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Old Sep 27, 2009, 12:58 AM
just Some Useless Geek
Chicagoland
Joined Oct 2008
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Way cool, Jim! So, the steps acted as reflow stops? This is very similar to the anti-reversion exhaust header technology that came out a couple of decades ago. Race cars and high performance street cars use exhaust headers with a series of concentric cones facing the exhaust port to block the backpressure wave from the exhaust system. Think of a stack of coffee cups with the bottoms cut out, and you can see the effect. The air wants to flow from the big end towards the small end, but doesn't want to fight the apparently smaller opening of the small end towards the big end.

I wonder if there is some kind of unified theory of airflow, sorta like the mystical Unified Field Theory that Green and Hawking and guys like that have been chasing for an eternity?
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Old Sep 27, 2009, 06:18 AM
Onward through the fog.
Cybernaught's Avatar
Bohol Philippines
Joined Aug 2008
1,566 Posts
Quote:
Originally Posted by A Useless Geek
My latest Dart is a Smart Dart XS modded with a rear-biased sweep and a KFm2 on the top. I screwed it up, though, and went 40% across the width of the wing instead of 50% as the rule states. Also, since I am a completely lazy cuss and only wanted to make something really simple and easy I just laminated one sheet of O-C pink on top of another to make the step. Results can be found here.

You'll notice that I also added the winglets (tip plates) that seem to be all the rage. Actually, I was just interested in protecting the wingtips from landing rash without adding a bunch of tape out there. The winglets are EPP and hang below as well as stick above the wing.

From a casual examination of the results I can see that the (buggered up) KFm2 creates a bunch more lift than the flat plate wings on my other SDXSs. In fact, the only other planes in my hanger that fly anything like my Smarty Pants are the ones with hollow airfoils like the Slow Stick, Pico Tiger Moth, Minium, etc. Way cool.

To my way of thinking the KFm2 is the airfoil of choice for simple and quick builds. It is far less demanding of time, material, expertise, and craftsmanship than any hollow airfoil or scale airfoil wing build. As per my experience, it is also far more forgiving than any "proper" airfoil design; my Smarty Pants was off by 20% and it still gave a significant improvement in generated lift! Try that with a scale wing. Heh.
Nice vid... Flies pretty good in that tight area. KFm2 is a good foil and simple enough to build.
tnx fr sharing,
Steve.
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Old Sep 27, 2009, 08:59 AM
gpw
“There’s no place like Foam”
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United States, LA, New Orleans
Joined Jul 2002
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Well tested in Many planes ... Many KF devotees ... Really strange question is why don't we see it in the various Foamie kit designs , or in the mags ???
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Old Sep 27, 2009, 09:07 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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A Useless Geek wrote:

"..went 40% across the width of the wing instead of 50% as the rule states..."

There are not any rules that have to be adhered to. I built a KFm3 wing and accidentally put the back step at 86% instead of 75% and it turned out to be my best wing for speed and snappy performance with the Blu-Baby 33.

"..I just laminated one sheet of O-C pink on top of another to make the step. Results can be found here..."

I couldn't imagine doing it any other way. That is one of the gateways to the 15 minute builds.

If anyone wants to see a slow, stable, yard flier that looks like an airplane instead of nutball, they need to look at Marty's video here:

http://www.rcgroups.com/forums/showt...2#post13068482

Watch it at 1:10 seconds when he takes in between the two trees and then brings it back up and over into an Immelman. All in like slow motion and with little or no apparent increase in speed. Wonderful!

Jack
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Old Sep 27, 2009, 09:22 AM
just Some Useless Geek
Chicagoland
Joined Oct 2008
2,539 Posts
Quote:
Originally Posted by jackerbes
Watch it at 1:10 seconds when he takes in between the two trees and then brings it back up and over into an Immelman. All in like slow motion and with little or no apparent increase in speed. Wonderful!
Ah, yes. Wonderful. If only you had seen the look of panic on my face as I was doing that...

I was too busy talking for the camera and not paying enough attention to the trajectory of the plane. Since the turning radius on the Smarty Pants isn't all that good I just bailed out straight up instead of hanging the dumb thing in a tree. It has more power than it knows what to do with, so climbing out straight up wasn't a problem.

However, I must agree that the very gentle handling characteristics of this plane (mostly due to its low wing loading and KFm2) made this seemingly acrobatic maneuver easy to accomplish. This is only the second design into which I've incorporated the KFm2, but I can't see making any other general sport flyer without using it.

Before the weather breaks I'll be building a fast park jet using the KFm4. I want to go fast without a lot of CF bracing, and the KFm4 seems like the answer to prayer.
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Old Sep 27, 2009, 09:38 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..Really strange question is why don't we see it in the various Foamie kit designs , or in the mags ???..."

My guesses are:

1 - Because there is virtually no established avenue or manufacturing scheme for cutting, packaging, and merchandising our foamies or their foam components as kits.

2 - Because the kits would be so much cheaper than the other stuff on the LHS shelves that it would undermine the sales of higher cost items with bigger markups.

3 - Because the vast majority of the corporate RC hobby empires do not really understand or smell any potential for profit in scratchbuilt foamies, KFm, or the stong allure that is there for a lot of us RC'ers.

4 - As soon a RC builder bought one the cat would be out of the bag and everyone he knows would soon be right where we are, busily avoiding buying anything at the LHS because we can do it better/cheaper/quicker without a trip to the LHS and have more fun at it.

We are a fringe market that is hard to intrude on or capture because, for the most part, we do not have to go to them for our primary building materials or designs.

