Embraer RJ Stall Fences

S

sierra9000

New Member
I been working at American Eagle, working the ramp around Embraer 135 and 145 RJ's. Toward the leading edges of the wing mounted underneath the wing are stall fences. I would like to know what the purpose of them is. I have some ideas. I am assuming the are in place to produce more desirable stall characteristics but how do they work?
 
When I hear stall fences, I think of those 3-6 inch high, full chord "walls" sometimes put on older swept-wing designs to control span wise flow, In this case, I think you're talking about the small yellow tabs on the outer third of the underside of the wings, those are very small "vortilons". You could just say these are vortex generators which create vortexes which direct airflow in such a way as to energize the boundary layer at higher angles of attack and keep the airflow straighter (preventing spanwise flow, like a stall fence)and attached to the wing surface longer. In this case, they are placed in the area ahead of the ailerons in order to keep aileron control deeper into the pre-stall and stall area.
 
Your explanation makes sense to me. I've asked several of the pilots about the stall fences and they have yet to give me a reasonable and understandable answer, they all give different explanations. They are not very small, they protrude down from underside of the leading edge and there are three fences on each wing. They are shaped in a manner in which the tips (closest to the leading edge) are extremely sharp.
 
They are very small in comparison to the other most common vortilon, on the DC-9/MD80. Look sometime at its vortilon, about 10 feet out from the wing root. It's about 6 feet long, a foot high, and about 6 inches wide.
 
The story goes that Douglas engineers, when testing the DC-9, found that it had a significant problem with span wise flow that they had not seen on the DC-8. So one said, "Maybe the engine pylons on the 8 are preventing the spanwise flow. Let's add a pylon (with no engine) to the DC-9 wing to see if it helps." So they did and it helped, but no one really knew why at the time. They kept cutting back the size of the "pylon" on the DC-9 to see what the minimum size required would be. At the same time they discovered that it was the vortex generated by the "pylon" that was giving the desired effect. So the smaller "pylon" was named a "vortilon", short for "vortex pylon." Don't know if this is a true story or not, but it is the one I've heard, and it seems plausible.
 
My understanding of this is that those stall fences are designed to channel air long the wing. Specifically the becuase the wing is a swept wing the airflow has the tendency to move outward toward the tip as it travels. So in this situation the air begins to move laterally and the relative wind direction changes slightly. The stall fence is put in its place to channel air more directly along the wing chord at the same location as where either the flaps, the ailerons, or the roll stabilizers begin on the trailing edge. This fence channel the air bacwards to that that flight control will have air moving along it parallel the direction of flight, not laterally outboard on the wing.

That is probably as clear as mud. Let me know.
 
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