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Old Mar 20, 2012, 05:30 PM
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Originally Posted by dave1993 View Post
there is a reason we gravitate to the undercambered single surface
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Originally Posted by dave1993 View Post
so we can go on and on about dragonflies
...
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Originally Posted by dave1993 View Post
and rogallo hang gliders but my comments generally refer to the kind of wings most rc hobbyists are concerned with.
Forget about 747's. And forget about hang gliders (though modern single-surface hang gliders, e.g. Wills Wing Falcon, have exactly the kind of cambered, single-surface airfoil that park flyers "gravitate to" as you put it-- and believe me I know from direct experience that it is a really bad idea to screw up the airflow over the top surface!) This idea that the low pressure on the bottom is the most important, are you just talking about the cambered "single surface wing"? Or just the flat plate? Both? Exactly why would you think the dragonfly article is not relevant? The author is specifically discussing both the flat plate case and the cambered "single surface" case. That's why I posted the links. Or it's not relevant to the cambered "singled surface" because it also mentions flat plates, and it's not relevant to flat plates because it also mentions the cambered "single surface" airfoil? Sorry but that sort of circular argument seems to be the direction that this discussion is heading.

So I remain curious-- what are your reasons for thinking the bottom surface is the most important (or that more high-pressure is generated below than low-pressure is generated above?) with flat plate and/or cambered single-surface airfoils? Enquiring minds want to know....

I'm not saying it is or isn't the case. I just want to know what is your reason for believing it to be the case. Simply because that seems to be consistent with the observation that flat plates and single-surface cambered airfoils work well in low-speed flight? If that's your reason, fine, but I'm not seeing that that (valid) observation really rings true with the idea that high pressure on the undersurface is the main source of lift with these kinds of airfoils. As far as I could see by looking carefully at the pressure plots, the dragonfly link and the other link with the 3 airfoils and the pressure plots for each, suggested that more than half the lift came from the low-pressure area on the top surface, even for flat plate and single-surface undercambered airfoils.

I'm not saying anything about Newton vs Bernoulli, cause vs effect, downwash versus pressure differences on airfoil surface, that argument is boring and has been going on forever ( http://www.twitt.org/miller.htm ) and I think anyone who is arguing one vs the other is wrong and not seeing the whole picture . Just asking what your reasons are for believing there is more high-pressure on the bottom than low-pressure on top, on the kinds of wings that you are interested in.

Steve
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Old Mar 20, 2012, 05:48 PM
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Originally Posted by Tom Harper View Post
Dave,

It seems that way.

From a design standpoint, lift is a given - fixed by weight. Drag is a budgetary item.

Tom
Careful not to fixate on absolute values when coefficients are more useful. It's a meaningful question to ask whether we are trying to design for high lift or low drag. Of course we are talking coefficients, not absolute values. Certainly lift ends up being exactly equal to weight in level flight, but that doesn't mean we might not want to design for high lift (meaning high lift coefficient) to allow smaller wing area and/or lower flight speeds (w/ associated lower power demand), etc. Even if (for some reason) we talking about an absolutely fixed weight and therefore an absolutely fixed lift vector (in level flight.) I'm sure you knew that but anything can be misconstrued in this thread it seems so thought would emphasize-- someone is going to claim you are saying "lift is a given, just reduce drag". Yes that's true if are talking fixed values not coefficients-- but we usually aren't, or shouldn't be. L/D (which is also (Cl / Cd) )is often a very key consideration, for example.

Steve
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Old Mar 20, 2012, 05:53 PM
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Originally Posted by richard hanson View Post
A lot of info can be gathered by using a simple model -the VAPOR- andthen spending a few hours in a gym ,flying the model at all speeds and workable angles of attack.
Any plate which produces a pressure difference top to bottom is a wing it can be flat or curved or painstakingly shaped to an exact shape as dictated by load/speed n size
ALL of the othe r info , Bernoulli ,Coanda Newton etc., describe some of what is going on and is necessary if one likes to calculate stuff.
Bottom line- lift is simple a result of pressure differential at work
trying to decide wether the top does more than bottom seems to me to be a waste of time
Monday I was doing trimmed ,hands off flight and the model was ever so slightly climbing.
I added a touch of down trim - the climb rate increased
Not sure what is the point of this post-- was it surprising to you, that the climb rate increased when you added some down trim? Naturally only one a-o-a gives the max climb rate, you simply started out a higher angle-of-attack and then fixed things to be closer to optimum (closer to max climb rate). We would all agree that we don't need to understand (or agree on) the deeper theory to complete this sort of task....

