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Jan Carlsson

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Have made an alternative wing to this Beauty, to get the benefits of composite wing, now the question, how is the trailing edge made thicker? I mean how far is the extra thickness smoothed out to the true airfoil(s)? What is the minimum practical thickness of trailing edge? .125"?

Using freese stile aileron and single slotted flap, can the drag be lowered by making the control surface thicker at the leading edge/max thickness of aileron/flap? Any one tested this?

http://www.bd5.com/BedeDesign09.jpg


With this new airfoils the span and wing area is a little larger, to keep the stall speed the same. What to make out of the wingtips? Hoerner style or up bend/back swept like Melmoth2?

Jan
 

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orion

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The practical thickness really depends on the scale of the wing. 1/8" is a good ballpark number to use but I've seen examples of 1/4" thick trailing edges with no decrease in performance - on the contrary, some years ago I ran across a bit of data that suggested two improvements: Slightly reduced drag, decreased control forces.

Essentially you design the wing with the full length chord airfoil but when you build it you shorten the trailing edge to get the practical thickness that you need to achieve a sufficient structure back there. The thickness then, especially in composites, pretty much becomes a function of the thickness of your laminate. The interesting thing though is that despite the fact that you essentially shortened the chord, at low sped the plane still behaves as if it still had the full gross area.

Regarding the larger leading edge radius of the Frise Type ailerons and slotted flaps, it won't really cut down your drag by a whole lot but it will do a better job of keeping the flow attached at higher angles of attack and/or with larger deflections of the control surface. But the latter applies only to the aileron - due to the geometry of the deflected flap, the larger radius will not provide any real benefit there.

But keep in mind that ideally your control deflections will be much greater trailing edge up than with it moving down (differential motion). As such, they may not move down far enough for this to have any noticeable effect.

And for this scale airplane, make the tips so they look good - one particular design or another will most likely have very little real effect on the airplane's performance or efficiency.
 

Norman

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NASA-TM-X-2336 reports on TE thickness of a supercritical airfoil. It concludes that a TE thickness of 1%c decreased transonic wave drag but increased form drag at sub-critical Mach. I also remember that NASA blocked open the drag rudders of the X-4 on some of its later flights to reduce the dead band in yaw response and unexpectedly found that it was a bit faster. In fact the one supersonic flight of the X-4 was with all trailing edge surfaces blocked open, thus creating not just a thick TE but a hollow one. The X-2 had also had blunt TEs on the ailerons to "preserve transonic effectiveness and preclude buzz"

MD developed a feature called a "Divergent Trailing Edge" for the MD-11 and (maybe) the C-17. Boeing is retrofitting some of their older airliners with a "trailing edge wedge". They're kind of like Gurny flaps adapted for transonic flight.

Those industrial sized windmills have some really thick airfoils with blunt TEs at the root but I think thats more about structure than aerodynamics.

As I recall some people making sailboat keels are making the TEs thick for damage resistance and they said 0.03 is the maximum thickness you can go to before drag becomes significant.

In any case the TE should be square not rounded
 

Jan Carlsson

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Thank you Orion and Norman.

Speaking about boats, I hade a visitor the other day, the boss of a large salboats manufacture, he had a large rudder with him on the trailer, I comment that it was the old NACA 0015, and he said yes, it was what they used on both the keel and rudder, the trailing edge was cut at an angle of 30 deg, and about 3 mm (1/8") thick, the chamfer was there to control the trailing edge turbulens, as he said that others it rool over from on side to the other, back and fort.

He will travel with me next week on my boat, it will be an interesting day,
I am getting tired of anwer the question: how they get the water back up the locks again? and if it is frezing in winter?

Have to rush, to work.

Jan
 

wsimpso1

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Some folks recommend making the TE by just truncating the wing at the point where the foil thickness equals your minimum laminate thickness.

Others recommend making it thicker, particularly on the ailerons to improve on-center feel and eliminate the dead band near center - both the result of boundary layer thickness. Gurney flaps are supposed to do the same sort of thing... Some folks (Harry Riblett among them) feel that the top surface of flaps and ailerons should be slightly raised and curved to improve effectiveness and delay stall when deflected down. I have not heard from any "authorities" on these ideas, but I sure would like to know if any wind tunnel work has been done to settle these topics.

Billski
 

orion

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I think I recall that the issue of increased LE radius for plain ailerons and plain flaps was actually discussed herein before and that it was referenced with a NACA report, although at this point I can't find it. Some of this boundary layer control work dates back to pre-WWII and I think there was also a bit published more recently. The concept does work but is limited to only a few control surface configurations that can take advantage of the slightly "compressed" flow. I used it in one of my current projects on the elevator.

And yes, the trailing edge chamfer idea works well also, although that usually results in a somewhat thicker end (1/4" or more). When you chamfer the top and bottom skin-to-trailing edge junction you essentially create a small fixed trim surface which acts to reduce the surface's control's force gradient.
 

orion

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The trouble is that none of the previous references really apply to the size and scale of the small airplane. Most construction simply accepts that the thickness at the trailing edge will be whatever it will be, based on material selection and construction type. So, with aluminum you'll generally have a very thin trailing edge and with composites it will be a bit thicker. In neither case do you have to reloft the airfoil - you just build it as you need to.

