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RPM and Thrust
Name: Bilal
Status: Student
Grade: 9-12
Location: CA
Country: United States
Date: Summer 2009
Question:
I have search the web and I was unable to
find anything, but I was wondering whether there
was a relationship between RPM and thrust. Would the
relationship be different between a plane`s
propeller and a Helicopter's rotors If there is some
sort of formula please tell me that as well.
Replies:
Hi Bilal,
Unfortunately things are a lot more complicated than you may be
hoping. All helicopter blades, and most airplane propellers have
variable pitch that allows thrust to change without RPM changing.
However, for a fixed propeller pitch, and for moderate changes in RPM,
thrust will be approximately proportional to changes in RPM. There is
no actual formula (at least not a simple one) that exactly equates
thrust change as RPM changes, since propellers tend to be optimized
for a relatively narrow range of RPM for best efficiency. You can
notice this with helicopters, whose blades are always operated at
constant RPM, and the pilot changes the collective pitch to increase
or decrease thrust (lift). When operated outside of their designed RPM
range, a propeller's aerodynamic inefficiency starts to increase,
complicating any hope of finding a simple formula that exactly equates
thrust with RPM.
Regards,
Bob Wilson
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Update: June 2012
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