Propeller twist, angle of attack, and even thrust.

[Pfactor8]

To somoneone who can explain things better than FAA knowledge books (which I usually find easy to understand, but not this time):

From the Pilot's Handbook of Aeronautical Knowledge (pg. 3-23): "'Twisting,' or variations in the geometric pitch of the blades, permits the propeller to operate with a relatively constant angle of attack along its length when in cruising flight." Then next sentence says that the blades are twisted to keep thrust equalized.

As I understand it the tips of the propeller are moving faster than the core and must have a lesser angle of attack in order to keep thrust equal along it's length. Shouldn't it say that the angle of attack (with the blade angle) must vary to keep thrust equal? What is the difference between angle of attack and blade angle? The relative wind making the difference here is that produced by the prop rotating. The relative wind component caused by the forward motion of the blade is constant for all sections of the blade so it can be ignored (right?). Level cruise flight also here.

I am going to get drunk now and agonize over this because I don't have a girlfriend.

 

[Grabo172]

You're pretty close on this one...

The first thing we need to clear up is the relative wind issue... The reletive wind for the prop is NOT the same as the relative wind for the wings. The relative wind for the prop is in the plane of rotation. (roundy roundy)

So the blade angle SETS the angle of attack, and therefore, the thrust (lift in the forward direction) of the prop blade.

The further from the hub you get, the faster the blade speed, and therefore the blade angle is lowered to lower the angle of attack for that portion of the prop to maintain the thrust equal to that of the inner parts...

(I think I'll get drunk now too after typing that!)

 

[mtsu_av8er]

I'll just get drunk!

 

[BrettInLJ]

[ QUOTE ]
The relative wind making the difference here is that produced by the prop rotating. The relative wind component caused by the forward motion of the blade is constant for all sections of the blade so it can be ignored (right?). Level cruise flight also here.

I am going to get drunk now and agonize over this because I don't have a girlfriend.

[/ QUOTE ]

The relative wind from the forward motion may be the same throughout the length of the blade, but it cannot be ignored. The relative wind at a certain point on the prop should be thought of as a vector, made up of an arrow representing the relative wind from the forward motion (the length of this arrow will be the same no matter what point of the prop you are looking at) and also an arrow representing the relative wind from the turning velocity at that point (a shorter arrow as you get closer to the hub).

When you combine add these two vectors and draw a third arrow to complete the "triangle" this value represents the relative wind at that point. It is this vector of relative wind, that the blade pitch/angle of attack is adjusted to, so I think that is what the book is saying. Of course a fixed pitch prop would then only have a constant angle of attack across the length at a certain airspeed. I may be confusing you and myself even more, so someone correct me if this is not completely correct.

 

[kellwolf]

[ QUOTE ]
I am going to get drunk now and agonize over this because I don't have a girlfriend.

[/ QUOTE ]

Shoulda gone to FL instead of PHX. John's not exactly a good substitute ya know.

 

[Pfactor8]

[ QUOTE ]
You're pretty close on this one...

The further from the hub you get, the faster the blade speed, and therefore the blade angle is lowered to lower the angle of attack for that portion of the prop to maintain the thrust equal to that of the inner parts...


[/ QUOTE ]

That's what I thought! So the angle of attack does vary as you move out from the hub?

Brett: [ QUOTE ]
Of course a fixed pitch prop would then only have a constant angle of attack across the length at a certain airspeed.

[/ QUOTE ] The vector from the wind from forward motion is the same though, so the vector from the wind relative to the "spin wind" is what is varying. Is the AOA different at the tip of a prop compared to the hub AOA?

Kellwolf: I know man! Now we aren't going anywhere though because of an electric fuel pump not working on 737. Hopefully Sat. morning. See you in August dude.