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Old Nov 16, 2011, 08:20 PM
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Here is the Finaish Whiterspoon report on stepped airfoils

The work done by Finaish and Whiterspoon was at 500,000 Reynolds numbers. Here are their results regarding stepped airfoils... I would be interested and appreciative in any observations or comments regarding this material. I value all opinions.

Dick
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Old Nov 16, 2011, 09:04 PM
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Hmmm...
The following quote from the paper would not seem to be what KF fans want to hear:
The drag coefficient values for all cases with steps were higher than for the airfoil without steps. As step depth increased, drag increased with it. No drag reduction was achieved using any combination of steps.
I think I also detected some woolly thinking regarding L/D, but I'll leave that to others for now.
Also to be noted is that the test sections bear little resemblance to those 'we' use.
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Old Nov 16, 2011, 10:26 PM
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It looks like i've kicked up the traffic a bit here... one of the first things to do is figure out what types of airfoils to test. Everyone has got their own way of slapping these together, so i'll have to come up with a systematic way of defining them (step locations, thickness, LE radius...).
Concerning many of the previous research done on stepped airfoils: it is remarkable that nobody saw potential in the extremely high stall angles which can be achieved...
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Old Nov 16, 2011, 11:06 PM
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A lot of those high stall angles are not applicable in our world.
I see those results of high angle of attack at high airspeed and think of the model ripping round in a tight loop and getting itself back to a more normal AoT in a flash.
Perhaps a pylon racer may 'push' its wing that way but most models with a normal wing loading and moderate speed would not come anywhere near it.
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Old Nov 17, 2011, 12:57 AM
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Quote:
Originally Posted by thefamoushat View Post
it is remarkable that nobody saw potential in the extremely high stall angles which can be achieved...
My assumption is that the drag associated with those very high AoA is just so high as the make it impractical for any real world purpose. Also despite some of the claims made it's a simple fact that in the real world we know KF airfoils (at least all the ones tested here and in other threads) do actually stall at fairly normal angles of attack (though the stall may be gentle). There are many videos that show a KF wing stall, including the tufted wing experiments shown earlier in this thread.

For instance check out Roger's tufting tests: http://www.rcgroups.com/forums/showp...&postcount=495
At about two minutes into the video Roger starts doing some stall tests. The wing stalls in a totally normal way and at a normal AoA, much as you would expect of a 'standard' airfoil. Not trying to knock KF airfoils but simply pointing out the plain fact that all testing that has been done by members here does not support the claim that KF airfoils remain un-stalled up to 50Deg AoA.

Steve
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Old Nov 17, 2011, 02:19 AM
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This has been discussed before in here somewhere, but could someone mention the fact that air doesn't scale? Wind tunnel tests at real speeds with real wings will always bear different results to tests performed on scale models at scale speeds, will they not?
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Old Nov 17, 2011, 05:54 AM
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Originally Posted by A Useless Geek View Post
This has been discussed before in here somewhere, but could someone mention the fact that air doesn't scale? Wind tunnel tests at real speeds with real wings will always bear different results to tests performed on scale models at scale speeds, will they not?
It has been mentioned over and over ... every time someone invokes "Reynolds Number" (or "Re") that is what is meant.

In other words, the way to compare "apples to apples" is to make sure we include the Re of the test as part of the discussion ... and for a comparison to really be "apples to apples" the Re must be the same.

FWIW some wind tunnels are setup to manipulate the Re so that a small model can be tested that has the aerodynamic characteristics of the full-scal aircraft.
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Old Nov 17, 2011, 06:12 AM
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Originally Posted by Whiskers View Post
I totally agree with the claim, "... airfolis are not really relevant at the reynolds numbers we as hobbyists are concerned with."

