Creating aerodynamic twist by using a trailing edge slot?

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Grimace

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I'm designing a wing for a very light, mid-performance aircraft with a heavy emphasis on being easy/cheap to build. Sort of a love child between the Hummelbird and the AR5, if you will, with a little bit of Cri-Cri and Questair Venture thrown in. I know those are very different aircraft. Basically, small wing, small fuselage and fixed gear (cheap and simple to build)... then take this tiny little thing with high-performance-type dimensions and tame it down as much as possible. Some design specs:

Engine: 38-60hp
Gross weight: 650lbs
Wing Area: 36sq ft
AR: 9
Taper ratio: 1 (no taper)
Airfoil: NACA 63-218

Flipping through Abbot & Doenhoff many years ago, I began thinking about using a slot just ahead of the ailerons as a way of increasing the CLmax and also the stall angle on the outboard portion of the wing without resorting to twist. Obviously, this would come with a drag penalty at the high end, but the benefit would be greater simplicity in building the wing due to there being no span-wise changes in a "hershey bar" wing.

As we all know, as a wing approaches stall, the "challenge" is to maintain aileron effectiveness. The slots, as far as I can tell, would do this in two ways. First, they would have the increased CL and about a 2 degree higher stall angle as compared to the rest of the wing. Second, when deploying the ailerons, the down aileron makes use of the slot, helping to offset the increase in camber caused by the downward deflection... while the up aileron effectively closes off the slot, but is less likely to stall due to the decreased camber.

As far as I can reason, this should make the aircraft more spin-resistant. Of course, I know this wouldn't work on something like a Lancair or even a KR2... but I'm shooting for a rather modest cruise and in the slightly lower speed range (cruise under 175mph), it looks like a rather attractive option.

Does anybody know of any aircraft that uses this approach to creating aerodynamic twist? Anybody have any thoughts on it?
 

addaon

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If simplicity is your goal, Junkers ailerons (or flaperons) may be more fitting. That way, all wing ribs are identical, and wing trailing edge structure is easier.
 

Grimace

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If simplicity is your goal, Junkers ailerons (or flaperons) may be more fitting. That way, all wing ribs are identical, and wing trailing edge structure is easier.
I almost dismissed your idea out of hand... but thinking more about it, it may actually be pretty well in-tune with the goal of the airplane. I am going to do more research, figure out what kind of drag penalty that would impose... but I'm thinking I may want something with less drag. Granted though, the wing is tiny and maybe the extra flap surface would be more useful than the drag reduction. It's always a trade-off, eh?

What I'm planning on right now though is double-slotted flaps to reduce the stall speed from ridiculous to "a little high". My thinking is that when your chord is only 24 inches, building extra flaps for the back isn't such a daunting task. In fact, going back to the simplicity idea, the second flap could pretty well fit into the slot provided for the aileron... so the wing shape could still be constant, with the slot provided for the outboard ailerons being filled in by the second flap on the inboard portion.

It's all about trade-offs, naturally.... I'll have to further consider the flaperon to get a little more area out of the wing (which is as small as it is because each wing skin can pretty much be built with a single sheet of 1/4"x48"x96" foam), but the "soft goal" is to be cruising somewhere in the neighborhood of 170mph on 50-60hp. It sure would be kinda cool to see something like flaperons on something of a miniature "fast glass" airplane...

In a lot of ways, this is not exactly "fast glass". I'm not looking for any speeds over 200mph, engines won't be much over 100 lbs. So I am well aware that some aerodynamic drag penalties just won't be as bad on this plane as it would be on something faster... on the other hand, we're working with pretty small engines, so every little bit helps.
 

gschuld

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I hope that this post does not steer the subject of your thread off track, but I was looking into solutions to the very same problem. Below is the LoPresti Fury, and those sure look like stall fences mounted just inboard of the ailerons. Something to consider. I don't recall seeing stall fences like these on small low wing high speed GA aircraft these days.:lick: They actually look pretty good, and drag should be fairly minimal. Just a thought.

