Slotted Split Flap

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DaveK

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I ran across something I’ve never seen before, what I can best describe as a slotted split flap. I saw a plane that looked like a Tailwind W10, but metal wing and bigger. Nothing special there, but the flaps where airfoil shaped with hinge axis well forward of flap leading edge and flush with bottom of wing. So flap rotates down opening a slot, but doesn’t translates back like a normal slotted flap since the hinge axis isn’t below the wing. The typical aileron torque tube found on Tailwinds was there in front of the retracted flap leading edge.
Anybody see a flap like this? Any data on how it performs?
 

Vigilant1

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The top surface of the wing showed no sign of a flap at all (like a normal split flap), but instead of a simple solid hinge underneath the flap is instead on arms that result in a slot between the undersurface of the wing and the top of the flap when deployed?

I don't recall seeing an arrangement like that. I, too, wonder what the intent was and how it performs. It would seem that the flow through the flap could reduce the turbulent low pressure area on the back side of a regular split flap (between the top of the deployed flap and the underside of the wing), so maybe this was intended to reduce drag when the flaps are open.

Perkins and Hage cited some NACA reports indicating (for the airfoils and flap sizes studied) that a split flap at 45 deg deflection provides a higher Clmax than a plain flap, and provides a better L/D at Clmax, too. That always looked counterintuitive to me--the open split flaps just look so draggy. With the added advantage that the split flap leaves the top of the wing totally smooth, they seem to offer some potential advantages for some applications.
 

DaveK

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This one wasn’t really a split flap though. The top skin went back part way, but didn’t cover the entire flap when retracted. I’d imagine it would have a bit less drag and similar lift to a split, but that is just a guess. I’d think that you get some lift on the leading edge of the flap due to the flow through the slot, but still get a pretty big detached wake from trailing edge of wing and the top of flap.

One advantage is it is very clean when retracted and simple.
 

jedi

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This one wasn’t really a split flap though. The top skin went back part way, but didn’t cover the entire flap when retracted. I’d imagine it would have a bit less drag and similar lift to a split, but that is just a guess. I’d think that you get some lift on the leading edge of the flap due to the flow through the slot, but still get a pretty big detached wake from trailing edge of wing and the top of flap.

One advantage is it is very clean when retracted and simple.
Sounds like a good idea to me. Lots of potential. The Cessna 310 has a split flap. I too thought when reading about split flaps that they would have a great deal of drag. I was always impressed with the effectiveness of the C-310s split flap. The one you describe sounds like a possible improvement.

The exposed upper surface of the rear of the flap could be rigged as a negative flap position for additional aerodynamic effect. In a flying wing it could provide the reflex to help stability and could even be variable to effect a pitch trim capability.

I can see where the application to a swept wing could have additional benefits in the drag reduction area but won't go into details at this time.
 

DaveK

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I dug out my copy of Theory of Wing Sections after a good search (just moved) and for a true split flap having a slot is detrimental. Significant loss of CLmax. A split flap with an airfoil shaped instead of flat shaped flap?
 

Vigilant1

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I dug out my copy of Theory of Wing Sections after a good search (just moved) and for a true split flap having a slot is detrimental. Significant loss of CLmax.
I think I see what you are looking at (it's page 202 in my edition).
" The effect of a gap between the wing surface and a split flap with a nominal chord of 0.20c hinged at 0.80cfrom the leading edge of the section is sown in Fig 109. The data shows that the loss of maximum lift coefficient associated with the gap is considerably grater than that caused by removing te same are from the trailing edge of the flap."
This doesn't surprise me. I figured the biggest impact of the slot would be to reduce drag (by reducing the extent of the low pressure zone behind/above the deployed split flap and below the main wing's trailing edge). Unfortunately, TOWS doesn't talk about the impact the slot has on drag.
In most of the designs I've considered, the primary utility of flaps is as a means of controlling approach angle (glide path). In this role, their drag is welcome, so "conventional" split flaps would be fine. The fact they are simple to fabricate and (compared to most plain flaps) leave the wing with fewer gaps/air bleed through when they are retracted are also attractive features.
 
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