Flap hinge position

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addaon

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Does anyone have a source for aero data on the tradeoffs between a top-hinged and bottom-hinged simple flap? A NACA TR would be most excellent, and I'm sure there is one, I just can't stir it up. Obviously the bottom-hinged flap will be "better" in many ways (larger chord with downward deflection, smoother flow, etc), but I'd love to see some wind tunnel numbers, especially for the sharp-hinge-in-flow issue (the difference in chord is easy to deal with).
Screen Shot 2012-10-12 at 4.02.08 PM.jpg
 

Aircar

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Can't help with a NACA link but experience with upper and lower surface hinged plain flaps on sailplanes shows little difference --the HP series have been the most studied with the plain flap inboard (lower surface hinge) being deflected up to+ 90degress as airbrake and the ailerons usually being top hinged to get higher up (-)deflections for differential aileron throw (but some pilots built theirs with all lower surface hinges to get a smoother profile with positive flap ) the V tail is a case of upper surface hinging but with opposite delections for rudder so that the hinged side reverses in effect -if there were any big difference in lift on each side it would be apparrent but seems not to show up --drag will depend on whether the nose of the flap is radiused correctly and whether any sealing strip fairs the gap well (there have been some issues with flexible seal shim material buzzing when on the cambered side and with bleed air if the hinge is not taped sealed or vents from the ends )
 

Dan Thomas

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Does anyone have a source for aero data on the tradeoffs between a top-hinged and bottom-hinged simple flap? A NACA TR would be most excellent, and I'm sure there is one, I just can't stir it up. Obviously the bottom-hinged flap will be "better" in many ways (larger chord with downward deflection, smoother flow, etc), but I'd love to see some wind tunnel numbers, especially for the sharp-hinge-in-flow issue (the difference in chord is easy to deal with).
View attachment 20227

The bottom-hinged simple flap will increase area only marginally, and the gap it leaves on top will make a mess of airflow. the old Aero Commander Larks and Darters had that; ugly. If you really want bottom hinging I'd be inclined to copy the system used on Super Cubs and the like.

Dan
 

StarJar

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With top hinged, you'd need a larger gap than say on an aileron, because of more deflection. As far as the sharp hinge line, just like a river makes makes short-cuts, I think the air would do kind of the same thing. And with flaps down, usually drag is not totally unwelcome.

But I would like see such tests also, if someone can find some.
 

Topaz

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Does anyone have a source for aero data on the tradeoffs between a top-hinged and bottom-hinged simple flap? A NACA TR would be most excellent, and I'm sure there is one, I just can't stir it up. Obviously the bottom-hinged flap will be "better" in many ways (larger chord with downward deflection, smoother flow, etc), but I'd love to see some wind tunnel numbers, especially for the sharp-hinge-in-flow issue (the difference in chord is easy to deal with).
Have you looked at Theory of Wing Sections? I know there is a section on flaps, but I don't recall if it addresses this question.
 
F

fly scared!

I don't like the idea of bottom hinges too, the extrados profile is too critical.
Unless it's mainly a drag device; in which case possibly the bottom hinge would keep the extrados clean (with proper sealing).
 

Toobuilder

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The bottom-hinged simple flap will increase area only marginally, and the gap it leaves on top will make a mess of airflow. the old Aero Commander Larks and Darters had that; ugly. If you really want bottom hinging I'd be inclined to copy the system used on Super Cubs and the like.

Dan
Looking at the RV-8, it has a simple piano hinge on the lower surface and a nice radiused rub strip on top to fill the gap. Both upper and lower surfaces are tightly sealed regardless of flap position. For a simple flap, it is an elegant example.
 

addaon

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Yep, I was assuming an RV-style top surface for a bottom hinge, as drawn; you get a much nicer top surface that way than with a bottom hinge (radius = thickness of wing at flap hinge chord, so ~ 6% for me). The bottom surface is kinked (radius = 0) for any hinge position within the profile of the wing and downward deflection.
 

addaon

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Nothing in TOWS, and 677, while close, is measuring slotted flaps, which are a completely different aerodynamic setup (since the top surface is always radiused, and because of the energy from the slot, there's no disruption of flow at a discontinuity/kink on the surface).
 