George




Wing Design Features - Stall Fences


These are sometimes used on swept wing aircraft which suffer from undesirable tip stalling effects. This is due to the presence of a spanwise flow component which has a thickening boundary layer as the flow moves out to the tip. Since separation is more likely to occur with a thick boundary layer, a stall condition is most likely to occur first of all at the tip. This will produce a large roll effect due to the long moment arm. A stall fence, as shown in Fig 1, disrupts this spanwise flow component and help to alleviate the tip-stall problem.

 

Grimace

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That's interesting (and a pretty picture too!)... a stall fence could also serve as the mounting location to support the flaps and the aileron pivot point, killing 3 birds with one stone....
 

gschuld

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That's a lot of birds;). It just seemed to be such a logical approach to keep a tapered wing twist free yet improve the tip stall issues. I wonder how effective it really is? I mostly see these stall fences on more swept wing planes(airliners) not small, non swept wing small GA aircraft. Perhaps LoPresti is on to something?:lick:

Cruising at 170mph on 38-60hp, that sure will need to be an aerodynamically clean plane. It sounds like your talking about some tiny wings. The AR-5's wings were a good bit bigger than 36sq ft, right? I'm curious, do you have any more finalized specs, drawings, etc. to share? 650lbs gross, wow. At my size, I would need to build an airplane that weights 410lbs including fuel! He He!



George
 
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Norman

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gschuld

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Good point, and no actually, I don't know of any "Rutan products" around these parts. I guess that I don't get out much.:lick: Those canard planes have pretty well swept wings, so their use of stall fences is more inline with their typical use. Cool planes though.

George
 

Grimace

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BTW here's some information on Junkers flaps:External airfoil flaps

Wow... great link! Thanks


Edit to add... looks like most iterations of the junkers-type flap are out due to too much drag. However, there's some info there on the slotted flap which would be applicable to a slotted aileron... so the info still is going to be incredibly helpful....
 
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Grimace

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Cruising at 170mph on 38-60hp, that sure will need to be an aerodynamically clean plane. It sounds like your talking about some tiny wings. The AR-5's wings were a good bit bigger than 36sq ft, right? I'm curious, do you have any more finalized specs, drawings, etc. to share? 650lbs gross, wow. At my size, I would need to build an airplane that weights 410lbs including fuel! He He!
Obviously the 170mph would be more towards the 60hp side and less towards the 38hp side of the range. ;) And yes, the AR5 had ~55sq ft of wing area, and got a 53mph stall speed with plain flaps on 50% of the wing. I'm planning on smaller ailerons and a double-slotted fowler flap arrangement to make up more of the difference. Of course, when the side of the fuselage is only 8 feet from the wing tip, you can make an astonishingly compact and efficient structure. There's really not going to be much besides a spar and some push/pull tubes inside the wing. As another point of comparison, the AR5 weighs about 650 gross.

Let me see what I have laying around me regarding specs... I'm pulling from different sources here, so anything calculated may have changed...

Seats: 1
Engine: 38-60hp (under 110 lbs)
Empty Weight: 222-355lbs
(depending on whether you look at the "heavy" or "light" estimates on my spreadsheet. I think we both know which are more likely to be accurate ;) )
Gross Weight: 650lbs is the target... design value is 700
Fuel: 10-12 gallon header tank

DIMENSIONS
Length: 12' 6"
Height 5'6" (but almost half of that is just ground clearance for the prop and gear)
cockpit width: 24" at shoulders

WING
Area: 36sq ft
Span: 19 ft
AR: 9
Taper ratio: 1 (no taper)
Airfoil: NACA 63-218


Each wing skin will be made from a single sheet of 48"x96" foam; wing chord is 24 inches. The construction will be fiberglass with graphlite sparcaps. Construction will be wet layup with vacuum bagging (rather than carving) used to form most contours.

Landing gear will be steel tube, though I'm working out a Mooney-like "donut" system to provide some shock absorption... which probably will not be necessary on such a light plane.

I do not yet have a detailed analysis of the fuselage, but "small" is an apt description.

