. per second, well within the limits for a wooden prop. This is typical of a Rotax Engine turning at 5550 rpm. Propeller blade diameter can be found easily knowing the engine horsepower by using the following formula Prop Diameter, ft. = 1.83*hp^(1/4) that is, 1.83 times the forth root of the engine horsepower. For instance, for an engine of 50 hp the 4th root of 50 is 2.66 multiplied by 1.83 gives a diameter in feet of 4.87 feet or 58.4 inches. For the example of the 40 hp engine the required propeller diameter will be Prop Diameter = 1.83 * 40^(1/4)= 1.83*2.515 = 4.6 feet or 55 1/4 inches. Three blade propeller things change slightly. The multiplying factor now becomes 1.5 instead of 1.83 as in the two bladed prop. In the case of the 50 hp engine the diameter becomes, Prop Diameter = 1.5 * 50^(1/4) Prop Diameter = 1.5 * 2.659 Prop Diameter = 3.989 Ft. or 47 7/8" substantially smaller. There are many other considerations regarding propellers, however, the above should be used as a guide to understanding more of the technical aspects of what props to use and how to verify if your selection meets the criteria set forth by these equations.
The first article on propellers dealt with propeller diameter. Since it requires a certain
amount of pitch to the propeller to cause forward motion, this section will deal with the elements
required to determine the necessary pitch for any particular application. P = pitch in inches |