Dan Thomas

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Looking at the RV-8, it has a simple piano hinge on the lower surface and a nice radiused rub strip on top to fill the gap. Both upper and lower surfaces are tightly sealed regardless of flap position. For a simple flap, it is an elegant example.
OK. The Lark/Darter example I gave had a square-nosed flap hinged on the bottom. A real dirty arrangement.

The Lark and Darter were supposed to compete with the 172 but sales were dismal. The airplane was heavier; the Lark had to have 180 horses to keep up with the 150-hp 172. You sat really upright and high in it, and it had a flat plywood floor. The cabin was of welded steel tube, which held the wings and gear and engine and so on, and covered in aluminum so it looked like a 172. Sort of. The tube made it pretty strong but I don't know how crashworthy it was. The gear legs were thick fiberglass leaves. Some of them had the forward-swept fin, like a Mooney.

Dan
 

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Head in the clouds

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Yep, I was assuming an RV-style top surface for a bottom hinge, as drawn; you get a much nicer top surface that way than with a bottom hinge (radius = thickness of wing at flap hinge chord, so ~ 6% for me). The bottom surface is kinked (radius = 0) for any hinge position within the profile of the wing and downward deflection.
Here's how I've arranged the flaps for AussieMozzie. I'd have liked Fowlers but didn't want the complexity and weight of tracks and track mounting points, and I couldn't have under-wing hinge points because when the wings (double) fold they fold bottom surfaces together so I cannot have any protrusions on the bottom surfaces.

The diagram shows the flap rib without skins on, so imagine the skins wrapping around the t/e and coming together at the upper right point and then riveted face to face, the skins continue forward so that they contact the false spar when fully retracted.

The curve on the upper surface is not quite radial to the hinge point so the gap on the upper surface opens a little as the flaps deploy.

Also - I may have a series of holes in the lower wing skin behind the rear spar and forward of where that lower skin meets the false spar (in the zone of the false spar support rib). Then a similar series of holes in the same zone but in the false spar itself. With the flap fully retracted the flap skins in contact with the false spar will sit against a seal (rubber strip). As the flap opens it will allow air to pass through the holes in the lower skin and false spar, past the l/e of the flap (which are no longer sitting against the seal) and exit through the small gap between the flap and the t/e of the upper surface of the wing sheeting, energizing the boundary layer like a fowler slot. That's the theory anyway, the holes are optional at this stage and can be added later after I set up a test piece and see if it provides any benefit.

OzMoz flap configuration.jpg
 

addaon

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Yep, that's nearly the exact configuration I was planning for originally. However, I'm discovering that various parts of the geometry become much simpler if I top-hinge. The issue is that, due to other aspects of the design, I will be flying with moderate (-15° to +3°) flap deflection at all times (although high-speed cruise is in the range -3° to +3°). Thus, I'm trying to figure out what the aero trade-off is with flaps deployed and a top-hinged surface.
 

Head in the clouds

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Yep, that's nearly the exact configuration I was planning for originally. However, I'm discovering that various parts of the geometry become much simpler if I top-hinge. The issue is that, due to other aspects of the design, I will be flying with moderate (-15° to +3°) flap deflection at all times (although high-speed cruise is in the range -3° to +3°). Thus, I'm trying to figure out what the aero trade-off is with flaps deployed and a top-hinged surface.
If you're wanting reflex for speed then wouldn't you want to reflex the ailerons also? In which case couldn't you run a flaperon torque tube with a mixer? If the TT ran in several bushings within a semi-circular false rear spar it would be a very clean installation whether deflected up or down.
 

addaon

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I'm not wanting reflex for speed, and I'm not worried about the aerodynamics when reflexed (positive deflection). It's the aerodynamics with the negative deflection (downward) that are the issue, and that's what I'm trying to find references on.
 

