Captured Vortex Airfoil

[mlbco]

I've been experimenting with a strange low RE airfoil on my rubber powered RC model that has a cavity on the upper surface to stabilize the laminar bubble and promote effective boundary layer transition. Has anyone heard of others trying an airfoil like this? I saw a drawing of something similar in Zaic's 57-58 yearbook by Henry Jex. Jex apparently wrote about this in Soaring magazine in the 50's but I can't find the original article. Does anyone have a copy? I sort of re-invented the captured vortex airfoil in 1984 when I built some FF models with this section and they had particularly good performance.

My experimental model uses a NACA 6409 modified for the vortex cavity and a curved re-attachment surface. I designed the cavity by eye and there are no calculations backing up any of this. Flight tests show a low sink rate but I need to compare it against an unmodified section with simple boundary layer trips. The modified section may be no better than one with a good boundary layer trip.

I'm also planning to put a recording altimeter on the model to get an estimate of sink rate in still air.

Steve Morris

[nmasters]

The Lanier Vacuplane was the first example that I know of, I think he started in the 1930s. In the early '70 Richard Kline and Floyd Fogleman managed to patent a step at 50% chord. You can find all the relevant patents on the rexresearch site.

--Norm

[JetPlaneFlyer]

To test the effectiveness or otherwise of the cavity why not simply tape over the cavity with masking tape or similar and see how the performance compares?

PS, I dont think that the Kline Fogleman airfoil really bears comparison because their airfoil, as covered by their patent, had the step on the bottom.

Steve

[nmasters]

PS, I dont think that the Kline Fogleman airfoil really bears comparison because their airfoil, as covered by their patent, had the step on the bottom.

That was just because they had some weird misguided idea that it would perform better inverted above Mach one. Many of the models from back in the '70s had the step on top. And the wind tunnel tests I mention on my web page were tested both ways with different configurations showing better results one way or the other. Anyway the vacuplane presseds everything else

--Norm

[mlbco]

Norm,

The Vacuplane is interesting, although I can't find pictures that show the CV airfoil, just many variations using slots used to augment lift on low AR wings:

http://www.rexresearch.com/lanier2/lanier2.htm

I'm interested in any experimental results using notched airfoils, particularly 2-D low RE data that might apply to model airplanes. The Vacuplane concept looks a bit more like an idea in search of a theory and isn't in the same flow regime as my models.

Thanks,

Steve

[nmasters]

Yeah the Laneir patents look like BS to me but but there were some planes built and flown so even though he apparently didn't have any understanding of what was actually going on he produced enough results to sell it to some investors, for a couple of decades anyway.

Most of the papers I have are based on subjective observations by a group of guys who are keeping a very long thread going in the foamy scratch built forum.

One piece of hard science I can offer is a paper by Fathi Finaish and Steven Witherspoon that was published "Journal of Aerospace Engineering" Jan '98

If you want it PM me your e-mail and I send it

--Norm

[Brandano]

Those wing profiles look just like an huge slotted flap without the increased camber

[Cory]

If you opened those slots through to the bottom you might get some great S.T.O.L. performance like a Fieseler Storch or a Zenith CH 701!

[nmasters]

But they weren't slots. They were boxes, closed at the bottom. I've never seen any real data but apparently he had help from the university of Florida so I assume there was at least some kind of competent oversight

[Cory]

I was talking about mlbco's plane in post #1.

[kcaldwel]

Steve,

Have you looked at the Kasper wing stuff? I remember the little bit of wind tunnel testing that was in Soaring a long time ago.

http://www.twitt.org/KASPBIBLIO.html

First page of the wind tunnel test, I don't have access to the rest. Max Cl supposedly 3.15!

http://pdf.aiaa.org/preview/1977/PV1977_310.pdf

Kevin

[nmasters]

Max Cl supposedly 3.15!

I believe that that was deduced from the speed during a dynamic stall of the glider, not wind tunnel tests. Nothing unusual really just a wing with a helicopter rotor section that was designed to stall every half revolution. The high CL only lasts a few seconds (11 from Kasper's own description)

[kcaldwel]

That isn't the Kasper wing that was in Soaring, or that supposedly used to do near vertical 100 fpm descents to landing. He had an ultralight that was in all the hang gliding mags of the day. It had a movable spoiler like thing on the top surface that was supposed to generate a captured vortex, and generate really high Cl.

