Originally Posted by mclarkson

I was never entirely sure, reading those diagrams, just what the 7-12% referred to. The chord? The span? Something else?

It is the wing thickness at the thickest point (top of a step to bottom of the wing normally) expressed as a percentage of the root chord. There is a slightly newer version of the KFm variations figure over on post #2 of the ** Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion ** thread and that shows the percentages for the KFm4 as 6-12% rather than the 6-9% on the figure in this thread:

http://www.rcgroups.com/forums/showthread.php?t=1117276

But it is all very forgiving, you might find that the thinner wing is better for higher performance flying (speed and aerobatics) and the thicker wing better for slower flight.

I have a wing with a 12" chord, for example. So a KFm4 would need to be around 1.25" to 1.50"? So, were I making it from 1/4" FFF, I'd want to use multiple layers? (Otherwise, I'd only be 0.75" thick, correct?)

It's thickness would be .72" to 1.08" using the 6-9% figure, or .72" to 1.44" using the 6-12% figure. Those are not dimensions that have to be met for it to work, they are approximations of what has been tried and is known to work.

So the 1/4" FFF in three layers would give you close to the 6% thickness. If you added a 1/8" spar under the foam at both of the steps, that would make the thickness 1/4" more for a 1.0" thickness and a percentage of about 8%. Two 1/4" thick spars would take up to 1.25" thick and you'd be at 10% of the chord.

For those examples, the step heights would have gone from 1/4" to 3/8" to 3/4" if the foam were resting on top of the spars. I prefer (for the chord lengths on the wings I have built) that the steps be 3/8" to 1/4" or so.

Here is an example of the 3/4" step height resulting from FFF and a 1/4" spar

3/4" step height

=============== <---top layer of foam and top of step
...........................[ ] <---1/4" square spar
====================================== <-- center panel

That could be reduced to 1/4" step height by just letting the FFF rest even with the top of the spar and the spar becomes the step

1/4" step height

============[ ] <--- top of FFF even with top of 1/4" tall spar
==============================================

It could be changed to a 3/8" step height by using a 1/4" x 3/8" spar with the 3/8" side vertical

3/8" step height

============[ ] <--- top of FFF even with top of 3/8" tall spar
....===...............[ ] <--- filler strip added between spar and leading edge
==============================================

When the filler strip is added in it can be FFF thinned to 1/8" or so, or a strip of the thinner than FFF Dollar Tree store Readiboard foam could be used.

When there are small voids between two layers of FFF, like that unfilled 1/8" gap in front of the spar on the 3/8" step height you can just let the foaming action of the PU glue fill that small void. When the PU glue has foamed and set up, the PU foam, the two layers of FFF, and the face of the spar will all be solidly joined.

Here are a couple of threads with an examples of a KFm3 wing builds:

Building a 32" KFm3 Flat Wing w/Ailerons - http://www.rcgroups.com/forums/showthread.php?t=1335888

** Blu Baby 33" KFm3P Polyhedral Tip Wing Build ** - http://www.rcgroups.com/forums/showthread.php?t=1474665

The KFm3P is not an official designator, it is just a name I gave to it when I added the polyhedral tips. And I think that is the best flying KFm3 wing I have made. It is suitable for a basic trainer if the ailerons are left off or not used, with ailerons it seems to fly the same as a flat wing. I like it because it does away with the center joint required to add dihedral to a wing. It really simplifies the build and produces a very nice flying wing.

Jack

Thanks, everyone. I've built several KFm wings with 1/4" FFF and they seem to work okay. I was just wondering about scaling things up or down and, as I said, wasn't sure what the percentages referred to.

Thanks again.

Don't know if this has been mentioned before but there is an application for the iPad that does a visualisation of the solutions to the Navier Stokes equations. You can play around with various parameters and output representations including pressure, speed, vortex field etc.

http://algorizk.com/windtunnel/

Just for fun I put a KFm4 into it.

It's not really a serious tool, it is based on incompressible flow and there is no control over the Re number so it is more a fun visualisation but it would be interesting to see what it makes of the KFm variants compared to a conventional airfoil.

John

John...

I fooled around a bit with these crude profile drawings that I drew with my finger on my iPad.

It's a great program for doodlers.

Thanks for the tips.

