blade angle and angle of attach

[fanman]

hello

fanman here, i was just looking at the altons blade caculators and was wondering what the difference is between the blade angle in degrees and the angle of attack whiich is 4 degrees? i have a few different books on this subect, and they all try to explain it, but i never caught on to what they meant. ive built several blade sets and they all work ok i guess, i always guessed that they were both the same thing

thanks fanman

[Warrior]

Hello Fanman,

I'm no expert on the subject but as I understand it, the blade angle in degrees is the angle of the blade compared to a plane parallel to the blade root.

The angle of attack is the angle between the blade and the relative wind.

The relative wind is the trigonometric sum of the realwind and the headwind. The realwind hits the blades perpendicular to the plane of rotation, the one we measure with the anemometer where as the headwind is the "wind" that hits the leading edge of the blade as it's turning, so in other words the headwinds' direction is in plane of rotation. The realwind & relative wind are 90 degrees with respecto to one other.

As the blade speeds up or slows down so will the headwind, so your angle of attack is constantly changing with blade speed & wind speed.

Hope that helps...

Warrior

[Warrior]

The realwind & relative wind are 90 degrees with respecto...

Sorry that should have read realwind & headwind are 90 degrees with respecto to eachother

[SamoaPower]

[finnsawyer]

This gets us into the nub of the issue.  While the angle of attack can be constant for a given design, the blade angle as you show it can not.  It was implied from the original posting that the blade angle like the AOA would be a characteristic of the blade.  The definition that was given satisfies that condition, but the problem is that blades can be made with different amounts of twist.  That is, one can follow the desired twist further toward the root.  This would give a different blade angle as per the definition.  So, it appears the poster should go see how the term "blade angle" is used by the design software.  Is a blade angle given for each station along the blade or is it a single number that somehow characterizes the blade?

In reality both the AOA and the blade angle can vary along the length of the blade.