Maybe that paper I referenced isn't the tunnel testing that was in Soaring, thought it was...

This is the Kasper wing I was thinking of, although my foggy memory has the ultralight wing looking a bit different than this:

http://www.google.com/patents?id=IM47AAAAEBAJ&pg=PA2&dq=3,831,885&source=gbs_selected_pages&cad=0_1

http://www.google.com/patents?id=IM47AAAAEBAJ&pg=PA2-IA1&dq=3,831,885&source=gbs_selected_pages&cad=0_1
Kevin

[mlbco]

Kevin,

I've seen Kasper's work and I read his book sometime ago on flying wing design theory. I have my doubts about his claims for performance and I don't fully agree with his philosophy of flying wing design, but he did some very interesting and crazy flight demos including tumbling the wing on-demand. His work, like many others, seems to focus on the properties of stabilized vortical flow over wings in order to augment lift. I'm hoping to find a more effective way of transitioning the boundary layer of a low RE airfoil and not have large-scale separation over the wing, just a stable vortex in the cavity. It seems there isn't much experimental work in this area so far.

Thanks,

Steve

[kcaldwel]

Steve,

Sounds interesting! Keep us posted on your results if it isn't proprietary.

I thought you'd be well aware of the Kasper wing stuff. He always did seem to make some pretty outrageous claims, but I had also heard his flight demos were fun to watch.

I stumbled across your slewed wing stuff a while ago. Very interesting!

Kevin

Edit: It just occurred to me where else I had seen captured vortex on low Re wings - Dragonflies! You've probably seen this stuff too Steve, but just in case:

http://www.iop.org/EJ/article/1748-3190/3/2/026004/bb8_2_026004.pdf?request-id=869b5d15-0177-4f2b-8837-e5ea73d1617d

http://scidok.sulb.uni-saarland.de/volltexte/2008/1636/pdf/Modelling_and_Manufacturing_of_a_Dragonfly_Wing_as _Basis_for_Bionic_Research.pdf

[nmasters]

I'm hoping to find a more effective way of transitioning the boundary layer of a low RE airfoil and not have large-scale separation over the wing, just a stable vortex in the cavity. It seems there isn't much experimental work in this area so far.


In the '60s someone named "Ringleb" did some work on how snow cornices form. Fabio Goldschmeid later used the results of that research for the slot geometry on his suction BLC low drag body. In the drawings I've seen it's labeled a Ringleb cusp. You might find something with those names. I have a beautiful picture of a water tunnel test of a vortex flap on a flat plate at Re=1500 but I don't want to post it on a public bulletin board (I've violated enough copyrights today)

--Norm

[mlbco]

Kevin,

Those papers are very interesting, Thanks! I haven't been following the academic aero research lately and hadn't seen these.

The first paper looks at gliding flight but I think the dragonfly airfoil also benefits during flapping motion (unsteady flow) in ways not considered in the paper. The vortices "filling in" the airfoil shape is very similar to my model, although at much lower Re.

Thanks,

Steve

[nmasters]

"An Experimental Analysis of Vortex Trapping Techniques"
Journal of Fluids Engineering SEPTEMBER 1999, Vol. 121 / 559

[kcaldwel]

This might be of some interest. I found it when following up on Norm's snow cornice reference:

http://staff.polito.it/luca.zannetti/Int_j_fld_res_29.PDF

There are lots of papers on dragonfly flapping flight, and it does look like the trapped vortexes are a key part of that. That paper on gliding dragonfly flight is interesting from the model perspective. Could have applications for the tiny RC planes, and maybe even the indoor 3D stuff.

Kevin

[nmasters]

Yep, that's it--

A sharp slightly upswept lip with an elliptical hollow of major radius roughly equal to the step hight if the step were square and a bull nose transition from the bottom of the trough onto the aft surface (re attachment bump). I tried to get the KF foamy builders to notice this geometry when they were talking about possible ways to improve the efficiency of their stepped wings but apparently it's too hard

--Norm

[nmasters]

Hmm...

I see that that radius is more like 2.2 times the step hight.

[BMatthews]

It sure would be interesting to see this tested on Michel Selig's wind tunnel for low speed.

Just a note to keep in mind that free flighters have been turbulating for many decades. The amount of turbulence needed to "glue" the airflow to the surface is not really that much and trip strips of raised thread or thick tape or actual invigorators or even turbulator strings of elastic thread so they "thrum" in the airflow and are mounted out in front of the leading edge have all been used for this same purpose without the need for big V's in the surface.

