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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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