At the high end of contest flying, where 1000% effort is deemed worth 1% improvement, airfoil selection and construction may well come into play.
i should probably phrase it as "LESS relevant" instead. i.e. toilet paper rolls and pool noodles make crummy wings.

generally anything that promotes separation improves stall. as mentioned my friend had similar results with a row of tiny vortex generators (angled fins) across the top. the more the merrier. i got kf type stall resistance with a simple strip of triangle cross-section placed just in front of Cl. both resulted in considerably more drag than kf and also lacked its wing strength benefit.
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Old Nov 17, 2011, 06:51 AM
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Originally Posted by dave1993 View Post
generally anything that promotes separation improves stall. as mentioned my friend had similar results with a row of tiny vortex generators (angled fins) across the top. the more the merrier. i got kf type stall resistance with a simple strip of triangle cross-section placed just in front of Cl. both resulted in considerably more drag than kf and also lacked its wing strength benefit.
Actually anything that 'prevents' separation improves (as in helps delay) stall. Stall is by definition what happens when you have flow separation on the top surface of the wing, so anything that promotes separation also promotes stall.

Vortex generators and turbulators are both devices specifically designed to prevent separation, not promote it. We speculate that KF steps might have a similar effect at low Re numbers but that's as yet unproven.
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Old Nov 17, 2011, 07:02 AM
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Quote: i should probably phrase it as "LESS relevant" instead. i.e. toilet paper rolls and pool noodles make crummy wings.
Well, yes.
I took it to mean the thing had to approximate a wing section, but it is a fact that a model size wing is far more tolerant of aerodynamic outrages than is the case with a full-size wing.

Geek, we do not have to worry about scale effect. It seems that this proposed testing can be done on full-sized model wing sections at the low airspeed models typically use.
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Old Nov 17, 2011, 07:11 AM
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Geek, we do not have to worry about scale effect. It seems that this proposed testing can be done on full-sized model wing sections at the low airspeed models typically use.
Not sure what you mean by this.

If the purpose of the test is to examine properties of KFm sections/wings at "model airplane Reynolds Number(s)," then the person conducting the test must ensure the Reynolds Numbers match (i.e. the Reynolds Number of test section/wing matches "model airplane Reynolds Number(s)").
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Old Nov 17, 2011, 07:28 AM
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Originally Posted by JetPlaneFlyer View Post
Actually anything that 'prevents' separation improves (as in helps delay) stall. Stall is by definition what happens when you have flow separation on the top surface of the wing, so anything that promotes separation also promotes stall.
while its true that stall (or at least huge drag) occurs on full separation its also true that separation on 50% or so of the upper surface is maximum lift.

imo the big advantage of kf is not so much that it delays stall (which can be very sudden and violent on laminar flow wings) but rather causes a more gradual onset and easier recovery. the ultralight guys refer to it as "mushing". i think separation occurs with kf at very low speeds which does improve lift and makes the plane feel so much more "friendly".
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Old Nov 17, 2011, 08:03 AM
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Results of the KF on the tip of a prop.

When the KF was applied to the tip of a prop it increased the thrust by almost 9%.
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Old Nov 17, 2011, 09:01 AM
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Originally Posted by Whiskers View Post
Geek, we do not have to worry about scale effect. It seems that this proposed testing can be done on full-sized model wing sections at the low airspeed models typically use.
It's more than airspeed.

Reynolds number is dependent on both airspeed and airfoil chord, and *EVERYONE* has to worry about the scale effect.

Lots and lots of work has been done comparing the differences in wing behavior at high and low reynolds numbers by Selig, Drela, and others, and even the difference between typical model types are important if you're talking aboyt how a wing performs.

Flat plates work quite well if the reynolds numbers are low enough.

That said-- most sport/aerobatic models are so over powered that they fly under the F-4 school of thought, so wing performance isn't terribly important to them. Other factors such as stall onset behavior and ease of construction matter much more.
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Old Nov 17, 2011, 09:08 AM
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Originally Posted by JetPlaneFlyer View Post
it's a simple fact that in the real world we know KF airfoils (at least all the ones tested here and in other threads) do actually stall at fairly normal angles of attack (though the stall may be gentle). There are many videos that show a KF wing stall, including the tufted wing experiments shown earlier in this thread.
Interesting- I hadn't seen that.

Mathematically, though, isn't stall when Cl drops to zero, (which generally occurs when flow separation occurs?)

The neat thing about Kf is that the Cl doesn't do that, but rather ralls off gradually.
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Last edited by rdeis; Nov 17, 2011 at 12:48 PM. Reason: typo
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