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Old 12-02-2005, 11:17 PM
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da Rock
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Location: Near Pfafftown NC
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Default RE: Transitional Pitched Prop???

Since the outboard tips obviously are moving at a greater airspeed, they may be less angled to prevent creating too much "lift" at the outboard ends
Sorta....

Somebody used to sell a prop pitch gauge. It would show you the "pitch" at each station you chose to measure from the hub out to the tip. I got one in the drawer downstairs, but since shifting over to flying just R/C haven't had any reason to pull it out. whatever...

As the stations on a prop move out toward the tip, each station does see greater airspeed. But the books actually describe what happens and why the pitch flattens out a bit different than the object being the lift generated at each station. Turns out that the airfoil of most props also changes as it goes to the tip. And they say that because of the speed increase toward the tip each section sees a different angle of attack. That idea caught me out too. But it was clearer when they said that a prop unloads in the air compared to when run without forward motion on the ground, because it was seeing less angle of attack due to the forward motion when flying, that the AOA each station sees is a function of it's speed, and what the entire prop sees is the also a function of forward speed and they add up. That made it easier for me to see that speed had something to do with AOA.

They say that the reason a prop has less pitch toward the tip is to balance out the airfoil versus the AOA (since both are changing as you move toward the tip) so that each station of the prop will operate to the optimum. I guess the idea in prop design is to keep each station pulling with an efficiency that results in the prop pulling whatever the pitch rating says. And in keeping any stations from stalling and others from not producing their share of lift.
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