~ Dick

I made a crude wind tunnel with this app for the iPad.

I have said that I thought that there was a kickback or slight push behind the step which is not being measured or included in any testing.

It shows up in this little test, however.

Thought you might find it interesting.

~ Dick

Tim from Calgary has posted a terrific video that visually shows the characteristics of different KFm airfoils. The various flights speak for themselves.

KF Airfoils - Flight Testing (Amazing Results!!) (5 min 8 sec)

The persistent problem with simulations is that they are only approximations of what actually happens. This is why experimental planes are flown with tufts on them and cameras hung out of every orifice. Heh. Of course there are much more sophisticated sensing mechanisms in place today, but you get my drift.

So far nobody has had the financial and scientific backing to do a really thorough study of the KF in the realm in which we're interested. But then again, empirical results are sufficient to prove that the KF provides so many benefits that we don't really need to know all the details, do we? It's just nice to pursue the science for further study.

AUG, we never bothered with the science of the KF , It worked so well in the practical application (the flying ) , scientific proof was never needed (for me ) ... and it worked well on disks and LAR wings too ... We’ve been flying KF planes for years now , and it’s still my Absolute Favorite airfoil for a Foamie !!!

Nice to understand how things work , better to experience it ... !!! JMHO

Have an apple in my hand , I can write volumes how it tastes , or you could just take a bite ...

no thanks, im a wintel dude. maybe a pc w/linux if really pushed.

gpw..Nice to understand how things work , better to experience it ... !!! JMHO

Have an apple in my hand , I can write volumes how it tastes , or you could just take a bite ...

dave..no thanks, im a wintel dude. maybe a pc w/linux if really pushed.

:-)

I was always bigblue even though I have seen where the apple is better for a lot of the graphics and audio programs. I guess they are called apps, now. and bill gates is a mobster! he has impeded progress in more ways than could ever be determined. what a brutal businessman.

Yet it is a good point gpw makes.
Works well to give it a try even though some of us enjoy trying to understand precisely how it accomplishes what it actually does.

[QUOTE=rusty case
Works well to give it a try even though some of us enjoy trying to understand precisely how it accomplishes what it actually does.[/QUOTE]

Rusty... A few years ago I came up with a conjecture as to how I think the KFm3 Heavy Lifter generates lift. I put this conjecture out there in the hopes that someone would counter with a different opinion.

The KFm3 has two steps on the upper surface @ 50% and 75%. They cause two vortexes to form which spins the air molecules faster than the air below thus reducing air pressure over the upper surface of most of the cord. So from leading edge to trailing edge you are able to obtain lift. Perhaps this is also why the CG is so forgiving.

On the other hand... The true airfoil generates lift 25%-35% from the leading edge back. The remaining portion of the chord delivers very little lift as I understand it.

BUT, this is just my conjecture. My opinion. If someone has a better explanation I would like to hear it. There is always more to learn as far as I am concerned.

~Dick

I generally prefer a Devil's advocate viewpoint, Dick, because anyone can easily slap someone on the back and present attaboys!
At least some other considerations might be made if a prospect is contested...

Unfortunately, I do not have the training in aerodynamics that would constitute any sort of basis to make presentations, either way. Any longer, I don't have the wherewithall to pursue detailed education.

From what I glean here and there in my travels I can see a strong case in favor of the KFm step... nothwithstanding, it is a very simple build method that adds strength to a model airfoil in a nearly foolproof construction method.

Best
rustycase

Originally Posted by gpw

AUG, we never bothered with the science of the KF , It worked so well in the practical application (the flying ) , scientific proof was never needed (for me ) ... and it worked well on disks and LAR wings too ... We’ve been flying KF planes for years now , and it’s still my Absolute Favorite airfoil for a Foamie !!!

Nice to understand how things work , better to experience it ... !!! JMHO

Have an apple in my hand , I can write volumes how it tastes , or you could just take a bite ...

Wanna go flying this weekend?



Took out a few planes the other day, dusted them off... I've got the glue-gun warming to reinforce a few loose seems.

Just need to order some batteries...

Let me know when you're ready, and I'll be good to go.

Has anybody seen Advanced Theory/Science around here lately?
He seems to be missing...

Wiskers, you are welcome to add some. Done any testing lately?