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Old Jan 10, 2012, 12:29 PM
Grumpy old git.. Who me?
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Originally Posted by DUMBERTHUMBS View Post
Would you guys please recommend me an airfoil for a 96" motor glider.
Thanks
I'll be careful what I say on this as I don't want to get anyone's 'back up'.. But I think it's fair observation, with which most would agree, that for gliders the typical laminated type KF airfoil is at a slight (in some cases not so slight) performance disadvantage compared to a really good 'normal' glider airfoil.

Of course they have the advantage of being quick, cheap and easy to build, so the choice is yours. I'd expect that multiple small steps would work better for a glider than one big one, and it's important to sand in a nice 'streamline' leading edge profile.

Some testing I did on my own glider: http://www.rcgroups.com/forums/showthread.php?t=1542598

Steve
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Old Jan 11, 2012, 11:09 AM
Jack
USA, ME, Ellsworth
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'..I'll be careful what I say on this as I don't want to get anyone's 'back up'..."

As long as we keep the discussion polite and constructive, our "backs are not up", right?

I certainly agree with your assessment that KF winged gliders are at a disadvantage when compared to the airfoils that are most often used for gliders. But I only with that if you throw out all considerations except performance.

And, for the record, I don't think any of the regulars here have ever said that they expected it to be much or any different than that.

I have found that scratch built foamie motor gliders can be built with wing loadings that are very similar to those found many of the gliders that are in use in sports and even lower echelon competitive gliding events.

And my personal findings on Big Blu 96's performance was that, with it's 3 fps or so still air sink rate. it doubled or even tripled the sink rates of the gliders in a sailplane performance analysis study I was using for a comparison. But that was only my first build, that is not a very refined design.

I'm new to gliders, only have a scant basis for comparison, but I'll go out on a limb here and say that I think that there is plenty of performance and room for joy and satisfaction to be taken from the building of KFm winged gliders.

You are to be complimented for your meticulous and well thought out and documented comparison on the Elf DLG and the KFm3 clone of that.

I was really glad to see you build that and followed the thread (lurking quietly) from end to end. That threads stands as one of the most (if not the only) fairly objective comparisons we have for a plane built with a KFm wing that emulates the planform of a non-KFm wing. And for which we have some data with a comparison of the two in flight.

I know the results are subjective but I consider your assessment in the comparison summary in post #88 to be a honest and objective description of what people can expect to get when they build a KFm winged DLG.

"..This works out that the KFm duration is 61% of the Drela..."

But I would caution them that they also have to be as careful in their material selection, as meticulous in their workmanship, and work at it as hard as you did if they expect to be able present or claim the same results you got.

Some of the other reasons that we build in foam and build gliders in foam were not addressed as a result of the comparison. Things like cost, speed of build, ease of building, and the like.

But I would not be dishonest if I told someone that you can build a KFm3 winged DLG for less that $10 (my guesstimate) that has 61% of the performance of a DLG that sells for $199.

That is worth something to some of us. And then there are things like the joys of having built it instead of having bought it, the joys taken from flying it, and other simple satisfactions like that.

I know we (or was it just me?) have left you a little thin skinned about the way we reacted to some of your comments here. If you want to see the other side of the coin on that, read this recent thread over on the Sailplanes Talk forum:

"aerofoils for very slow" - http://www.rcgroups.com/forums/showthread.php?t=1564581

That is, to me, a classic example of the negative and irrational reactions that people the "sailplane community" bring to almost any and every discussion of gliders with KFm wings.

I read a question asking for opinions on "..fairly thin ,flat bottomed sections for simplicity of building..." and posted a suggestion that the KFm airfoils might be worth looking into.

My post drew a quick comment that that KF airfoils "..did not deliver on their promises..." and an implication that my suggestion had no merit. An attempt to discuss that with a person who appears to consider himself to knowledgeable about gliders drew little more than additional misinformation about KF airfoils and confirmed that person's full and complete ignorance of where and how the KF airfoils are being used in RC.

