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Old Feb 17, 2012, 09:36 AM
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Originally Posted by Dickeroo View Post
Maybe we can close the tailgate before the vortex gets out?
Dick, to trap and retain the vortex, it would need to be short, but perpendicular to the surface, like a pickup truck's tailgate. A smooth ramp like your sketch will redirect the vortex a little, but will probably not retain the vortex.

There's an article in Wikipedia on Gurney flaps:
http://en.wikipedia.org/wiki/Gurney_flap

Gurney flaps go on the underside of the trailing edge. They make a bulge in the airflow, thickening the boundary layer on the underside, but also creating an effect that tends to pull the upper surface flow downwards with it. It's most effective in cases where the upper surface flow is having trouble staying attached at the trailing edge. The thicker wake under the wing increases drag, but if done just right, the decrease in upper surface wake is greater, and overall the total wake is reduced, reducing total drag.

This is not quite the same as what's needed to retain the vortex on a K-F airfoil, but the shapes and proportions should be similar.

Don't know if it would work, and it would probably require some optimizing (just like winglets and turbulators) even if it does work, but it might be worth a try.
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Old Feb 17, 2012, 09:58 AM
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Don...

Thanks for analysis on the vortex. You have a very extensive knowledge base on this subject. I have a question that I would like to present to you regarding the vortex is two situations.

First, on Richard Whitcomb's coke bottle fuselage. Why is it that there dosen't seem to be a problem for the vortex in the waist of the fuselage? Air has to get trapped in the waist which helps to reduce the sonic boom. So why doesn't it become unstable?

Second, on Whitcomb's supercritical wing he has a small cusp at the trailing edge on the bottom of the airfoil. This has to trap a very small amount of a vortex yet doesn't require any suction to keep it stable.

Any of your thoughts would be greatly appreciated.

Dick
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Old Feb 17, 2012, 10:05 AM
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Originally Posted by Cap_n_Dave View Post
Unfortunately this is a somewhat complex topic that can't be easily addressed in a short post on an internet forum.
...
This is all fairly well documented stuff, suggest you get a textbook to learn more.
Well, yeah, tru dat... But if i wanted to look it up, i wouldn't have posted, and you guys are happy to provide the answers, so why not?
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Old Feb 17, 2012, 02:50 PM
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Originally Posted by Don Stackhouse View Post
No. Thin wings are required for supersonic flight, the F104 being one fairly extreme example (3% thick, no camber to speak of, leading edge radius 1/64", so sharp that they had to put covers on them on the ground so mechanics wouldn't get injured if they walked into one).

However, for subsonic flight, much greater thickness is better. Full-scale high performance sailplanes frequently have airfoils 15% to 18% thick, including outboard on the wing where the structural challenges of high aspect ratios are no longer a problem.

There are some outstanding 18% NACA 65 series laminar flow sections with unusually high section L/D's and good stall characteristics, if you are working with full-scale Re's. For full-scale general aviation aircraft, typical thicknesses run 12% to 18%.

Those thicknesses at model airplane Re's are a disaster. For model sailplanes, going thicker than about 8% to 9% can actually reduce max lift
You say something is not right and them post information that shows that the original proposition was correct.
The truth is that the Wrights determined that thick wings are not good, and history shows that to be true.
The Spitfire flew better than the Hurricane largely because of the thinner wing on the Spitty. Typhoon and Tempest... same story. In a case like the B-17 and B-29 there were obviously other great differences but the later design featured a thinner wing presumably because the designers thought that thinner was better.
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Old Feb 17, 2012, 03:18 PM
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i think theres confusion caused by not specifying the airspeed. there is no doubt in my mind that for slow flight parkflyers and foamies a thin single surface undercamber can not be beat for lift. certainly better than plate, clark-y, aquila, us35, or exotic glider profiles. there are faster airfoils and slower ones but for lift at speeds we have tested raskins 4-40 uc was tops. jets, pylon, 3d, etc who knows.
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Old Feb 17, 2012, 03:26 PM
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Whiskers, if what you claim was true, then all of the manufacturers of full-scale aircraft since the 1930's are being intentionally stupid.

In the case of the Spitfire vs. the Hurricane, they were getting into the realm of transonic flows, so supersonic issues apply.

However, although there are variations in thickness among those examples, some thinner and some thicker, they are all thicker than what is optimum for our Re's, and for the Re's in the Wright's wind tunnels.

When you get into supersonic operations, such as the F104, the rules get more complicated. Airfoils for high Mach numbers are not suitable for full-scale subsonic aircraft, but also not generally very good for low-Re model applications.
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Old Feb 17, 2012, 03:39 PM
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Dick, there are not any trapped vortexes in those examples. The high pressure flow under the wing keeps the flow attached in that cusp on the underside of the supercritical airfoils. There is also attached flow across the "wasp waist" of the area-ruled fuselage.

Area ruling is another thing that applies to supersonic flight. A major part of the drag of a supersonic aircraft comes from the shock waves. You get a shock wave every place there is a sudden change in the cross-section of the aircraft. Note, you have to consider the entire aircraft cross-section, not just the individual parts.

Travelling aft along the length of the plane, when you get to the wing, the wing causes a sudden increase in the aircraft's total cross-section at that point. By slimming the fuselage in the area of the wing, we can remove enough cross-section from the fuselage to cancel out the wing's added cross-section.

