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Old May 18, 2009, 03:35 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Why do you choose the airfoil you do?

This isn't a poll, because I was looking more for a discussion. It stems from a comment I saw in another thread about the KF style airfoil. I don't want this to become a contentious debate; rather, I hope we can all be respectful of others' views. We all like different planes, flying styles, and are at different levels of accomplishment in our hobby.

For me personally, I have to admit, I don't care for the "look" of most KFm airfoils, except the KFm1 (that's the one with the step on the bottom). It looks "clean" from the top, like a "normal" airfoil. It's probably why I also like the 4-40 undercambered airfoil for very small planes - it "looks" better.

But dangit, if my Stevens Akro aerobat with a KFm2 (step on top) doesn't perform nicely for me - stalls are controllable, flies inverted just as nicely as upright, and is very strong. Plus, it was super easy to make. And the GWS IPS powered glider I just finished up flies like a dream - has a KFm1 on it. My "Lazy Boy" glider has a KFm1 - it's a VERY strong wing (yay carbon fiber!), and flies very well. I've built a KFm4, but haven't finished that plane yet - the wing is extremely strong given that I put a hefty carbon fiber tube in it.

IMO, all these planes fly better than flat plate wing planes I've flown though aren't much harder than a flat plate to build, yet none of them require the difficulty of creating tapered, fully-airfoiled wings (both in chord and thickness) either. If I want to do that, I'll stick build a plane - which I've done, several times now. I enjoy that too.

But this is about foam.

So what are your thoughts on this? Do you go with KFm for simplicity, because it gets you out there flying faster? Like the way the KFm handles? What about the 4-40? Flat plate? Full "real" airfoils? Are the aesthetics just as - if not more - important than the flying characteristics?

To all of the above: Why or why not? Or does it matter at all? If you design planes, do you choose an airfoil based on the type of plane you're designing?

Again, please keep this civil and respectful of others' views. We can agree to disagree without being disagreeable. Smile before hitting "Submit post."
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Old May 18, 2009, 04:26 PM
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earlwb's Avatar
USA, TX, Grapevine
Joined Dec 2008
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Yes it is the simplicity and building speed to get out flying faster. if a flat plate wing with maybe a KFm airfoil type speeds up the process, then go for it. Now one could easily build a Clark Y type of air foil too. But since the KFm airfoil is a natural for foamy RC planes, it is a winner hands down.
But it all boils down to what you yourself want to do, this is a hobby, it allows for creativity, and ideas and experimentation. But most of all fun.
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Old May 18, 2009, 04:36 PM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
4,044 Posts
I perform an incredible amount of calculations and tests.
Finding the most efficient form for the planform and power plant,
I sculpt an exquisite avionics art form.
Not willing to leave well enough alone,
I pick up my sanding block to "touchup" a rough spot or three
When finished, I install the wavy clark y wing.
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Old May 18, 2009, 05:16 PM
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IdahoAir's Avatar
South Central Idaho
Joined Mar 2009
205 Posts
It builds fast and it's simple. When you wreck aircraft as I do, fast and simple is good...

I also think it's a fine wing design for a neophyte foamie builder, like me, to master before moving on to more complex wing forms. I just finished a BB 42, a Divinity 42 wing and completed my first scratch built design based on a Aeronca C-3, all this weekend... The KFm style wing contributed greatly to the effort...

Cheers.
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Old May 18, 2009, 05:33 PM
Flying high
billwhit's Avatar
Seattle, Wa
Joined Dec 2002
2,263 Posts
I just built a Tuffy to use for aerial video and was looking for nice and stable slow flight. I started by trying the KFm. It flew well, but it felt like a faster wing than I wanted so I built another one Clark y. This wing flies much slower, and I can always swap it for the KFm if I like!
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Old May 18, 2009, 06:23 PM
it WILL fly! someday....
Richard_s's Avatar
Slovakia
Joined Aug 2008
448 Posts
I do both, KF and real airfoils. I like stall characteristics of KF, but hate the drag. I just love to glide and float . For that, I slap together quick and dirty ClarkY by bendig 2-3mm foam over hardwood spar (constant height, Im lazy ), and hotglue trailing edge together. No sandig. Simple, quick, flies great. I plan to build universal fuse and set of wings (flat, airfoil, clarky) as many others did, to feel which wing does what in air.
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Old May 18, 2009, 06:26 PM
My plans are in my blog
Rusty-Gunn's Avatar
Kotzebue, Alaska
Joined May 2006
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When I first got into R/C five years ago I had the notion all airfoils ought to be Clark Y types like real airplanes. I figured flat plat wings was the stupidest thing I ever saw, and the K-F 'explosion' hadn't arrived yet.
I bought ready-made foam wings with a "real" airfoil and flew them, until I developed my three-bend airfoil, using sheet foam, in an attempt to create a poor man's Clark-Y type. See my blog.
I got into using flat plate wings because "everybody" used them, and I learned it wasn't the stupidest idea, just my attitude about it was stupid. I've used flat plate (I tend to think this is redundent) wings ever since.
Then the K-F airfoil became a big thing a few years ago, and by then I had a supply of one-skin FFF, so I got into the K-Fm types simply because I wanted to stiffen up the FFF wings, and discovered the K-F airfoil works wonderful, so I began using it a lot. I highly recommend them.
So, for me, it depends on what type of plane I'm building. Slow flyer Cub types will use a three-bend or K-Fm, prop jets use a flat plate for speed and biplanes use flat plates for simplicity.
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Old May 19, 2009, 08:21 AM
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lake flyer's Avatar
Joined Nov 2005
4,404 Posts
I build a lot of scale planes and therefore I usually try for a "scale " airfoil.

