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Old Sep 19, 2012, 09:16 PM
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Originally Posted by richard hanson View Post
What made the biggest differences was always - weight reduction.
As is usual with all of aeronautics. Lightness trumps all other design factors. The one exception is if you have stupidly powerful engines like we have now since the advent of lipos and brushless. Then thrust to weight trumps everything else.

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Originally Posted by richard hanson View Post
My gliders --anything like a Clark Y worked about the same
They work about the same until you have to compete. Then it will be obvious just how bad the Clark Y is at gliding. From poor penetration all the way to being very insensitive to thermal signs. Again, like the F1B rubber powered free flight mentioned above, it's competitions that forces people to develop airfoils. That's why you see lots of talk of how to use and interpret fluid dynamics software in the glider forums.

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Originally Posted by richard hanson View Post
The indoor aerobatic types -flateplates work great- -I can carefully shape 1/16" thick foam to taper or round edges but there has been no change in performance from a simple square cut flat plate on the.these models
Actually, there have been some work done that have shown that flat square leading edges perform better at indoor aerobatics because they can be stalled earlier and more predictably. 3D aerobatics are all about flying post-stall.
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Old Sep 19, 2012, 10:41 PM
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thermal huning is now out for me- -baaad eyes at distances
indoor stuff a blast. do a bunch of it .
My old TOC Bucker just sits in the corner --
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Old Sep 21, 2012, 11:08 AM
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My gliders --anything like a Clark Y worked about the same
The thing is, with a glider, pretty much all we have to make a difference is the wing section. And reducing weight isn't the game changer it can be in powered aircraft, as we are relying on gravity to give us the momentum to move the wing through the air. Too light, and the model simply won't fly fast enough for either the task in hand, or to give a useful enough maximum LD for thermal seeking, cross country etc.

Witness, that in F3b competition and in F3f competition, it is desirable (and necessary) to increase the wing loading of the model in order to maximise either speed, penetration into wind, or LD (ie in F3b distance).

Again, look at the weight of the fastest DS models. If the model is too light, it won't perform (in that situation)

So, in that regard, wing section selection is very critical. A glider equipped with a Clark Y wing would possibly not fair too well in an F3f competition in a 40mph wind.

So, again, design for the task.

Look at the wing sections employed for DLG, a Clark Y would be far to draggy to acheive the launch speed/height required.

Have a peek at the wing sections the likes of Joe Manor are using on DS models like the Dynamic. It isn't your typical "glider" wing section, it is specifically designed for maximum performance in a particular role. Clark Y on a 400mph+ DS glider? It won't work.

We could make a list of applications where Clark Y would be the wrong choice, just as we could make a list where it would be entirely appropriate. The fact that in YOUR experience, the gliders which you have flown have been the type where that section works, does not mean that across the board, the choice of wing section is irrelevant to performance.

I am quite sure you have a great deal of experience and that experience has lead you to certain conclusions, but all of us have varying degrees of experience, and my mediocre 20 odd years of flying gliders of all descriptions informs me that wing section selection is of pretty high importance. In fact, most of my own design models have started from the basic principal of selecting a wing section most suited to the "mission profile" and then designing the model around that.
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Old Sep 21, 2012, 11:15 AM
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Makes perfect sense to me
My gliders were never competitive types -
My competitive powered stuf tho - prooved that cross sections were pretty much non critical as th speed range and maneuverability soon prooved that building for min weight trumphed almost all other factors
Now ,we simply go for thinnest section which will stand the loads - bending n twisting
on the tiny stuff - flat plates do the best job.
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Old Sep 21, 2012, 12:16 PM
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Richard, I think we can concur on most of what we're discussing, I am not too well versed in power flying, and especially aerobatics, so I am more than willing to believe your point that weight is more important.

Tiny stuff and flat plates? Again, I think I can concur with that also, or at least, I am willing to believe that a flat plate is as good or better than anything else we currently have in our armoury.

So all in all, I think we are all on the same page with the notion that every "task" is different in how critical wing sections are. In some scenario's, the choice is vital, in others, other criteria such as weight, rigidity, weight distribution and quite possibly ease of building (which for sport flyers, is probably up there among the prime motivators) take on more importance, to a point where airfoil selection may become a minor concern.

Quite possibly, the performance differences between sections, in particular scenarios, is too insignificant for us to notice, or at least, for ME to notice. I can say that, in gliding certainly, the difference can be quite palpable, if not measurable. As an example, a slope glider with an E374 wing section, feels completely different to one with an E205. Even if the performance differential isn't dramatic, which in the case of two models I had which were otherwise very similar, the "feel" is very different. Now, I don't know what that "feel" is or how it could ever be quantified, but it was an almost tactile feeling. A "personality" if you like.

