On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square hollow corner where the wing meets the fuselage. Due to the pressures in that area, sooner or later the air can't follow the contour, especially at high angles of attack in slow flight. The "standard" cure for this is a fillet to provide a contour the air can follow. Putting a turbulator/vortex generator ahead of the separation can entrain more air into the boundary layer there and possibly reduce the effect somewhat. I once put some fillets on the root of a 1-26 and reduced the separation enough to lower the stall speed about 4 mph if I remember correctly. It is worth noting that this is a touchy area on certificated aircraft.
UH