And when he played the personal insult card and accused me of being closed minded about the "facts" he had presented so I simply went away. I couldn't see any reason crap up that thread arguing the point. Besides, when you argue to distance yourself from idiots, it can be difficult to set yourself very far apart from them.

Are we just reacting to responses like that here? Are we getting even? Is this all the normal give and take as far as the open mindedness of the respective communities here?

Or is there not the slightest chance that the KFm winged glider can ever serve any purpose in the "glider community"?

Jack
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Old Jan 11, 2012, 11:47 AM
United States, FL, Hollywood
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kfm 9 look kool, hope it works good with 25% cord ailerons spanwise.... thank you very much guys
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Old Jan 11, 2012, 01:20 PM
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Originally Posted by jackerbes View Post
you can build a KFm3 winged DLG for less that $10 (my guesstimate) that has 61% of the performance of a DLG that sells for $199.
i suspect you can build one for $10 that has 90%-95% of the performance by leaving out the kfm.
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Old Jan 11, 2012, 01:47 PM
Grumpy old git.. Who me?
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Jack,

Thanks for the supportive words regarding the glider testing I did. it was really just a piece of fun to satisfy my own curiosity, well short of true definitive scientific test. But i did do my best to do the test in an even handed way. I plan on doing some follow up testing with the steps covered with film but flying opportunities are very limited this time of year in these parts.

Regarding the exchange with Don Stackhouse... Don is a very knowledgeable guy being the designer of propellers for real aircraft (including some very famous ones like the Rutan Voyager) and also having a business designing and selling RC gliders. So he does have impressive credentials and is worth listening to.

However I think the reason why you are likely never to reach any common ground with the likes of Don is because he comes at the debate from the diametrically opposite side. For him performance is almost everything, ease of construction, ability to build using cheap readily available materials and with little specialist tools or expertise are all issues that come much lower down his list of priorities. The typical foam KF builder usually puts a very different slant on the priorities, he's willing to trade performance for cost, ease and speed of construction. For Don this simply wont compute.

Even if you did 'corner him' on the ease/cost of construction argument, he would probably argue that he could almost as quickly build a balsa wing with a conventional airfoil that would perform better, or he could hot wire cut a foam wing even quicker out of the same materials and at the same cost as you could do a KF. Fact is he probably could because he has all the tools and has been doing it professionally for many years, but of course that's not true of everyone.

I suspect on this one that the opposing players will just have to agree that it's something that they cant agree on What is useful though is having some idea of just how much or little performance you might trade for convenience, which was the purpose of my little test.

At the end of the day the most important thing (in fact the only important thing) is that we enjoy the hobby. What it takes to enjoy the hobby is different from one person to the next, but that's ok.

Steve
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Old Jan 11, 2012, 08:11 PM
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Steve: I think you nailed it on the "coming at it from diametrically opposite sides" and the slight chance of any common ground. But one item in the original poster's request is really important here, and that's the "motor glider" aspect. In my limited experience, and I think, in the Blu Sail, BluGuppy, and Jack's Big Blu, having the motor to get the plane up into the thermals and back to home if needed adds a lot the the fun aspect and mitigates the inefficiencies in the KFM airfoils. So if he wants to just go out and have fun flying power off in lift for a few bucks with a plane he's built, he's going down the right track, if he wants to compete in national soaring contests, maybe not.....

The longest flight I recall for a OneSheetGlider (KFM2) was 53 minutes with only a few of that power on in fair lift ( I say fair lift, as it was at a metropark here in MI where there doesn't seem to be consistent lift anywhere around). So for fun flying and thermal hunting KF does it too.
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Old Jan 11, 2012, 10:18 PM
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Magnificent performance on a budget can be had by flying your 'not so efficient' glider on a hint of power, just enough to make the sink rate similar to the Big $$$ sailplanes.
Maybe it's not the go for everyday, but it's interesting to experience a little taste of how the good ones go. (Sorta kinda, a bit...)
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Old Jan 21, 2012, 12:35 PM
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Against my better judgement I'm going to chime in here, seeing as Jack has already been making disparaging remarks here behind my back.