Jack
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Old Sep 27, 2009, 09:49 AM
gpw
“There’s no place like Foam”
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OK, I'll go along with that ....
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Old Sep 27, 2009, 10:26 AM
PunchDrunk ex-Pug try'n fly'n
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United States, MN, Minneapolis
Joined Jan 2009
1,014 Posts
Quote:
Originally Posted by jackerbes
...We are a fringe market that is hard to intrude on or capture because, for the most part, we do not have to go to them for our primary building materials or designs..... Jack
How true, now when I go to my LHS the guys ask me "where ya been?"
I do still stop by once a month or so for little things like props and collets.
jp
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Old Sep 27, 2009, 11:35 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..Ah, yes. Wonderful. If only you had seen the look of panic on my face as I was doing that..."

I don't underestimate the skills involved there, you're not a rookie and it shows.

"..I must agree that the very gentle handling characteristics of this plane..."

And there is a strong attraction there, I'll bet if you had 15 minutes, a little more room, reduced the throws and travel a little, you could have someone that has never flown before flyint it and considering taking up a new hobby.

"..Before the weather breaks I'll be building a fast park jet using the KFm4. I want to go fast without a lot of CF bracing, and the KFm4 seems like the answer to prayer..."

Is that going to be an EDF or a pusher prop? You may be able to do it without any CF spars. I built a 32" Divinity flying with Elmer's foam board with the paper peeled. That is a 1/8" or 3mm foam that is a Depron-like foam (or so I'm told).

It flies at about 10 oz. was built without spars. I laid nylon filament tape parallel to the leading edges under the KFm4 strips and the strength and stiffness has been fine so far.

I flew it in my front yard (a little more open than your area but with electrical lines and lots of trees). I smacked a electrical line with it at some speed and also did a moderately hard nose first landing and the plane was undamaged. My last Divinity 32 broke the wing on an impact like that. That plane is not as much of a slow, gentle flier, you have to have pretty light thumbs.

Build details - http://www.rcgroups.com/forums/showp...&postcount=570

Jack
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Old Sep 27, 2009, 02:08 PM
just Some Useless Geek
Chicagoland
Joined Oct 2008
2,539 Posts
Yeah, I'm just not that enthused about wings. I have a Mini Swift, Swift II, and a Maxi Swift in pieces parts. The Mini and Deuce just never did thrill me; wings have their own dynamics that I really don't like.

My plan is to build another XYZ or something similar using a fairly hefty Turnigy heli motor in the back. If I extend the root braces as KFm4 steps on top and bottom I should be able to do without the CF altogether. We shall see.
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Old Sep 28, 2009, 09:24 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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"..My plan is to build another XYZ or..."


OK, I give up, I can't find a build called an "XYZ" here in the forum or on the net. What is that?

Jack
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Old Sep 28, 2009, 05:28 PM
"Watch out for...CRUNCH!!
Modarius's Avatar
United States, UT, Provo
Joined Jul 2008
218 Posts
Hi, I have a forward swept wing and when I input my parameters into the calculator in post 930 I get a negative number for my step filler thickness.
Major wing chord- 8.5"
Minor wing chord- 5"
Panel Length- 16"
Total sweep (one panel)- -1.25"
Center position %- 13%
Calculated MAC- 6.9"
Base foam thickness- .335"

If you guys wouldn't mind checking my work, that would be great! As a side note I entered the sweep angle as negative.
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Old Sep 28, 2009, 06:54 PM
Crash Master
Gene Bond's Avatar
Indianapolis, IN
Joined Sep 2001
16,632 Posts
Try it with a positive sweep, and it should be as you would with a Clark-Y or any other conventional foil.

Of course you'll need the negative sweep for CG/MAC calcs...
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Old Sep 28, 2009, 07:23 PM
"Watch out for...CRUNCH!!
Modarius's Avatar
United States, UT, Provo
Joined Jul 2008
218 Posts
There is no place that I can see on the KF step calculator to enter sweep. The calculated MAC stays the same whether I enter the sweep as positive or negative. Is there something that I am missing? I don't want to brag but I tend to do well in math and this has me stumped.
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Old Sep 28, 2009, 08:20 PM
Jack
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Joined May 2008
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I tried to use that calculator once but I am really weak on math. There more I studied the less help it seemed to be to me.

Since I start out with foam of a known thickness (1/4" nominal for FFF foam, usually a little less) and have or can get a variety of spar materials of various known thicknesses, the process evolved down to one of putting the foam and spar pieces in various arrangements and looking for a combination that did two things for me. First the spar or spar pieces were in an arrangement that would stiffen the wing, and secondly, they did it in a manner that got the wing thickness close to the percentage value I was trying to attain.

So the variation was not one of cutting fillers of a specific thicknesses but that of taking the thickness that was as close as I could get to the the recommended value.

The choices boiled down to things like if I had a 1/4" x 3/8" balsa spar I could lay the balsa with the 1/4" side vertical and the foam resting on top of that, or I could put the 3/8" side vertical and butt the foam up against and flush with the top of the 3/8" face. The first way would give me a wing thickness of 3/4" and the second would be about 7/8" thick.

I used foam filler strips to help get me to acceptable heights for the wing thickness and the step heights when there was multiple steps like on the KFm3.

As an example, the attached photo is the root of a KFm3 BB 33 wing with a 1/4 x 1/2" basswood spar. The spar was used with the 1/2" side vertical, the foam butted into the face of the spar and was flush with the top for the 50% strip. Then a filler strip to get two layers of the 1/4" (nominal) foam on the back of the spar to set the height of the first step, and that went back to a single layer of foam at the75% end of the step.

The narrow filler in front of the spar was to support the foam and provide more material that would be bonded together by the PU's foaming action. That wing turned out very strong and durable, it outlived several fuselages.

So my actual wing thickness there was just under 3/4" on a wing with a 8" chord, that worked out to 10% against a called for 9-12%

I'm not trying to talk you out of using the spreadsheet but it just complicated things for me.

Jack
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