** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science ** - Page 57 - RC Groups
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Feb 18, 2012, 09:58 AM
jackerbes's Avatar
And again, you cannot leave out the consideration that the discussion here is heavily weighted towards building with readily available and/or inexpensive and/or more experimental materials that often are a major factor in determining things like wing thickness, step heights, and the like.

We are not, for the most part, taking specifications tor optimum performance and trying to build to those specs. We are fully willing to just have something close that flies nicely.

The primary focus as the KF airfoil has advanced in RC use has been in powered flight. It is very easy to offset drag with power and it is not a cardinal sin to do it intentionally or habitually just because it is what we enjoy doing.

Most of my KF wing build have been layered in increments of 0.25"/6mm or thickness and two or three layers. The thickness was usually dictated by the components. The mid chord or high point thickness tends to range from 11% to 16% and it works fine.

Maybe drag is an essential component of KF wing building. I've been involved in a number of fun things where drag was an essential component of the fun. Let you imagination run with that one a little...

But springer is going to sort it all out with his wind tunnel and we'll have some real answers...

Last edited by jackerbes; Feb 19, 2012 at 12:57 PM.
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Feb 18, 2012, 10:00 AM
springer's Avatar
"But springer is going to sort it all out with his wind tunnel and we'll have some real answers... "

dream on!
Feb 18, 2012, 10:23 AM
RC beginner
Originally Posted by jackerbes
Maybe drag is an essential component of KF wing building. I've been involved in a number of fun things where drag was an essential component of the fun. Let you imagination run with that one a little...
little imagination required. if you spend any time in the f3p area youll find drag can even be an asset in competition. they spend considerable effort designing in drag inducing structures.
Feb 18, 2012, 10:26 AM
“There’s no place like Foam”
gpw's Avatar
I think a good bit of the success we have with our little flat plate park fliers, is due to the tremendous Power now available and the ridiculously Low weights we can achieve... Wing loadings in the Blimp category almost, compared to RC model weight years ago ... ... No wonder they fly ... Airfoil or not ... we just used the KF all along because it Greatly improves the stall characteristics at these ridiculously low park flier speeds... a little more draggy than a flat plate , but some time’s that’s appreciated too for the way we fly ... But the KF is Not as draggy as the most popular under-cambered airfoil .. so as with anything else in aviation , a Good Compromise... and another major benefit , it strengthens an otherwise thin flat plate wing ... on many reasonably sized Foamie’ planes, eliminating the need for a spar ...

But then I’m not a scientist , I just build and fly a lot !!! All for FUN !!!
Feb 18, 2012, 11:45 AM
Registered User
Don Stackhouse's Avatar
as implied the only reason for thicker is structural integrity. theres also a cosmetic factor because people simply dont like struts or bracing wires as much as straight sexy cantilever. human psychology is a big factor in aerodynamic engineering just like it is with automotive.

for example the wire braced quicksilver ultralight and its copies remains the highest performance airframe many decades after its design. its single surface 4-40 uc wing is only 1 dacron fabric thick (few mils). thin is good for models AND full size if the wing can be made strong enough. its that annoying "frontal area" thing that keeps cropping up.
Don't think so.

In terms of aerodynamic performance, as well as overall performance, the Quicksilver was surpassed many times, in fact there were planes that came before it that had better performance.

Single-surface airfoils can be made to work, but are not the highest performance, including at very low Re. This is particularly true if you need an airfoil that performs well at a range of airspeeds and lift coefficients.

In my experience (and this happens to be an area where I have a LOT of experence, four and a half decades of it, at Re's as low as about 70, yes, that's seventy, not seventy thousand!), designing a really good low Re airfoil is significantly more challenging than at higher Re's. Thickness and camber both need to go down compared to higher Re's, but the amount of each, and especially the way they are distributed along the chord, becomes increasingly critical. Too little thickness can be as bad as too much, and zero thickness is definitely not the optimum.

The Quicksilver's airfoil is really rather crude, with a poorly controlled fabric section in the middle, but a large round tube in a sewn-in pocket at the leading and trailing edges. There are ribs, but they are few, and widely spaced. The airfoil on my Rogallo hang glider could give it a run for its money in terms of 2-D airfoil performance.

There are a few more recent versions that have a double-surface wing, and that are a few MPH faster on (in some cases) 20% less horsepower.

There is far more to aerodynamic performance of an airplane than just the airfoil shapes, but airfoils do play their part.

As far as wires vs cantilever, an externally braced wing is structurally very efficient, but aerodynamically quite the opposite. I recall reading about a wind tunnel test of a 12" thick Clark Y airfoil compared to a 1" diameter round wire. The drag of the wire was far greater than the twelve times thicker airfoil shape. It also left a lot to be desired in terms of its lift-making ability.