The goal, after all, is to promote the air's sticking to the surface of the wing when flown near to the stall point.

To properly test your wing the key would be to make a normal version of the same wing and then test glide it inside a building and see which version either goes further (better L/D) or takes longer to settle to landing (slower sink speed). Otherwise it's all just personal impressions.

[mlbco]

Here are some mostly crazy thoughts on possibly beneficial effects from this type of section when compared to the typical turbulator strip:

1) Reduced skin friction drag due the "missing skin" and the viscous drag acting in the thrust direction within the channel. Maybe more channels are better than one? How much skin can one get rid of before performance suffers?

2) "Hyper-energized" boundary layer. The air recirculates in the vortex channel and could possibly develop a turbulent boundary layer similar to a much higher Re flow that then favorably influences the pressure recovery region of the airfoil. Is it possible that air circulating around in a channel can make an airfoil seem effectively longer?

3) Passively adaptive pressure distribution for the outer flow. The cavity can be shaped to produce a passively adaptive pressure distribution that favors overall performance as angle of attack changes.

I'm not for a minute suggesting that any of this is achievable, but it's food for thought.

I suspect that my model doesn't have a highly stable vortex in the cavity and probably has a hysteresis behavior at stall. The cavity dimensions probably need to be tuned for optimal performance. I'll know more after I install the altitude recorder and continue the flight tests.

Steve

[nmasters]

Well Kline and Fogleman, and there advisers, would have agreed with 1 and 2. I don't know if they got far enough into the science of the thing to think about 3. Richard told me that all the other folks involved from those early days are dead. Some of them did have appropriate academic credentials and there was some wind tunnel testing but probably not much more than needed to give the patent application some validity. It's interesting that a shear layer in air is much more stable than a boundary layer on a surface. What I mean is that the air stream won't separate from the 'surface' of the vortex. I visualize it like a roller bearing with a flexible surface. If the surface moving over the bearing is uneven the flexible bearing surface can flex with the bumps and waves to maintain smooth contact. Then from that I jumped to a Flettner rotor. Did you know that in a wind tunnel test a Flettner rotor developed a CL of 9 (not 0.9, 9). The test was stopped at that level because of concerns that the tunnel equipment might be damaged. That thing was turning a really large volume of air through a very large angle. Which lead me to the idea that a step, or as SABB called it a “vortex ditch” could make a very powerful addition to a flap, either leading edge or trailing edge. Then I gave up on it because I don't build anymore and most of the active modelers who are interested in stepped airfoils seem to think anything more sophisticated than a folded piece of styrofoam is too hard

--Norm

[Brandano]

I doubt you'd get reduced skin friction, if anything you'll get increased skin friction. Keeping the vortex energized will subtract energy to the relative airflow, and this will be wasted as surface friction within the vortex channel. I am not completely sure about what the aim of the exercise is, but while it might work as a high lift device, I doubt it will help reducing drag. It might raise CL, but if it works it will probably lower LD as tradeoff, same effect as deploying flaps. On the plus side it can be used in addition to flaps as Norm pointed out.

[kcaldwel]

Brandano,

You should read the dragonfly wing paper I referenced above - negative shear over the vortex regions (in effect negative drag), and as good or better L/D, with a higher max Cl, albeit at very low Re.

Kevin

[nmasters]

Most of the drag from large flap deflections is pressure drag from separation, not friction.

[BMatthews]

Brandano,

You should read the dragonfly wing paper I referenced above - negative shear over the vortex regions (in effect negative drag), and as good or better L/D, with a higher max Cl, albeit at very low Re.

Kevin

For the super small wings of a dragonfly the reynolds numbers is what makes it all work. Whether or not it's applicable at the reynolds numbers of your FF model is the question.


This is where it would be interesting if we, as the model building community, got together and all agreed on a design that would be a flying wind tunnel for testing stuff such as this along with other ideas. The test model would be a glider so that power issues are not part of the question and we would ballast the model to the same weight and launch in a manner that pushes the model into a stable glide with no excess. Duration and distance would be measured over something like 20 to 40 flights and from that along with temperature and local altitude we should be able to draw some conclusions on how well various ideas such as this actually work. The other option would be to produce test sections and have Michel Selig test them in the next series of airfoils for lift-drag performance. Frankly I think the standard test glider would be just fine. We'd be looking for clear differences after all.

