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Old Feb 02, 2013, 12:12 PM
KlonWarz
Joined Dec 2012
455 Posts
pop...

how is the emoticon thingy done for sigh ?

we're just not allowed to motor on down the road any longer, with something missile - looking hanging out the passenger side window, are we... gosh, they might send out a swat team!

Way back then I had encounters with enforcement people over class a & b estes rockets.
It worked out ok when I showed them they were paper, weighing only a few ounces...
Today, with goofy shoes on airliners, and all, I think I'd have a bit more trouble... lol
Best
rc
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Old Feb 02, 2013, 03:52 PM
Build straight - Fly twisty
Whiskers's Avatar
Australia, QLD, Little Mountain
Joined Feb 2010
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maguro wrote: Trying to determine stall behavior by throwing a glider is not going to give you any usable data. If you want to do a stall test, take the glider up to altitude (tow or carry it), and then do stalls in still air by radio.
Another test for another time by another guy, perhaps.
I did get usable information from my stall tests, and it could be summarized thus:

When this model was induced to stall it maintained its heading, did not yaw and remained level in the roll axis. From this it can be inferred that for this particular test model the KF and Ca sections stalled in a very similar manner.

The points that have been made about accurate profiles and rigid structures are valid for testing aimed at collecting numerical data.
The little plane I made satisfies me that it's true enough for me to rely on it to indicate 'behavioral characteristics.'
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Old Feb 02, 2013, 04:24 PM
Old_Robin
United Kingdom, England, Oxon
Joined Aug 2012
51 Posts
Whiskers, I really like the glider with two different sections. Excellent test. Well done.
I am sad there are people (not you) peddling the stupid idea that the airfoil section produces lift. The ONLY and I mean ONLY thing that produces lift is angle of attack.
The section changes the characteristics of the drag induced, and the speed and behaviour at stall, but produces no lift per se. Wings have depth only for the reality of mechanical strength, viz sails work rather well on a yacht. Am I a heretic? Regards Robin
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Old Feb 02, 2013, 05:07 PM
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I'll go along with you to the extent of agreeing that every wing section has a zero-lift angle.
When the angle is changed in the positive direction, lift in produced.
When the angle is changed in the negative direction, negative-lift (?) is produced.
So yes, no AoA = No Lift.
However testing and experiences over many years show that factors such as camber-line, thickness ratio, position of maximum thickness etc. determine the L/D performance of the airfoil.
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Old Feb 02, 2013, 05:08 PM
Registered User
Joined Jul 2012
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Quote:
Originally Posted by Old_Robin View Post
Whiskers, I really like the glider with two different sections. Excellent test. Well done.
I am sad there are people (not you) peddling the stupid idea that the airfoil section produces lift. The ONLY and I mean ONLY thing that produces lift is angle of attack.
The section changes the characteristics of the drag induced, and the speed and behaviour at stall, but produces no lift per se. Wings have depth only for the reality of mechanical strength, viz sails work rather well on a yacht. Am I a heretic? Regards Robin

I have to disagree. There is a reason why airfoils are shaped a certain way. If, as you claim, "[the] only thing that produces lift is angle of attack," then why can airplanes fly level?

Of course if you mean "effective angle of attack" then yes, the effective angle of attack is directly related with lift until stall point.
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Old Feb 04, 2013, 08:38 AM
RC beginner
New York
Joined Oct 2008
6,054 Posts
old-robin is right. airfoils matter less at the reynolds numbers and subsonic speed we are concerned with. the problem is many fall for that old story from flight school regarding bernouli etc:



in fact flat plate dynamics, coanda, and newton prevail with the type of models we fly.

