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Old Nov 16, 2011, 08:41 AM
Jack
USA, ME, Ellsworth
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RE: Trashing the KF airfoil as a pastime.

The worst offenders and the relative rankings of their offensive statements are always in scale to the individual's perceived importance and renown in the aviation arena. And the more they seem to know about mainstream aviation the more they seem to need to criticize things KF. Especially so if they are eating at the trough of public monies and supposed to be imparting knowledge to others.

It is the open minds and simplistic needs of us nihilistic pleasure seekers that has taken this where it is now and I don't even want to try to define exactly where it is or what contribution it has made to society. But I see the KF as it is being used and enjoyed now and here as pleasurable and worthwhile and not worthy or deserving of any negative criticisms.

In the idle hours I have thought that I have seen glimpses of a place where real airplanes might be able to use it and benefit from it. And that might be in a more constructive purpose than the simple joys of model building. I just haven't put it all together yet...

Jack
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Old Nov 16, 2011, 08:43 AM
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Originally Posted by dave1993 View Post
there are few actual naysayers. in fact ive never heard a single contradiction to the true advantages (wing strength and improved stall). it is also untrue that rigorous studies are lacking. numerous government and academic evaluations were done although it does take time to dig the data out.

as i mentioned before the fellow who got me started in this hobby built dozens of kf wings, top and botton, of varied percentages with careful measurments of lift-to-drag/glide-ratio. i was there. in ALL tests kf exhibited increased drag over flate plate and other common airfoils and no major improvement in lift. on the other hand kf improved stall characteristcs big time.

i built a few myself and found the plane much more fun to fly. my kf wings were significantly stronger too. however getting to altitude on a given mah were slightly inferior to flat plate and markedy less efficient than the famous 4-40 uc.

its unfortunate that religious zeal often overshadows real world benefits of this wonderful wing configuration.
Dave...
You might want to view these videos of a side by side comparison between a KFm3 airfoil and a conventional airfoil. The results were surprisingly similar.

ENDURANCE TEST:
Endurance Test, Flight 1 (11 min 0 sec)


STABILITY TEST:
Stability test part 2 (13 min 40 sec)


Dick
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Old Nov 16, 2011, 09:11 AM
RC beginner
New York
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dick,

unfortunately im unable to view flash videos but would appreciate a description of whats shown if you could. or a link w/pics if there is one.

i do have your book and couldnt believe my eyes when i built the little curvy wing paper airplane. it was like magic.
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Old Nov 16, 2011, 10:27 AM
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Originally Posted by Dickeroo View Post
Usefull Geek...

It is also interesting that Richard Whitcomb really bashed and trashed the KF airfoil concept, yet he uses a coke bottle or wasp waist fuselage to reduce sonic boom. The depression in the fuselage allows some of the displaced air to fill in the body of the fuselage thus relieving some of the pressure build-up. He also has a slight cusp on the trailing edge of his super-critical wing. He claimed that the KF airfoil was the worst airfoil he ever tested. The worst? Hmmmmmmmmm.......
Dick:
I would suggest that it might be more productive for you to separate emotional issues (Whitcomb bashing your invention) from science issues (the results of Whitcomb's work).

The area rule to which you refer is about a smooth transition of cross-sectional area in order to mitigate/eliminate transonic compressibility effects. If there is a sudden increase in cross section, there is a concomitant increase in flow velocity - at high transonic speeds this results in a bubble of air going supersonic, with extra drag as a result. You have used similar phrases ("displaced air filling in the body to reduce pressure effects") to describe your airfoils ... I find this comparison dubious at best.

I would be interested in learning more about the transonic or supersonic properties of your airfoils, generally speaking any corner on a body in a supersonic flow will generate shock waves - which lead to energy loss (drag and to some extent structural heating). How does your airfoil avoid/mitigate this issue?

If, in fact, "thefamoushat" is a university student who has access to a very low-speed tunnel, it might be interesting to have him do a study and publish the results. It would anecdotally appear that the stepped foils have good properties at very low Reynolds numbers ... would be nice to see some work done to corroborate that.
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Old Nov 16, 2011, 10:41 AM
RC beginner
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thanks for the pm dickeroo. as i said it was a privilege to talk to the man who got me started with little planes.

note it is accepted that air is compressable or non-compressable fluid like water depending on supersonic speed or not. also airfolis are not really relevant at the reynolds numbers we as hobbyists are concerned with.
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Old Nov 16, 2011, 12:21 PM
Grumpy old git.. Who me?
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Originally Posted by Dickeroo View Post
I would like to point out that the wind tunnel testing that was done many years ago was totally incorrect.
Dick,
I doubt that the NASA results were 'incorrect' as such. I'd be reasonably sure that they were accurate and 'correct' for the actual airfoil they tested under the conditions they tested it at.
ok they tested a pointed nose airfoil which was by conventional thinking was sure to produce bad results at subsonic speeds, but they copied the shape direct from the patent document, which i understand was based on the airfoil used on your vary famous paper plane, so it's easy to understand why they went with that shape.

I think this comes back to the point i made earlier; unless someone comes up with definitive coordinates of what each KFm airfoil actually is then test results of any type are always going to be highly variable.

Steve

Please note.. no criticism of anyone or any airfoil implied in above (or previous) post
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Old Nov 16, 2011, 12:24 PM
just Some Useless Geek
Chicagoland
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Hmm.

Perhaps we are still talking at cross purposes here. Is the intent of this thread to chase down the effects of the KF on general aviation aircraft, or are we limiting the discussion to radio control sizes and speeds only? I thought that we were doing the later, since it is obvious that nobody here has the kind of moolah required to produce large scale or near supersonic studies.

