That is an interesting study and comparison. I don't pretend to be an aeronautics expert or even really understand all of what is said there. But the primary things that are compared are the lift and drag coefficients between a NACA 4415 airfoil and the same airfoil with a step added to the bottom at 55% of the chord (that compares well to the 50% we would normally use).

The KF version of the wing still had the same cross section shape as the NACA 4415 with curved surfaces on the top and bottom so it was not quite the same as the flat plate wings we most often build.

The NACA 4415 airfoil is much like the Clarke-Y airfoil other than it not being flat bottomed, it is more symmetrical and alike on both top and bottom.

But we could (and some have I'm sure) make our wing more closely match the test foil were willing to expend the extra time and work...

The comparison data was taken across a range of angles of attack that would be realistic for typical flights.

If you look at the graphs in the study, the wing with the KF step had more lift and it also had more drag. And it looked liked the amount of the lift increase (if the increase was expressed as a percentage) was fairly significant while the increase in drag was a much smaller percentage.

So, as we have pretty much learned, for a small increase in power the KF wing can easily match and even exceed the performance of conventional foils.

And I'll admit that the one area where the KF wing has a small performance penalty is that of the wings cross country speed in a power off glide. So it won't match up to high performance glider airfoils.

But, other than for competitive glider flying venues, it is still a great wing for use on gliders that are intended as glider flying trainers or to introduce the flier to process of glider flying.

It would be a wonderful thing to get these scientists to do comparison to some of the KFm variants as the KFm1, while a good wing, has been out performed by some of the other variants. And it would be interested to know how adding more steps or moving steps from the bottom to the top or both top and bottom changes the numbers.

Jack

An interesting concept.

https://www.academia.edu/37458225/IR...paration_delay

In the late 1970s a son did a science fair project which involved an airfoil similar to the KV.....it was called the "terraced airfoil" At the time, RC high start and towline gliders were sure that a "new" form of lift had been found by terracing the top of a wing. It was hard to deny because the planes zoomed up at an observed faster rate of climb.
He used the small rudder / elevator Ace Guppy glider with the Ace foam wings. The left wing had several 1/8" x 1" wedges cut away starting at 30%.....the right wing was stock. On a long sloped hill he gently launched the plane and when it became stable, he slowly applied UP elevator and in every test flight, the right standard wing stalled well before the terraced wing. I videoed the tests for his presentation.
Next he made a simple lift /drag device and used a nozzle blowing airflow only at the leading edge of small section of both wing styles. The standard wing stalled at a normal 12 degree angle, while the terraced stalled at an amazing 20 degrees. Both airfoils created the same lift but the standard had the regular forward "bump" while the terraced was flatter and covered more of the chord.
conclusion was that the terraces hindered reverse boundary airflow as the AoA increased. This allowed the wing and plane to tow up like a kite. There was no magical new lift. From the pilot's point of view it certainly looked like more lift.

Originally Posted by Jim Petro

In the late 1970s a son did a science fair project which involved an airfoil similar to the KV.....it was called the "terraced airfoil" At the time, RC high start and towline gliders were sure that a "new" form of lift had been found by terracing the top of a wing. It was hard to deny because the planes zoomed up at an observed faster rate of climb.
He used the small rudder / elevator Ace Guppy glider with the Ace foam wings. The left wing had several 1/8" x 1" wedges cut away starting at 30%.....the right wing was stock. On a long sloped hill he gently launched the plane and when it became stable, he slowly applied UP elevator and in every test flight, the right standard wing stalled well before the terraced wing. I videoed the tests for his presentation.
Next he made a simple lift /drag device and used a nozzle blowing airflow only at the leading edge of small section of both wing styles. The standard wing stalled at a normal 12 degree angle, while the terraced stalled at an amazing 20 degrees. Both airfoils created the same lift but the standard had the regular forward "bump" while the terraced was flatter and covered more of the chord.
conclusion was that the terraces hindered reverse boundary airflow as the AoA increased. This allowed the wing and plane to tow up like a kite. There was no magical new lift. From the pilot's point of view it certainly looked like more lift.

Jim... Thank you for this report. Can you post your video of this experiment?

~ Dick

I wish that was possible, but you have to remember this was some 40 years ago. The camera was hand held but the recording on tape was done in a briefcase size DVR. This was on loan from the high school. He also had to borrow a TV....remember how big even "small" color sets were at that time?
After all the fair was done, I tried to make a copy of the tape but did the erase instead of copy.
We also borrowed a PC computer one Christmas break and this was my first try of one. A half hour later, my son came to where it was set up to see why I was looking at a dark screen........he brought all the manuals and not one told where the ON/OFF switch was located.........it was unmarked and on the back side.
We had some pretty exciting experiments for the science fairs. It was at a time of hands on and not the type put on now.

