Portage Planes R/C Club - FYI Prop Speed

Links Page Club Picture Area Upcoming Events Club Info New Member Info Grabber Green Members Only Area HOME		FYI - Super Sonic Prop Speeds - A Bad Thing Here is an article posted to the newsgroup regarding prop speeds, the original author is Robert Williams. According to the propeller manufactures I have consulted, the efficiency of a propeller is dramatically reduced as the tip speed approaches the speed of sound. In fact after 80% of the speed of sound, increases in RPM have almost no affect on thrust. Thus, it is important to keep tip speeds below 75%-80% of the speed of sound. To calculate the tip speed of your prop, use the following formula: [(Prop length in inches) X (Pie=3.14)] / (12ft./inch / (5280 ft./mile) X (RPMs) X (60 min/hour) = Tip speed in miles per hour. For example if I have a 30 inch propeller turning 6000 RPM the tip speed is: [[(30inches X 3.14)/(12 inches/ft)] / (5280 ft/mile)] X [(6000 RPM) X (60 min./hr)] = 535 miles per hour or (535 miles per hour)/ 760 miles per hour) = 70% of the speed of sound. Therefore I am within the efficiency limits. (back to the links page) \Email

FYI - Super Sonic Prop Speeds - A Bad Thing

Here is an article posted to the newsgroup regarding prop speeds, the original author is Robert Williams.

According to the propeller manufactures I have consulted, the efficiency of a propeller is dramatically reduced as the tip speed approaches the speed of sound. In fact after 80% of the speed of sound, increases in RPM have almost no affect on thrust. Thus, it is important to keep tip speeds below 75%-80% of the speed of sound.

To calculate the tip speed of your prop, use the following formula:

[(Prop length in inches) X (Pie=3.14)] / (12ft./inch / (5280 ft./mile) X (RPMs) X (60 min/hour) = Tip speed in miles per hour.

For example if I have a 30 inch propeller turning 6000 RPM the tip speed is:

[[(30inches X 3.14)/(12 inches/ft)] / (5280 ft/mile)] X [(6000 RPM) X (60 min./hr)] = 535 miles per hour or (535 miles per hour)/ 760 miles per hour) = 70% of the speed of sound. Therefore I am within the efficiency limits.

(back to the links page)

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