The original poster in the thread Jack was referring to was specifically looking for airfoils that would improve penetration without hurting his low speed performance. A K-F airfoil would not help either of those parameters.

Steve, thanks for your comments, which were fairly close to the mark in many respects:

Quote:
Originally Posted by JetPlaneFlyer View Post
...Regarding the exchange with Don Stackhouse... Don is a very knowledgeable guy being the designer of propellers for real aircraft (including some very famous ones like the Rutan Voyager) and also having a business designing and selling RC gliders. So he does have impressive credentials and is worth listening to.
For the record, I've been systematically studying low Reynolds number aerodynamics in general, and airfoils in particular, since 1967. During the period that our company's main focus was on competition sailplanes, 1.5 meter RCHLG's in particular, for a number of years running roughly half the field at the NATS flew our designs. Airfoil trends that we started persist in Thermal Duration sailplanes to this day.

Quote:
However I think the reason why you are likely never to reach any common ground with the likes of Don is because he comes at the debate from the diametrically opposite side. For him performance is almost everything, ease of construction, ability to build using cheap readily available materials and with little specialist tools or expertise are all issues that come much lower down his list of priorities.
Well, not exactly. All of those properties are very important in our designs, not just performance. In fact, we put more priority on handling qualities than raw performance. If the average flier cannot fly the airplane consistently to the limits of its performance capabilities, then the plane's performance is not useful to that pilot. It's like "altitude above you", you can't use it and definitely can't count on it when you need it.

Likewise, if the plane is so "edgy" in its behavior that trying to fly it is like nailing jelly to a tree, your mental capacity will be consumed by the task of flying the plane, leaving little capacity left to maintain situational awareness, plan strategy for the rest of the flight, or even recognize the signs of entering lift or sink.

Also, such an airplane will not be fun to fly for most people , and an airplane that is not fun to fly is not likely to get flown very often. Contrary to what some folks might want to believe, most of the modelers I know exhibit relatively low levels of masochism.

The same holds true for building. A model that is easy to build is likely to be more popular in the marketplace than one that has a reputation for being difficult to build.

So, ALL of those parameters are important. The trick is coming up with a design that satisfies all of those requirements without compromising any individual parameter any more than absolutely necessary.

Quote:
...Even if you did 'corner him' on the ease/cost of construction argument, he would probably argue that he could almost as quickly build a balsa wing with a conventional airfoil that would perform better, or he could hot wire cut a foam wing even quicker out of the same materials and at the same cost as you could do a KF. Fact is he probably could because he has all the tools and has been doing it professionally for many years, but of course that's not true of everyone.
Well, almost true. Yes, I have a fairly well-equipped shop, although not nearly as fancy as you might think. Furthermore, for my kit designs to be successful, I have to design for customers who have an absolute minimum of equipment.

The fact is that there are wing construction methods that are as easy to construct, or even easier, than a K-F airfoiled design, and that do not involve the significant performance penalties inherent in the K-F sections. For example, the Jedelsky type wings are much better in performance than K-F sections, and are at least as easy to build. The wing design we use in our Roadkill Series models is close to the construction simplicity of a K-F section, and has much better performance. The Depron foam construction used in the Pibros delta-winged glider is essentially a stack of foam that creates what amounts to a K-F shape, but then covers the top of that with a curved sheet of foam that bridges over the steps and yields a smooth airfoil shape that gives conventional airfoil performance with nearly the same simplicity as a K-F section construction.

