I agree with Lee in that everything should be grist for the scientific mill. Show me the data!

That being said, I am quite positive that KF airfoils will prove to have some value in airfoil design. To what extent that value is, I cannot say. But every time I see a bird fly, I am watching a stepped airfoil in action. In my experience, betting against several million years of evolutionary adaptation is generally a bad idea. YMMV.

Hey all,

I have started a new thread here:

http://www.rcgroups.com/forums/showt...1#post17331516

I would love to get everyone input on the proposed testing as well as on the builds.

Thanks
Jon

Friends,

Here is an interesting article & links about using very low profile stepped discontinuities, such as the denticles on the surface of a sharks skin, to minimize drag. (To date, most experimenting with KF stepped discontinuities has been done by scratch-build foamie experimenters on a rather course scale when compared to what works so well for a shark, and allows a shark to swim faster and more silently through the water.)

My implementation of the ~2mm deep stepped discontinuities with the rounded profile to the leading edges / walls of the vortex pockets [on the DANCER III 62" MH32/KF3P wing prototype] shows a lot of promise as far as reducing drag (in comparison to the same wing with deeper stepped discontinuities.) I'll get back to further test flying once we get into the Lake Havasu, AZ area by the end of this week. I'll post updates as appropriate.

Here's the DANCER discussion thead:
http://www.rcgroups.com/forums/showt...=860461&page=3

VIKING

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December 5, 2007
Shark Skin Research Could Reduce Airplane Drag By 30 Percent

By Mary Grady, News Writer, Editor



It may seem obvious that the surface of an airplane should be as smooth as possible to minimize aerodynamic drag, but that's not really the case. A bit of roughness can break up the boundary layer and improve efficiency. Sharks, with skin formed of rough scales called denticles, can slip through the water at speeds of up to 60 mph with minimal drag. This week, The Lindbergh Foundation awarded a grant to Dr. Amy Lang, at the University of Alabama, to study whether the surface texture on the skin of fast-swimming sharks, capable of bristling their scales when in pursuit of prey, could be mimicked and used to reduce the drag on aircraft. "If we can successfully show there is a significant effect, future applications to reduce drag of aircraft and underwater vehicles could be possible," said Lang. The technology has the potential to increase aerodynamic efficiency up to 30 percent, with savings of billions of dollars and substantial reductions in fuel burn and emissions.

Dr. Lang will perform water-tunnel experiments to measure the flow over and within a bristled sharkskin model (2 cm size scales), which achieves similarity with real sharkskin (0.2 mm size scales) by a corresponding scale down in velocity of the experiments. She will also obtain drag measurements over a sharkskin model in a Couette flow facility containing high-viscosity oil. Her work is also supported by the National Science Foundation.


http://www.avweb.com/avwebflash/news..._196715-1.html

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http://aem.eng.ua.edu/FM-ACE/projects.htm



Projects

Boundary Layer Control

Dr. Amy Lang, alang@eng.ua.edu, http://aem.eng.ua.edu/people/lang/lang.asp

Department of Aerospace Engineering and Mechanics

This research is primarily interested in 2D patterned surfaces with micro-cavities where vortices embedded within the cavities of the microgeometry lead to the formation of a partial slip condition thus favorably increasing the momentum in the boundary layer close to the surface. Dr. Lang is investigating the biomimetic microgeometry of butterfly scales (see image). The flow over the scales, about 100 microns in length, is very low Reynolds number. We can scale up the geometry to ~1 cm and work in high viscosity oil.

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http://aem.eng.ua.edu/people/lang/lang.asp

