Martin Gregorie wrote:

On Sun, 18 Jul 2004 12:45:02 -0700, Eric Greenwell
wrote:


I was talking about the separation on the top surface at high AOA
during a "stall situation". I now realize you were talking about laminar
flow separation on the bottom surface, which isn't related to the stall
situation.


Actually, I was talking about upper surface separation at low speed -
thermalling regime. Our oldest club Discus has zigzag turbs just ahead
of the aileron hinges and those were the turbs I was thinking about.
I'm not sure how common there are, come to think of it, because the
other club ship (Czech-bult with tiplets) doesn't have them.

Turbulators on the top of the wing are uncommon. Except for a Speed
Astir, I haven't seen any, not even on Discus(es?). Generally, I believe
the separation that occurs while thermalling is not laminar flow
separation (which would start around 60% or so on the airfoil), but
turbulent flow separation starting at/near the trailing edge at the
onset of stall (flying too slowly).

I forgot about the lower surface turbs, but I think their placement is
due to airfoil shape rather than anything else. I've only seen them in
front of the narrow undercambered area under the TE and assumed they
were to stop separation in the undercamber dish at the top end of the
speed range.

I hope I didn't cause too much confusion there.


For the modern laminar airfoils, the transition (from laminar flow to
turbulent flow on the bottom of the airfoil) is at least 80% or more. On
my ASH 26 E, the turbulators are on the flaps and ailerons at about 95%.


Interesting - I've never seen a 26E close enough to know what its
airfoil looks like. Does it also have a somewhat hooked trailing edge?

Well, it is a flapped ship, so the trailing edge can deflected down 10
degrees or so. The flap and aileron seem to have a slight concavity on
the top side. The ASW 27 is essentially identical, and they both use
blow turbulators, like the ASW 20 models.


The transition from laminar flow to turbulent flow on the top of the
airfoil is sooner, perhaps in the 60%-80% range. There is rarely a
laminar flow separation, though the Speed Astir is a well-known example.


Could it have been more of a problem on the early glass?

I haven't heard that it was. I think it was eventually found on the
Astir because it performed so far below expectations, that much effort
went into discovering the cause.

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Eric Greenwell
Washington State
USA