Blade Flapping

[TaIk2Kevin]

Question: In a Fully Articulated Rotor System the individual Blades flap up and down independently to compensate for dissymmetry of lift. What actually makes them do this. What makes the blades Flap down while retreating and up while advancing??

[joker]

Talk2Kevin,

 

Velocity!

 

I'll start with a simple explanation:

 

In a forwad moving helicopter, as the blade advances, the airflow over the blade increases.

 

The amount of lift that blade produces is (among other things) directly related to the speed of the air over that blade.

 

Therefore the lift on the advancing blade increases. It flaps upwards.

 

On the otherhand, the opposite is happening to the retreating blade. The speed of the aircraft moving forward effectively subtracts from the overall speed of the air over the blade. This reduces the lift and thus the blade flaps downwards.

 

Hope this helps. If not, ask again, and I'm sure you'll get fuller explanation, maybe with some nice diagrams.

 

Joker

Edited January 22, 2007 by joker

[TaIk2Kevin]

Makes sense to me, But It is still a little hazy. It’s hard to visualize it.

 

When the Helicopter is producing lift conning occurs it leads me to think that the pressure on the bottom of the airfoil and the weight that the airfoil is carrying would be enough to keep the rotor blades turned up and flapped up the entire time. Unless the combination of centrifugal force and the amount of wind velocity decreases so much to the extent that it can no longer flap as high and inversely flaps downward. I don’t know, the explanation makes sense I’m just trying to understand it completely.

 

[nsdqjr]

Question: In a Fully Articulated Rotor System the individual Blades flap up and down independently to compensate for dissymmetry of lift. What actually makes them do this. What makes the blades Flap down while retreating and up while advancing??

 

Coning and flapping are two completely different subjects, here's the flapping bit:

 

Velocity does play a part in the flapping of the Rotor system like the other post states; however, the retreating blade has some very specific differences from the advancing. As the blade begins to retreat, 5 different aerodynamic regions develop. From the center out they are: Reverse Flow, Negative Stall, Negative Lift, Positive Lift, and the Positive Stall. Without getting uber technical, the airflow in the Negative Lift Region along with the decrease in rotational relative wind, causes the retreating blade to flap down. It actually flaps down hard enough in some rotor systems to produce a very slight amount of upward inflow, this wouldn't happen if the down flap was only a product of reduced velocity. Everyone knows that the retreating blade flaps down, angle of attack increases. Advancing blade flaps up, angle of attack increases. The reason that flapping works is because it changes the amount of induced flow, which alters the angle of attack. Advancing blade flaps up, increases induced flow, decreases angle of attack. Retreating blade flaps down, reduces induced flow, in some cases creates a small amount of upward inflow, and increases the angle of attack. I'm totally fascinated by this particular subject, good times!

Edited January 21, 2007 by nsdqjr

[Guest rookie101]

You may find wikipedia to be of some help, if you haven't already checked it.

 

http://en.wikipedia.org/wiki/Dissymmetry_of_lift