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May 20, 2009, 08:02 AM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by Hepdog
IMHO if you can't have full and true laminar flow - the next best bet is to separate it as soon as is possible.
Another question: Why not put a step on the KFm closer to the LE then? Why not at 20% or 30%? Has anyone tried that? What's the effect?
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May 20, 2009, 09:11 AM
treefinder
springer's Avatar
Quote:
Originally Posted by gpw
Airfoil choice depends on what kind of plane you're flying and what conditions you expect to fly in...
Having built & flown many many planes in the past, we've found the simple under cambered airfoil (foamie) has been the best for slow floaty type of flying .... best suited to calm or low wind situations or Indoor flying... Lightly loaded UC airfoils can fly really SLOW.... We use the BendFoam jig to make simple UC airfoils... GeneBonds 4-40 airfoil seems to work the best for us..
For general outdoor or windy conditions, the KF airfoils are really hard to beat... We've Hot wired/laminated/stick and ribbed/vacuum bagged /etc. many different airfoils and while they 're very nice and airplane "looking" (full scale comparison), The simplest KF airfoil works BETTER in our "scale" air... We MUST consider "scale effect" when designing/building our smaller Foamie models ... What works in the real world ,doesn't seem to translate well to our size range... Small vs large bird thing!!!
I bought "The Book" many years ago , but never translated it well till we got into Foamies, where the material itself was the attraction...and the KF not only gave us a simple ,build able , airfoil with good efficiency, but also the KF addition provided extra support to the wing structure, eliminating the need for heavy spars/rods/skewers.... A much appreciated construction benefit.... Once we tried it we never looked back.... IMHO , the KF concept IS the ULTIMATE Foamie airfoil!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!.... Strong words, but I assure you, well tested.....
Anyone who has not tried it should certainly consider doing so!!!!!!!!!
Being a builder of "disposable Foamies" (just stuff for FUN) we've always used just two variations of the KF, the simplest too...
For stall resistance we like the 40% on the bottom ... for best glide we like the 50% on the top... we don't use spars /etc, just stick one flat piece of foam onto another...The absolute easiest /quickest way to make a wing .... best of all , it works GREAT!!!! Thanks Dickeroo!!!!!
My experience exactly! (at least as far as the KFM goes, have only built a tiny fraction of the planes gpw's done ). Just flying the mite this morning in almost still air, (simple KFM 50% top step) 2s, 2410-09, 9x7.5 prop, half to 1/3 throttle and she's just floating around nice and slow, good control, power on or off. Such a simple wing and works so well!
May 20, 2009, 03:24 PM
Slow is beautiful
Alex Slowfly's Avatar
Another half to my 2 cents worth...
I do not believe in Bernoulli anymore...
Just to nail down the still air side of the rope.... flying with 5 to 7 oz/sqft max. at speeds in the 5 to 10 m/sec. range, KF-wings are very stall proved but too fast. I tried top and bottom steps (50 and 40% of chord) and understand the air pocket theory that makes absolutely sense in every aspect. Nevertheless with all the KF experiments I have the direct feeling (at the sticks) that quite a lot of energy is NOT going into lift in any way (hard to explain - you just realize that like you realize the weak performance of a simple flat plane).
Where the (strong) undercambered wing has a very direct feedback of its lifting and drag behavior... you just feel that on the sticks.

Conclusion on my side: for very calm air - we have that most of the time where I live - it's simply not possible to beat a "Frog-like" undercambered wing. Talking about Re-numbers of 50'000 and below....

Keep that experiences coming.... very interesting thread.
May 20, 2009, 05:16 PM
Registered User
Dickeroo's Avatar

This gentleman does not believe in Bernouli's Principal


His name is Gene Gent and he is a former pilot with a long history of working in aerodynamics. His links are quite interesting. I have spoken with him several times. He presents his views from a different perspective.

http://www.aeronautics.ws/index.html

http://www.aeronautics.ws/blunder1.html

http://www.aeronautics.ws/optimization.html
May 20, 2009, 05:33 PM
Out the Window
High Flyin''s Avatar
Lets say you have a 5 inch wide wing with a 1 inch wide ailerons and you want to put a step at 50% on a KFm wing, do you add the aileron to the total width? Would 50% of cord be 2.5 inches or 3 inches?