Quote:
I suspect on this one that the opposing players will just have to agree that it's something that they cant agree on What is useful though is having some idea of just how much or little performance you might trade for convenience, which was the purpose of my little test.
Just about all of the sailplane fliers I know would immediately and absolutely reject a design that includes a 30% sacrifice in performance. A design that has a minimum sink rate of 3 fps would also be received with little or no enthusiasm, and with good reason. In the weak lift common in many parts of the country (such as where I live), such a design would not be able to work many of the average thermals. If you brought the sink rate down by going to an extremely low wing loading, the penetration would be so poor that the plane would not be able to fly home in even a modest breeze. Oh, BTW, every serious sailplane flier I know of, including the electric sailplane enthusiasts, would consider using power to make up for a basic shortfall in L/D and/or sink rate to be "cheating". It is absolutely against the rules for any and all sailplane competition work, including electric sailplane competition, even for the most impromptu local club contest.

In addition, a powered airplane that suffers from a lower L/D requires more power to meet its mission requirements, and that means more weight and cost in the powerplant to make up for it. This of course means more weight, size and cost in the airframe to carry that bigger powerplant, which also means more power required to make up for the increased airframe weight and whetted area. It's obviously very easy for that situation to snowball. I know of at least one very high-profile turboprop full-scale airplane that got into just such a trap, in their case due to a poor aerodynamic integration of the airframe with the powerplants. The poor aerodynamic efficiency required a large increase in installed power and fuel burn to meet their flight performance guarantees, which impacted their operating economy. In the end the plane was a very expensive commercial failure.

Regarding stall characteristics, from what I've seen the K-F sections with their steps on top generally have "good" stall behavior because they are essentially already stalled. Looking at the performance penalties, L/D, etc., they do indeed fly that way, similar to the performance of conventional airfoils when flown beyond their stall. For example, the once common Eppler 205 has a very gradual trailing-edge stall that allows some airplanes to operate part way into a stalled condition. I have used this on E205 airfoiled models as a method for glide slope control on landing, something like using spoilers. The L/D in that condition is similar to what a K-F section would provide. The negative side of this is that it's very easy on these planes to slow down too much in a thermal turn, and lose large amounts of precious altitude before realizing it. We had a similar problem with one of our early versions of the Monarch 1.5 meter HLG where the stall was too gentle, allowing pilots to get into an altitude-wasting "mush" in a thermal turn. We altered the airfoil at the root to create a more well-defined but still gentle stall break, and the problem was solved.

The bottom line is that an airfoil with an excessively gentle stall is not necessarily an advantage.

Eventually K-F sections will suffer a leading edge stall in front of the most forward step, so it is possible to have a definite stall break (perhaps a nasty one). Of course the drag just prior to that will be so high that it's doubtful too many designs would ever push themselves that far, unless they are doing extremely high alpha work. In addition, some tailoring of the plane's stall characteristics is possible through things like planform changes, stall strips, etc..

Note also, K-F sections most certainly do not have a monopoly on benign stall characteristics, nor is it safe to assume that any and all designs with K-F sections will inherently have such characteristics. Because it is possible to do some tailoring through things like planform, it is also possible to accidentally come up with a design that has undesirable characteristics.

So, K-F sections can be easy to build and cheap to build, but there are other options that can claim the same. K-F sections generally have good stall characteristics, but it would be very dangerous to assume that they ALWAYS have good stall characteristics, and in any case, there are other sections that also have equally good stall characteristics, especially when planforms with significant 3-D flow are considered. A Pibros is quite stable, predictable and controllable at angles of attack approaching 90 degrees, using a conventional airfoil. The fact is that there are other design and construction options that provide the claimed benefits of a K-F section, but without the performance penalties.

The key to good airplane design is to thoroughly understand the plane's "mission profile", then use the design features that working together do the best overall job of performing that mission. I personally have no allegiance to any particular design or construction concept, tail design, airfoil concept, materials choice, or whatever. I do my very best to not allow what amounts to "religious" arguments to cloud my judgement, I do my very best to make my design decisions on a coldly objective basis. I use WHATEVER combination of design and construction features does the best job of achieving that design's requirements.