If done just right, when you plot the cross-sectional area at each point along the length of the plane, you get a smooth curve. This reduces or eliminates the shock wave intensity at all the intermediate locations, leaving just the ones at the ends of the nose and the tail. This minimizes the wave drag.

When they added the extended upper deck (i.e.: a longer "bump" on the top of the forward fuselage behind the cockpit) to the Boeing 747, it smoothed out the cross-sectional area between the cockpit and the wing. This reduced the wave drag, and the plane went faster despite the extra weight and whetted area of the extended bump.

There's a good article about it in Wikipedia:
http://en.wikipedia.org/wiki/Area_rule
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Old Feb 17, 2012, 07:07 PM
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Originally Posted by Whiskers View Post
The truth is that the Wrights determined that thick wings are not good, and history shows that to be true.
I'm sorry to pile on here, but there is a huge difference between subsonic flight (where thicker airfoils are "good") and supersonic flight (where thin airfoils are necessary).

People study this in undergraduate, graduate, and post-graduate schools ... you're not going to master the knowledge in this field by reading a bunch of stuff online, etc.
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Old Feb 17, 2012, 07:08 PM
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Originally Posted by Dickeroo View Post
First, on Richard Whitcomb's coke bottle fuselage. Why is it that there dosen't seem to be a problem for the vortex in the waist of the fuselage? Air has to get trapped in the waist which helps to reduce the sonic boom. So why doesn't it become unstable?
As Don mentioned, there is no vortex there.

Are you polling people or something?
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Old Feb 17, 2012, 08:27 PM
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No. I'm not polling people. I asked Don because he appears to be very knowledgeable and since I didn't know the answer I asked him. I am never afraid to ask questions because that is the way I learn things. If you ask two doctors for an opinion, you often get two different answers in which case you have to decide which one works for you. I'll go along with what Don says.
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Old Feb 17, 2012, 08:35 PM
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Originally Posted by Cap_n_Dave View Post
I'm sorry to pile on here, but there is a huge difference between subsonic flight (where thicker airfoils are "good") and supersonic flight (where thin airfoils are necessary).
Huh? Where are all these thicker winged subsonic planes?
Every where I look I see thin sections, and every new plane seems to go thinner than ever before.
You have to go back to designs of the 20s 30s to find really thick wing sections, and after that it's as thin as the structure (or other non aerodynamic requirements) will allow.
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Old Feb 17, 2012, 09:23 PM
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As I said earlier, full-scale general aviation sections typically fall in the range of 12-18%. Modern R/C sailplanes and smaller powered RC models should be around 8-9% or less for best performance, and the trend is in that direction. At lower Re's such as Mosquito class sailplanes and higher-aspect ratio DLG's, the thickness needs to be even less, maybe 4-6%. Older model designs (Nostalgia-class vintage) used thicker sections (such as the 12% thick Clark Y, which is a good full-scale airfoil, but not so good for models), and suffered for it.

One thing that seems to be lacking in this thread is some appreciation of the quantitative aspects of airfoil designs. There is lots of discussion of K-F this and K-F that, but what sort of studies have been done regarding the effects of subtle changes in thickness, or camber, or how those are distributed along the airfoil?

A change of less than half a percent in thickness or even less than that in camber, can have a profound effect on airfoil characteristics. The airfoil properties needed at a wing tip are generally different than what's needed at the root, and subtle changes in thickness and camber are needed to properly deal with that. On our Chrysalis series sailplanes there are typically three different baseline airfoils at different places along the span, each optimized for the local requirements in that portion of the wing, with non-linear blending between those baseline airfoils, and that's before the corrections to each individual rib shape to compensate for covering sag.

One-size-fits-all is generally NOT the best approach to wing or tail design, especially if you're not even keeping track of what the exact size is. In design work it's difficult to know which way to go unless you first make sure you fully understand where you've been.
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Old Feb 17, 2012, 09:40 PM
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I'm confused.
I'm saying thinner sections are better and you, Don, are saying that's not right; thinner sections are better.
I agree about the woolly nature inherent in the term KF airfoil, but it's early days and perhaps Springer's wind tunnel may be a good start to looking at this from what we, the denizens of Scratchbuilt Foamies, regard as the real world.
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Old Feb 17, 2012, 10:36 PM
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One more time:

Thinner sections are better for low Reynolds numbers (models).

Thicker sections are better for high Reynolds numbers (subsonic full-scale aircraft).

The Wrights' wind tunnel gave them an answer that applied to low Re, which they then applied to high Re, saddling the early full-scale aviation industry with thin wings that had less lift, poor stall characteristics, and required elaborate and very draggy external bracing.
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Old Feb 18, 2012, 08:38 AM
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as implied the only reason for thicker is structural integrity. theres also a cosmetic factor because people simply dont like struts or bracing wires as much as straight sexy cantilever. human psychology is a big factor in aerodynamic engineering just like it is with automotive.

for example the wire braced quicksilver ultralight and its copies remains the highest performance airframe many decades after its design. its single surface 4-40 uc wing is only 1 dacron fabric thick (few mils). thin is good for models AND full size if the wing can be made strong enough. its that annoying "frontal area" thing that keeps cropping up.
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