The only exception is when you want it thicker than what the material selection dictates, at which point you can reloft the foil as some of the references above suggest, or you can truncate the trailing edge of the given sections. For a small airplane, personally I'd tend towards the latter.
 

Jan Carlsson

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Back from the canal, added 16 locks to the about 60000 under my belt.

This might be little off topic but,

Last winter I rebuilt the rudder to a so called Shilling rudder of my own design! (I used DesignFoil) before it was just a plane steel plate 10 mm thick, now it is 25% thick at 20% chord.
The rudder stock is now at 25%
I also added end plates at top and bottom of rudder.
Before it was at 21,5% (shortened the chord at trailing edge)
I didn't make a fish tail this time, I wanted to test it first without, the hydraulic steering is hand cranked (pumped) so I was afraid that the steering would be to heavy, but it become the opposite, now it is a little heavier to center the rudder then giving full rudder, so it need some tweaking. Will ad a small fishtail and at same time add some area at the trailing edge, chord is now 800 mm, so if adding 50 mm (2") the rudder stock will be at 23,5%
(With fully powered stearing and joystick they put rudder stock at 40%)

The Improvement is dramatic, it really steer better in the sharp turns in the canal, it also need less rudder input when going straight and level :)
I still have the same +-40 degree rudder angle, but will modify this to +-60-70 deg, but need a bigger diameter cylinder or longer stroke, at these large angles the rudder will act as a "bow thrust" making the stern go side ways with very little forward motion.

The boat is 73´ by 13´
The locks is 4"-8" wider

Jan
 

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Jan Carlsson

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On a wing and aileron I built the trailing edge was chamfered starting (ending) about 1/2" in from the trailing edge, on the top skin only, the rudder had both sides chamfered as Orion mentioned. this make the trailing edge strong and straight (if built straight) when the top and bottom skins is joined like this as an tab.
I guess this is Rutan style?

Jan
 

Norman

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The trouble is that none of the previous references really apply to the size and scale of the small airplane. Most construction simply accepts that the thickness at the trailing edge will be whatever it will be, based on material selection and construction type.
It's pressure drag, isn't it? Does Reynolds number matter with pressure drag? Anyway I found two more references. One is the attached illustration that I found in another windmill thing. I don't have the referenced document but since Horner is one of the authors I'd imagine the data is relevant to GA.

The other is in "Airfoils At Low Speeds". Three of the airfoils were tested with their trailing edges thickened by about 0.8%c. All three suffered drag increases of 4 to 5%. This was low Re wind tunnel testing but the airfoils were more realistic
 

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orion

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Well, issues of transonic drag (first set of references by Norman) and considerations used in applications such as in wind turbines (often characterized by sections with very large thicknesses) certainly have interesting bits of analysis and yes, application, but when it comes to applying those ideas to the small airplane, the benefits may be questionable and may actually result in penalties that a more simplistic approach might avoid.

Regarding the drag increase for sections with a thickened trailing edge, that tends to be a function of how those sections were treated. If by "thickened" we mean a modified loft at the back end that actually increases the thickness at the trailing edge over the baseline shape, then yes, of course it's draggier - it's not only an issue of pressure drag but also one of base drag. Some of the characteristics may be a function of the Reynold's Number and some a function of the base shape.

The modification I've been referring to however is a simple truncation of the basic section. There was an exercise back in school that looked at treating the sections in this manner and the results were interesting in that it did allow for a more structurally stable trailing edge while at the same time delivering a very slight decrease in drag. The analysis revealed that by shortening the section we have eliminated a small amount of wetted area, which reduced the skin friction component of the drag number. True, the newly introduced base drag of the blunt shape increased the drag number but the increase was smaller than the decrease caused by the reduction in area so the net benefit was positive. But the break-even point seemed to be somewhere around 1/4" - if the blunt trailing edge was any thicker the penalties increased, overcoming the net benefit. The only exception to that was when the shape utilized the 30 degree wedge (tapered from top and bottom), but I don't recall the details of this mod.

One of the benefits of this type of modification is that for a given theoretical chord, it allowed you to truncate an airfoil with a slightly larger chord. As such, you could design a wing a fixed physical area but one that ended up with a thicker section, which would have structural and volume advantages. True, this may be minimal but if one is tweaking the small numbers for any benefit, properly done, this one gave benefits with no penalties.
 

Norman

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Well I'm an idiot and I have proof.:lick: While looking through the appendix of and old web page of mine, for an unrelated reff, I ran across NACA-TN-2074 "Aerodynamic characteristics at Reynolds numbers of 3.0 x 10(exp 6) and 6.0 x 10(exp 6) of three airfoil sections formed by cutting off various amounts from the rear portion of the NACA 0012 airfoil section"
 
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