Probably a dumb question, but I have just usually followed the plans.

Ray
May 20, 2009, 05:56 PM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by Alex Slowfly
I do not believe in Bernoulli anymore...
I used think lift was only Bernoulli. Then I did some reading (can't remember where now), and am convinced that lift is both Bernoulli AND Newton.

Maybe one day I won't believe Bernoulli anymore, but I'm not there at the moment.

Also, good links that you provided, Dick - thanks for sharing them! Although I confess, at some point, it becomes too much physics for me and my mind turns to mush.
May 20, 2009, 06:02 PM
it WILL fly! someday....
Richard_s's Avatar
Guys, read something about Coanda (effect). That might add something to the equation
May 20, 2009, 06:20 PM
Jack
jackerbes's Avatar
Tony65x55 drew that figure with the four basic variations. In that figure the percentages are of a chord length that includes the ailerons. I think I know that because the question was asked in the Blu-Baby thread. And if you look at the plans for the Blu-Baby and some of the other Tony65x55 KFm wings, the step locations verify that the percentage includes the ailerons.

And I think those percentages are intended for aircraft with a conventional wing and fuselage configuration but I don't know that for sure.

I say that because Tony65x55's plan for the Zagnutz flying wing measures the chord from the tip of the nose to the front edge of the prop slot cutout. That actually omits a little bit of the trailing edge from the chord measurement and, of course, the elevons.

And that seems to be a fairly standard or understood method for measuring the chord length on pusher configured flying wings. At least on the wings that I have come in contact with.

But there does not seem to be any hard and fast rules for your questions as far as I know. In your case the difference will be 1/2" in the step location and the wing will undoubtedly work either way. If you really want to explore it, you could build it both ways and then try to figure out if one is better than the other and in what way. If your plane is a fairly standard wing and fuselage configuration I would include the ailerons in the measurement.

Jack
May 20, 2009, 11:41 PM
Balsa + glue + bluesky= fun
leaktech's Avatar
Quote:
Originally Posted by Richard_s
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.
Not sure it's the same thing but what you describe sounds like a free flight dethermalizer. FF ships have no elevator just a stab. When the timer goes off it trips the stab and a rubber band lifts the TE of the stab straight up. The model is usually cruising at glide speed> immediate and aggressive stall stops all forward movement> model floats to ground balanced on CG.
James
May 21, 2009, 07:57 AM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
Quote:
Originally Posted by leaktech
Not sure it's the same thing but what you describe sounds like a free flight dethermalizer. FF ships have no elevator just a stab. When the timer goes off it trips the stab and a rubber band lifts the TE of the stab straight up. The model is usually cruising at glide speed> immediate and aggressive stall stops all forward movement> model floats to ground balanced on CG.
James
Here's a question for someone with solid knowledge of aerodynamics to answer: Does the center of lift and center of pressure need to be in the same place for this to occur? Otherwise, wouldn't the plane glide instead (or have a "traditional" stall)?
May 21, 2009, 10:33 AM
SG Talon... Super Genius.
I am not sold on the kfm wing.

I have been building planes for going on 30 years now and i have built every style of wing known to man.

When stumbled across this site about 4 years ago i started building foam and was introduced to the flat plate wing. The simple fact is there is nothing simpler, lighter, or easier to build than a flat plate.

I have built a lot of identical planes in the last couple years and i am finding that single piece wings are better than anything else.

When it comes to 3D planes the flat plate simply performs better. I built 4 3DBatix Extra 330's. My first one was exactly to the plans with a flat plate wing, 2 were KFM, one was Symmetric airfoil. The best flying one - hands down - was the Flat Plate. The others added too much weight with not enough increase in performance. Every one of these planes had the exact same gear in them too.

Sure they don't glide as nicely as the KFM or Airfoil when the power is off but who flies a 3D plane with the power off?? And really how much less glide do you really get? It is not THAT much less.

When it comes to trainer style planes i have tried them all too. All 3 styles, Flat (Cambered), KFM, and 2 surface Airfoil. The best performing Trainer 1 i have built (out of about 8 now) was the simple cambered wing.