SGER: a biomimetic surface roughness geometry for boundary layer control


PI: Amy Warncke Lang
PROJECT SUMMARY

This grant will support exploratory, experimental research to investigate the boundary layer flow over a biomimetic roughness geometry. The surface mimics the formation of a roughness microgeometry that can be observed on the skin of fast swimming sharks, conjectured by scientists to have the capability of bristling their denticles (scales) when in pursuit of prey at increased swimming speeds. It is theorized that such a surface geometry may lead to the formation of a three-dimensional array of cavity vortices forming between the denticles, and thus a complex partial slip condition over the surface may result strongly affecting the transition to turbulence in the boundary layer. Depending on the Re of the flow (based on the cavity height or size of the denticle) and the thickness of the boundary layer, the result could be either skin friction reduction or enhancement at the surface. Results from this study may give insight as to why fast sharks, such as the Shortfin Mako (Isurus Oxyrinchus) believed to achieve speeds upwards of 60 mph, have smaller denticles than slower shark species. Another implication is that sharks with larger denticles may not be able to achieve higher speeds due to a sudden increase in drag when attempting to swim past a certain speed. Additionally, the fact that on a single shark the size of the denticles can vary corresponding to regions over the shark’s body where the flow has been accelerated (smaller denticles) or decelerated (larger denticles) due to body curvature would also be explained. The intellectual merit of the project lies in the potential understanding and application of a means by which nature has already worked out a solution for the reduction of skin friction over a solid surface, resulting in the control of boundary layer flows and their transition to turbulence. Not only would this new method of boundary layer control be discovered, leading to new technological innovations resulting in energy conservation, but the implications regarding a greater understanding of the biology and evolutionary development of sharks would be significant. The broader impacts of such a method of flow control include: drag reduction (e.g. reduction in fuel requirements and/or increased range for aircraft, ships, submarines, etc.), separation control, and mixing and heat transfer enhancement (e.g. cooling of compute hardware components).


Research Laboratry :

To perform these studies she has purchased a water tunnel (shown below) with a 15 inch wide by 30 inch tall by 9 foot long test section that is capable of speeds up to 1 ft/s. This tunnel has been designed to run at very low turbulence levels (0.4% or less) and also has a two dye injection system. The laboratory also includes a Time Resolved Digital Particle Image Velocimetry (TR-DPIV) system capable of capturing images of the flow at 1000 fps. This system acquires velocity data within a two-dimensional plane by imaging and tracking particles that have been illuminated in the flow through the use of a laser sheet.

Students:

Currently Dr. Lang advises one Ph.D student (Pablo Hidalgo) as well as several undergraduate researchers in her lab. She is always looking for promising graduate students and if you are interested in attending the University of Alabama and performing research with her then please email her at alang@eng.ua.edu

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As promised here is the new KF test airframe. It is all blue foam with the occasional cf stiffener. It has a much more powerful motor than the old Slow Stick airframe, and provides room for lots of test hardware in an aerodynamic shell. The wing is the same one from the Slow Stick airframe with a little paint added. Now if it only flies well.

The over wing hatch was designed so it could be modified to accommodate different KF wing types. When it was all just blue foam that idea seemed a good one. Now that I have it all painted, the idea of cutting up the hatch doesn't seem like such a great one. Oh well...

The wing attaches with pins in the front and screws in the back. The pitot and static tubes are visible in the second photo just inboard of the red stripes.

The third photo shows the plane with the front and over wing hatches removed. The blue tubes will attach the pitot and static tubes to the ports on the Eagle Tree once the initial flight testing is done. Just aft of the step is a hole that provides access through the wing to the lower hatch. The last photo shows this hatch. It was made as large as possible so the battery could be moved around for CG testing. The Eagle Tree transmitter will go in here as well.

Let me know what you think.

Roger

This will be a very exciting test bed. It looks to me like you've made the steps concave. This would conform to the shape of the vortex as Bruce Stenulson pointed out in one of his posts. Let us know how it all goes. Thanks for doing such a terrific build.

– Dick

Dick, these steps are flat, but it would be interesting to sand them to a concave shape and compare the performance. Yet another test to add to the list.

Roger

Now I know you guys love the science of this , but just speaking from the Simple side of the Force, The KF was originally considered (by me) to improve the performance of a Flat Plate Foamie airfoil ... And although I've never built the more complex KF shapes , the addition of the KF plate to a simple flat wing , vastly improves the performance , and strength of the wing .. and I now make the KF a Primary consideration in All new builds/designs... So even the simplest addition of the KF works very well !!! I know you guys are going for Max performance, on your more exotic builds....but I'm just extremely Happy with the much improved flying on my ultra simple flat Foamies !!! Thanks again Dick !!!!

