Frise Aileron Slots

[Nelson L.]

Hey guys. On a trip in Puerto Rico, short on time, so it'll be a short post. Was catching up in Stick and Rudder, but I just can't seem to wrap my mind around Frise Ailerons. When an aileron is down (for lift), how does the slot (as result of the offset hinge on the aileron) nullify the wing-stalling tendencies? Online resources (ie google) weren't much help... Wouldn't the air just go up through the slot and not hit the aileron itself? What are the airflow patterns there, and how is the wing's lift formed?


No replies from me until I get back on Sunday.

[Peter Grey]

Simple answer is by allowing high energy air from below the wing to the top of the wing to help stabilize the boundary layer.

This is similar to why slats are useful.

[Nelson L.]

Isn't the boundary layer the place where the air meets the top of the wing (i.e. the lower parts which come into contact with the wing) and causes either a laminar or turbulent flow? My understanding of this isn't that great (its actually rudimentary at best), so I'm having trouble understanding why stabilizing the boundary layer would decrease stall speed. I know its something about "flow separation", though exactly why is beyond my grasp (too many lift/drag coefficients and numbers to think about). Do you (or anybody for that matter) know of a source/link which goes into more detail?

Edit:

Actually, upon further research and the ensuing brain damage, I've decided I'll take Langewiesche's word on this...

[chevyrules]

Isn't the boundary layer the place where the air meets the top of the wing (i.e. the lower parts which come into contact with the wing) and causes either a laminar or turbulent flow? My understanding of this isn't that great (its actually rudimentary at best), so I'm having trouble understanding why stabilizing the boundary layer would decrease stall speed. I know its something about "flow separation", though exactly why is beyond my grasp (too many lift/drag coefficients and numbers to think about). Do you (or anybody for that matter) know of a source/link which goes into more detail?

Edit:

Actually, upon further research and the ensuing brain damage, I've decided I'll take Langewiesche's word on this...

An airfoil stalls once the boundary layer separates from the wing correct? This happens typically because the airflow slows down to a point where the boundary layer can't remain attached and separates( and once enough of the boundary layer separates, stalls the airfoil. This occurs once the wing exceeds the critical angle of attack)



The slot in the frise aileron operates on the same principle as slotted flaps. From the PHAK Chapter 5,

Slotted flaps increase the lift coefficient significantly more than plain or split flaps. On small aircraft, the hinge is located below the lower surface of the flap, and when the flap is lowered, a duct forms between the flap well in the wing and the leading edge of the flap. When the slotted flap is lowered, high energy air from the lower surface is ducted to the flap’s upper surface. The high energy air from the slot accelerates the upper surface boundary layer and delays airflow separation, providing a higher CL.

The faster airflow from the bottom of the wing speeds the top boundary layer back up thus delaying separation thus the wing keeps on producing lift at a higher angle of attack.

Hopefully that explains it better. And I am sure Peter could do a better job elaborating.

[RyanK]

Here's a wind tunnel video from the 1930s with some slotted flap examples in the last couple minutes. The principle is the same as a down deflected frise aileron.

https://www.youtube.com/watch?v=q_eMQvDoDWk

[NameCoin]

Isn't the boundary layer the place where the air meets the top of the wing (i.e. the lower parts which come into contact with the wing) and causes either a laminar or turbulent flow? My understanding of this isn't that great (its actually rudimentary at best), so I'm having trouble understanding why stabilizing the boundary layer would decrease stall speed. I know its something about "flow separation", though exactly why is beyond my grasp (too many lift/drag coefficients and numbers to think about). Do you (or anybody for that matter) know of a source/link which goes into more detail?

Edit:

Actually, upon further research and the ensuing brain damage, I've decided I'll take Langewiesche's word on this...

The boundary layer is a region of airflow around an airfoil where viscous (friction) effects are dominant. Within the boundary layer, the flow can be categorised as laminar (relatively smooth and slow-moving) or turbulent (chaotic and fast-moving). These are sometimes a lot of work to analyse theoretically. At the outside edge of the layer and beyond, friction can be ignored, so the airflow is subject to "simple" classical equations, like the Euler equations.

The following image shows the pressure distribution of over the top of an airfoil at two angles of attack.



You can see that the near the front of the airfoil (after the leading edge), the pressure at each point is negative and reaches a negative peak. After the peak, the pressure rises. The rising portion of this graph is called an adverse pressure gradient. A higher angle of attack comes with a stronger gradient. Fluid in the boundary layer has to travel against the rising pressure, which slows the fluid. If the fluid does not have enough energy / velocity to overcome the pressure gradient, it tends to separate from the airfoil, eventually leading to the stall condition.

A slotted flap / aileron allows high pressure air from underneath the airfoil to join the boundary layer on the top. This adds energy needed to overcome the adverse pressure gradient, therefore delaying flow separation.

[Keith Smith]

took me a while to realize that higher on the Y axis was negative pressure (the scale goes from positive to minus, bottom to top), after that the text made sense. That's a great explanation of why the airflow eventually separates after exceeding the critical angle of attack.

[Nelson L.]

Thanks for the explanations guys - that makes sense as long as I don't contemplate the math that went into those equations. Boundary Layer, at least for me, has been demystified to the extent that I'm willing to go. Those images certainly went a long way in terms of comprehension.

For Brain Damage - http://en.wikipedia.org/wiki/Boundary_layer

I applaud the person who can understand all those numbers...