I know the weight seems low, but I used Raymer's(?) spreadsheet and scoured wicks and airfraft spruce to put accurate weights on all the components. The "heavy" empty weight includes a 25 pound battery, disk brakes, a second fuel tank, 16 lbs of gear leg, 4 pounds of radios, and a 10 pound nose gear retract mechanism. Maybe my 40 pound fuselage weight is a little optimistic as a "max" weight, but for a monocoque structure of this size, I think it's a reasonable estimate
 
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DaveK

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Those little fences just around the flap / aileron gap are also seen on the Embraer 120. The one I was on the other day had a small triangular little fence just along the end of the flap.
 

gschuld

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Grimace,

Very interesting! I have always been fascinated by Ken Rands original KR-1. Small, simple, lightweight, and pure, so to speak. The comparison between your specs and Ken's aren't that far off. The most obvious is the wing chord difference. I believe that the KR-1 had at or near a 48" chord at the root tapering to a bit less than 36". Don't hold me to those specs, but that is certainly what was used for the later KR-2, just stretched the wingspan to suit the gross weight needs. 24" just seems so small to me. I would imagine a 36" root and tapering from there would seem to be a much more "normal" proportion to even a plane as small as you wish to build. Negardless, I am in no way qualified to judge your project, just a very interested observer. Well, if Ken Rand could accomplish similar goals 41 years ago, I figure you have a good shot at it:lick:. Below is just a little light reading for comparison if you have not already read it. Please keep us informed...:)

Rand Robinson KR-1 - N1436

The KR-1 is the creation of Ken Rand, EAA #30184, Huntington Beach, California and EAA Chapter 92. Back in 1968, Ken and Stuart Robinson, EAA #71345, started two homebuilts based almost equally on the Taylor Monoplane and their control line model airplanes.
The fuselage is the familiar plywood and spruce box. Two ladder-type sides are built up of 5/8" X 5/8" spruce stock and are covered with 3/32" plywood from the firewall to just aft of the cockpit with 1/16" used from there to the rudder post...just like most wooden airplanes from the early de Havillands to the Volksplane.
The "backbone" of the airplane is a 5 foot 5 inch center section (actually the main and secondary spar carry throughs), which ties the fuselage sides together. This “backbone” serves as the support for the seat bottom and the retractable landing gear with its retract/locking mechanism, attach point for the side-mounted stick and trim control, and, of course, the mounting points for the outer wing panels.
Up front, the little 1200 cc VW engine has been somewhat modified for aircraft use. The crank has a 30 taper to match the tapered hub, the rear main bearing has an oil groove added, the oil breather line is relocated, thin wall exhaust stacks replace the auto equipment, a Revmaster injector carburetor is fitted, and the Wolfsburg ignition is replaced by a belt driven single magneto.
The retractable landing gear is operated by a single handle, which pivots the whole assembly through a fore and aft range of about 90 degrees. Two spring-loaded latches with detents lock the gear in the "up" or "down" position. To retract, the wheels move straight back and up into wells in the center section leaving about 1 1/4" of the go-cart wheels exposed much like the early, conventional geared Bellancas. The only shock absorption comes from the tires. Whatever flexing the horizontal gear assembly has is from the pivot points outward, and the seat cushion.
The brakes used on the prototype KR-1 are simply tire scrubbers that are intended for differential ground steering and a little braking on the landing roll. Hydraulic go-cart brakes can be used, if desired.
The landing gear legs, including the wheels, are about 17 inches long, which means the leading edge of the wing is about the same height off the ground and the trailing edge literally brushes the grass. The tail wheel is a dolly caster bolted to a length of auto leaf spring. This extremely low-to-the-ground stance is one of the striking aspects in the appearance of the KR-1.
The engine cowl, fuel tank, fuselage turtle deck, vertical and horizontal tail surfaces, and outer wing panels are largely constructed of polystyrene foam! Slabs of polystyrene are glued in place, are trimmed and sanded to the desired profile, and have a layer of Dynel cloth epoxied on to form an amazingly tough and, when sanded, smooth exterior.
The vertical fin is two upright wood spars with a profile rib at the top and bottom. The rest of the fin is polystyrene foam -- including the leading edge! The rudder and elevator are even simpler; there is a leading edge wood spar plus a rib at each end and the rest is foam and Dynel, including the trailing edges, which are knife edged.
The turtle deck was built up by gluing on the polystyrene foam slabs and sanding to shape with no bulkheads or bracing of any type. Before the Dynel and epoxy application, you could smash the whole thing to bits with one half-hearted swipe of your hand, but after the Dynel covering had cured, Ken proved its strength to a slightly incredulous FAA inspector by standing on it!
In the area between the instrument panel (also of PS foam) and the firewall, an integral 7 1/2 gallon fuel tank is built in ... of PS foam/Dynel/epoxy, naturally.
The tight fitting cowling is formed around the VW engine by simply gluing the blocks of foam to the engine, shaping, etc. The builder then saws it off, splits it where necessary, bonds in fasteners and snaps it back in place.
Even the spinner is made with foam/Dynel. Ken sawed out a circular piece of wood, glued foam blocks to it, put the whole thing in a lathe and turned it to the shape he wanted and then laid on the Dynel. Sanding, cutting out the prop blade holes and drilling a center retaining screw hole completed the job.
PS foam has little to do with the strength of the finished product. It is merely a filler and, most important, a built-in mold or form for the final shape of the layer of Dynel and epoxy. This outer shell is incredibly light and strong
The wing is composed of two 60-inch built-up wooden spars with a rib at the inboard and outboard ends. Two foam ribs are installed at the Y3 and 1/3and 2/3 positions between the end ribs for support and shape only. A thick plank is glued on to form the leading edge and the remainder of the wing is planked with one-inch thick slabs of PS foam, sanded to shape and covered with the Dynel and epoxy.
The ailerons are simply sawed out of the wing and are reinserted in the same space, attached to piano hinges that have been bonded in the wells. A spruce strip is installed in the leading edge of the aileron for mounting the hinge.
In summary, Ken Rand's KR-1 was one of the really significant homebuilts at Oshkosh '72 which pioneered the way for many new composite designs. The $500 total cost of the prototype and the prospect of a short construction period were the motivating factors, which made the KR-1 a successful design.
SpecificationsLength 12' 9"Wing Span 17' 0"Total Wing Area 62 sq. ft.Empty weight 375 lbs.Gross weight 750 lbs.Useful load 375 lbs.Baggage capacity 20 lbs. maxTake off distance 350 ft.Landing distance 900 ft.Stall Speed 52 mphMaximum Speed 200 mphCruise Speed 180 mphRange 1400 miles Rate of Climb (light) 1200 fpmRate of Climb (gross) 800 fpmService ceiling 15,000 ft.Engine VW 1834Fuel 8-30 gal.Fuel consumption 3.8 gphLanding gear Fixed conventional or trigear, or retractable conventional
 

