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Old Oct 11, 2011, 08:48 AM
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South Central Idaho
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I think Rich Thompson summed it up very nicely in his ad-hoc "Kline-Fogelman Airfoil Comparison Study" that he published back in February of '08:

"There may be some who criticize the use of subjective data simply because it is subjective. My response to that criticism is that in my 30+ years of flying RC, I've observed one thing that remains true of all RC pilots. If you hand the controls of a great flying plane to any RC pilot, they immediately know they have a great flying plane on their hands. If the you hand the controls of a poor flying plane to any RC pilot, they immediately know they have a poor flying plane on their hands. This knowledge is completely independent from what one might observe in a video or by watching nearby, because this information is based on their feel, and is totally subjective. I claim that it is the subjective information about how a plane handles that is most significant to RC pilots." ~ Rich Thompson (KAOS2)

Cheers.
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Old Oct 11, 2011, 08:53 AM
fix-it-up chappie
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Valley Village, CA
Joined Jan 2002
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Quote:
Originally Posted by JetPlaneFlyer View Post
PS.. There is an easier way to prove, one way or another, the efficiency of KF airfoils. Rather than spend ages building wind tunnels and doing tests that some might not believe anyway; why not just build a really high performance KF wing glider (say a 1.5m contest spec discus launch glider). Then go along to an organised contest and beat all the other models. That's how you could prove the claims to be true. After you did it a couple of times no one, including me, could argue. No amount of wind tunnel testing however well done would have the same impact.

Steve
This is exactly right... if one were to assume that the best use of an airfoil is in a high performance glider, or that the goal an any airfoil is to be more efficient.

I think almost everyone here agrees that a KF airfoil might find its value in trading efficiency for stability. At least that is my subjective view on it.

Put it this way, we have all kinds of cars and trucks. Many of them are a great vehicles, but they are all engineered around a core concept in which they perform best. While my Prius happens to be highly efficient when it comes to gas consumption, it would be inefficient to use it to haul scrap metal, or race it on high speed track.

In this context, your comment above is like saying "take your car to a formula 500 race, and if it places well, everyone wil agree is a good car." Not every car is a race car, and the metric of a "race" is not always the best measure of its value.



Quote:
Originally Posted by JetPlaneFlyer
'knowing' something to be true without any solid evidence to support that belief is faith not science.. Perhaps KF should be declared a religion and Dick Kline a deity?
LOL.
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Old Oct 11, 2011, 08:56 AM
fix-it-up chappie
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Originally Posted by viking60 View Post
Friends,

Flying is very much a subjective experience and process. It's all part of why I enjoy it.

.....
Well said. I think you write as well as you build. Excellent post.
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Old Oct 11, 2011, 10:49 AM
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BTW, regarding KFm wing performance in gliding wings, nah ... A decent symmetrical or Clark-Y outperforms a KFm ... I've been messing around with different Kfm configurations for a "thermal seeker" and none of them beat a more traditional airfoil for glide performance.

That being said, for a sloper, they can work pretty well ... A top step KFm3 adds a bit to the aerobatic capabilities, stall recovery and agility of a sloper ...

Cheers.
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Old Oct 11, 2011, 11:17 AM
Grumpy old git.. Who me?
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Aberdeen
Joined Mar 2006
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Quote:
Originally Posted by jackerbes View Post
"..It's a strange statement to make that..."

His statement is probably not any stranger than say, someone who would come to a place where people are primarily discussing the actual results of building KF wings and their experiences in flying them, without having actually done any of the same building and flying,
Jack,
Check the title of the thread....
I'll give you a reminder, it's
Quote:
Discussion ** Kline-Fogleman (KFm) Airfoils - Advanced Theory/Science **
Coming here to discuss theory and science of KF airfoils is surely not so strange This thread is not "where people are primarily discussing the actual results of building KF wings and their experiences in flying them". There are many other threads for discussing building and flying so if people want to discuss practical building and flying then they should take it to those threads. I can understand those who are not interested in theory and science, that's not a problem, many find it very boring, they don't have to read this thread or post in it.

Here's a question for you: Why come to the theory and science discussion thread and then make posts purely to criticise someone for discussing theory and science?
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Old Oct 11, 2011, 11:22 AM
Grumpy old git.. Who me?
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Quote:
Originally Posted by tolladay View Post
This is exactly right... if one were to assume that the best use of an airfoil is in a high performance glider, or that the goal an any airfoil is to be more efficient.
Tolladay,
That's why i qualified my statement by saying it would prove the 'efficiency' of the airfoil. The point here is that's pretty much all a wind tunnel will do for you. It will give you lift and drag and not much else. A wind tunnel wont prove any of the more subjective claims like stability, CG position etc.. Which is one of the reasons i stated why a wind tunnel test is probably not going to give the answers people seek.
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Old Oct 11, 2011, 11:24 AM
fix-it-up chappie
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Valley Village, CA
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Quote:
Originally Posted by JetPlaneFlyer View Post
Tolladay,
That's why i qualified my statement by saying it would prove the 'efficiency' of the airfoil. The point here is that's pretty much all a wind tunnel will do for you. It will give you lift and drag and not much else. A wind tunnel wont prove any of the more subjective claims like stability, CG position etc.. Which is one of the reasons i stated why a wind tunnel test is probably not going to give the answers people seek.
Agreed.
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Old Oct 11, 2011, 02:01 PM
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I built an 8 foot sailplane using a KFM3 airfoil (The Blu-Guppy). It proved that a sailplane could be built using an airfoil that supposedly only useful on airplanes with large thrust-to-weight ratios. The Blu-Guppy flew well. It was a pleasure to fly, but it would in no way compete with my 30 year old Cirrus or Flamingo designs. That wasn't the point. The idea was to show that an all foam KF airfoiled sailplane would work reasonably well. This it did. The Blu-Guppy makes a fine beginners sailplane. A number of Blu-Guppys are currently flying around the world, so obviously the design works.

