Why does a NACA 643-418 airfoil have a flat region in its lift curve?

[Thusithatck]

I have plotted a lift curve of a NACA 643-418 airfoil section which has a flat region in the lift curve approximately from 11 degrees to 16 degrees. Beyond 16 degrees it completely stalls. Can some explain to me the reason behind this flat region in the lift curve.

The experiment was a force balance method which doesn't record pressure on the airfoil surface. Therefore it is unable to get an idea of this flat region.

I suspect as the flow on the upper surface being completely turbulant from 11 degrees to 16 degrees. Beyond 16 degrees the flow compleyely seperates.

 

[boneh3ad]

Turbulent flow doesn't really have anything to do with separation in the sense you seemto be implying. Turbulence has negligible effect of lift.

Mor likely is that your airfoil is just getting more separated as you increase your angle and it happens to be correlated with te increase in lift otherwise generated by the angle increase.

 

[rcgldr]

You didn't mention drag polars, but I assume they continue to increase during the lift flat spot range. Perhaps the lift from the bottom surface is compensating for the decrease in lift at the top. It's also possible that small vortices are being formed, but still producing lift, delta airfoils rely on this effect, reaching angle of attack around 20 degrees or so before stalling. Not having seen the air foil I don't know why there is a flat zone, but doing a web search I see similar data being mentioned or questioned.

 

[RandomGuy88]

The flat top is due to the type of stall of the NACA 643-418. It is known as trailing edge stall. The other main types of stalling are leading edge stall and thin airfoil stall, you can also have a mix.

In trailing edge stall the boundary layer begins separating a short distance from the trailing edge and this separation point moves upstream as the angle of attack increases. As the angle of attack increases the suction near the leading edge is continuing to increase (stronger suction) this effect wants to increase the lift. But the point of separation near the trailing edge is moving upstream as the angle of attack increases and this effect tries to decrease the lift. So these two effects essentially cancel each other and you get your flat top. Eventually the trailing edge separation dominates or the flow separates at the leading edge and your lift decreases.

Turbulence can effect the lift. A turbulent boundary layer is thicker than a laminar boundary layer and therefore reduces the camber of the airfoil which reduces the lift.

 

[boneh3ad]

I am not saying you are wrong, but how does it reduce camber? The greater displacement thickness would cause the airfoil to appear slightly thicker to an inviscid code but not by a huge amount.

 

[RandomGuy88]

The boundary layer is thicker on the upper surface near the trailing edge. This effectively removes some of the curvature of the airfoil (reduces the camber). This is true whether the boundary layer is laminar or turbulent but if it is turbulent then it will be even thicker. You are right that the displacement thickness is going to make the airfoil thicker but the extra thickness is not distributed uniformly so it modifies the camber. The difference isn't huge but for some airfoils it doesn't take much to modify the lift curve. The change in lift probably won't be large either unless the boundary layer is tripped early resulting in an excessively thick layer.