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Old Feb 16, 2012, 01:12 PM
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Regarding the Mythbusters show on the drag of a pickup truck:

Yes, they found that the drag was lower with the tailgate closed. This was because the tailgate trapped the vortex over the bed of the truck, stabilizing it. With the tailgate open, the vortex was unstable and kept slipping out the back. The overall drag of the pickup was greater than a smoothly contoured automobile shape the same size, but with the tailgate closed and the vortex stable, its drag was lower than with the tailgate open, and an unstable vortex.

This is a major problem for the K-F sections in their present form. They correspond to the "tailgate-open" configuration, and (as a number of different studies by various people in various countries have shown) the vortex is unstable. It keeps slipping out the back, and has to re-form, driving up the drag in the process.

This does suggest an interesting possibility: what about adding a "tailgate" to a K-F section, to stabilize the vortex just as it does for a pickup truck? Something similar has already been tried successfully on conventional airfoils, the "Gurney flap" or "wicker bill" used on the wings of open-wheel race cars, such as F1 and at Indy. This was invented by Dan Gurney while consulting with Dr. Robert Liebeck of McDonnell-Douglas (one of the fathers of modern computer-optimized airfoil design) on high-lift race car wings. The way it works in that application is not the same as the problem on a K-F section, but it's similar enough in the way it does it that it might be a worthwhile experiment.

I doubt it would completely solve the low section L/D problem. At our Reynolds numbers ("Re") in particular, skin friction is a major culprit in profile drag, and the overall flow path around all the local flows is inevitably longer than for a conventional section, which increases the skin friction. More skin, more skin friction. However, if it can at least help stabilize the vortex, it might help.

Dick Kline (very nice gentleman, BTW) and I have had some private discussions via e-mail, including about a study he found on an airfoil with vortex-containing pockets in the wing surfaces, with and without suction in the pockets. The researchers found that they did have to use considerable suction in the pockets to stabilize the vortex and keep it from getting continually sucked out. There was the question of whether, even if the vortex pockets did help maintain laminar flow and lower drag, the power needed for the suction would exceed any possible gains from the laminar boundary layer (in full-scale airfoils it's all about keeping the flow laminar as long as possible to reduce drag; for us it's the opposite, we need for it to be turbulent as soon as possible so it will stay attached and not separate prematurely). That does not sound encouraging for the possibilities of improvement from a "Gurney flap" at the aft edge of the step in the airfoil, but it might still be worth investigating. The height of the Gurney flap would be critical, and the contour between it and the surface ahead of it might be critical as well.
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Old Feb 16, 2012, 01:33 PM
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Regarding wind tunnels:

"Reynolds number" is a numerical way to measure what modelers commonly call "scale effect". It's the density times speed times some characteristic length, such as chord, divided by the absolute viscosity. "Kinematic viscosity is the viscosity divided by the density, so you can also use the speed times the length, divided by the kineatic viscosity.

For example, in "ISA Standard Atmosphere" sea-level air

Re = 778 x speed (MPH) x chord (inches)

If you want to test something in a wind tunnel, you need to match the Re in the tunnel to the Re in the "real world"example you want to compare it to. If your wind-tunnel model is half the size, then your wind tunnel must generate twice the airspeed, so the Re's are the same. The amount of power you need to do that could be surprising. For serious testing in a tunnel for our kinds of speeds, with a 3' x 4' working section, you need around 40 horsepower.

Getting decent wind-tunnel data at our Reynolds numbers is significantly more difficult than for full-scale Re's. A good low Re wind tunnel needs to be carefully contoured, with multiple carefully designed screens in the inlet section to eliminate even the tiniest traces of turbulence. A cardboard box with a couple electric fans is going to generate data, but it's not likely to be reliable, usable data.

Find a copy of "Soartech 8: Airfoils at Low Speeds", Michael Selig's doctoral thesis. There's some discussion in that of what it takes to get good data at low Re's. Even so, what he did started having problems below about Re = 100K. Going lower than that (as many of our smaller, slower models do) becomes even more difficult. I'm not saying don't try, just recognize that it will be difficult, and proving that you've succeeded in generating reliable data will be even more difficult.

Model tests in free air avoids a lot of that, but then you have to deal with the problem of how to find (and verify) that you have really smooth air with no wind or updrafts or downdrafts, and how to actually measure your data reliably. You would be surprised how much convection is actually out there on any given day.

It's easy to generate impressive-looking numbers. It's far more difficult to generate ones that actually mean something. Not impossible, but difficult.
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Old Feb 16, 2012, 01:56 PM
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I notice that in NASCAR truck racing, the pickup trucks all use a cover over the truck bed with the tail gate up/closed.
This (goes faster) reduces drag??
Resembles a KFm3 configuration??
=> two smaller K-steps (tonneau cover) have less drag than one big KF-step (tailgate open)
=> extrapolate to "improvements" from KF-variants with multiple KF-steps (??)
=> infinite number of steps => conventional airfoil shape (??)

some videos: http://etonneaucover.com/news-DoTonn...rsSaveGas.aspx
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Old Feb 16, 2012, 02:11 PM
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Quote:
Originally Posted by Don Stackhouse View Post
Getting decent wind-tunnel data at our Reynolds numbers is significantly more difficult than for full-scale Re's.
although not an aerodynamic expert like yourself, i must say that dont seem to make a lot of sense. help me out here.

it strikes me that its easier to gather reliable info for models since airfoil size and speed (hence re) are exactly the same. imo it would be more difficult to get full scale data where these may not be the same because scaling is required for practical space reasons.

im not that enthusiastic over cardboard wind tunnels either but for different reasons. im just not a big fan of theory, simulation, and laboratory testing in general. careful field observation and measurment seems more dependable.

i agree about turbulence etc but the diy tunnel does appear to be somewhat useful for testing small airfoils. there is a formula for calculating free space around the sample under test and iirc my buddy took that into account when deciding on chord vs housing size.

im just not sure testing airfoils is that useful to determine how rc planes will perform. what is the formula for calculating "fun per buck"?
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Old Feb 16, 2012, 02:35 PM
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Getting good data at model Re's requires getting unusually uniform and turbulence-free flow in the tunnel. That's more difficult than what's needed at full-scale Re's.

