C208 Flap question

[trolleydriver]

A question for all the aeronautical engineers.

The outboard trailing edge of the grand caravan has a piece of aluminum bent at 90 degrees to the air flow??? Does anyone know why/what it does? I assume it has something to do with the stalling characteristics; the short body van doesn't have it?? Any ideas :confused:

[Harry Cooper]

Sounds like a flettner strip - however you usually only see those on ailerons and elevators. They are used to control control surface trail or float in a certain direction. If its protuding upwards into the airflow the result will be a downward moment on that part of the surface and vice versa. I can only guess that the outboard section of flap has a tendency to float and instead of adding extra weight to the structure by re-inforcing and/or stiffening the control surface they took an aerodynamic fix instead.

[youngmic]

Hi,

If memory serves me correct (and often it doesn't). From the 208 course in Whitcita.
It was an after thought to aid in the prevention of flow seperation over the flap section, you will also see rubber VG's on the leading edge of the flap outboard section as well. Hope I'm right there, little while since I've been near the van.

Not quite sure how it effects boundary layer control to mitigate flow seperation, perhaps it is a similar principal to that of the trailing edge "cup" on high performance boat props. Maybe.

Love to hear from others how the trailing edge strip helps boundary layer control on a flap, if in fact it does. I believe trailing edge strips on control surface are to reduce control forces, blunt control trailing edges achieve the same to I believe.

Regards
M

[EngineOut]

It serves to effectively lengthen the chord of the wing (flap section in this case) by delaying the point at which the upper and lower air streams 'rejoin'. I assume on the van it is there for when the flaps are down, and not for the cruise condition. I can only assume it is there to slightly lower stall speed. It is probably something to do with the fact that singles have to have a certified stall speed in the landing configuration of 67kts or less (I think, from memory). As the 208B was a stretch/IGW version of the original 208, the 208B probably needed to generate that little bit more lift for certification. The flap leading edge VGs are there to re-energise the boundary layer to prevent/delay separation.

[Dave Incognito]

If you have a look from behind, you will notice that the strips are directly behind, and also the same length as the spoilers...

[trolleydriver]

Thanks for the info guys.

Quote 'If you have a look from behind, you will notice that the strips are directly behind, and also the same length as the spoilers...'

Interesting point, but the shortbody van also has spoilers, but no added strips :confused: