** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 25 - RC Groups
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May 20, 2011, 12:05 AM
just Some Useless Geek
Quote:
 Originally Posted by JetPlaneFlyer So what they found was that without 'active vortex control' it was impossible to trap a stable vortex in the step/cell.
Yeah, this was something that Dick and I were talking about previously. He suggested that the shape of the wing could be changed on the fly to accommodate changes in altitude, airspeed, AoA, etc. I opined that the wing's shape could be changed through computer-controlled actuators utilizing multiple interlocking and overlapping surfaces. We didn't get too far in this discussion, but the science of the KF points to a fully active solution as being the best way to exploit the step.

Also, it seems pretty obvious that the larger the Reynolds number is the more adjustment would be needed to make the step fully functional. On little 200 gram airplanes the step is not nearly so critical, which is why the bulk of the planes we throw together with a couple sheets of FFF work so well.
May 20, 2011, 01:31 AM
fix-it-up chappie
Why not just make the front half of the top of the airfoil lift up? We're used to "D-tube" construction. What happens if the top half of the "D" can rotate? The hinge point would be the leading edge, and the back part, presumably at 50% of the chord, would raise up just like old speed brakes. Instead of lifting a flat blade into the air, like speed brakes, they would be lifting the front half of the airfoil.

Here. A picture is worth 1000 words.

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May 20, 2011, 02:46 AM
Grumpy old git.. Who me?
Quote:
 Originally Posted by A Useless Geek Yeah, this was something that Dick and I were talking about previously. .........
Actually the wind tunnel testing done was at quite low Re numbers.. 15m/s (about 30mph) velocity and about 300mm (12") chord. That gives an Re of roughly 300k, which is about right for a medium size model in normal flight. Admittedly small slow flying models will be operating in a much lower Re but what effect such Re that would have on the results, if any, is speculation. I don't think there is any reason to assume the vortex would be any more stable at lower Re?

The vortex cavity shape used in the test was actually arrived at as being the best shape to trap a vortex (it says that in the paper). Presumably then they considered a simple square step and decided, or proved, that it couldn't trap a stable vortex. Even the optimised vortex cavity they tested wouldn't hold a stable vortex without suction so it seams highly unlikely that a simple square step wouldn't have much hope of doing so.

As far as I've gathered from all that I've read the idea that the KF airfoil traps a vortex is basically speculation. The 'pretty picture' images often shown for a vortex sitting neatly behind the KF step are just 'photoshop' type drawings, they are not taken from wind tunnel testing or even computer simulation (and computer simulation cant be trusted for this type of analysis anyway).

Based on this testing maybe someone should be considering vortex cavity suction and/or blowing.. They got good results once they used this form of active control in the tests. Of course actually doing it an an plane (even a real plane let alone a model) is very complicated and tends to be unreliable, which is why boundary layer suction is not widely used in-spite of the known positive effects... this takes us very far from simple foamy models.

Steve
 May 21, 2011, 05:39 PM My hovercraft is full of eels. Hmm. I believe this is the wrong thread. Never mind this post!
May 21, 2011, 06:24 PM
Boffin
Quote:
 Originally Posted by tolladay Why not just make the front half of the top of the airfoil lift up?
Kasper patented that idea back in the '60's. It generated a lot of excitement but no one was able to replicate his results in the wind tunnel or full scale. People have been trying to capture a vortex for a few generations now, and no one has been successful yet with a practical design.

Rick.
May 22, 2011, 03:41 AM
Grumpy old git.. Who me?
Yeah, the Kasper wing did feature a similar hinged upper leading edge 'flap'.. The patent also covered a 'clam shell' type split flap arrangement on the TE. The idea was that these two flap devices trapped first a vortex above the wing, similar to what has been claimed for the KF, and the split TE flap trapped another vortex behind the wing, which theoretically had the effect of making the wing both thicker and wider in chord, improving lift and enhancing slow flying ability.

It was patentedd in 1974: http://www.google.co.uk/patents?hl=e...page&q&f=false which interestingly is around the same time as the KF patents. Not sure if Dick Kline had any knowledge of the Kasper design or vice versa.. Dick?

Like many patents the Kasper wing sound fantastic but simply didn't work. Wind tunnel testing on the design was done by Kruppa in 1977 which showed, rather like this latest trapped vortex testing, that it was impossible to trap a stable vortex in the way Kasper claimed. All the flaps did was cause turbulence and drag.

