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Old Feb 19, 2012, 12:12 PM
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Tells me.

Be careful when you pull on that loose thread on your favorite white sweater, it might end up as a test instrument on somebodys wing....

Just having a little fun here, please someone, explain if we can learn anything by looking at this video.

Thanks,

Bill
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Old Feb 19, 2012, 02:19 PM
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Lol!
It just shows the basic flow of air around the step on the KF airfoil. The use of tufts to roughly give an image of airflow has been used for a while although it does have its limitations
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Old Feb 19, 2012, 06:30 PM
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Quote:
Originally Posted by A Useless Geek View Post
Cool. So, you haven't aimed your testing at some aspect of the KF that is specific to your target flying. That's okay, we've got lots of folks flying KFs who can maybe pick some testing target.

But I am a bit curious about the stall characteristics of the KFs being the same as the symmetricals. I presume you've flown both enough to know if there is a difference and have not detected one. Is there anything that stands out as differences between the wings? Are the planes really similar enough to allow A-B comparison? You know, built on the same pattern, to the same scale, yada yada?
I haven't done any testing as such, just observed the way they fly. Having spent much of my long life observing flying things of all descriptions I have good flight diagnostic skills.
On the KF stalling thing: If you look at the video JPF posted re KF v Drela wing you will see where he trims out the KF with a few clicks of up. When he goes too far the plane starts to do the familiar porpoising exactly as would be expected for most conventional airfoils.
Looking at the video recently posted here, when the plane is made to stall its behavior is not the least bit exceptional. Again it is about what you would expect from any old wing.
Mind you, all those tufts could have been killing the performance
I have some ideas for some tests and I will certainly post the results here if I get interesting results. Believe me, I am totally open minded on this. My only interest is in zeroing in on the truth.
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Last edited by Whiskers; Feb 19, 2012 at 06:41 PM.
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Old Feb 19, 2012, 06:47 PM
MK
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I apologize if this has been asked before. I tried searching this thread the best I could but couldn't find anything and I don't have time to read all 58 pages.

I'm building a Eurofighter Typhoon foamie and have added a KF4 airfoil. Does this do anything to the location of the CG? Will it move it forward, backward or keep it where it originally is?
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Old Feb 19, 2012, 10:50 PM
Jack
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Quote:
Originally Posted by MK View Post
I apologize if this has been asked before. I tried searching this thread the best I could but couldn't find anything and I don't have time to read all 58 pages.

I'm building a Eurofighter Typhoon foamie and have added a KF4 airfoil. Does this do anything to the location of the CG? Will it move it forward, backward or keep it where it originally is?
It will stay about the same. I say that because any CG calculators I have used were not intended specifically for KF wings but worked fine with them.

Jack
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Old Feb 20, 2012, 12:23 AM
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Quote:
Originally Posted by Whiskers View Post
Huh? Where are all these thicker winged subsonic planes?
Every where I look I see thin sections, and every new plane seems to go thinner than ever before.
You have to go back to designs of the 20s 30s to find really thick wing sections, and after that it's as thin as the structure (or other non aerodynamic requirements) will allow.
I guess it depends on your definition of "thick."

F-16 has a 4% thick airfoil.

Boeing 737 has 15%, 12% and 10% thick airfoils.

P-51 has 15% and 12% thick.

IMO the F-16 has a "thin section." I don't think you're going to find a General Aviation aircraft or a transport aircraft with a wing that thin.

I will note that these figures are not authorative but instead reflect about 5 or 10 minutes of searching on google; I would say that they seem reasonable to me based on my limited education.
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Old Feb 20, 2012, 12:52 AM
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What I'm saying is that now they don't design 'em like this.
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Old Feb 20, 2012, 08:29 AM
gpw
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The Tuft video was Super !!! It really showed how the KF kept the flow attached ... Curious the lower tufts on the first step hardly moved at all ...
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Old Feb 20, 2012, 08:31 AM
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kf experiments

Quote:
Originally Posted by Dickeroo
Dave...

I have seen a number of UAV videos where a KF airfoil is used. Jaron's design comes to mind. They seem to perform quite well. Can you tell me what you witnessed regarding your friend's type of UAV using a KF airfoil?
In what way did it perform so poorly. The glide factor is important and the KF needs power to perform well. It is extremely effective in high winds and probably superior to most other wings. I would just like to know the story behind what was tested. I'd really appreciate it.
Have you heard of the VortexCell2050 Project? They are using the same basic concept of trapping a vortex.
Any info would be greatly appreciated.
Dick Kline - The K in KF

Quote:
Originally Posted by dave1993
dick,

pleased to hear from you again. in regards to our tests you pretty much hit the nail on the head when mentioning the need for power. that is exactly what a solar uav (or any solar plane) lacks. you would not believe how much effort goes into squeezing out that last milliwatt. weight of course being the key factor.

as far as measurement technique, in addition to timing flights to lvc, we used a chainlink fence as a backdrop when videoing power off landings. it gives a very accurate indication of glide ratio and speed. the conclusion was there is slight penalty in both domains when comparing kf with raskins 4-40 undercamber. the plain uc airfoil high lift but so draggy compared to drela etc profiles id hesitate to say the kf "performed poorly" by comparison. its just the differenece was measurable and enough to eliminate it from consideration for that application.

the reason we chose these kind of wings in the first place was cost and ease of construction and i certainly cant criticize kf in those departments. i fly a 4' foamie kf often for my own pleasure and enjoy it significantly more than the simple uc. Fun with a capitol F. thanks again for helping develop and promote the design.

ps i actually own the very same plane we did tests on and can put up some pics if you like. it had removeable steps and was quite interesting in its own right. i built a couple copies myself since then so have several now (minus the pv stack of course).