Yes, thickness is important from a structural standpoint. However, its contributions to aerodynamic properties are important as well. And, just as with medicine, the exact dosage is critical. Too much, or too little, can both have negative consequences.

As far as power, yes, you can make anything fly if you put a big enough motor on it. However, generally speaking, it will fly farther, faster and longer, for less weight and cost, if you can design it to use finesse instead of brute force.
Last edited by Don Stackhouse; Feb 18, 2012 at 11:54 AM.
Feb 18, 2012, 03:18 PM
RC beginner
Originally Posted by Don Stackhouse
In terms of aerodynamic performance, as well as overall performance, the Quicksilver was surpassed many times, in fact there were planes that came before it that had better performance.
get ready for a long one. ot too.

we must agree to disagree. having spent literally hundreds of hours over the last few years with dozens of ultralight pilots conversing and watching them fly at one of the most popular large ul airports in the northeast i would have to say you are flat wrong.

for everything an ultralight is designed for the mx is noted for surpassing all others. its sales record speaks out for this. this includes planes marketed before and up to today and those costing many times more. for its short field landing, 10' takeoff roll, and general ability to clear runway trees, it is unequaled. literally. it drops almost like an elevator with spoilers deployed. nothing else can do that and survive.

with a rotax 277 it has no problem staying under 1.2gph which has never been achieved with any other man carrying fixed wing aircraft out of a hundred or so at that field. its almost impossible to stall, instead it just slows down and drops like a parachute. 8500' ceiling has been recorded at the airport which was better than most ga trainers there. probably because its so much bigger and lighter than the c150 and other small planes.

i also do not think you have a handle on the actual drag penalty of wires either. they are ugly but are the single reason the mx was the only ul of its time rated for aerobatics. although ive never done it myself ive seen two other pilots fly inverted for 10-15minutes at a time and do inside and outside loops. other things theve done are beyond simple words so i advise checking out some of the videos in the yahoo forum.

based on your comment about "faster" it looks like you may be confusing ultralight with sport pilot "doctor killers" or 2 seat exemption abusers where speed IS adored. this is not a factor in ul performance due to faa part 103 speed limit. i doubt you can find a single other legal ultralight with capabilities described above. many craft marketed as uls are in fact what are known as "ill eagles".

your statements about fabric stiffness also strikes me as way off base. contradicts direct observation of vector brand uls in flight and pilot comments about achieving incredible climb numbers when weather gets hot and the dacron "cups". not as incredible as the mx though even at ther best. i have witnessed altitude contests between the two at a small private field.

you may be knowledgeable in full scale arena and jpl type engineering but have lost some credibility with me on small planes and possibly rc. mostly based on that last comment about the rogallo kite. it is actually laughable to anyone who knows the truth. ive had direct experience with both full scale versions and models and had to rolleyes and chuckle over that one.

sorry to disagree. im sure youre much smarter than me in the general field of aerodynamics. but thats my 2 cents regarding uls.
Last edited by dave1993; Feb 18, 2012 at 11:16 PM. Reason: faa NOT fcc :|
Feb 18, 2012, 03:43 PM
just Some Useless Geek
Is this Yet Another Case in which the ivory tower science gets trumped (and tromped) by the vast array of empirical data available? Where testing has proven that the calculations are missing some factor that shows up out in the Real World? Where no amount of simulation can take the place of actually flying a test bed in real conditions?

Perhaps we need to have someone who can compare the theory and calculated effects against legitimately collected data from flight testing. I do not wish to poo-poo Mr. Stackhouse's observations about the effects of airfoil, drag, etc. in the area of high performance sailplanes, but, as has been pointed out several times before in this thread and others, the ultra high performance arena is out of the KF's realm. We need to accumulate science aimed at the kind of aircraft for which the KF is a good solution.

To that end I ask fellow participants to look for ways to test various specific theories about the KF and apply those to actual models to see if we can get some real data happening here. There's a lot of "science" being slung all over the place a little measurement to back this stuff up. Eh?
Feb 18, 2012, 05:25 PM
OpenTX University Staff
maguro's Avatar
Geek, collecting empirical data is the reason I built the first test plane, and why I'm going to build another.

Collecting worthwhile data is not as easy as one would think. One of the tests I attempted to conduct was to fly straight and level over the distance of our flying field (about 600 feet) and collect data on the current needed to fly at a constant airspeed. I used an Eagle Tree Seagull Pro setup to collect the data and provide real time telemetry of things like airspeed. I discovered that trying to maintain a constant airspeed of say 30mph over any distance was almost impossible. Airspeed easily varied by up and down up to 2-3 mph from one second to the next. Since data was delayed by about a second from the time it was collected to the time my wife saw it, and another say 2 seconds for her to relay the reading to me, I was already 3 seconds behind the airspeed, and the airplane has moved 1/4 the distance of the field, before my reaction time kicked in. Needless to say, I was way behind the curve.