Steve

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May 22, 2011, 08:58 AM
Registered User
Our first patent was granted in December 19,1972. Our attempt was to only patent a rather shallow step @ 50% on the underside. We did not try to define an entire airfoil profile because we knew that it worked (in small scale) on the top as well as on the bottom. We couldn't afford to patent all the different variations. We were unaware of the Kasper concept because it didn't come up in the patent search.

Years later, In 1986, Demeter G. Fertis was granted a patent on a stepped airfoil. Ironically, he had the same patent examiner that we had, Galen Barefoot. He seems to have done quite a bit of research on the stepped airfoil. He has a website here showing his interests: http://demetergfertis.net/

Note: Reynolds numbers used were 550,000.

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Last edited by Dickeroo; May 22, 2011 at 09:06 AM.
May 22, 2011, 09:18 AM
Registered User

New Airfoil-Design Concept with Improved Aerodynamic Characteristics

by Demeter G. Fertis, M.ASCE, (Prof., Civ. Engrg. Dept., Univ. of Akron, Akron, OH 44325-3905)

Journal of Aerospace Engineering, Vol. 7, No. 3, July 1994, pp. 328-339, (doi 10.1061/(ASCE)0893-1321(1994)7:3(328))

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Document type: Journal Paper
Closure: (See full record)
Discussion: by L. Lazauskas and et al. (See full record)

Abstract: The research in this paper is the result of an experimental study regarding a new airfoil-design concept, which is developed to produce greater lift coefficients over a much broader range of operational angles of attack, to improve or eliminate stall at virtually all operational airspeeds, to increase functional lift-to-drag ratios over a greater range of operational angles of attack, and to be adaptable for aircraft of both the fixed-wing and the rotary-wing types. The writer has combined his effort with L. L. Smith, and a U.S. Patent, entitled “Airfoil,” Patent No. 4,606,519, was obtained on August 19, 1986. Patents were also obtained or are pending in other countries. The experimental results, obtained by using the new airfoil-design concept, have been compared with experimental results obtained from a conventional NACA 23012 airfoil. Flight performance tests by using a 2.134-m (7.0 ft) model and remote-control devices, as well as flow-separation studies, were also performed. The results were compared with the ones obtained by using the NACA 23012 airfoil.
May 22, 2011, 09:31 AM
Registered User
Here is patent #4,606,519 issued to Demeter G. Fertis in 1986...

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May 22, 2011, 10:05 AM
fix-it-up chappie
Quote:
 Originally Posted by Dickeroo Our first patent was granted in December 19,1972. Our attempt was to only patent a rather shallow step @ 50% on the underside. We did not try to define an entire airfoil profile because we knew that it worked (in small scale) on the top as well as on the bottom. We couldn't afford to patent all the different variations. We were unaware of the Kasper concept because it didn't come up in the patent search. Years later, In 1986, Demeter G. Fertis was granted a patent on a stepped airfoil. Ironically, he had the same patent examiner that we had, Galen Barefoot. He seems to have done quite a bit of research on the stepped airfoil. He has a website here showing his interests: http://demetergfertis.net/ Note: Reynolds numbers used were 550,000.
That is one interesting paper. Has anyone replicated these results, besides the modeling community?
May 22, 2011, 10:15 AM
fix-it-up chappie
Quote:
 Originally Posted by JetPlaneFlyer Yeah, the Kasper wing did feature a similar hinged upper leading edge 'flap'.. The patent also covered a 'clam shell' type split flap arrangement on the TE. The idea was that these two flap devices trapped first a vortex above the wing, similar to what has been claimed for the KF, and the split TE flap trapped another vortex behind the wing, which theoretically had the effect of making the wing both thicker and wider in chord, improving lift and enhancing slow flying ability. It was patentedd in 1974: http://www.google.co.uk/patents?hl=e...page&q&f=false which interestingly is around the same time as the KF patents. Not sure if Dick Kline had any knowledge of the Kasper design or vice versa.. Dick? Like many patents the Kasper wing sound fantastic but simply didn't work. Wind tunnel testing on the design was done by Kruppa in 1977 which showed, rather like this latest trapped vortex testing, that it was impossible to trap a stable vortex in the way Kasper claimed. All the flaps did was cause turbulence and drag. Steve
Thanks for finding this. I like this implementation of the airfoil change over the simple diagram I drew.