PS did the climb angle increase too? If so, are you aware that the wing was generating less lift than before you changed the trim? (In absolute terms, not just as a coefficient)

Steve
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Old Mar 20, 2012, 06:07 PM
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Originally Posted by aeronaut999 View Post
Forget about 747's. And forget about hang gliders Exactly why would you think the dragonfly article is not relevant?
hard to forget when everyone keeps bringing them up along with references to that scale. and dragonflies are lovely but few rc models are that tiny and flap their wings to stay aloft. (some but very few, you out there brian?)

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Originally Posted by aeronaut999 View Post
So I remain curious-- what are your reasons for thinking the bottom surface is the most important (or that more high-pressure is generated below than low-pressure is generated above?) with flat plate and/or cambered single-surface airfoils? Enquiring minds want to know....
first i suspect top pressure isnt really that much different than below, specially since reading that link you posted. but mostly im not convinced low pressure above is as effective as air from below. this is from playing with a piece of foam in front of and in back of a summer fan. im always more impressed by a good blow compared to things that suck.

dont bother to say how that issue has been settled earlier in the thread. maybe its just me but im less impressed by aerodynamic theory than real world obesrvations. actually seems to be just me. maybe rh too (who is that guy anyway?? lol!).
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Old Mar 20, 2012, 06:10 PM
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OK thanks for answering-- just wondered if you had some access to secret knowledge that I was missing.

"Faith is an island in the setting sun, but proof is the bottom line for everyone"
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Old Mar 20, 2012, 06:20 PM
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there is really no such thing as proof at the REAL bottom line implied by postulations of hume and descarte. it could all be a dream including the apparently immutable faith of some in reason, logic, and the scientific method. (faith in reason... lol!)

i am personally fond of experimentation though. if nothing else a fun way to kill time.
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Old Mar 20, 2012, 06:56 PM
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Old Mar 20, 2012, 07:04 PM
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not for much longer gramps. lol!
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Old Mar 20, 2012, 07:12 PM
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Originally Posted by aeronaut999 View Post
Not sure what is the point of this post-- was it surprising to you, that the climb rate increased when you added some down trim? Naturally only one a-o-a gives the max climb rate, you simply started out a higher angle-of-attack and then fixed things to be closer to optimum (closer to max climb rate). We would all agree that we don't need to understand (or agree on) the deeper theory to complete this sort of task....

PS did the climb angle increase too? If so, are you aware that the wing was generating less lift than before you changed the trim? (In absolute terms, not just as a coefficient)

Steve
Well the result was a bit of a surprise at first- and it demonstrated that that the power was too high for existing trim. the flying speed was somewher under 3-4 m per sec .
I had been playing with obtaining minimumforward flying speeds .
IF I had added a bit of up -it would have slowed down
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Old Mar 20, 2012, 07:17 PM
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Interesting, what data are you gathering.

Tom
Probably nothing that is of interest to anyone not actually involved in flying small light stuff..
Apparantly this is not a hands on forum - g' by
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Old Mar 20, 2012, 09:28 PM
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oh..newwwwww... when rich came onboard i harbored huge resentment cuz he was stealing my limelight. but now that hes gone i just feel this huge emptiness.... i feel so ALONE
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Old Mar 20, 2012, 09:36 PM
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Dave, you say "undercambered" a lot. What does that mean to you? And to everyone else, what do you mean by single surface? Even a flat plate has an upper surface and a lower surface.

Also, I don't have much experience with flat plates but I think cambering it doesn't do much to help it's short comings (other than increase CLmax).
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Old Mar 20, 2012, 10:04 PM
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what do you mean by single surface? Even a flat plate has an upper surface and a lower surface.
not necessarily.

if you had experience with "really small light stuff" you would for example know about os film. os is short for one sided. invaluable wing material for the sub-gram models. we all know a sheet of copy paper has 2 sides. imagine though, a material so thin it has only ONE SIDE! hence "os", one sided, film. boggles the mind. you can probably verify what im telling you with a quick search here on rcgroups. im serious.

anyway that is the principle behind single surface airfoils. imagine the drag reduction with wings so thin they only exist in one dimension.

there is some truth to this. i have personally created wing films by touching the surface of water in a pan with a stick dipped in collodion-like polymer. thickness was judged by interference colors in this normally clear material. this part is true.

regarding the airfoil, is it possible you have not seen a single one of the photos ive put up here and scattered just about everywhere else on rcgroups? like just a few posts back for example? what many do not accept is that no other profile competes in terms of simple lift. not even drelas works of art and science. the late jef raskin, also developer of apple macintosh, is credited with first testing this out and determining a 4% thickness at 40% chord is optimum for rc models (4-40 uc).

google is your friend. check out duponts os film for duration and other sub-micros while youre at it.
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Old Mar 20, 2012, 10:48 PM
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You can spare me the sarcasm. I was asking what you meant by something. A simple "When I say single surface I mean very thin" would do. Now, when I say flat plate I mean infinitely thin, unless the context is regarding an actual piece of foam that must have thickness.

