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Suitable airfoil large thickness, high RE

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berridos

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Hi everybody
I am seraching for a simmetrical airfoil that has the particularity, that it is very thick (20%) and operates at high RE (7.000.000).
May somebody could give me a hint on the design characteristics that makes an airfoil suitable for large thickness or high RE environment?
As a starting point i was handling 63A015 and 64A015 (or Ribbletts sizlings) and resizing it to 20%.
But i would like to understand what parameter an airfoil designer would emphasize from the beginning if he knew it would be thick and operated at high RE.
 
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Victor Bravo

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Look up Martin Hepperle's airfoils, I believe he has done a lot of work in this area, large and slow stuff. Don't know of the Re is quite as high as what you are looking for, but Hepperle is definitely worth a look.
 

Jay Kempf

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Hi everybody
I am seraching for a simmetrical airfoil that has the particularity, that it is very thick (20%) and operates at high RE (7.000.000).
May somebody could give me a hint on the design characteristics that makes an airfoil suitable for large thickness or high RE environment?
As a starting point i was handling 63A015 and 64A015 (or Ribbletts sizlings) and resizing it to 20%.
But i would like to understand what parameter an airfoil designer would emphasize from the beginning if he knew it would be thick and operated at high RE.
What do you want to get out of such an airfoil? Re can be either come up based on chord size or intended speed... Are you looking for drag counts, stall performance, targeting a certain CL, or just wanting thick because it will be lighter with a taller shear web and smaller spar caps?

Any of the NACA symmetrical airfoils can be scaled to what you want within reason. Farther back max thickness is normally lower drag as well as cusped (hollow) trailing edge. Farther forward max thickness, blunt leading edges tend to hang on longer without stalling so higher CL slightly (without flaps). Blunt trailing edges and thick have more docile stall than sharp. But all that depends on planform in the end anyway. A bad airfoil can make a good wing when you add a little twist, sweep and taper.
 

cluttonfred

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The Snyder Arup series operated at an average Reynolds number of 5-11 million between slow and high speed flight, and all but the first one (which used Clark Y) used a modified M-6 (12%) airfoil. Using that as a proven, successful example, you probably want an airfoil with modest thickness (~12%) fairly far back (25-35%), modest camber (2-2.5%), and a little reflex. Fishing around on Airfoiltools.com turns up a lot of pretty old airfoils: M6 (GOE 677), M12 (GOE 676), TSAGI 12%, CLARK YH, NACA 25112, etc. A few of those are pretty flat-bottomed so NACA 25112 is probably a good place to start.

 

cluttonfred

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The Snyder Arup series operated at an average Reynolds number of 5-11 million between slow and high speed flight, and all but the first one (which used Clark Y) used a modified M-6 (12%) airfoil. Using that as a proven, successful example, you probably want an airfoil with modest thickness (~12%) fairly far back (25-35%), modest camber (2-2.5%), and a little reflex. Fishing around on Airfoiltools.com turns up a lot of pretty old airfoils: M6 (GOE 677), M12 (GOE 676), TSAGI 12%, CLARK YH, NACA 25112, etc. A few of those are pretty flat-bottomed so NACA 25112 is probably a good place to start.

PS--I am assuming you meant semi-symmetrical rather than truly symmetrical.
 

berridos

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Thanks for the inputs. Guess i will write hepperle a letter to comunicate the semipregnancy.
I am working on the conversion of the verhees delta into a vortex lift delta. I will the stay by the concept of the verhees but its really a new plane.
Symmetrical because a delta needs a perfect stable cm and i dont want to take the risk of a reflexed airfoil.
20% because i sit inside the airfoil and the concept requires that thickness to avoid a fuselage
7.000.000 RE because the central section has a huge chord and will fly minimum at 200kmh, hopefully 240kmh.
The further the max thickness section , the better, as the design is strongly nose heavey and the pilot could sit further back.

In the second stage i am researching how to sharpen the leading edge in order to improve vortex eddies.
The planforms that i will test with 3 rc models are already approximately defined.

One of my existential doubts is if all that theory regarding short buble stall, longbubble stall or trailing edge stall has any importance once i sharpen the leading edge and use a sweep higher than 50-55. I guess those concepts disappear and thats why i want to select the above airfoil for cruise speed conditions. Do deltas have any special cruise cl condition i should be aware of, or is 0,3 ok?
With the used planfrom added to vortex lift i will test the design without the 1,5º washout that the verhees uses.
 
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Victor Bravo

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I believe this will be a perfect candidate for 1/3 scale model testing, to confirm/deny whatever you are trying to create, before you build the full scale Delta.
 

berridos

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Yes the experiment is too wiered to approach it without researching previously with rc models. The problem is vortex lift hasnt got scale effects and the rest of the flight regime has the common scale effects. So a bit of frankenstein conclusions could be the result.
On the other hand i am not rc pilot. 35 years ago i built a dozen rc models but never learned to fly them. Hope i find a good testpilot.
 