drela notwithstanding i have experimented with many airfoil types including kf, flat, classic usa35, 4-40 undercamber, etc and the difference it minimal as whistlers demo shows. if you need to trailer your model to the field then things like that may start to be significant but not so much at the size most of us deal with. aoa is king.
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Old Feb 04, 2013, 09:49 AM
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I fully agree that Park Flyer type planes are remarkably tolerant in regard to airfoils.
However the 'family type' of an airfoil can not be disregarded.
Aerobatic aircraft, of all sizes, perform best with symmetrical-section wings simply because other types of wings are inferior when inverted.
It would be far from ideal to use a curved plate wing on a plane that was required to fly well inverted. If section-shape really didn't matter, this would not be an issue.
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Old Feb 04, 2013, 10:27 AM
RC beginner
New York
Joined Oct 2008
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true. like the discussion about kf for competition gliders, as requirement for performance increases so do little details like airfoils and streamlining. f3p contest guys would certainly fair poorly with uc wings. but im convinced aoa is a far more important issue for the other 99% of hobbyists. comparatively speaking.
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Old Feb 04, 2013, 02:53 PM
Old_Robin
United Kingdom, England, Oxon
Joined Aug 2012
51 Posts
I am happy we have a concensus. I was convinced by the fact that KFm-2 is basically KFm-1 upside down, and KFm-6 is entirely symmetrical.
So fit a spar into the thinnest wing section you can, round the front and taper the back and that is very nearly the best wing section for models. Regards Robin
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Old Feb 04, 2013, 03:05 PM
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Quote:
Originally Posted by Old_Robin View Post
I am happy we have a concensus. I was convinced by the fact that KFm-2 is basically KFm-1 upside down, and KFm-6 is entirely symmetrical.
So fit a spar into the thinnest wing section you can, round the front and taper the back and that is very nearly the best wing section for models. Regards Robin
Robin....

I thought you might like to know that the wind tunnel tests we conducted at Notre Dame and LaGuardia School of Aeronautics both showed that the KFm2 had higher L/D ratios than the step on the bottom (KFm1). So, in that regard they are not exactly the same. These tests were done in 1969.

~ Dick
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Old Feb 04, 2013, 04:38 PM
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Originally Posted by Dickeroo View Post
Robin....

"... the KFm2 had higher L/D ratios than the step on the bottom (KFm1)..."

~ Dick
This is exactly the result I would expect.
Hmmm! Perhaps I should to a KFm1 and KFm2 hybrid test.
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Old Feb 04, 2013, 07:24 PM
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Just a guess on my part....

The KFm2 traps a rotating vortex behind the step that lowers the air pressure over the upper surface. Since this is occurring behind the step in the second half of the chord, the COG has to be further back.

I don't have a clue as to how the KFm1 generates lift.
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Old Feb 04, 2013, 07:26 PM
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Joined Apr 2007
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I tested the KFM2 on an 8 foot sailplane. While it flew very well it would not hold it's own against my foam Fox sailplane of approximately the same wing span, wing area, and weight. The increased drag of the step makes a difference in "hang time" compared to the same wing without a step no matter what airfoil you use. I still think the stall characteristics of the KF step are better, but I don't have any data to prove it.

Roger
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Old Feb 04, 2013, 09:09 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Quote:
Originally Posted by Dickeroo View Post
Just a guess on my part....

The KFm2 traps a rotating vortex behind the step that lowers the air pressure over the upper surface. Since this is occurring behind the step in the second half of the chord, the COG has to be further back.

I don't have a clue as to how the KFm1 generates lift.
There would be a vortex beneath the wing on the KFm1 wouldn't there? And the wing area behind the KFm1 step keeps bumping into the vortex continuously as it flies and that holds the wing up or slows the descent.

That is the explanation I use to momentarily silence the folks that bring up the Bernoulli effect or say that the Bernoulli effect seems to deny that the KF1 wing can work as it does.

Other than that, I don't have a clue either...

Jack
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Old Feb 04, 2013, 09:20 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Quote:
Originally Posted by maguro View Post
I tested the KFM2 on an 8 foot sailplane. While it flew very well it would not hold it's own against my foam Fox sailplane of approximately the same wing span, wing area, and weight. The increased drag of the step makes a difference in "hang time" compared to the same wing without a step no matter what airfoil you use. I still think the stall characteristics of the KF step are better, but I don't have any data to prove it.

Roger
When my 96" Big Blu glider with a KFm9 wing (three steps on top) was still flying I did an sink rate evaluation one day. For that plane I considered that, without thermal lift, I would get sink rates that were around 2.8 FPS. And a study I had read about glider performance indicated that if I had a "real" glider I could get sink rates that were in the range of 1.0 to 1.5 FPS by spending a *lot* more money.

The details of my tests and a link to the performance study are here: http://www.rcgroups.com/forums/showp...2&postcount=39

I was perfectly happy with the amount of performance I got from Big Blu and thoroughly enjoyed flying it. Regardless of the opinions of the "real" glider pilots and their "real" gliders.

Jack
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