I see some issues with Kasper's design, mostly at where his separation is. I'm not anything like an expert, but I suspect creating a step at less than 50% of the chord might be an issue.

I have a question to you, one that may simply show the depths of my ignorance. Perhaps you might have an answer. What if the vortex is a chimera? That is, what if one could produce an airfoil with some slight benefits, especially at higher AOA, without producing a vortex? In other words, I believe what I am asking is if a vortex the only way to explain the benefits of a stepped airfoil?
 May 22, 2011, 10:39 AM High Altitude Flyer Tolladay & friends, turbulating the boundary layer airflow to minimize airflow separations is a technique that's been in use on sailplanes for a long time. Based on my personal experimentation, I'm inclined to theorize that a shallow stepped discontinuity can accomplish this while minimizing drag. (There are a LOT of variations which can be implemented, and some of them may be more beneficial than others.) Whether or not a continuous recirculating vortex is trapped or not is hard to say at this point... but that type of vortex "trapping" may not be necessary to still derive a benefit from the implementation of the stepped discontinuity. (They do turbulate / "energize" the boundary layer airflow, which definitely has benefits....) VIKING Last edited by viking60; May 26, 2011 at 04:45 PM.
May 26, 2011, 04:39 PM
High Altitude Flyer

# Vortex generator strips being used on full-scale aircraft

Friends,

I spoke to the owner of a (full scale) Cessna 182 in Arizona earlier this year who told me that he had installed " VG Strips" on his aircraft's wings. He said, "It was like adding power steering to the aircraft- the handling & control response was far more precise."

From the web site: http://www.microaero.com/

"Micro Vortex Generators are placed in a spanwise line approximately ten percent aft of the leading edge of the wing. They control airflow over the upper surface of the wing by creating vortices that energize the boundary layer. This results in improved performance and control authority at low airspeeds and high angles of attack.

Diagram of Micro's Vortex Generators and Boundary layer air flow (below).

A Micro Vortex Generator

Creates a tiny vortex in the airstream over an airfoil. This vortex energizes the normally stagnant boundary layer of air on the wing's surface. An energized boundary layer is more resistant to flow separation than a stagnant boundary layer. The result is that airflow "sticks" to the wing better, permitting flight at lower airspeeds with improved control authority."

(Food For Thought.....)

VIKING

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 May 27, 2011, 02:30 AM Grumpy old git.. Who me? viking, No argument from me about the benifits of turbulators and/or vortex generators.. i've used both on models of my own and they do work, no question, the science behind boundary layer turbulation is proven and universally accepted. i can even add turbulators to my airfoils in my airfoil analysis program X-foil and the benifits at low Re are clear to see... now the question would be: Does a KF step act like a boundary layer turbulator/vortex generator? The answer to this one IMHO is that a conventional deep KF step could not act as a boundary layer turbulator. It's just way too deep. for our small models the boundary layer is typically fractions of a mm thick. All a large step will do is cause the boundary layer to flow off the edge and into a large chaotic mass of separated airflow.. This is the very opposite of what a turbulator is intended to do.. A turbulator/vortex generator's only purpose is to ensure that the thin film of boundary layer air remains firmly attached to the wing surface. It's pretty much inconceivable that the boundary layer could remain attached to the surface as it flows around a large square edged KF step Not only is it hard to imagine that it could be so, there is lots of wind tunnel testing that proves it not to be the case. Perhaps very VERY small steps of no more than a couple of mm or so could act like turbulator strips. This may well partly explain the relatively good performance of the multiple 'micro-step' glider airfoils that Davereap has been experimenting with lately.. However I would argue that there comes a point where the steps are so small that the airfoil is not really a KF, it simply becomes just a conventional airfoil with turbulator 'ridges' something that was tried and tested long before the KF idea was ever conceived. Steve
May 27, 2011, 06:42 AM
Registered User

# Too deep or not too deep. That is the question.

This is the drawing from our original patent which shows a very shallow step. It is the step that we received a patent on. Not an entire airfoil profile. The height of the step has varied greatly in the RC. It all depends on what its intended for.

In 2006, Tony Bernardo brought it out of the shadows into the RC Groups. Without Tony, it would probably still be sitting in the dark. Thank you, Tony65X55.