Quote:
Originally Posted by dave1993
hi dick,

in a day or two ill move this to a post instead of email if you dont mind due to inability to attach photos here and aslo others may benefit. heres a link to the plane i mentioned (sans kf) from a discussion in the radio forum showing autopilot and airframe.

http://www.rcgroups.com/forums/showt...2#post19933907

unfortunately i dont have photos of the removeable steps atm because they are in a different location i will put some up some after i go home this weekend.

i dont really know about kfm4 because our work was done before the various code names were developed in that kf megethread. after some preliminary testing we settled down to steps on top and had variable attachments from 20% to 70% iirc.

thanks for showing interest in our project.

Quote:
Originally Posted by Dickeroo
Dave...

I'd really like to see the planes that you've built with the KF airfoil. Have you ever tried a KFm4?

Dick

here are photos of one of our test wings as promised. they show the removeable kf steps that are held on with double side tape. even the #0 step at the leading edge can be taken off. as you can see step position can be changed in ~7% increments.

not shown are recesses in the underside which hold coins to insure wings with and w/o steps weigh the same. also not shown is a mylar strip which extends from the steps to the trailing edge for testing the thicker foil but no step. as far as we knew as many variables as possible were accounted for during the experimentss.

i may be able to provide samples of the data from our kf lift/drag tests using the chainlink fence. failing that i may be able to replicate those tests when weather improves.

for curiousity sake ive included pics of a wing equipped with vortex generators which we found had similar slow flight and stall characteristics to the kf. main differences were:

1. lighter than kf

2. kf was easier to build

3. placement and spacing far more critical than a step

4. lacks kfs wing stiffening ability

we also tried a simple thin triangular strip across the top. like the vg it duplicated flight characteristics of a full kf step and was lighter but lacked the wing stiffening feature too. imo all-in-all the real kf beats out these alternatives for rc foamie use because of this.
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Old Feb 20, 2012, 08:50 AM
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My conjecture on the KF lift factor.

As far as I know, no one has speculated as to how the KFm2 generates lift.
Most airfoils produce lift from the leading edge back to 30%-50% on the normal airfoil. Very little lift is produced from 50% back to the trailing edge.

I believe lift on the KFm2 is due to the rotating air behind the step that lowers the air pressure thus making most of the length of the airfoil, from leading edge to trailing edge, generate lift. And, thus Bernouli's principle is still in play.

Perhaps this also explains why the CG needs to be a little further back on the KFm2 airfoil.

Mind you, I'm just guessing if that's allowed.

– Dick
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Old Feb 20, 2012, 09:42 AM
gpw
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Just the Facts Please !!! hahahahahahahahahaha
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Old Feb 20, 2012, 10:14 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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dave1993 wrote:

"..here are photos of one of our test wings..."

Is the step configuration seen the photos as it was flown and tested? As far as the number of steps and their approximate locations and spacing?

And is the wing also undercambered?

Jack
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Old Feb 20, 2012, 10:56 AM
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Quote:
Originally Posted by jackerbes View Post
Is the step configuration seen the photos as it was flown and tested? As far as the number of steps and their approximate locations and spacing?

And is the wing also undercambered?
that was one of many wings involved in the tests. some were fixed step and some had polyhedral but that is the one ive been using myself at a local soccer field. also used recently to start playing with autopilot rtl similar to the one in that radio forum thread. thanks to the kf it floats like magic.

original step numbers in the reply to dicks pm were not accurate. measurements this weekend show step #0 is 30% and the others at 7% intervals ending at 65%.

wing is raskins 4-40 uc which has yet to meet any competition in terms of lift. even exotic glider foils failed in this respect.
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Old Feb 20, 2012, 11:33 AM
Jack
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Thanks for the details, Dave.

I was not sure from the photos if it had much curve or not. I'm not much of a curved airfoil guy, at least not curved on the bottom anyway. The much-beloved UC wing for the BB 33 is considered by most to be of the 4-40 type or family if I remember right And it did produce a superb flying plane for Tony65x55's original intent of an indoor flyer.

But it was pretty obvious that it wouldn't do as well in my then inexperienced and heavy hands and outdoors in Maine so I moved on to the KFm2 and KFm3 for my first BB wings.

If I ever find an indoor flying venue I'll probably reconsider it all...

Jack
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Old Feb 20, 2012, 11:46 AM
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ive never flown indoor myself (unless you consider those wonderful toy helis) but do agree uc is not an optimum wing for outdoors. specially in wind or performance gliding. its only advantage is lift which becomes the single minded goal for low cost solar flight. this requires huge sacrifice in speed and penetration. but there is no choice if you need to stay in the air with cheap heavy solar cells.

initially claims of increased lift was what motivated us to try kf. quite disappointed to find no benefit. there seems to be an increase because higher aoa w/o stall does provide more lift. but only at the expense of disproportionate drag which did not work in our favor due to the already abysmal uc.
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