Originally Posted by gpw

Now I know you guys love the science of this , but just speaking from the Simple side of the Force, The KF was originally considered (by me) to improve the performance of a Flat Plate Foamie airfoil ... And although I've never built the more complex KF shapes , the addition of the KF plate to a simple flat wing , vastly improves the performance , and strength of the wing .. and I now make the KF a Primary consideration in All new builds/designs... So even the simplest addition of the KF works very well !!! I know you guys are going for Max performance, on your more exotic builds....but I'm just extremely Happy with the much improved flying on my ultra simple flat Foamies !!! Thanks again Dick !!!!

Thank you, GPW.
I am thrilled that the KF airfoil concept offers such a wide variety of experimentation for all different kinds of configurations. And, that it is inexpensive to build and also saves a great deal of time in the build.
I can see that a lot of the RC people enjoy building as much as flying. The fun is in seeing what is in your imagination and putting it into reality. For me, the big kick is always in the process. Once it is finished, your mind then turns to "What else can I do with this idea?"

"What else can I do with this idea?"... My idea was , it worked so Well , just keep using it !!!! Never tried to "understand" or improve it , just accepted that it worked SUPER !!! But then i'm not a scientist , just a humble builder of small flying thingies'... Like most of us , I guess !!

There have been a lot of experiments with "sharkskin surfaces" and full size aircraft have flown with large areas of wing in similar materail, advantage small if any, and almost impossible to keep the surfaces clean ; when dirty with bug remains and dust etc, considerable degradation of performance.
Surface roughness can be critical, there have been several fatal crashes of aircraft taking off with frost on wings.
Lastly, dolphins swim rings round sharks ; with smooth skins.

My friend, Bob Tilden, sent me this video which shows an inflatable wing which has a series of indented ribs running the length of the wing. The wing is then placed in a smoke tunnel and compared against a conventional airfoil. You can clearly see how the ribbed configuration prevents separation of airflow whereas the conventional wing is unable to hold the airflow closer to the surface of the airfoil. Quite possibly, a similar thing is happening with the KF steps.

Inflatable wing aerodynamics (2 min 36 sec)

Originally Posted by Dickeroo

My friend, Bob Tilden, sent me this video which shows an inflatable wing which has a series of indented ribs running the length of the wing. The wing is then placed in a smoke tunnel and compared against a conventional airfoil. You can clearly see how the ribbed configuration prevents separation of airflow whereas the conventional wing is unable to hold the airflow closer to the surface of the airfoil. Quite possibly, a similar thing is happening with the KF steps.

Interesting video. I noticed that the regular airfoil did much better at the 100k Re number, and that the inflatable airfoil broke up the flow along the top (the smoke is more blurred near the airfoil than with the regular airfoil), but holds much tighter to the airfoil. I don't know if the slightly more disturbed flow helps or hinders.

The 50k Re number test shows the inflatable airfoil to be much more efficient.

My KF test airplane has an 11" wing chord. Flying at about 25mph on a standard day (59 degrees F @ seal level) the Reynolds is number of a bit over 200K. My sailplanes cruising at 23 mph fly at an Re of 160,000.

The KF test airplane would have to fly above 30,000 feet to get the Re down to 100K and at 50,000 feet for an Re of 50K. This makes sense if you plan to fly on Mars where the air density is so low.

As an FYI, at the KF test planes speed and size the Laminar boundary layer is 0.118 in thick and the turbulent boundary layer is 0.2415 thick. I don't know it that is of any use to us, but I thought it was interesting.

Roger

Originally Posted by tolladay

The 50k Re number test shows the inflatable airfoil to be much more efficient.

Yes, rough surfaces do better than smooth surfaces at 5 digit Reynolds numbers because the flow on the smooth surface won't transition naturally so it needs to be tripped. Nothing new about that. The multiple humps of the air-mattress wing are a bit new but not all that much. They're probably getting laminar separation on the first hump with turbulent re-attachment on the second hump and enough boundary layer stress on each successive hump to prevent the boundary layer from settling back down to laminar. Yes, at really low Re the boundary layer can go back to laminar, that's why some really small free flight planes use multiple trips.

--Norm

Apologies!!! Not meaning to get too off topic ... The guys at the TT and Teardrop trailer forum are on again about drag reduction ...
The problem is the shape is just too FAT to let the air smoothly roll off ... not like a long thin airfoil ...
So , I was thinking , maybe applying the KF principle to these would be of some benefit ... and may even allow better handling in tow ... Works Great on planes eh !!!
Any thoughts/opinions ??