Which could lead us into a discussion concerning the metaphysics of model aircraft design
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Old Sep 21, 2012, 01:12 PM
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Is is common to see BIG differrences in design- for the same task-
I built models for competition-in F3a- TOC IMAC US Pattern etc-- fo many years
These were flown by different parties . results ranged from 1st thru whatever --the thing noted tho was that there was big differences in design concept in TOC/ and "follow -the - leader in other classes.
I have tossed the tx to others -on planes which I setup for my tastes and th results ranged from "that's perfect!" to -" No- too tailheavy!" or " No- too noseheavy"

in other cases th flier liked to holda certian amout of down or up trim as they flew - so getting a "ideal " setup was simply "ideal for whom?"
Some guys swore that a sharpLeading edge was needed for clean snaps - till they flew a model setup with blunt LE but more aft CG-
When I hear comments about "perfect airfoils - I wonder "perfect-for whom?"
Even the perfect Spitfire wing -ended up with clipped blunt wings and the "unbeatable" Mustang airfoil was replaced --
I must sound jaded
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Old Sep 24, 2012, 11:46 AM
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I'm thoroughly confused by the whole premise of this discussion. The fact that flat plates might work the best for a specific size aircraft doesn't mean that airfoil sections don't matter at small sizes- it just means that a thin airfoil is best for that particular model. I believe the reason I (and probably many others) like flat plates for small 3D stuff is that the airfoil has what would typically be considered 'poor' stall characteristics for most other types of models- the airfoil has very low drag at very low angles of attack but flow separates from the leading edge at even moderate angles of attack, which helps significantly when we're trying to slow down quickly. For example, it's more difficult to quickly transition from fast level flight into a vertical hover without gaining altitude if you have a 12% airfoil regardless of the aircraft's weight, because the wing is able to pull a higher Cl without creating a huge amount of drag. However, the thicker airfoil will often have nicer high alpha characteristics with less wing rock (in my experience.)
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Old Sep 24, 2012, 09:19 PM
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Originally Posted by RC_AV8R_181 View Post
I believe the reason I (and probably many others) like flat plates for small 3D stuff is that the airfoil has what would typically be considered 'poor' stall characteristics for most other types of models
That's exactly the reason. As I mentioned, flat plates with square leading edges stalls earlier. And that's a good thing when you're doing post-stall-flight (also known by it's popular name as 3D aerobatics).

Indoor competition pilots started with flat plates simply because it was the simplest structure and less structure = less weight. Initially some pilots round the leading edge of their wings for reduced drag. But over the years they've stopped rounding or beveling the leading edges because square leading edges stalls earlier and in a much more controllable manner.

There is something funny that happens however when the wingspan approaches 1 meter. The increased Re numbers at the larger sizes significantly reduce the effectiveness of flat plates such that they cannot compete with even the most simple eyeballed symmetrical airfoils. But that's only when you care about the performance. I've seen people build very large planes with flat plate airfoils.
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Old Sep 24, 2012, 09:55 PM
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Another interesting thing that happens at very low Re is that optimized airfoil designs start to resemble flat plates with slightly bent leading and trailing edges. I remember the paper mentioning that slight variations in the shape of the airfoils does not affect their performance much.

I don't remember which paper it is from. I just took a screenshot of the airfoils for reference. But it is interesting to note that it looks similar to WW1 airfoils. Compare the airfoils to the RAF15, the airfoil used by the Sopwith Camel.

Another interesting thing to note is that the optimized airfoils look nothing like the airfoils used by typical rubber powered indoor duration planes.
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Old Sep 25, 2012, 11:34 AM
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Many of us just got tired of trying to "discuss" anything with Mr. Hanson, who never adds anything but his opinion.

It will not be possible to observe many differences in airfoil performance on indoor foamies, with external bracing, drag panels, very light wing loading, and lots of power. Drag differences of airfoils optimized for L/D or max Cl, even at the correct Reynolds number, would be lost in the huge drag from the bracing and drag panels. Plus they won't fly upside down that well, so they won't be very popular for indoor aerobatics.

That is one thing you have to be careful of at low Re scales: airfoils optimized for stand-alone performance may not give the best aircraft performance. If the airfoil is so thin that has to be externally braced, then any drag reduction will be more than lost. A thicker airfoil that provides some structural depth and allows a wing to be cantilevered with internal structure may allow the overall aircraft L/D to be much better, even though the thicker airfoil doesn't perform as well at low Re. Assuming you are interested in aircraft L/D or max endurance.