I have heard people scoff at the flat plate a lot but in my experience, and i have built a lot of planes, you can't beat flat plate or cambered wings.
May 21, 2009, 10:48 AM
Out the Window
High Flyin''s Avatar
Quote:
Originally Posted by jackerbes
Tony65x55 drew that figure with the four basic variations. In that figure the percentages are of a chord length that includes the ailerons. I think I know that because the question was asked in the Blu-Baby thread. And if you look at the plans for the Blu-Baby and some of the other Tony65x55 KFm wings, the step locations verify that the percentage includes the ailerons.

And I think those percentages are intended for aircraft with a conventional wing and fuselage configuration but I don't know that for sure.

I say that because Tony65x55's plan for the Zagnutz flying wing measures the chord from the tip of the nose to the front edge of the prop slot cutout. That actually omits a little bit of the trailing edge from the chord measurement and, of course, the elevons.

And that seems to be a fairly standard or understood method for measuring the chord length on pusher configured flying wings. At least on the wings that I have come in contact with.

But there does not seem to be any hard and fast rules for your questions as far as I know. In your case the difference will be 1/2" in the step location and the wing will undoubtedly work either way. If you really want to explore it, you could build it both ways and then try to figure out if one is better than the other and in what way. If your plane is a fairly standard wing and fuselage configuration I would include the ailerons in the measurement.

Jack
Thanks Jack for the information. I guess I should include the Ailerons in my figures for a conventional tractor wing.

I would like to be able to build a couple and see the difference, if any. I am such a SLOW builder, I feel lucky to finish one of a style.

Thanks,
Ray
May 21, 2009, 03:10 PM
Bernoulli+Newton=Lift
magic612's Avatar
Thread OP
This is an interesting discussion. I appreciate everyone being respectful of others' views about various airfoils. I think what I really like about not only this site, but this sub-forum section in particular is the camaraderie of everyone helping everyone else out. If someone doesn't like what another person has done with one thing (KFm, undercambered, flat plate, full airfoil, whatever), it's cool that we're pretty much okay with going our own way - yet it seems that we all still benefit immensely from others' thoughts, plans, and insight.

I just wanted to say thanks to everyone for the civil tone we've displayed so far, and I hope we keep it that way.
May 21, 2009, 04:21 PM
Sopwith Camel's Cousin
Quote:
Originally Posted by lake flyer
"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 . "

That is a stall.
I think 2 senses of "stall" are being used.

One sense (the 'classical' one) of stall is when the wing is in
part of the lift/drag curve where increasing drag (like by increasing
the angle of attack) produces decreasing lift (and not increasing lift).
The plane is probably in a stall for this sense of stall.

But one definition of 3D (including parachutes, harriers, etc.) is that
all of 3D occurs when the plane is in this sense of stall.
Spins and snap rolls also are done when a plane is in this sense of stall.
So a plane flying a 'parachute' or 'harrier' is also in this sense of stall.

Some stalls are easier to fly with (like mild root stalls)
than others (like sudden tip stalls).
Since a plane is always in a stall (in the classic sense) for
3D (and spins and snap rolls), people doing such flying may reserve
the term 'stall' for those stalls that are hard to fly with.
This is probably the sense of stall being used by 'jackerbes' in post #29
and 'Richard_s' in post #28.

p.s. All the views 'calmly' being placed in this thread is proving to be quite helpful to me.
Last edited by flying-llama; May 21, 2009 at 04:36 PM. Reason: add Richard_s comment
May 21, 2009, 04:44 PM
Registered Plan-a-holic
Hepdog's Avatar
In the 3D sense of "stalls" and flying at and beyond the stall, the KFM is - in my humble opinion, gentler and smoother during the transition.

Case in point - I re-winged my trusty old Furious bipe with FKM top on top and bottom on bottom. A much smoother flyer around the stall area - no more snappy snappy and much less wing rock in medium harriers. More weight indeed - more foam. But I'll take an ounce for better flying.

As to other airfoils, unless you are building a 3m+ glass sailplane, I'd stick with Clark Y (or X) or fully semitrical if required. I just can't see how anyone can scratch build any fancy airfoils nearly close enough to spec to matter at the smaller sizes. Some of these airfoils only change by fractions of a mm - paint can do that.


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