However, bear in mind that even at hobby-level Reynolds numbers what we're discovering in these experiments can apply to various commercial and military applications. Consider the plans for airborne cellular antennas supported by solar-powered UAVs. Think also about military UAVs that need to remain airborne through conditions of violent wind and other difficulties. The stuff we're working on can directly impact these markets. You'd think somebody would be interested, eh?
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Old Nov 16, 2011, 01:08 PM
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Quote:
Originally Posted by A Useless Geek View Post
......You'd think somebody would be interested, eh?
There is a lot of interest in vortex trapping airfoils (which is what the KF step is supposed to do). There has been some big budget wind tunnel testing done on such airfoils within the last few years. I've linked these test results a couple of times in this thread already, here's a link to my last post regarding these tests: http://www.rcgroups.com/forums/showp...&postcount=355

I appreciate that the airfoil tested was not a classic KF step design but the design intent is identical. Tests were over a range of Re numbers, some low enough to be relevant to R/C models.
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Old Nov 16, 2011, 01:32 PM
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Hox does the Kf step compare with turbulation?

If the Kf step's main advantage is delaying stall, it would seem to do something similar to adding a trip strip to the airfoil, which also delays stall?
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Old Nov 16, 2011, 01:46 PM
Grumpy old git.. Who me?
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Originally Posted by rdeis View Post
Hox does the Kf step compare with turbulation?

If the Kf step's main advantage is delaying stall, it would seem to do something similar to adding a trip strip to the airfoil, which also delays stall?
Good question but as there (to the best of my knowledge) has been no low Re scientific testing of a KF airfoil vs. a turbulated 'conventional' airfoil no one can really give a definitive answer.

From a theoretical point of view it's hard to see how the airflow can quickly re-attach to the wing surface if it goes over a deep KF step, and unattached flow creates drag. This might explain why the KFm airfoils with multiple small steps appear to have lower drag than single deep step designs? It's quite possible that these smaller steps might work in a similar way to turbulators.
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Old Nov 16, 2011, 02:22 PM
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In 1986, Demeter Fertis was granted a patent for a stepped airfoil with the step on top. He had the same patent examiner that we had fourteen years earlier, Galen Barefoot. This patent contains all of his wind tunnel data as well as all the claims he made for the stepped concept. Could someone give me their viewpoint on the data expressed by this patent? Does any of this data answer some of the questions brought up in this thread? I would appreciate your opinion concerning the claims that are made in this patent.

Dick
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Old Nov 16, 2011, 02:33 PM
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Dickeroo:
Interesting, thanks for posting.

1. Looks like the Re would be fairly high ... no way to tell definitively because no dimensions are provided regarding the length of the 'foil. I would say that based on the cartoon the section tested would be about the chord length of a Cessna type aircraft.

2. There are numerous mentions of a lower L/D ratio but improved stall performance. This is where it gets hairy for larger airplanes, they are looking for better L/D at cruise - the only place they care about stall performance is takeoff and landing. If you could make a variable geometry wing that could incorporate the step (that is the same weight or less than current flap/slat/spoiler configurations) then maybe Boeing would be interested.

3. The comparison does not illustrate performance when compared to wings/foils with high lift devices (flaps/slats). You posted one earlier but it does not show a KFm that also has high lift deevices. Could be an avenue for exploration ... problem would be that the L/D is not so great, so I'd think if you'd want a step, it would have to be variable geometry.

4. No explanation of the tests made to generate the L/D graphs.
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Old Nov 16, 2011, 02:58 PM
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Yep, the main problem i have with the graphs used by Fertis in his patent are that there is no explanation of where they came from. I don't even see any statement definitively stating they are taken from actual wind tunnel tests, they may purely be Fertis's thoughts on how the airfoil 'might perform'. Any proper scientific testing would have full details of the apparatus used to perform the tests, when they were done, under what conditions and by whom.

taken on face value the graphs look good but there again Fertis has a vested interest in making his invention look good. I'd be looking for some better backup for his data, otherwise they could for all we know have been made up on the spot.
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Old Nov 16, 2011, 03:06 PM
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Dave and Steve...

Thank you both very much for your input. I very much appreciate it

Dick
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Old Nov 16, 2011, 07:40 PM
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Quote:
Originally Posted by dave1993 View Post
thanks for the pm dickeroo. as i said it was a privilege to talk to the man who got me started with little planes.

note it is accepted that air is compressable or non-compressable fluid like water depending on supersonic speed or not. also airfolis are not really relevant at the reynolds numbers we as hobbyists are concerned with.
I totally agree with the claim, "... airfolis are not really relevant at the reynolds numbers we as hobbyists are concerned with."
I've lost count of the number of times guys have turned up at the field with wings that would horrify anyone with even a smattering of aeronautical theory.
Usually they were nice bird-section RTF trainer wings that had been strengthened with numerous CF rods taped on the top and bottom.
A bit like if you attached a number of spaced 6 inch PVC pipes to the wing of a real Piper Cub.
I'd would not want to be on board that Cub, but the models flew just fine.
It shook my world, but even though I have seen it many times I still could not bring myself to do it. <shudder>
Another one sanded his KF steps so they had rounded trailing edges. It flew in a very standard way, neither remarkably good or bad just... er... normal.
And look at the sharp LE, monstrously fat symmetrical wings some of the SPAD guys build. I haven't seen one of them fly but they keep building them so they must perform OK.
The list goes on, and so does the debate.
At the high end of contest flying, where 1000% effort is deemed worth 1% improvement, airfoil selection and construction may well come into play.
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Last edited by Whiskers; Nov 16, 2011 at 07:48 PM. Reason: Added the rider.
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