Head in the clouds

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I'm not wanting reflex for speed, and I'm not worried about the aerodynamics when reflexed (positive deflection). It's the aerodynamics with the negative deflection (downward) that are the issue, and that's what I'm trying to find references on.

Ah - I missed the point, I've always thought of positive deflection of flaps being downward and negative being reflex.

I think the only factor that affects it is how tall your rear spar is, the taller the spar the greater will be the reduction of chord length for any given downward deflection angle.
 

addaon

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I'm just not convinced that the chord effect is the dominant effect... and I suspect (based on your own flap arrangement) that you're not either.

I mean, draw your flap at 15° downward deflection with your current (my preferred) bottom-hinge, and then try it with a top hinge. Doesn't that discontinuity look scary at the top? That has to cause some more detached flow than the bottom hinge setup does...
 

Head in the clouds

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I'm just not convinced that the chord effect is the dominant effect... and I suspect (based on your own flap arrangement) that you're not either.

I mean, draw your flap at 15° downward deflection with your current (my preferred) bottom-hinge, and then try it with a top hinge. Doesn't that discontinuity look scary at the top? That has to cause some more detached flow than the bottom hinge setup does...
Funnily enough I wrote about the detached flow thing just before and then deleted it, thinking that wasn't your concern.

I'm pretty sure that there are two very different schools of thought regarding the purpose and use of flaps. And depending on the mission presumably.

The first school wants more lift without significant extra drag and so the variable geometry offered by Fowler or Junkers systems satisfies that admirably, albeit with a significant weight and/or complexity penalty.

I do believe that all simple top or bottom hinged flaps with a kink or gap at the top will cause detached flow with even moderate flap deflection when approaching mid alpha. So they won't provide as much lift as flaps which promote attached flow because only the bottom surface will be generating the lift, perhaps only one third as much extra lift as a Fowler system would. And they produce far more drag because of the flow separation, perhaps similar to a split flap.

So I think it depends, to a large extent, on what you wants your flaps for, Fowler/Junkers will give slower flight with less risk of getting behind the power curve and 'simple' flaps will provide some extra lift but lots of extra drag which makes glide-slope control easier and shortens float and ground-roll without brakes.

I'd choose Fowler any day and use slipping for glide-slope control but it isn't a perfect world... so I've used the bottom hinge and top curve thing to encourage attached flow to a higher alpha than with the 'kink' in the top surface. By the time I reach very high alpha I should be on short final so will welcome flow separation then, for a shorter roundout and roll :ponder:
 

addaon

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I do believe that all simple top or bottom hinged flaps with a kink or gap at the top will cause detached flow with even moderate flap deflection when approaching mid alpha. So they won't provide as much lift as flaps which promote attached flow because only the bottom surface will be generating the lift, perhaps only one third as much extra lift as a Fowler system would. And they produce far more drag because of the flow separation, perhaps similar to a split flap.
This is exactly my concern. I agree, but I need a better approximation of the actual quantitative effects to evaluate the trade-off.

(These are elevons, so Fowler isn't an option because of need for upward deflection, and Junkers too high drag at zero deflection. Elevon deflection is downward, flap style, at low speeds [yes, I know this means a statically unstable aircraft], so extra lift is critical and extra drag is not desired. TOWS plain flaps fit all requirements numerically, so now I'm in the detail of exact flap geometry.)

So, back to the original question, and bringing everyone back in -- anyone have an actual source with wind tunnel data (or other hard data) for this issue?
 

Head in the clouds

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I hadn't realised it was a flying wing, so is there some reason why you can't centre hinge the elevons on a torque tube? I went through most of the options for my next build (my Avatar) and couldn't find a cleaner solution (or any published data either).
 
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