flapping/angle of attack question????
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flapping/angle of attack question????
A dumb sounding question but....
could somebody explain in 'simple' terms why when a rotor blade flaps up the angle of attack reduces. I know the it relates to the relative airflow but I cannot picture it.
I have searched through here but could'nt find anything simple enough!!!!lol
Any links to some good visual diagrams would be helpful!!
Thankyou very much in advance!
could somebody explain in 'simple' terms why when a rotor blade flaps up the angle of attack reduces. I know the it relates to the relative airflow but I cannot picture it.
I have searched through here but could'nt find anything simple enough!!!!lol
Any links to some good visual diagrams would be helpful!!
Thankyou very much in advance!
Draw an aerofoil cross section, inclined with the leading edge higher than the trailing edge. Draw a horizontal longish arrow point towards & touching the trailing edge of the aerofoil.
This represents the relative airflow towards aerofoil, giving an AoA with no vertical movement.
Now add a small downwards pointing arrow that touches the tail of the horizontal arrow. These are the components of the airflow the aerofoil will experience if it moves up while moving forward ie a relative airflow from the front and a relative airflow from above.
The combined effect of those airflow components is the vector sum of them - and that's what the aerofoil experiences. Draw an arrow from the tail of the small, vertical arrow to the head of the long horizontal arrow. Compare the AoA now to the original.
This represents the relative airflow towards aerofoil, giving an AoA with no vertical movement.
Now add a small downwards pointing arrow that touches the tail of the horizontal arrow. These are the components of the airflow the aerofoil will experience if it moves up while moving forward ie a relative airflow from the front and a relative airflow from above.
The combined effect of those airflow components is the vector sum of them - and that's what the aerofoil experiences. Draw an arrow from the tail of the small, vertical arrow to the head of the long horizontal arrow. Compare the AoA now to the original.
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Or to visualise how it actually happens:
Before the blade flaps up or down let's imagine that the air is flowing horizontally towards it. This will give a certain angle of attack.
Now if the blade flaps, it is moving upwards relative to the air. So the air is moving downwards relative to the blade. This downward motion of the air reduces the angle of attack.
If the blade flaps down, the opposite happens. The blade moves down relative to the air. So the air is moving up relative to the blade. This increases the angle of attack.
The key to it all is that whatever the blade does relative to the air, the air will do the opposite relative to the blade.
Before the blade flaps up or down let's imagine that the air is flowing horizontally towards it. This will give a certain angle of attack.
Now if the blade flaps, it is moving upwards relative to the air. So the air is moving downwards relative to the blade. This downward motion of the air reduces the angle of attack.
If the blade flaps down, the opposite happens. The blade moves down relative to the air. So the air is moving up relative to the blade. This increases the angle of attack.
The key to it all is that whatever the blade does relative to the air, the air will do the opposite relative to the blade.
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Thankyou both. I tend to overthink things so I understand it initially then I over analyse things!!!!
I had got it into my head that the flapping also altered the pitch of the blade instead of just looking at the change in relative airflow. I have drawn it out and it all seems clear in my head now!!!!
Appreciated
I had got it into my head that the flapping also altered the pitch of the blade instead of just looking at the change in relative airflow. I have drawn it out and it all seems clear in my head now!!!!
Appreciated
Last edited by General Mutley; 15th Jan 2010 at 09:15. Reason: adding info!