KF airfoils have their niche. They can't go out and replace existing airfoils on competition sailplanes, nor are they likely to appear on the next airplane to come out of Bert Rutan's workshop (Yes I know Bert retired). The one place we know where KF airfoils work is on R/C models, particularly on foamies where the KF airfoil is particularly easy to construct.

I personally have not been trying to prove the KF better than another design. I've been trying to understand how well KF variants perform in relation to each other, and how an MH32/KFM(2/3/3a) airfoil, stacks up against the MH32 with no step.

Testing is currently on hold, while I rebuild the test plane. I was doing low level aerobatics wit it, and pulled when I should have pushed.

Roger
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Old Oct 11, 2011, 03:09 PM
just Some Useless Geek
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Okay -- Sorry About That

(Edited to de-thermalize)

Once again I'd like to ask those who have accumulated scientific data on the KF to present here. We're having a difficult time objectively quantifying the characteristics of the KF, so rather than continuing to get everybody involved in this thread wet why don't we take a look at some numbers? If you have science then please bring it. If you don't have any science then please...don't contribute to the noise level.

For most of us here the only "science" we have is that which we've accumulated through experimentation and comparative evaluation. We have proven that the KF provides a stiff wing -- relative to a flat plate. We have proven that the KF provides a lower stall speed -- relative to a flat plate. We have proven that the KF provides better overall stability -- relative to a flat plate.

We have not quantified these results for the most part. We don't even know how to go about quantifying these factors, nor even what units in which to express measurements. Okay, there's your cue -- if you'd like to positively benefit this discussion then do some measuring and come back here with numerical results.

Sweeping generalizations about the scientific method aren't helping us create a foundation of data. Complaints about subjective analysis in place of objective measurements aren't getting us anywhere, either. If you want to use a wind tunnel to compare one wing shape to another then fine. However, as has been pointed out to no end, a model airplane isn't a set of mathematical symbols on a piece of paper -- it's a collection of surfaces all reacting to the air in phunky ways that are, at best, difficult to measure and even more difficult to convey in objective terms. Why do y'all think this board is so full of airplane reviews? Why do you think those reviews are chock full of subjective analysis, with almost no space devoted to objective measurements?
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Old Oct 11, 2011, 03:36 PM
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South Central Idaho
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Quote:
If you don't have any science then please STFU!!
Nice attitude to have ... unsubscribing ...
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Old Oct 11, 2011, 05:08 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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Geek! Calm down. Relax. Such an outburst is unseemly from a fine intellect such as yours.
Speaking personally, I am here to seek information and express an opinion and not to bash the thread.
Discussion (as this purports to be) is: Consideration or examination by argument, comment, etc., especially to explore solutions. Also informal debate.
Now, let's continue the informal debate.
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Old Oct 11, 2011, 05:19 PM
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United States, TX, League City
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Quote:
Originally Posted by Whiskers View Post
Geek! Calm down. Relax. Such an outburst is unseemly from a fine intellect such as yours.
Speaking personally, I am here to seek information and express an opinion and not to bash the thread.
Discussion (as this purports to be) is: Consideration or examination by argument, comment, etc., especially to explore solutions. Also informal debate.
Now, let's continue the informal debate.
THANK YOU
Wise words
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Old Oct 11, 2011, 08:57 PM
treefinder
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SE MI
Joined Oct 2004
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to quote the geek: "For most of us here the only "science" we have is that which we've accumulated through experimentation and comparative evaluation." Ya know, that is pretty much what folks have been doing since we picked up the first "tool" and tried it out. And No less than the Wright brothers, Santos-Dumont, Curtiss, etc did a lot of the experimentation and comparative evaluation that got us well on the way to where we are now. So I'd not be too down on the kinds of experimentation that folks have been able to do with the resources available to them.

On the glider thing, I've now built three different wings for the OSG, (KFM5a undercambered, KFM2, and conventional based on a 7035), flown them a lot, in various wind conditions, and my go to wing is the KFM2. I also have a Gentle Lady with the wing/airfoil it came with and an Aspire with it's supplied wing. While both fly ok, I find i stay up longer, find lift better and generally enjoy the time more with the OSG and KFM2. (I suppose that a part of the duration can be attributed to the fact that the OSG is in the sub 8oz range and the other two are pushing 2lbs) I suppose that's more anectodal data to quote Dogbert!
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Old Oct 11, 2011, 10:48 PM
Build straight - Fly twisty
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Australia, QLD, Little Mountain
Joined Feb 2010
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Yup! For thermaling (in calm conditions) nothing is better than adding lot of lightness
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Old Oct 11, 2011, 11:30 PM
In the 20' glider range
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Boise, Idaho
Joined Jul 2009
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Geek. I am sorry you think that way about this thread. That really stifles the amount of information you could have learned. Humans don't set out to find numbers, they find an idea and the numbers support it. We are still finding that idea. We will
Eventually get the numbers here. But until that no one is going to "STFU".


I hope that isn't your attitude towards life, if so, good luck living it happily.
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