I purchased a RUBY for flight stabilization, but the development of the RUBY is way to slow to make it of any use to us. A number of other options for flight stabilization exist, but I just don't have the money to buy yet another one, in hopes of getting something that will allow me to get accurate data.

Unless we have a way of maintaining level flight, a slight climb or descent will cause the airspeed and motor current to vary enough to make detecting differences in the efficiency of KF airfoils very difficult. Maybe someone knows an expert IMAC or AMA pattern pilot, who can fly the dozens of test flights for me. Those guys get very good at keeping an airplane on a specified track.

Feb 18, 2012, 05:56 PM
Build straight - Fly twisty
Whiskers's Avatar
What a beautiful summery of the problems we face here. Real-world testing is plagued by variables, and variables are the last thing needed when acquiring data.
Getting back to the bros Wright (I'm a big fan) they first tested by holding sections into the wind. Too many variables. Then they mounted a test rig on to a bicycle and peddled at a set speed. Too many variables.
Then they built their wind-tunnel and acquired so much good data that it renewed their faith that flight was achievable. Previous to that they had started to think it was all too hard.
Feb 18, 2012, 06:49 PM
springer's Avatar
Maybe we need to look at data collection more like a process control problem, run many tests under the "same" conditions and build xbar and R charts to find mean values and ranges, and compare them between airfoils.
Feb 18, 2012, 07:18 PM
just Some Useless Geek
I'm with Mike. I think repetition is the key here. If we collect the same data the same way as well as we can then it's the sample size that will eventually give us a useful source for analysis. I have one airplane design (the Longhorn) that I've only built seven copies of so far, but amongst those seven copies the planes have accumulated somewhere around 400 or so flights of no less than five minutes apiece. The observations of the several pilots who have flown the bulk of those flights are, of course, subjective, but the overall performance envelope of the plane is clear as day to anyone observing the plane fly under the control of someone experienced. You know what a Longhorn is capable of before it even leaves the ground.

If we can focus on repeating certain experimental conditions to collect relative data on some aspect of the KF's behavior then we can eventually make pretty definitive statements about the KF based on the "science" of bulk empirical data. Some of our members here have already started doing that with interchangeable wings, yada yada. So, if somebody is flying slope planes and flies a KF versus a non-KF on the same slope at the same time and does this dozens of times then the final analysis is still useful, even if the science of eliminating variables isn't completely rock solid. The simple fact that the data has been accumulated under the same conditions so many times leads to a relatively clean conclusion. N'est cé pas?
Feb 18, 2012, 08:04 PM
Build straight - Fly twisty
Whiskers's Avatar
Yes but what actual data will be collected and how will it be measured?
I'm in good balance here. I have 3 aerobatic planes with KFm4 wings and 3 with symmetrical section wings.
They all fly fine. The KF wings stall just like the others. All are stable. Bewilderingly the same really, given the different style of the planes.
One of the KFm4 wing things is a little indoor plane, the name 'Ugly as L" says it all, and it needed more wing area to enable slower flight.
I stuck 4 inch symmetrical non KF extensions on the tips of the 30 inch wing, and as I was doing it I wondered if that would make for tip-stalling.
No such problems have been encountered (the back of a retracted basketball net was encountered, but that's another story )
So my impression over hundreds of flights is that the ease of building and the lack of sanding are the big advantages of the KFm. As far as flying goes I've noticed no advantage.
I am still very open to be convinced otherwise.
Last edited by Whiskers; Feb 18, 2012 at 08:09 PM.
Feb 19, 2012, 11:10 AM
just Some Useless Geek
Cool. So, you haven't aimed your testing at some aspect of the KF that is specific to your target flying. That's okay, we've got lots of folks flying KFs who can maybe pick some testing target.

But I am a bit curious about the stall characteristics of the KFs being the same as the symmetricals. I presume you've flown both enough to know if there is a difference and have not detected one. Is there anything that stands out as differences between the wings? Are the planes really similar enough to allow A-B comparison? You know, built on the same pattern, to the same scale, yada yada?
Feb 19, 2012, 12:04 PM
Registered User
Dickeroo's Avatar

KF testing

Both Roger McClurg and Mike Springer have done some testing with threads on the step foil. Here is an example provided by Roger. What does it tell us?

KF Wing Flight Test (5 min 48 sec)
Feb 19, 2012, 12:10 PM
Registered User
And if anyone wants to see this as well, here is the corresponding external video of the flight. You may want to watch both videos at the same time.
Flight of KF Test Aircraft (5 min 51 sec)

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