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Old Oct 10, 2012, 09:51 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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It is not uncommon for "scientists" to have an agenda, to want to find facts that support a belief, and things like that. And then when the results of a supposedly scientific study are summarized for presentation to laymen, they can also manipulate the wording of their conclusions to support a desired conclusion.

As a layman with an interest focused on building and flying, I find the report interesting but see little or nothing in the images that leads me to think that it has much in common with KF wings as I know them.

Jack
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Old Oct 10, 2012, 09:55 AM
Smells like SCIENCE!!!
ibillwilson's Avatar
Canada, ON, Toronto
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Quote:
Originally Posted by JetPlaneFlyer View Post
FWIW...My personal take on it is that the flat upper surface of the KFm1 operates part stalled almost all of the time. Hence no sudden onset of stall and no sudden drop in lift. Stall becomes a more gradual process. The down side is that operating partly stalled most of the time drastically increases drag.

Others may disagree, and that's perfectly fine, however I've yet to see any sensible explanation as to how a step on the bottom of an airfoil could prevent flow separation on the top of the airfoil... After all, the air flowing over the top of the wing has no 'idea' what the bottom of the wing looks like
If the trailing edge vortex is affected at all by the existence of the bottom step, then separation and reattachment on the top of the airfoil could be affected. Also, I don't get why you say why the top surface is always partially stalled... Would you say that about a flat plate?
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Old Oct 10, 2012, 03:19 PM
Grumpy old git.. Who me?
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Quote:
Originally Posted by ibillwilson View Post
If the trailing edge vortex is affected at all by the existence of the bottom step, then separation and reattachment on the top of the airfoil could be affected.
The trailing edge doesn't make a vortex* In streamline flow the air from the top of the wing and bottom come together smoothly in a laminar flow 'sheet'. You can see this in any wind tunnel airfoil visualisation. In turbulent or stalled flow there is chaotic turbulence, but still no vortex.

* exception is when an airfoil first starts to move when 'starting vortex' forms, but this is left behind on the runway.
Quote:
Originally Posted by ibillwilson View Post
Also, I don't get why you say why the top surface is always partially stalled... Would you say that about a flat plate?
I'm speculating on viable reasons why a KFm1 might have softer stall than a conventional airfoil (assuming in fact it has). It's true that flat upper surface airfoils do experience massive separation. At low AoA this separation may re-attach near the trailing edge, so the wing isn't fully stalled, but come some critical angle the 'separation bubble' bursts and a conventional stall ensues. If you take a look at the airflow visualisation images in the article linked a few posts back there are some excellent images showing detached flow on flat plates at very small angles of attack, then full stall at slightly greater AoA (images attached).
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Old Oct 10, 2012, 04:33 PM
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That first image shows the Virtual Airfoil I'm often raving on about.
It's why a flat plate works better than you'd think at 'our' sizes.
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Old Oct 10, 2012, 04:55 PM
Smells like SCIENCE!!!
ibillwilson's Avatar
Canada, ON, Toronto
Joined Jun 2012
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Quote:
Originally Posted by JetPlaneFlyer View Post
The trailing edge doesn't make a vortex* In streamline flow the air from the top of the wing and bottom come together smoothly in a laminar flow 'sheet'. You can see this in any wind tunnel airfoil visualisation. In turbulent or stalled flow there is chaotic turbulence, but still no vortex.
My understanding was that the situation is different at the low Reynolds numbers that our models experience. There are some nice images of trailing edge vortex wakes in the paper linked below.

http://goo.gl/yX2c9 (PDF of "On vortex shedding from an airfoil in low-Reynolds-number flows", Yarusevych, Sullivan & Kawall, 2009.)
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Old Oct 10, 2012, 06:20 PM
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Quote:
Originally Posted by Mitchell Covell View Post
I think that you are right on this Jetplane. In the range of Re. #'s where separation occurs before transition to a turbulent boundary layer I suspect that there would be little if any difference in performance between a flat plate and an otherwise similar Kfm1.
In the few wind tunnel tests that have been performed a vortex does form in the space behind the step just as the original hypothesis assumed and the pressure distribution has a funny bump upstream of the step with a dip downstream then a spike a bit farther aft. I hate to keep harping on the same old data but new guys keep coming into the discussion and nobody else mentions that there was a good, professional, test of a few step configurations. There's a synopses of that test here.

--Norm
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Old Oct 11, 2012, 01:12 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
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Quote:
Originally Posted by ibillwilson View Post
My understanding was that the situation is different at the low Reynolds numbers that our models experience. There are some nice images of trailing edge vortex wakes in the paper linked below.

http://goo.gl/yX2c9 (PDF of "On vortex shedding from an airfoil in low-Reynolds-number flows", Yarusevych, Sullivan & Kawall, 2009.)
I only speed read the paper but it was clear that the vortexes that they refer yo are rolling vortexes that form in the detached flow area (of normal streamline airfoils) and are shed downstream. The vortices are neither formed at the TE nor do they attach themselves to the TE. The article also does not even speculate at any benefit from these shed vortices.

I'm unclear on how you would conclude that this could have a positive 'anti-stall' effect?
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Old Oct 11, 2012, 01:27 AM
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Quote:
Originally Posted by nmasters View Post
In the few wind tunnel tests that have been performed a vortex does form in the space behind the step just as the original hypothesis assumed and the pressure distribution has a funny bump upstream of the step with a dip downstream then a spike a bit farther aft.
--Norm
Norm,

All the testing I've seen concluded that vortices did form ion the step area but they were not stable and were constantly shed downstream. The net result generally being an increase in drag over the non-stepped version.

