** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 121 - RC Groups
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Dec 14, 2017, 06:18 AM
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Quote:
Originally Posted by gpw
WOW!!! That Space Ship One is very COOL eh !!!
Have 'ya started cuttin' foam yet?
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Dec 16, 2017, 02:13 PM
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Burt Rutan's Car-top Test Rig


Seek and ye shall find...
Here is some information on Burt's car-top 'wind tunnel.'

Burt Rutan 5 of 8 First Home Built Airplane (8 min 23 sec)
Jan 25, 2018, 07:19 AM
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Dickeroo's Avatar

KF Implemented Flying Wing paper


Here is a paper produced by Dakshina Fernando....

http://www.dakshinafernando.com/#publications

KF IMPLEMENTED FLYING WING
WDT Fernando

Abstract— In modern aviation the main emphasis is pointed towards the direction of improving the efficiency of operation. The design of the aircraft is solely concerned on providing the most effective operating conditions. Designing and developing a new aircraft design needs to be encompassed with the best corporation of forces acting upon to bring out the most efficient flight. In any aircraft design the major apprehensions are on the basic coefficients such as lift and drag. Respectively with the combination of forces acting upon will bring out the collective operating perspectives of the aircraft. With the aid of a 3D designing software and with the assistance of manual designing the aircraft is modelled. The comprehensive use of K – epsilon model in a 3D domain with second order upwind discretization have promptly provided the results as satisfying. The working conditions are to be complied with the general operating conditions of a major airliner used aircraft. The conventional designs are optimized up to a crucial level that have reached the peak with the rising edge of technology. The computational fluid dynamic methods need to be carried out in the rudimentary operating conditions in order to ascertain the condition of the design in the natural constraints. The analytical data have taken a path toward a considerably profound aspect ratio, though the basic design is optimal in operational conditions. Apart from the upper and lower boundaries of operation envelop, the calculations need to be conducted from an inferior level that can bring out its primary stages of operation and build up towards all the conditions with future simulations. Which can be subjected with further computational power towards more effective simulation data finding.

Keywords: Flying wing, Kline Fogelman airfoil, Computational Fluid Dynamic
Jan 25, 2018, 05:21 PM
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The 2017 COMSOL Conference in Rotterdam


https://www.comsol.com/paper/downloa...iak_poster.pdf

My mistake... the second page is shown first.
Jan 25, 2018, 05:26 PM
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The 2017 COMSOL Conference in Rotterdam


https://www.comsol.com/paper/downloa...iak_poster.pdf
Jan 26, 2018, 08:07 PM
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Michael V's Avatar
Quote:
Originally Posted by Dickeroo
https://www.comsol.com/paper/downloa...iak_poster.pdf

My mistake... the second page is shown first.
So what we see here is:
- All Kfm have drag penalty
- Kfm1 and inverted 3 (steps on bottom) have a slight lift benefit (it basically creates an "undercamber" contribution, but still with drag penalty.)
- Kfm3 doesn't show any lift benefit as sometimes claimed (not surprising to me though, as I've said the benefit comes from increased AOA tolerance, not absolute lift increase)

Not unexpected as far as I'm concerned, it recoups my observations.

I am surprised to not see Kfm2 have a better lift performance with AOA increase versus non Kfm, but it is hard to discern where the curve points are unfortunately.
I also wished they had studied higher AOA all the way into stall as I would expect the curves to diverge and Kfm have an advantage then (maybe slight), again from observations.
And I also wished they had plotted the x/c -Cl diagrams, again, I would expect to see benefits from Kfm there (better attachment)
Jan 26, 2018, 08:39 PM
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I agree with your observations Michael.
The high AOA at high airspeed as tested in wind tunnels (virtual in this case) are seldom experienced in the foamy model world. Mostly the plane changes direction and prevents that from occurring.
I have had it happen with a relatively large and highly loaded model stalling out in high G manoeuvres, but not with more *reasonable* planes.
Jan 26, 2018, 09:31 PM
Jack
jackerbes's Avatar
I can't help but think that these studies would be more interesting and more related to the realities of what is actually going on in the hobby if they tested wings that were built like we build them with flat sheets of foam.

They always show a wing cross section that looks like a symmetrical or Clarke-Y airfoil with a step added by removing material to form the step or steps. The attached image would be a more realistic shape if they are trying to shed light on what we hobbyists are actually doing.

The image is an accurate portrayal of the cross section of a wing built with FFF (Dow Protection Board III) sheet foam that is about 1/4" thick. And the foam comes in fan folded 24" x 48" sheet and one of the folds was used for the leading edge. The other steps were formed with stacked pieces of foam, not by removing material from thicker sheets of foam.

And some of the other things tested and mentioned as if of some importance are not really present in our builds. Almost without exception, we mount wings with zero incidence and they spend very little if any time at all flying with the wing positioned and advancing at anything but a zero degrees AOA.

The higher AOA's will be seen momentarily when the planes are stalled but the typical KFm stall behavior has little in common with stalls with the symmetrical or Clarke-Y type airfoils.

What I usually see when a gliding level and a stall is induced by slowly raising the nose is an small increase in the AOA as speed is lost and it increases until the stall happens. Then the nose drops, the speed picks back up, and the glide resumes until the next stall.

I don't think the drag in a KFm wing is a "penalty", it is a characteristic. The way the word is used in some of the reports the writers seem to have some other airfoil in mind as being superior and they seem to be focused on describing and convincing us as to what makes the KFm wing inferior to some other wing.

It is a very simple matter to use power to overcome drag so unless you are in a situation where you can't have or do not have enough of it, it is never a problem. We foamie builders simply use power as needed in quantities that will overcome the drag.

