Propeller sizing worm can
12-27-2007 | 06:28 PM
#27
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From: Jonkoping, SWEDEN
RE: Propeller sizing worm can
ORIGINAL: HighPlains
Several interesting questions.
Long life on an engine has more to do with staying on the rich side and operating in a clean environment than RPM. If anything, low RPM is harder on the engine than higher. If you insist on lower RPM operation, use additional oil. Low RPM operation also makes the needle setting much more critical.
As to the prop size difference between 40 and 60. An 11" prop has 21% greater area than the 10", while at the same rpm has a 10% greater tip speed. When you increase speed by 10% it takes about 30% more power. Multiply the two factors (1.33 x 1.21 = 1.6). Not an exact equation and certainly not the actual equation, but close enough to give an idea of why.
Aircraft fly on horsepower. The rate of climb is directly related to hp, while the speed is related to a cubic function. The trick is to find a prop that allows the engine to develop the power while effectively transferring that power to thrust. At best a prop is only 85% efficient in transferring power, and can be lower. It is usually best to set up an engine for sport flying to operate at a RPM two to three thousand below where it is rated for peak power. This allows the engine to unload and operate closer to peak power.
Several interesting questions.
Long life on an engine has more to do with staying on the rich side and operating in a clean environment than RPM. If anything, low RPM is harder on the engine than higher. If you insist on lower RPM operation, use additional oil. Low RPM operation also makes the needle setting much more critical.
As to the prop size difference between 40 and 60. An 11" prop has 21% greater area than the 10", while at the same rpm has a 10% greater tip speed. When you increase speed by 10% it takes about 30% more power. Multiply the two factors (1.33 x 1.21 = 1.6). Not an exact equation and certainly not the actual equation, but close enough to give an idea of why.
Aircraft fly on horsepower. The rate of climb is directly related to hp, while the speed is related to a cubic function. The trick is to find a prop that allows the engine to develop the power while effectively transferring that power to thrust. At best a prop is only 85% efficient in transferring power, and can be lower. It is usually best to set up an engine for sport flying to operate at a RPM two to three thousand below where it is rated for peak power. This allows the engine to unload and operate closer to peak power.
The power absorbed by a propeller is proportional to RPM^3 and diameter^5 (!).
As for setting up the engine to operate at a RPM two to three thousand below where it is rated for peak power, one should bear in mind that for most sport engines manufacturers peak bhp figures are based on open exhaust measurements. Typically, when a standard muffler/silencer is fitted the peak horsepower RPM is reduced by several thousand RPMs.
Over the years I have found that most sport engines perform best if set up to produce somewhere between 10-12 kRPM static when a standard silencer is used.
12-27-2007 | 08:20 PM
#28
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From: Houston, TX
RE: Propeller sizing worm can
10X6 has a 5 inch radius. Pi X R X R =78.5 sq. inches of disk area .
12X6 has a 6 inch radius..................=113 sq. inches of disk area.
Comparing the disk area of the 10x6 to the disk area of the 12X6 78.5 / 113 = 70 %
Comapraing the displacement of the 0.46 to the dispalcement of the 61 46/61 =75 %
Seems to add up.
12X6 has a 6 inch radius..................=113 sq. inches of disk area.
Comparing the disk area of the 10x6 to the disk area of the 12X6 78.5 / 113 = 70 %
Comapraing the displacement of the 0.46 to the dispalcement of the 61 46/61 =75 %
Seems to add up.
01-07-2008 | 09:21 AM
#31
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RE: Propeller sizing worm can
Pilotfighter. When you calculated the disc area did you forget to remove the area covered by the spinner since it adds no thrust? Lets say we use a 2" spinner just to keep the figures consistent.[8D]