addaon

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My design is yet another that's trying to match the performance of the KR-1 (although my major limitation is doing it in aluminum). The KR-1 is a pretty hot bird... that claim of 45 kts stall seems a bit low, having seen KR-1s land. And that's with 60 ft^2, rather than 36 ft^2.
 

Topaz

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Do remember that kitplane marketing back in the days of the KR-1 tended to be a bit... "optimistic" in terms of performance numbers, at both ends of the envelope. In my reading about the KR-2, the builder community (if not Rand-Robinson) admits that the airplane, as designed and with the original engine, is not capable of the speeds that are claimed for it. I've heard the same rumored about the KR-1.

Back in those days (60's-70's), the performance figures were whatever the kit/plans companies read (or thought that they read, or wanted to read) on the airspeed indicator in the cockpit of the prototype. A poorly placed pitot or static port could result in a very fast airplane - on paper. I've heard reports of some (unnamed) designs "mistakenly" putting Vne as a "top" speed, or even as a "maximum cruising speed."

Any discussion of the KR-1 should also take into account that Ken Rand was quite a small, light guy. About 125#, if I'm not mistaken. The airplane is tiny, and even if it were capable of hitting the published numbers with Ken in it, it's highly doubtful that it would do so with a 200# 'average' guy today, even if the latter could shoehorn himself into the cockpit.

So do take the numbers, especially for older designs, with more than one grain of salt.
 

addaon

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Absolutely. On the other hand, engines have gotten bigger and lighter since then, and my understanding is that an "overpowered, overweight" KR-1 built and flying today achieves the KR-1 advertised cruise speeds, at the cost of stall speeds in the 55 kts range.
 