What exactly are you suggesting the drag is on a flat plate? It will only be better than a simple airfoil such as a NACA 0012 for very small angles. The 0012 will have slightly more shear stress, but the flat plate will suffer from separated flow much sooner.
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Old Mar 20, 2012, 11:01 PM
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single-surface / double-surface / undercambered undersurface

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And to everyone else, what do you mean by single surface? Even a flat plate has an upper surface and a lower surface.
Of course your statement above is correct-- but some things there need to be a shorthand for-- it's clunky to say "thin, constant-thickness, cambered airfoil"-- ok from now on I'll say "TCTCA" instead of single-surface cambered airfoil... (actually I won't!) -- ok, "curved plate" would be best wouldn't it. There you go. Curved plate.

The single-surface/ double-surface terminology is familiar if you come from a hang glider/ ultralight background-- the single-surface airfoil is literally made from one single layer of cloth, except for the narrow pockets that the battens slide into, to hold the airfoil shape. You can make a "conventional" rectangular wing this way, as well as a delta or swept hang glider wing built on a Rogallo-style airframe. The double-surface airfoil is made from two layers of cloth, with air space in between. (The air space between the layers or "surfaces" is also good for containing structural components so that they are not exposed to the airflow.) Also, in the case of hang gliders, we speak of "60% double surface", etc, meaning that 60% of the wing area (the first 60% of the chord) is comprised of two layers of cloth and the remaining 40%-- the last 40% of the wing chord-- is comprised of a single layer of cloth. Sure, "single-layer" versus "double-layer" would be a more accurate terminology, but doesn't roll off the tongue quite as well.

No joke, someone once asked me if the double-surface area should be counted twice when figuring the wing area.

By this terminology, I believe the Wright brother were using double-surface (double-layer) airfoils right from the start in their full scale aircraft (though they certainly tested curved-plate airfoils in their wind tunnels.) In contrast, modern hang gliders evolved from the earlier Rogallo wings where the straight leading-edge tubes, and also the keel tube running along the aircraft center line, were the only structural or shape-giving parts in direct contact with the wing or "sail", which was a flexible single layer of cloth. With so little defining structure, there would be little point in two layers of cloth.

With regard to indoor free-flight models, I'm having a hard time believing that there is any significant profile drag reduction afforded by using some ultra-ultra thin film covering. Doubling or tripling the covering thickness will make a trivial drag difference compared to the drag of the frame structure that defines the leading edge and trailing edge. I suspect that the real benefit of the thin film lies in weight reduction.

In hang gliders, the benefit of the single-surface style is better roll control due to more effective flexing (deformation) of the surface in response to aerodynamic loads and also in response to shifting of the hang glider structure (keel tube). Also lower inertia in roll. So, better roll handling. Also lower pitch inertia and better ability to generate lift at extreme angles-of-attack, so better behavior in the landing "flare". Also, lighter weight so easier to carry the glider around on the ground. Basically, it's what you typically learn to fly on. Double-surface gliders have lower drag (better glide ratio for same weight and same lift force), lower drag coefficients (lower drag force for same airspeed), much better lift/drag ratios (same as Cl / Cd ratios), and probably higher lift coefficients (less wing area needed for same stall speed at same pilot weight?) (The last is hard to say for sure as the design stall speed is often a little higher with the double-surface, and the wing area is also much lower, so hard to say for sure that the lift coefficient is higher, but I suspect it probably is.-- the differences in wing area are pretty dramatic -- say 170 square feet for my "single surface" glider vs 135 square feet for my 70% "double surface" glider.)

If you want to read about the benefits and drawbacks of an undercambered lower surface (but not necessarily a simple curved plate of constant thickness), see this link: http://www.homebuiltairplanes.com/fo...t-fannies.html . Yes there probably are some errors, but looks like generally good experience-based advice.

This link notes how free-flight airfoils designed for one speed only tend to have undersurface undercamber, and how these airfoils evolved into flat-bottomed airfoils for RC glider use: http://www.gliders.dk/airfoil_history_class.htm

I'm sure I'm rehashing old ground here and you are well aware of the pros and cons of various airfoil shapes.

Yes I'm aware that by the aerodynamicist's system of specifying airfoils by the shape of the camber line and the variation of thickness along the camber line, an "undercambered airfoil" would mean something completely different than what we mean when we talk about an undercambered (concave) bottom surface. Also I'm aware that by the aerodynamicist's system of specifying airfoils by the shape of the camber line and the distribution of thickness along the camber line, it's an arbitrary or accidental distinction as to whether the undersurface happens to have any undercamber or not-- (change the thickness, and suddenly the "same" airfoil that had no undercamber on the bottom surface, now has some undercamber on the bottom surface.) The wind or airflow may beg to differ?

Steve
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