Propeller tip would be stalled?
Thread Starter
Join Date: Oct 2013
Location: Seoul
Posts: 14
Likes: 0
Received 0 Likes
on
0 Posts
Propeller tip would be stalled?
Having read FAA's PHAK chapter 6, I have a question about propeller which has the same angle of incidence from the hub to the tip.
Below is from the page 6-5 of PHAK
Stalling occurs only when critical AOA has exceeded. Does the AOA of propeller increases as the airspeed of the airplane increases? I can't understand. Could someone help me understanding this?
Below is from the page 6-5 of PHAK
A propeller blade designed with the same angle of incidence throughout its entire length would be inefficient because as airspeed increases in flight, the portion near the hub would have a negative angle of attack while the blade tip would be stalled.
The prop's (for any position along the blade) AoA is vector sum of the blades rotational velocity, and the aircraft's forward speed. The blade section has a much lower speed near the hub compared to the tip due to the smaller diameter for the same RPM. Hard to show with a text a diagram so bear with me...
HUB vs TIP: Vertical lines represent rotational speed downwards, horizontal represents forwards speed. Slash is prop section. Imagine a line between the full stops to picture the resultant airflow vector.
hub at a greater AoA
.\
|
|
|
|__.
tip at a lesser AoA
.\
|
|
|
|
|
|
|
|
|__.
If the tip is at some reasonable +ve AoA, then the hub is at an excessive AoA
If the hub is at some reasonable +ve AoA, then the tip could even be at a -ve AoA
HUB vs TIP: Vertical lines represent rotational speed downwards, horizontal represents forwards speed. Slash is prop section. Imagine a line between the full stops to picture the resultant airflow vector.
hub at a greater AoA
.\
|
|
|
|__.
tip at a lesser AoA
.\
|
|
|
|
|
|
|
|
|__.
If the tip is at some reasonable +ve AoA, then the hub is at an excessive AoA
If the hub is at some reasonable +ve AoA, then the tip could even be at a -ve AoA
Join Date: Dec 2013
Location: Sydney
Posts: 24
Likes: 0
Received 0 Likes
on
0 Posts
Propeller tip would be stalled?
The PHAK extract says the hub has a lower AoA than the tip. Tinstaafl's explanation says the hub has a higher AoA than the tip.
I think I need to draw myself a diagram and see if I can make sense of it...
I think I need to draw myself a diagram and see if I can make sense of it...
Your original question was for a blade without twist. In the real world, blades are twisted, to try to keep a more or less equal AoA. Go out & look at a propellor and you'll see.