It's sort of like the way that very small model engines, like the Cox .010, are more expensive than bigger ones like the Cox .049, because the smaller size requires closer tolerances.
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Old Feb 16, 2012, 02:39 PM
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A new thread has popped up called Amazing KF and several people describe their experiences when they applied the KF steps to their different aircraft.

http://www.rcgroups.com/forums/showthread.php?t=1595442

Looks like they were pleasantly surprised.
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Old Feb 16, 2012, 02:54 PM
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Don: great education, i still don't need to read the books! (well....maybe) anyway, so yousay a 3'vertical x4' horizontal section is required for eliminating the scale/wall effects? Not sure why so much horsepower is required if the wing section is exactly the "real" size and chord and the flow speed is exactly (well, in the range of) the real world flying speeds?
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Old Feb 16, 2012, 03:40 PM
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Way back in 1901 the Wright Brothers did splendid work with their 6' long by 16" square wind tunnel.
With this instrument they were able to prove that the flat plate espoused by some 'experts' and the other weird and wonderful sections embraced by others, were far from efficient.
Over a two month period they tested about 200 wing configurations, and at the end of that time, for the first time ever, there existed a truly scientific study of aerodynamics.
With this data they produced an airfoil, which they used for both the wings and propellers, that enabled them to fly on minimal horse-power; and put them about 20 years ahead of the rest of the world.
So yes, a relatively simple, quite small wind tunnel can indeed enable good work to be done.
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Old Feb 16, 2012, 05:08 PM
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Originally Posted by springer View Post
Don: great education, i still don't need to read the books! (well....maybe) anyway, so yousay a 3'vertical x4' horizontal section is required for eliminating the scale/wall effects? Not sure why so much horsepower is required if the wing section is exactly the "real" size and chord and the flow speed is exactly (well, in the range of) the real world flying speeds?
that was my point but in his next post don explained that its more difficult to minimize turbulence on a small scale which does make sense. my friend was satisfied with the data he got for his full size aircraft. he settled for something similar to the gossamer albatross shape. but he concluded airfoils in general were less important for small models.
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Old Feb 16, 2012, 06:09 PM
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G: I was just reading up on them, trying to figure out how their balance systems worked, found an interesting java applet from NASA that helps. But your point is well taken, I'm still not sure i would need a 3'x4' tunnel section to test 6 or 12 inch span sections of a bunch of different foamie wings. That of course brings up another issue, The wrights only used 4" span sections if I recall in a 16" tunnel. I suppose aspect ratio in the sections is something to be considered too. Downloaded a paper on rules for low speed tunnels, so will peruse that and maybe get some guidance.

Dave: I noted the point, and alsoI saw in one of the sites that the open ended tunnels were susceptible to uneven flow/turbulence, but I would think that it should be possible to see it (smoke) and try different straighteners to eliminate it. After all, this whole exercise is pretty much just another "tinkering" like building a hot wire cutter, or a wing baking jig. (bit more complicated, of course, but just another thing an old retired guy can play with out in the barn, at least until the pallets of foam show up....)
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Old Feb 16, 2012, 06:10 PM
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Quote:
Originally Posted by Dickeroo View Post
A new thread has popped up called Amazing KF and several people describe their experiences when they applied the KF steps to their different aircraft.

http://www.rcgroups.com/forums/showthread.php?t=1595442

Looks like they were pleasantly surprised.
Dick: I noticed it too, funny how folks just continue having these totally unexplainable pleasant experiences with KFM's....
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Old Feb 16, 2012, 06:22 PM
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theres nothing unexplained about it because kf wings are pleasant to fly. the problem is many confuse the friendly stall and slow flight charateristics with increased lift and less drag which are not among its advantages.
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Old Feb 16, 2012, 06:48 PM
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I don't think I remember anyone ever claiming less drag as a KF benefit. But If you are flying a flat plate wing I think there is an increase in lift. I don't say that as an aeronautical expert, only as an impression I got. If a flat plate wing goes from that slightly high leading edge and riding on a cushion of air kind of flight to flying on the level and with the wing at zero incidence there must be some kind of increase in lift.

We always have some power so when you turn a flat plate into KF the small increase in drag is easily countered with power.

Jack
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Old Feb 16, 2012, 07:25 PM
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Originally Posted by springer View Post
Dick: I noticed it too, funny how folks just continue having these totally unexplainable pleasant experiences with KFM's....
True, but to play Devil's Advocate it should be noted that the pleasant experiences are no confined to the KF wing.
I recently read a blog where the guy was amazed at how well his plane flew with the wings salvaged from a Pattern model.
Of course a KF wing may have pleased him even more.
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Old Feb 16, 2012, 07:38 PM
Jack
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"..True, but to play Devil's Advocate..."

That's all right. As a community, on this thread and the building and flying thread, we fully addicted KF users have been accused of having made all kinds of outrageous claims for the KF airfoil.

Some of them are things we believe are true and the others are things we either know to be false or that we don't believe. But when any outlandish claim is made and attributed to the innocuous "They say..." source, it is really hard to get the conversation back on an objective level.

A few even seem to come here with no other objective than to try to tone down the excess of fun happiness.

Jack
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