Jack
Jan 26, 2018, 10:08 PM
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Whiskers's Avatar
Yes, the sections usually tested are not those used in the real world of the Scratch Built Foamie, and it would indeed be interesting to see tests performed on these real world sections.

I can't agree on the zero AoA however. The wing may be mounted that way, but in flight it will be at a positive angle. A symmetrical section, like a KFm4 would have zero lift at zero AoA.
But we know it does provide lift, so it follows that it's positive in flight.

The section illustrated in your post above would have a couple of degrees positive angle if the bottom surface was mounted flat. This, of course, is because the cord line drawn from the front of the nose radius to the bottom of the trailing edge is not parallel to the bottom surface.
Jan 27, 2018, 08:54 AM
treefinder
springer's Avatar
Interesting point, older bro. There was a discussion recently in the OSG thread stemming from a question about wing incidence. My original design mounted the wing (whether UC or KFM2) directly on the flat fuse top which is parallel to tail surfaces. In the discussion, the point that i and others round the LE and surface the wing to a smooth airfoil shape back to the step creates a forward point that is above the thrust line and therefore a few degrees positive incidence. While the glider in soaring flight appears to be level, the wing is positive. I would like to say I did that on purpose, but like much of the OSG design, it was a fortuitous accident.
Jan 27, 2018, 09:23 AM
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KFm2 UAV


I recently came across this on the net.... It seems to indicate some positive results for a UAV...

http://www.astech.com.mx/our-projects
Last edited by Dickeroo; Jan 27, 2018 at 01:23 PM.
Jan 27, 2018, 09:52 AM
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maguro's Avatar
Jack, I always build my KFM wings approximating a "known" airfoil, in an attempt to get the most out of them. Agreed that it is not typical of most KFM wings.

What I didn't see on the site was the Reynolds Number. It makes a huge difference in the results. The fact that the KFM has a huge drag penalty is nothing new. For most RCers drag is not an issue. The improvement in lift of the inverted KFM3 was news though. I'd like to see a lot more of their data.
Jan 27, 2018, 10:41 AM
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Michael V's Avatar
Quote:
Originally Posted by Whiskers
The high AOA at high airspeed as tested in wind tunnels (virtual in this case) are seldom experienced in the foamy model world. Mostly the plane changes direction and prevents that from occurring.
I have had it happen with a relatively large and highly loaded model stalling out in high G manoeuvres, but not with more *reasonable* planes.
Correct, and that is why I believe the Kfm3 experiment with the heavily loaded airplane (blue something) was showing better performance, because it was in stall mode (high aoa, low speed, and yet still controllable), and that is where a Kfm would show better attachment, thus maintain better control, leading to people believing it "has more lift", which is an incorrect conclusion IMO. In almost all cases you're right not enough inertia to stall the wing out of a foam plane.
Jan 27, 2018, 10:50 AM
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Michael V's Avatar
Quote:
Originally Posted by jackerbes
I can't help but think that these studies would be more interesting and more related to the realities of what is actually going on in the hobby if they tested wings that were built like we build them with flat sheets of foam.
Agreed, but they do it to have some reference point. So not fully valid for our purpose, but it gives some comparison point, again not fully representative

Quote:
Originally Posted by jackerbes
I don't think the drag in a KFm wing is a "penalty", it is a characteristic. The way the word is used in some of the reports the writers seem to have some other airfoil in mind as being superior and they seem to be focused on describing and convincing us as to what makes the KFm wing inferior to some other wing.

It is a very simple matter to use power to overcome drag so unless you are in a situation where you can't have or do not have enough of it, it is never a problem. We foamie builders simply use power as needed in quantities that will overcome the drag.

Jack
Agreed, and semantics. I didn't mean penalty in the sense it's bad, I meant penalty in the sense that there are solutions that can give me the same lift with less drag. So unless I'm interested in other characteristics of the Kfm (easy to build, smoother stalling behavior, higher AOA tolerance) then I'm not necessarily willing to pay that price. I'm a glider flyer, so for me drag is always a penalty since I don't have "free" power to spare. My Kfm planes always need more wind to fly (slope soaring).
As a power flyer, you're right, model planes nowadays are so overpowered to start with, that who cares about a little more power consumption, it won't make much of a difference, so for those drag is not really a "penalty", although it is (less flying time on the battery)
Jan 27, 2018, 10:58 AM
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Michael V's Avatar
Quote:
Originally Posted by Whiskers
I can't agree on the zero AoA however. The wing may be mounted that way, but in flight it will be at a positive angle. A symmetrical section, like a KFm4 would have zero lift at zero AoA.
But we know it does provide lift, so it follows that it's positive in flight.

The section illustrated in your post above would have a couple of degrees positive angle if the bottom surface was mounted flat. This, of course, is because the cord line drawn from the front of the nose radius to the bottom of the trailing edge is not parallel to the bottom surface.
Not only that, but Kfm airfoils are not exempt of pitching moment, therefore decalage will be necessary and thus most likely positive incidence in neutral flight of the main wing (hence lift).
Although it would be interesting to measure, we may have a case where the extra drag created by the step and bubble separation being slightly above chord line, where it may compensate the natural negative pitching moment of the airfoil due to aerodynamic shape, mhhh? Could we have a 0 pitching moment airfoil at 0 with lift? That would be interesting and then that could in turn be a case where the Kfm has a better L/D than a conventional shape in flight mode (the Kfm would fly at 0 AOA thus maybe less drag, than the conventional with some AOA), great also for flying wings. I don't think so, but interesting to find out.


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