# Autorotation - AOA

#### RodG81

##### Newbie
Hello Everyone,

I am learning about autorotation. I understand the basic concepts including the forces acting on an airfoil and how the blade is divided in a driven, driving and stall regions with points of equilibrium between them.

The regions are different because the direction of resultant TAF (in relation to the axis of rotation) is different in each one of them: aft in the driven region, forward in the driving region and aligned in the points of equilibrium.

If I understand correctly the direction of the TAF is directly related to the angle of attack (the angle between the chord and the relative airflow).

I have a question regarding the driving region. Is there a specific angle (or range of angles) that would be optimum to maximize the movement of the blade forward? I am thinking it as follows: Imagine the side view of the airfoil with the chord aligned with the plane of rotation. If you could direct an airflow to the blade ¿which angle would you choose to have the best performance of the driving region?

For example, the image shows the airfoil with an AOA of 17º

Thank you!

#### Attachments

• Example.pdf
72.6 KB · Views: 21

#### Jean Claude

##### Junior Member
Look a Lilienthal polar curve.
The Lilienthal curve is drawn by changing the reference directions. It refers to the direction of the chord, instead of the direction of the airflow.
You can see the best Cd for driving is just before the stalling

#### Tyger

##### Active Member
If I understand this correctly, it looks like the most forward "drive" is achieved in that part of the driving region nearest the stalled region, and that the further outboard one goes, the less drive there is, till it finally turns negative in the driven region.

#### XXavier

##### Member
If I understand this correctly, it looks like the most forward "drive" is achieved in that part of the driving region nearest the stalled region, and that the further outboard one goes, the less drive there is, till it finally turns negative in the driven region.

That's how I understand it, too...

From Wayne Johnson's 'Helicopter Theory':

#### Jean Claude

##### Junior Member
Yes, but this sketch is only during a axial descent. During the forward flight, the circles are shifted and enlarged to the retreating sides

#### XXavier

##### Member
Yes, but this sketch is only during a axial descent. During the forward flight, the circles are shifted and enlarged to the retreating sides

Sketch by Prouty, published in 'Vertiflite', for autorotation in forward motion...

Last edited:

#### RodG81

##### Newbie
Look a Lilienthal polar curve.
The Lilienthal curve is drawn by changing the reference directions. It refers to the direction of the chord, instead of the direction of the airflow.
You can see the best Cd for driving is just before the stalling
View attachment 1148244
Yes, I see, and it makes a lot of sense, thanks for the information!

#### Tyger

##### Active Member
Yes, but this sketch is only during a axial descent. During the forward flight, the circles are shifted and enlarged to the retreating sides
Shifted, of course, but why do you say "enlarged"?

#### Jean Claude

##### Junior Member
The stalled zone widens with forward speed as shown in naca report

Last edited:

Replies
19
Views
2K
Replies
50
Views
3K
Replies
1
Views
530
Replies
3
Views
2K
Replies
0
Views
280