If a K-F section was the best choice for a particular design, I would definitely use it, without hesitation. However, in my experience, for the vast majority of applications, there are other options that result in a better overall airplane for the mission. In the case of the thread Jack referred to, as I indicated above, a K-F section would not have provided the improvements the originator of that thread was looking for.
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Old Jan 21, 2012, 03:29 PM
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Steve Wrote: "I plan on doing some follow up testing with the steps covered with film"

We look forward to your report when you get a chance to do this. Flying conditions for any meaningful results are close to non-existent here too at this time of year. Many days (if the wind's not howling & the snow passing by horizontally at an impressive pace) there's 95% cold / sinking air with nothing approximating ideal 'neutral' air in which to try to do a test flight comparison. On the rare calmer sunnier days, a modest volume of warmer air may move through a flying area for a short period, followed by several more minutes of cooler sinking air again.

I'm anticipating, as others have already said, that covering the steps on the wing which you built will not noticeably increase it's performance in comparison to the DRELA wing.

I do wish you the best of success in your ongoing experimentation!

VIKING
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Old Jan 21, 2012, 04:18 PM
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Quote: Don Stackhouse: Oh, BTW, every serious sailplane flier I know of, including the electric sailplane enthusiasts, would consider using power to make up for a basic shortfall in L/D and/or sink rate to be "cheating". It is absolutely against the rules for any and all sailplane competition work, including electric sailplane competition, even for the most impromptu local club contest.
How can it be cheating if nobody is being cheated?
In Post #712 I was just remarking that you can get a 'taste' of high performance by adding a sniff of power to a not-so-good glider.
Don't get me wrong. I'd be totally outraged if it was done in a contest.
But having a quiet fly with a $20 powered glider, wondering what it would be like to have a $2000 sailplane up there... nothing wrong with it.
Thinking about it, I haven't done it often. It's more fun to try to get the best out of what you have and I enjoy a spectacular climb back to altitude as and when required.
My last sailplane contest was so long ago that there was no RC category (World-wide) and the models had to to built entirely by the entrant.
And 'built' means exactly that. Not like those RTF dudes that bang on about how they 'built' their already built plane.
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Old Jan 21, 2012, 05:06 PM
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You're cheating yourself. If for a similar effort and cost you can have something that doesn't need help from a motor to soar, that has excellent handling without compromising performance, then going with a concept that does require that kind of help is not the optimum approach, regardless of whether the criterion is cost, ease of construction, or flying capabilities.

All of the kits we presently sell are laser-cut wood construction. They fly great without a motor, or with a motor, have excellent soaring performance at all speeds without needing help from a motor, and are not very expensive ($50 for the 1.5 meter, $85 for the electric or the pure sailplane 2-meter). There are quite a few outstanding sailplanes available, kits or ARF's, for far less than $2000. There are also a number of excellent designs available for free on-line, such as Mark Drela's designs, that have excellent performance, relatively low materials costs, and that are easy to build.
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Old Jan 21, 2012, 05:12 PM
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The VortexcCell2050 Project

Final Report - VortexCell2050 (Fundamentals of actively controlled flows with trapped vortices)

Quality validation date:2011-04-14

Abstract
The 'Fundamentals of actively controlled flows with trapped vortices' (VortexCell2050) project aimed at delivering a new technological platform combining two cutting-edge technologies, the trapped-vortex and the active flow control. Trapping vortices is a technology for preventing vortex shedding and reducing drag in flows past bluff bodies. Active flow control is a form of control which requires energy input. The project outcomes will serve the designers of the next-generation thick-wing aircraft, and will also be applied in other areas where reduction in drag in a flow past a bluff body is desirable.

VortexCell2050 developed a tool for vortex cell design, collected a substantial amount of data on three-dimensional and actively controlled flows in vortex cells, and demonstrated the advantage of a thick airfoil with a properly designed vortex cell with active control over a thick airfoil without a vortex cell, thus opening a possibility of trying this technology in specific applications. VortexCell2050 also highlighted several promising avenues for further improvement of the vortex cell performance. The results of VortexCell2050 ensure European aeronautical sector a leadership in a small but critical area, the importance of which will grow in the future with an increase in aircraft size.