When building non scale , sometimes I build flat wings , and sometimes Clark Y or similar airfoils .

The full airfoils are (I think ) best , they are much stronger without extra weight , and better in torsional ridgitity , which is important for consistent flying characteristics .( when you apply ailerons , the wings do not twist as much as they do with flat or KF type wings )
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Old May 19, 2009, 08:50 AM
Postcards From The Purple Edge
tuppertn's Avatar
United States, OK, Sand Springs
Joined May 2007
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Quote:
Originally Posted by lake flyer
The full airfoils are (I think ) best , they are much stronger without extra weight , and better in torsional ridgitity , which is important for consistent flying characteristics .( when you apply ailerons , the wings do not twist as much as they do with flat or KF type wings )
Just to play Devil's Advocate, the KFm3 especially has fantastic torsional rigidity. The step from 50-75% essentially acts like a continous, full span rib that braces the back half of the wing. I've had a KFm3 upwards of 70mph on a propjet and it is flutter and twist free. I haven't tried the other KFm's in any really demanding applications. I do know that the KFm2 can flutter and twist if the cord gets much over 9" or so.

I agree that they don't look scale and that's always bothered me a little. Most of the time the ease of building and favorable flying capabilities make up for it.

Lake, sorry for being a dork about "real" airfoils over on the P-61 thread.

kendall
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Old May 19, 2009, 09:27 AM
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springer's Avatar
SE MI
Joined Oct 2004
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I haven't tried a flat plate airfoil. started with UC single surface, graduated to the KFC with fwd 40% doubled on a UC wing (which I still build on over 30" or so spans) Plenty strong for me. But I really like the KFM in all it's variants. For smaller wings I've done kfm2, 3, and the simple kfm2 (two layers laminated - 50%top step - It's a no brainer to make, strong and flys very nicely) 42" BB in kfm3 works really nicely as well, and as Kendall says is super strong. I've pulled out of loops and bent tips 3-4" (variable dihedral!) w/o problems.

So: 24-33" >single surface UC with skewers LE and TE; 33"+ >KFC for "gentle/floater", Simple KFM2 for general purpose (with and without ailerons)out to 40", KFM3 for ailerons, more agressive flying.
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Old May 19, 2009, 12:43 PM
it WILL fly! someday....
Richard_s's Avatar
Slovakia
Joined Aug 2008
448 Posts
I just maidened my Fantastic acro. 1m span, 540g, thin symmetrical 50% KFm profile. Absolutely stallproof. I couldnt believe - I cut throttle at about 30m above ground and pulled full up elevator. No wind. Plane landed to my feet like pancake, vertically fell from the sky . Amazing. Ill make a vid next time.
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Old May 19, 2009, 12:50 PM
Bernoulli+Newton=Lift
magic612's Avatar
Somewhere south of Chicago, IL
Joined Jan 2006
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Quote:
Originally Posted by Richard_s
Absolutely stallproof. I couldnt believe - I cut throttle at about 30m above ground and pulled full up elevator. No wind. Plane landed to my feet like pancake, vertically fell from the sky . Amazing.

That reminds me of a chuck glider plane I made a few weeks back (canard type plane): I'd stand on our fire pit, and launch it horizontally. It would fly with a slight bit of descent, but mostly level. As drag overtook it and it lost airspeed, it would fall straight to the ground - pancake style, as you described. Happened repeatedly.

It wasn't a KFm foil - just a flat plate since it was a 15" wingspan chuck glider, but your post reminded me of that experience. It made me wonder how/why that can happen...?

Amazing, indeed!
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Old May 19, 2009, 01:15 PM
it WILL fly! someday....
Richard_s's Avatar
Slovakia
Joined Aug 2008
448 Posts
I guess that drag and low mass of your glider caused that. In case of my KF wing, It has to do something with dynamic vortices created at the KF step. No matter what I did, I couldnt stall that plane today. Full up elevator with some airspeed and motor off, caused the same effect as if plane was tailheavy (I cant remember that word, somebody help me ). And without airspeed, plane wouldnt tipstall at all.
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Old May 19, 2009, 02:21 PM
My plans are in my blog
Rusty-Gunn's Avatar
Kotzebue, Alaska
Joined May 2006
5,416 Posts
Quote:
Originally Posted by Richard_s
(I cant remember that word, somebody help me
Its called 'lucky'.
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Old May 19, 2009, 02:44 PM
Postcards From The Purple Edge
tuppertn's Avatar
United States, OK, Sand Springs
Joined May 2007
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Quote:
Originally Posted by Richard_s
I just maidened my Fantastic acro...Absolutely stallproof...I cut throttle at about 30m above ground and pulled full up elevator. No wind. Plane landed to my feet like pancake, vertically fell from the sky...
This from another thread...
Quote:
Originally Posted by tuppertn
On some of mine I can hold full up elevator with no power and the plane just floats straight down like it's hanging from a parachute. You almost need to see it to believe it.
We actually call that the "Parachute Maneuver". I think it needs a lightly loaded plane to get the full effect. My 32oz, KFm3 FPV plane does it with a wing loading just under 8oz/sqft.

kendall
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