Lendo

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If you wanted to sit inside the wing I would suggest a good wing design expanding into a Blended Wing design, By that I mean the same Airfoil scaled-up in thickness for the fuselage. I used that concept for my Tandem Design, but it's not a Blended Wing design. It would be interesting to see how that worked out.
I will have to look at that - interesting!
George
 

Norman

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I don't think you can have a sharp leading edge on an airfoil that thick. You'll probably have to settle for a stall strip like they did on the DM-1
 

berridos

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The DM-1 was tested with pretty large plates protruding from the LE. Wouldnt call it conventional stall strips as i know them.
The verhees has a linear decreasing wing thickness from 20% at the center line, to 11% at the tip. I wonder if it would be positive to thin the airfoil much faster, as such a thickness is only required at the cockpit area and the spar height is anyway huge. Anybody finds pro and cons?
Regarding the sharpness/bluntness i am planning to build the rc-models with exchangable D section of the wing ,attachable to the wingspar.
I have seen several strongly sweeped airplanes that only have sharp airfoils at the first third at the root. My interpretation is that it is important to start the vortex at the root and once it becomes stong, inertia pulls the vortex down along the rest of the leading edge and sharpness becomes less important travelling down along the LE.
I will test in the rcmodel, 1/3, 2/3 and fully sharpened LE. And the same procedure for the double swept sections.
Regarding the sharpness i am still searching research that defines what LE radius to chordlength can be considered sharp. I am still pretty lost on this issue according to my chinese research method (copy, mix and improve))
 

WonderousMountain

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Eppler made a few hydrofoils with smol leading edge radius.
You'd likely get good performance at low lift coefficients.
Doing a rapid transition from Yuge root chord, to thin airfoil
may not give a lot of benefit. The change would be a few %
but build & section tailoring could suck time & precision.e837-il_l.png
 

Norman

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The DM-1 was tested with pretty large plates protruding from the LE. Wouldnt call it conventional stall strips as i know them.
The splitter plate served the same function that a modern stall strip dose, they both force separation at a specific AoA. The wind tunnel tests of the DM-1 may have been the first time this technique was ever tried. As is often the case when doing something new the details hadn't been worked out yet and in this case the part was much larger than necessary. A little piece of L-angle attached where the stagnation point is at alpha+7 degrees would do the same job as that huge splitter plate. An airfoil with an unfavorable pressure gradient at the leading edge (like NACA 6 digit sections) may also do the trick because that unfavorable pressure spike causes a front stall.


I have seen several strongly sweeped airplanes that only have sharp airfoils at the first third at the root. My interpretation is that it is important to start the vortex at the root and once it becomes stong, inertia pulls the vortex down along the rest of the leading edge and sharpness becomes less important travelling down along the LE.
That's pretty much it although the spanwise leading edge flow is caused by the low pressure of the outboard strips sucking on the stagnation point of their inboard neighbors, inertia doesn't play a very big part in it. A sharp LE at the root progressing to a blunt LE near the tip was one of the features described in Lippisch's 1946 paper On highly swept wings. He also suggested minimal camber at the root and higher camber at the tip. That paper is also the first use of the phrase "bell shaped lift distribution" that I know of.
 

berridos

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Hi
Someone got acces to:
Spiral vortex flow over a swept-back wing
or, On the Generation and Subsequent Development of Spiral Vortex Flow over a Swept-Back Wing
By poll?
 

berridos

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got it. Such a pitty that the reynolds numbers tested are always ridiculously small. I got 13 million at the root and 7 million average over the span.
According to XLFR5 changing the leading edge radius is critical for maxCL. The problem i see is that choosing a fairly sharp leading edge radius, requires the plane to fly at an pretty exagerated AoA in cruise.
 

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Norman

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The problem i see is that choosing a fairly sharp leading edge radius, requires the plane to fly at an pretty exagerated AoA in cruise.
That's why a stall strip is a better bet. At AoA below the forced stall the flow on the top surface will be pretty much undisturbed from the base airfoil. Unfortunately the bottom flow will be tripped to turbulent at low AoA by a conventional stall strip so the modified airfoil will have higher drag. Certain old-fashioned airfoils eg NACA 6xxxx have early front stall because of a little dip in the pressure distribution near the leading edge that produces a stall-causing spike. This spike produces an adverse pressure gradient and early stall at the front of the airfoil which is what Harry Riblett addressed with his GA sections. Look at the NACA 65(4)-221.
 

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berridos

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Is that adverse gradient in your picture the cause for the short buble continously mentioned at low AoA in the paper?
Top reply, thanks
 

berridos

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What your opinion on using a series 65 airfoil on a delta?
 
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Norman

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Is that adverse gradient in your picture the cause for the short buble continously mentioned at low AoA in the paper?
Top reply, thanks
I haven't read the paper but it probably is. Adverse pressure gradients are what causes bubbles. The normal transition bubble at the minimum pressure point occurs on all airfoils but the NACA 6xxxxx have an extra one at the leading edge. This LE pressure spike seems to not be a problem at high Re but at low Re it causes early stall and it's a front stall. That's why NACA 6xxxxx can have nasty low speed characteristics on small airplanes. The normal bubble farther aft can also be huge at low Re and cause a lot of drag but that one wouldn't be a problem for you because it's very small and flat at Re>1,000,000 as is the front bubble.
 
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