The software tool developed in the project optimises the vortex cell shape for the case of zero mean flow rate of the stabilisation system, as it was initially preconceived, while the developed control scheme requires a non-zero mean flow rate. Therefore, further developments should either generalise the optimisation tool to non-zero mean flow rate or achieve control with zero mean flow rate. The developed scheme of active control should be classified as an open-loop scheme, rather than the initially envisaged closed-loop scheme.

However, the numerical results obtained but not tested in experiment suggest that there are significantly more efficient control schemes. Further work should be concentrated on unsteady and feedback control schemes. The performance of thick airfoil with a vortex cell and control system designed and tested was observed to be better than the performance of the thick airfoil with control system but without the vortex cell, but only in a certain range of the angle of attack, and, more importantly, only when the flow past an airfoil with a vortex cell was in the more favourable branch of the hysteresis loop of the flow regime.

While methods of attaining the favourable branch were identified numerically, further experimental research should take care that the corresponding provisions are made in the design. The results obtained in the project provide a significant step forward as compared to the state of the art. In particular, the development of the software tool for optimising the shape of the vortex cell, the significant body of data on three dimensional effects collected, and the significant body of data obtained on the actively controlled flows past airfoils should be distinguished.

The results of the project were reported at about 40 conferences and in about 20 journal articles. The partners developed close links over the duration of the project, with many parts of the research done in genuine collaboration. As a group, the partners now have a world-leading position in the area of actively controlled flow with trapped vortices.
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Old Jan 21, 2012, 05:35 PM
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I can't be cheating myself. I know if I've got the prop going. I'm stupid, but not that stupid.
The plane I've done this with, infrequently and not for an entire flight, is an RTF that my family bought me as a gift. So I'm stuck with it, but I'm not such a complete elitist/purest that I would deny myself a small sample of what a really good sailplane can do. A bit like a Flight Sim.
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Old Jan 21, 2012, 09:28 PM
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Don and Steve: what you both say is all true, for most rc flyers, I'd guess (certainly i can agree with the science aspects), but I have to chuckle at the discussion on this whole page, it's what I see a lot on the builder's workshop and model science forum, but not typically here on scratchbuilt foamies forum. We're sort of talking past each other, I think. I suspect that's because more guys here are into the fun side of the hobby than the competition side. Personally, standing around watching others fly their planes in a contest while waiting for my turn is about as much fun as watching baseball, and that's slightly less fun than watching my dog sleep. I'm just not interested in competition. and I'm a card carrying member of the funperbuck club so cheap is good. I look at the 50 bucks for your 1.5meter kit, which I'm sure is an outstanding performer, and I see 3 sets of gear and a half dozen airframes in my chosen material. let's see, 1 or 3, hmm.... My "kit" costs a buck and a half if one has to buy the dowel for the LE spar. And several of us have spent fun afternoons all flying together each on a couple of 900mah 2s packs our OSG's outflying and out thermalling other guy's radians (not that that is a high performance machine, but it's way more expensive, eh?) Incidently, I have tinkered with wings on the OSG, currently have 4 (KFM2 square tipped, KFM2 rounded tips, KFM5a (UC) and conventional airfoil) The first and last fly essentially the same to my unprofessional eye, while the UC is only good for really light air, but fun with that. So, there I go, talking past you too.

I do like the mission profile statement, and that may be key, Perhaps ours are different than "normal" rc flyers. We are a little strange here, flying building insulation and posterboard....
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Old Jan 22, 2012, 07:58 PM
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Quote:
Originally Posted by Whiskers View Post
...The plane I've done this with, infrequently and not for an entire flight, is an RTF that my family bought me as a gift. So I'm stuck with it...
Yes, but with a few cents worth of craft paper and white glue you could smoothly bridge over the steps in the airfoil and get an increase in performance.
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