True that the Witherspoon tests did find some very narrow operating regime where a small benefit was observed for one discontinuity configuration (a kind of notch rather than a step). But that was in comparison to a symmetrical airfoil. non of the results they got showed numbers that would compare favourably to any reasonable cambered airfoil.

As for the claimed Top Flite tests..I take that with a large pinch of salt as they were never published and we have no idea of the test methods, or in fact it the tests even took place. FWIW the testing I did against a conventional Mark Drela airfoil showed a very marked reduction in performance for the KFm3 configuration tested. Other than the much increased sink rate and obvious increase in drag the KFm wing flew in a similar way to the Drela, and it exhibited normal stall behaviour: http://www.rcgroups.com/forums/showt...&highlight=elf
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Old Oct 11, 2012, 01:34 AM
China, Guangdong, Shenzhen
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Tests i n v o l v i n g variations i n the step geometry (locat
i o n , size and shape) indicate quite conclusively t h a t the l i f t i s primarily a
leading edge phenomenon which reaches a maximum when the step is not present.
Mhen the stepped airfoil i s turned upside down ( w i t h the step on tup of the
airfoil) there is a marked increase i n both ' i f t and lift/drag ratio b u t these
are s t i l l below the levels f o r conventional airfoils except a t very large
angles of a t t a c k .
Edward Lumsdaine
W i l l i a m S. Johnson
Lynn M. Fletcher
Judith E. Peach, 1974

It should be noted that these tests were carried out for 60,000 <= Re <= 135,000 and probably do not apply to the range of Re numbers associated with the formation of laminar separation bubbles.

We should be more careful about comparing apples and oranges.
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Old Oct 11, 2012, 01:36 AM
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Sorry about the errors - I cut and pasted from a .pdf copy of the article and many formating errors resulted.

Mitch
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Old Oct 11, 2012, 02:41 AM
Smells like SCIENCE!!!
ibillwilson's Avatar
Canada, ON, Toronto
Joined Jun 2012
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Quote:
Originally Posted by JetPlaneFlyer View Post
I only speed read the paper but it was clear that the vortexes that they refer yo are rolling vortexes that form in the detached flow area (of normal streamline airfoils) and are shed downstream. The vortices are neither formed at the TE nor do they attach themselves to the TE. The article also does not even speculate at any benefit from these shed vortices.

I'm unclear on how you would conclude that this could have a positive 'anti-stall' effect?
Earlier you stated that the trailing edge doesn't make a vortex, and then described streamlines coming together in a laminar flow sheet. I only referenced that paper because it illustrates something quite different. I realize it is not the trailing edge that forms the vortices, but as that paper illustrates, there is vortex shedding off the trailing edge.

Next, I didn't claim that the vortices coming off the trailing edge had a positive 'anti-stall' effect. What I suggested was that a flow phenomenon that affects those vortices could also affect separation and reattachment on the upper surface of the airfoil.

Think about a Gurney flap, for instance. It clearly has an affect on the flow distribution past the trailing edge of the airfoil, which then clearly affects the lift generated by the airfoil... probably by altering flow separation and reattachment.

My non-expert speculation is therefore that the KFm1 bottom-step affects the flow field past the trailing edge of the airfoil, resulting in a change to the nature of flow separation and reattachment on top of the airfoil.
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Old Oct 11, 2012, 07:37 AM
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JetPlaneFlyer's Avatar
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ib,

I think the debate is largely one of semantics.
In aerodynamics I don’t know of any recognised phenomena that is known as 'a trailing edge vortex' (other than the starting vortex) and that's what I said. To repeat: The trailing edge (at least on normal airfoils with sharp TE's) does not MAKE a vortex.. Vortices that may be made somewhere ahead of the TE simply go past the TE as they get carried along with the air. In separated flow these arent stable vortices like we get ot wing tips, they are unstable random tumbling of the airflow which (if you check my post) I refered to as 'chaotic turbulence'.. This random tumbling isnt what most people would consider a vortex, though if you want to be strictly accurate, yes this 'chaos' has vortices within it.

Anyway I guess the next question is 'does a bottom step actually shed vortices' and 'if it does do these have any effect on the flow separation on top of the wing?'
If I had to guess I'd say 'probably' to the first point and 'highly unlikely' to the second. I can’t see any logical explanation as to why vortices shed from the lower surface would prevent stall on the upper surface?

And don’t forget that the claim made for the KFm design is that the vortex doesn’t shed but sits firmly trapped behind the step. So the whole idea of vortex shedding is contrary to the usual claimed principal of operation of the KF airfoil.

Steve
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Old Oct 11, 2012, 08:47 AM
just gotta mess with it!
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North West Louisiana
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Quote:
Originally Posted by jackerbes View Post
Do we know or have acccess to anyone that has access to an wind tunnel?

Jack
Unlikely, and usually expensive, but with some small load sensors and perhaps an anemometer, you might be able to do meaningful comparisons with a wing mounted on top of a car using cruise control.....
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Old Oct 11, 2012, 04:56 PM
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You would have to be very mindful of the turbulence and disrupted airflow the car creates. Perhaps a bit forward on the vehicle may be good enough.
Then if you had exclusive use of a a perfectly smooth road and flat calm conditions, you may be able to get some meaningful data.
Variables are the killer...
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Old Oct 12, 2012, 05:05 AM
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France, RA, Cormaranche-en-Bugey
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I have a wind tunnel at home, a small one unfortunately.

I've never tested airfoils so far.

What you want to know?
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