Dragonflies use a corrugated airfoil, because the aero performance is quite good at very low Re, and it has minimum material and weight to carry the structural loads as well (attached).

http://michael.friswell.com/PDF_Files/C268.pdf

Or for smooth very low Re airfoils:

http://aero.stanford.edu/Reports/NDPAPER_kunz.pdf

The optimum airfoil depends on the aircraft requirements and overall design. There can be very big differences in the resulting aircraft performance, which is why so many airfoils have been designed - but a flat plate may well be optimum for a 3D indoor aerobatic airplane with external bracing, low wing loading, and lots of power for the weight.

Kevin
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Old Sep 25, 2012, 12:22 PM
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Originally Posted by slebetman View Post
I don't remember which paper it is from. I just took a screenshot of the airfoils for reference.
It's from Kunz and Kroo's Mesicopter work: http://www.niac.usra.edu/files/studi...rt/377Kroo.pdf

It should be noted that these airfoils are optimized for max L/D, which in part is why they have the strange camber break so far back. If I remember correctly, other work from Kunz implied that the this feature acts somewhat like a 'separation ramp' in that it prevents separation from moving forward as quickly as an airfoil with the high point farther forward. However, an airfoil optimized for lower Cl (perhaps like an F1d model) may very well have the high point farther forward with no sharp camber break, as separation will remain near the trailing edge anyway.
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Old Sep 25, 2012, 01:48 PM
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Originally Posted by RC_AV8R_181 View Post
It should be noted that these airfoils are optimized for max L/D,
I hope you guys don't mind me interjecting. The word "optimized" should always be followed by "for...", or something to that effect.

For example:
"The airfoil was optimized for minimum drag at CL=0.4"
"The airfoil was optimized for minimum drag over a range of CL's"
"The airfoil was optimized for maximum L/D given a constraint of thickness=12%"
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Old Sep 25, 2012, 02:42 PM
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Originally Posted by kcaldwel View Post
Many of us just got tired of trying to "discuss" anything with Mr. Hanson, who never adds anything but his opinion.

It will not be possible to observe many differences in airfoil performance on indoor foamies, with external bracing, drag panels, very light wing loading, and lots of power. Drag differences of airfoils optimized for L/D or max Cl, even at the correct Reynolds number, would be lost in the huge drag from the bracing and drag panels. Plus they won't fly upside down that well, so they won't be very popular for indoor aerobatics.

That is one thing you have to be careful of at low Re scales: airfoils optimized for stand-alone performance may not give the best aircraft performance. If the airfoil is so thin that has to be externally braced, then any drag reduction will be more than lost. A thicker airfoil that provides some structural depth and allows a wing to be cantilevered with internal structure may allow the overall aircraft L/D to be much better, even though the thicker airfoil doesn't perform as well at low Re. Assuming you are interested in aircraft L/D or max endurance.

Dragonflies use a corrugated airfoil, because the aero performance is quite good at very low Re, and it has minimum material and weight to carry the structural loads as well (attached).

http://michael.friswell.com/PDF_Files/C268.pdf

Or for smooth very low Re airfoils:

http://aero.stanford.edu/Reports/NDPAPER_kunz.pdf

The optimum airfoil depends on the aircraft requirements and overall design. There can be very big differences in the resulting aircraft performance, which is why so many airfoils have been designed - but a flat plate may well be optimum for a 3D indoor aerobatic airplane with external bracing, low wing loading, and lots of power for the weight.

Kevin
You obviously have no real experience with the setups I have mentioned - Your theorizing is fine - for those who simply like to postulate
Try stuff in actual practice and then comment.
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Old Sep 25, 2012, 02:50 PM
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I prefer experience over book-larning when it comes to toy airplanes, many of which operate below the threshold of investigation by full-scale researchers.
Ever tried ailerons on a Gentle Lady?
Miserable failure, wasn't it?
Why?
The downgoing aileron creates drag on the wrong side of the plane when turning, due to the slow speed.... high drag of the flat bottom wing.
But ailerons on a same sized GP Spectra work very well... due mostly to the shape of the airfoil.. the plane flies faster than the GL... Selig 3021.
Flat plates for small 3-D planes work very well, again because speed is not important, but the manuver enhancing drag is.
A true Clark-Y is a good airfoil for most purposes. There are many similar to it in use.
The Gentle Lady is pretty much the flat-bottom airfoil that many folks say is a Clark-Y.
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Old Sep 25, 2012, 03:10 PM
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One would think that in a model forum, the writer actually has some practical experience to back up their thoughts
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