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Old Nov 14, 2011, 09:35 AM
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Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
526 Posts
Tim, I couldn't quite decipher your last comment, but in regards to some things you've said in the past, let me say this:
Now that this discussion is purely about lift, we need to stick to it's definition. Lift is the force exerted on a body by the fluid it is moving through that is perpendicular to this relative motion (and perpendicular to a reference span direction).

In the case of a propeller/rotor positioned horizontally (hover), the flow is moving through the plane of the propeller (prop disk) at the same time the prop is spinning. So if you consider the relative velocity between a section of the prop and the flow that moves past it, you will find that this velocity is neither vertical nor horizontal. This means the lift produced by the section is neither vertical nor horizontal. As a matter of fact, the relative velocity is different for different sections of a prop (because the sections further out are spinning faster), which means they have local lift vectors that are pointing in different directions. Therefore, it does not make sense to discuss "the lift of the rotor". What you have been calling "lift" is "thrust".

The line from that website: "Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air" is meant to help middle school children picture what is going on. Lift is not defined relative to weight, to gravity, or some vertical reference. We are more sophisticated, so we can use the term "lift" as it is properly defined.
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