Inverted Junkers flap...

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Autodidact

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Has anyone ever tried an inverted Junkers flap? It seems to me that the low pressure on the bottom of the inverted airfoil of the flap would draw air into the slot and delay separation on the upper surface of the wing. Is anyone aware of any experiments along this line and if the paper(s) can be found?:

invertedjunkersflap.jpg

P.S.,
I'm thinking more roll control at the stall than high lift flaps; obviously you couldn't deflect these downwards much, I don't think, anyway.
 

batesjoe

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In a somewhat similar context, CTLS and Pipistrel aircraft make use of a "negative" flap deflection of approx. 5 degrees upward for cruise efficiency. I am not sure I completely understand the aerodynamics of this other that change of pitch moment on the aircraft reducing elevator load. If it is generally an effective means, would it not be more common in nearly all types of conventional aircraft?

Both the CTLS and Pipistrel aircraft are high-wing monoplanes. I have not seen or heard of low-wing aircraft doing the same thing.

How would I best determine if this can or should be done on other aircraft? In particular, the Spacek SD-1.

Thanks for the inputs!
 
H

Hole in the Ground

Surely more likely what will happen is that the aileron will be in the turbulent air when the wing stalls instead of hung out in the nice high energy air below?

edit: as norman said...
 

Dan Thomas

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Surely more likely what will happen is that the aileron will be in the turbulent air when the wing stalls instead of hung out in the nice high energy air below?
The aileron is part of the whole airfoil, not something flying on its own. If the part of the wing that stalls has the aileron on it, applying down aileron makes the stall worse and control reversal results. The aileron is not lifting or lowering the wing by deflecting air up or down; it is changing the camber and angle of incidence (and therefore angle of attack) of that part of the span. Lowering aileron increases camber, increasing the speed of the air over the top of the whole airfoil, and increases AoA, thereby increasing lift on that part of the wing. Raising aileron decreases the speed over top and decreases lift.

Flaps do the same thing. They increase camber, angle of incidence, and some of them increase area. They work together with the entire wing ahead of them.

Dan
 

Matt G.

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In a somewhat similar context, CTLS and Pipistrel aircraft make use of a "negative" flap deflection of approx. 5 degrees upward for cruise efficiency. I am not sure I completely understand the aerodynamics of this other that change of pitch moment on the aircraft reducing elevator load. If it is generally an effective means, would it not be more common in nearly all types of conventional aircraft?

Both the CTLS and Pipistrel aircraft are high-wing monoplanes. I have not seen or heard of low-wing aircraft doing the same thing.
Most, if not all, currently produced flapped gliders also have negative flap settings.
 

Autodidact

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I think they would be buried in the boundary layer at high AoA
That's what I would think, too, but aerodynamics doesn't always do what I would initially think. Thanks for the links. The Gamma is interesting because of the full span flaps. I wonder if they would keep the boundary layer attached a little longer. I would like to read a paper on this, but it seems a little scarce.
 
H

Hole in the Ground

The aileron is part of the whole airfoil, not something flying on its own. If the part of the wing that stalls has the aileron on it, applying down aileron makes the stall worse and control reversal results. The aileron is not lifting or lowering the wing by deflecting air up or down; it is changing the camber and angle of incidence (and therefore angle of attack) of that part of the span. Lowering aileron increases camber, increasing the speed of the air over the top of the whole airfoil, and increases AoA, thereby increasing lift on that part of the wing. Raising aileron decreases the speed over top and decreases lift.

Flaps do the same thing. They increase camber, angle of incidence, and some of them increase area. They work together with the entire wing ahead of them.

Dan​


Cheers Dan, I think you missed the part about these being junkers ailerons/flaps...
 

Dan Thomas

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Cheers Dan, I think you missed the part about these being junkers ailerons/flaps...
I was responding to one small part of his statement, namely this:

"....instead of hung out in the nice high energy air below?"

Dan
 

ultralajt

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Placing junkers ailerons (flaps) above trailing edge, you remove the main purpose of junkers ailerons (flaps)!!!!
The main reason for junkers flaps to be placed below TE is that at high AOA, they still fly in frresh undisturbed air flow, and working even when wing airfoil starts to stalling. This way you achieve full roll controll even in an early stall.
 

Norman

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Here's newsletter with an article I wrote about them:
<http://users.acsol.net/~nmasters/temp/Sept03Ltr.doc>

And here's the bibliography with links to all the NACA tech reports:
<External airfoil flaps>

You won't be able to download the NACA reports from the government server for a while because someone raised questions about sensitive information finding it's way to China through NTRS. Talk about closing the barn door after the horse has bolted. Oh, wait, NTRS is a glue factory full of used up old nags
 

Aircar

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Norman's link might be the same thing but there is a comprehensive NACA paper testing external airfoil flaps in every conceivable location around an airfoil (the "fowler" location and the "LE Slat" are the ones most adopted and found to be beneficial) - if I recall correctly it was done by Frances Rogallo (of the flex wing fame) and possibbly Fred weick et al . Paper NACA papers are no longer accessible at my closest university engineering library (now a foreign student common room...) and otherwise unsure of the title --it is buried deep in my files at home --I am now on a rented by the hour computer .

The Downlifting airfoil in proximity to the wing is a variation on the Mitchell wing external elevons (see Norman for more detail on that --hope your situation is improved also Norman..)

The MOST analogous arrangement like this though is in our friend John McGinnis' Synergy (Censors may delete this forbidden word -leaving a blank might suffice ) - the idea of 'supercharging' the wing by sucking the boundary layer off via a venturi near the trailing edge is of course also common to the Fowler flap --the FLAP Cl can be as high as five but NOT because of some "speeded up" airflow over it [ pressure and velocity will trade off just as Mr Bernoulli worked out centuries ago and you cannot speed up the local flow by first slowing it down to create pressure --lots of pilot aerodynamics texts say otherwise ] -perhaps we can have a non controversial discussion about this putative phenomenon without involving that which must not be named. ?


For a tailess aircraft loss of 'up elevator' is perhaps a good thing to limit or prevent stall (but NOT if it also destroys lateral control ) and following the descriptive 'logic' of the airflow being 'sucked' into the slot formed the operating in detached airflow criticism might not apply (probably the theory ) -= it is curious that you can keep the airflow attached at the aft end of an airfoil by either sucking away the boundary layer OR blowing air out at the same location -- some test aircraft actually redirected the air sucked from the inboard wing to the outboard section in a sort of "robbing Peter to pay Paul"
system -that worked . Not saying this is comparable but just the apparent contradiction involved. The Kutta condition around a trailing edge might need to be satisfied (cannot have 'two' trailing edges )
 

Autodidact

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Several nice papers in Norman's list. Rogallo, Weick, and others are in there. I haven't looked closely at them all, but NACA-TR-573 by Ira Abbott and Robert Platt does have the flaps above the trailing edge in some of the charts. Lots of info about slotted flaps as well in some of the other papers.
 
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