[mlbco]

Here is a paper that studies an airfoil with modifications similar to mine, but assumes active flow control and higher Re flow. Lots of math and no experimental results or performance predictions.


Steve

[nmasters]

Lots of math and no experimental results or performance predictions.

Yeah but that's your opportunity to record the hard data :cool:

[Bobber Bob]

Hi Folks...

A little "trick" for a "flying windtunnel" that we used to do, was make a "stock" wing panel,and then for the other panel, build a panel with the configuration that we wanted to test.

Now, we all knew that this was not a numbers test (data), but a "better then" test.

You put the plane in the test attitude you were interested in, and observered the reasults, and the needed trim changes and/or control inputs that were needed to correct for the differences between the wing panel.

With observation of the trim needed or control inputs, you could learn a lot, from a comparaitive point of view, we did.

Bob Reynolds
. "ComeUpHere"

[nmasters]

Here's a study of a step with an artificial sink at one end of the span. How strong a sink the tip of a real wing is I have no idea.

[mlbco]

I did some flight tests today with my new "How High" recording altimeter. I timed the flight from the maximum altitude until it landed while making big circles at minimum sink speed. The How High instrument records the peak altitude in feet and from these numbers I calculate the average sink rate. I first flew the wing with the vortex channel covered with tissue paper (NACA 6409 section) and then I removed the tissue and flew it with the vortex channel exposed. All flights were flown in the early morning calm on the same day. The standard 6409 section gave a sink rate of 105-110 fpm where and the captured vortex section averaged about 5 fpm less (100-105 fpm). I think my experimental error due to varying conditions is on the order of the difference measured between these two versions, so I can't say conclusively if there is any difference. The captured vortex airfoil seems to have a higher CLmax because the glide looked slower and the model was less prone to stalling during the climb.

One thing I hadn't thought much about before these tests is that the duration is not very sensitive to airfoil power factor (= CL^1.5/CD) and that any difference in airfoil performance would only produce small changes in the duration. My model weighs 1 lb and has a 58" span, and 348 sq-in wing area. Using drag data for the 6409 at Re=47K and parasite drag estimates for the rest of the model, I predict a min sink rate of 94fpm for the 6409 section (power factor=34). If the power factor is increased to 53 (56% greater) the sink rate is reduced by only 13%. For a 200 ft altitude the duration would change from 128 to 146 seconds (18 seconds increase). That's certainly a measurable amount of change, but it required a huge improvement in airfoil power factor to achieve it. I doubt I can measure small changes in power factor using glide tests unless I do an awful lot of tests in still air.

I still think this concept warrants further study. It's amazing that such a big notch doesn't seem to hurt performance and may slightly help it, at least for the 6409 section. A thorough wind tunnel test would probably reveal what, if anything, is really going on.

Steve

[nmasters]

This 1986 patent has some some polars

[Jim Petro]

About 1980, my son ran a 3 year science fair project on a terraced airfoil. Basically it is a series of spanwise cutaways on the top of the wing with the first at the high point and maybe 3 more evenly spaced rearward. The project started like Bob's with an R/C glider that had an Ace foam wing; one side was stock and the other side was terraced. I video'd the test flights on a long sloped hill as different nose up attitudes were applied. Real world flight responce matched the simple lift/drag charts from our simple shop wind tunnel.
The project originated from hi-start glider flyer reports that terraced airfoils gave higher launch altitudes. A terrace is cut vertically down maybe 1/8" and a horizontal cut rearward to free the triangular section.
Shop wind tunnel graphs showed no increase in lift compared to the stock airfoil. What caught our attention was that the stall angle almost doubled. With this and the videos, we concluded that the relative view watching the hi-start launch shows the glider climb without stalling at a much higher angle and perhaps getting more altitude.
A a series of pictures of a bird's wing feathers gave us the clue for what was happening; As the wing approaches the stall angle, air begins to flow from the trailing edge along the top to the front. A normal smooth wing has a very rapid catastropic loss of lift as the reverse airflow races up the wing and stops the lifting flow. Bird feathers curl up and block the reverse flow. The terraces each act as a series of dams to also impede the reverse airflow.
The terraced wing gives good lift while allowing a high alpha take-off or descent. It still has to have same airspeed, so don't think it can just crawl along.