Originally Posted by Jim Petro

I wish that was possible, but you have to remember this was some 40 years ago. The camera was hand held but the recording on tape was done in a briefcase size DVR. This was on loan from the high school. He also had to borrow a TV....remember how big even "small" color sets were at that time?
After all the fair was done, I tried to make a copy of the tape but did the erase instead of copy.
We also borrowed a PC computer one Christmas break and this was my first try of one. A half hour later, my son came to where it was set up to see why I was looking at a dark screen........he brought all the manuals and not one told where the ON/OFF switch was located.........it was unmarked and on the back side.
We had some pretty exciting experiments for the science fairs. It was at a time of hands on and not the type put on now.

And people talk about the, "Good Old Days."

Jim's post about the "cut in" wedges is very interesting. Unfortunately all my foam wings, so far are hollow.
Why not have cut in wedges just near the wing tip? Less drag than if they were full length and they may reduce tip stall.

Hmm, maybe it's time for me to make a hot wire cutter after all.

Chuck, I was wondering when someone would comment about placement of the terraces. The test model used an Ace constant chord molded foam wing so the terraced left wing was uniform from root to tip. One of the many questions was if the low air pressure sucks the air in the terrace out to the tip or draw it in from the tip? Another feature worth investigating was the unusual spread of the low pressure chord wise instead of concentration near the airfoil high point.
While I'm at the keyboard, I recently had some scrap paper covered foam board from a dollar store. My previous KV wings were Depron with sanded to shape LE. I got very nice LE with the foam board when I used a rod and pressed down hard on the inside at the LE. Smashed the foam almost flat but it rebounds somewhat. Then I folded the board and was surprised at the perfect constant smooth rounded LE this produced. It also makes the LE tougher when hot glue is applied in the pressed groove and then quickly folded. Probably would get similar results with the Depron and taped LE grooved and glued.

Sorry for the delayed reply Jim. My guess about the pocket of air or turbulence behind any step is that it wouldn't tend move sideways on its own. But would obviously be influenced by any sweep of the leading edge or wing. Air doesn't have a mind of it's own and all of it's characteristics have been noted and investigated over the years. Air likes to move in straight lines and is only changed substantially by solid objects or more air coming from a different direction, or of course, water. But a wave is usually created by wind in the first place and is merely a small moving hill after that and even a waterfall is near to rocks so, meh, makes little difference.

Originally Posted by Jim Petro

...conclusion was that the terraces hindered reverse boundary airflow as the AoA increased. This allowed the wing and plane to tow up like a kite. There was no magical new lift. From the pilot's point of view it certainly looked like more lift.

Yeah.

This is the anti-reversion principal I have been yapping on about for years. The same principal is used in firearms suppressors, automobile exhaust headers, liquid flow control diodes, etc. The effect is hard to measure and characterize, but easy to see with Instrument Eyeball.

Simulators can't plot this effect. Wind tunnel tests done with KF-modified Clark and NACA and such compound curve airfoils can't create this effect. It is astonishing that the simplest, cheapest, easiest-to-implement solution to a wing also shows the greatest impact of the KF on stability, lift, stall reduction, and C/G flexibility.

It is also astonishing that aerodynamics professionals refuse to acknowledge the KF. It is like refusing to admit the sun rises in the east.

Doh! I mis-read your comment Jim!
I think it's more likely that the low pressure behind a step encourages the air, at least close to the surface of the wing just before the step to acceleryate towards it. And likely the air close to the surface further behind the step to move forwards, thus adding to the low pressure over the whole chord.

Blimey!
Is this a Eureka moment?

Do the steps apparently encourage lower pressure over the whole wing more actively we previously realized? And by the same principal, they actively increase drag?
Dikeroo, where are you? Maybe you already thought of this.

Why didn't I think of this before?


Subliminal vortices?!

Hello Chuck...

I had not thought of that, but what I did think of was that the step causes the air to trip over and reverse direction inside the pocket. Its like turning a negative into a positive. That would mean a slight forward pressure push perhaps. I’m sure that aspect has never been measured though. I do feel that the air pressure in both pockets of the wings tends to want to equalize, thus creating better stability. Flying wings tend not to be very stable. But take a look at this one:

FPV49 V3 - with OpenAero-VTOL - Successful flight including transitions (3 min 27 sec)

Originally Posted by A Useless Geek

...............… aerodynamics professionals refuse to acknowledge the KF.

I think they do, but not as an acceptable airfoil to be used on 1:1 scale. I guess it's the same for a flattie airfoil too.

You are quite right about the KF being less useful on The Real Thing, Frank. For our purposes it is priceless, but for something large enough to carry a 100 Kg payload the KF starts to lose its effectiveness. I guess this is why "perfeshunuls" poo-poo the KF as being beneath their attention threshold. Hmph.

Advanced Theory/Science aside...…………….

The KF is KING!!

I haven't used my foam cutter in some time now. Sloping here has been almost but dead because of climate change. So, no more combat carnage on the slope and cutting cores for the guys. All my latest power birds are crowned with the KF.

Here's my latest KF wonder. It's has a span of 22" and weighs in at 5.5 ounces. I can do extreme things with it and heard a comment on Monday from the peanut gallery that they were amazed by the airfoil. I was flying in attitudes that another airfoils would have stalled and it would have been splash ..... get the boat.