Grimace

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The empty weight for the KR1 is listed at 375. I believe there's at least 120 pounds or so of weight that can be removed.

First there's the VW engine that's 50 lbs heavier than similarly-powered 2 stroke options. There's probably 25 pounds to be removed by going with a graphlite/FG spar on a wing that's half the size. And, I figure, that the glass structure on the KR1 is under-utilized. I'm guessing there's 45 pounds to be removed from there by going with a composite cored structure. I know that composite construction, in practice, isn't lighter than other methods when an appropriate safety-factor is used. However, my impression is that the KR2 adds the weight of the glass without taking advantage of its strength, relying on the underlying wood structure. So there's got to be some weight to be cut out there as well. So that puts my target (target meaning my goal, not my expected) weight at 255 empty.

With regards to the small chord, the Re number on homebuilts is pretty small regardless of the chord you choose, so I don't think it's a reason to avoid using a small chord. I'll tell you the reason I think we don't see chords this small is that they give you a really narrow CG range due to the small MAC. Obviously, not such a great thing on an airplane that flies in many different configurations. However, as a single-pilot airplane that doesn't get swapped around much, you should be able to nail the CG down pretty perfectly. So while I know it seems like a very small chord, I don't think it's too radical of an idea. I think most designers just want more CG range than this wing would provide.
 

Grimace

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As a side note, just throwing it out there for the sake of those who are curious about why the wing is so small... I have a second wing design planned for after the plane is proven. Same shape, same size, but...

...includes a 30% leading edge flap on the NACA 63-218. The idea being you can droop the leading edge, but yet still retain laminar flow on the first 30% of the chord while in the cruise configuration. The droop of 30% of the chord would allow me to mount the pivoting hardware directly to the spar. In addition, the small chord prevents the moment arm on the leading edge flap from getting out of hand, allowing the structure to remain relatively efficient despite the change of configurations.

To my knowledge, there isn't much research into leading edge flaps much larger than 15% chord. So it's very much an unknown... and far more than I wish to tackle as a first project. Such a project would likely involve "wind truck" testing and other absurdities... and will be saved for a later time...

But that is where this project is heading with its tiny little wing... ;)
 

Norman

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My understanding is that leading edge devices are incompatible with extensive natural laminar flow. Any gap that is wider than some % of the boundary layer thickness will end the laminarrity right there (sorry I don't remember the percentage off hand). Same goes for steps although a forward facing step is less disturbing.
 

Grimace

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My understanding is that leading edge devices are incompatible with extensive natural laminar flow. Any gap that is wider than some % of the boundary layer thickness will end the laminarrity right there (sorry I don't remember the percentage off hand). Same goes for steps although a forward facing step is less disturbing.
That's my understanding as well... hence drooping the entire front 30% of the chord, rather than just the 15% more commonly considered by NACA in their studies. That way, there's no gap or seem anywhere on the skin during that 30% or so of the wing that experiences laminar flow in the cruise configuration.

And I know it sounds like a lot... it's like, "jeez! the whole front third of the wing is going to move! In addition to the rear quarter (flaps and ailerons)!" 55% of the wing will move. That sounds kind of insane... but here's how I'm looking at it... The chord is only 24" so the leading edge droop only accounts for 8 inches. And the trailing edge controls are only 6 inches or so of chord. So you really only have 14 inches of stuff flapping around...
 

Topaz

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...With regards to the small chord, the Re number on homebuilts is pretty small regardless of the chord you choose, so I don't think it's a reason to avoid using a small chord....
Lift loss due to Reynold's number effects shouldn't be dismissed so lightly. Despite the fact that "everyone's doing it", the loss comes at a critical point in the flight envelope - low speed - and if you're going to run a chord this narrow, then an airfoil that's appropriate to that Reynold's Number should be used. The BD-5 got into trouble in this way.

Curious as to why you're using an 18% thick section at all? And that one in particular? Maximum lift is not particularly high, drag isn't particularly low, and the drag bucket isn't particularly wide. It's very unusually thick for an application like this.

What factors brought this airfoil to the fore for you?
 
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