stunthanger.com
General control line discussion => Open Forum => Topic started by: sleepy gomez on April 05, 2012, 08:37:59 PM
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Why doesn't one of the top flyers spend the time to get a biplane competitive.
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Have you not heard of the Biplane Curse?
Whenever a top flyer attempts to get a biplane tune up, he gets struck by lightning. (Or at least shocked badly enough to go to the hospital).
Heavens, man, it's happening to one of us right now!
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I suspect its the same reason there are few competitive biplanes in pattern and sport aerobatics . They are difficult to trim, heavier than a mono and generally not worth the extra effort of transporting them and assembling them. They look cool though.
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Why doesn't one of the top flyers spend the time to get a biplane competitive.
Take a look here, John Miller has been working on this for a while...
http://stunthanger.com/smf/index.php?topic=19379.0
I imagine that any further development will be put on hold for a while since John recently suffered some severe electrical burns. See the posts here in the Open Forum.
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Have you not heard of the Biplane Curse?
Whenever a top flyer attempts to get a biplane tune up, he gets struck by lightning. (Or at least shocked badly enough to go to the hospital).
Heavens, man, it's happening to one of us right now!
That might be funny in a year or so...
Brett
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That might be funny in a year or so...
I sincerely hope so. It kinda just came out...
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Hey Tim,, Knowing you, I know full well it was not intended with malice or mean spirit,, I too share your uh,, Humor? so I understand,,
that said, John is a pretty cool guy and a friend,, and it kinda hit a chord when I read it too,,
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It was one of those "dang, I shouldn't have hit the enter key" moments.
I don't even know if I've met John, but from what I know about him through here I like him, and I'm hoping that he'll get better. Being sick is not fun, and this doesn't sound like an easy thing to bounce back from.
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Why doesn't one of the top flyers spend the time to get a biplane competitive.
But back to your original question -- John Miller was working hard at it before his accident; his plane was certainly good looking but he kept reporting issues. It is hard to tell at this distance how many of them were perhaps because a biplane is more limited than a monoplane, and how many of them are just because a biplane is different, rendering much of the collected wisdom invalid.
I suspect that the ultimate answer is that you'll never quite make a biplane as competitive as the typical mid-wing monoplane, but if someone can get it up to the level of a semi-scale stunt warbird, the advantages in presentation and uniqueness may carry the day, or at least make a 4th with a biplane as satisfying as a first with Yet Another Impact Clone.
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Generally, wing loading and drag.
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Bob Whitley was researching a HyperBipe stunter . Don't know if he's still working on it or not, but if he is I would be interested in it.
A local guy here in the St. Louis area had a .38 Special profile from Brodak several years ago, and it flew well enough for him to enter intermediate class contests with it. The profile WWI combat models that SIG put out years ago flew OK also. Might not be top 20 at the NATS airplanes, but still fun. Doc Holliday and a couple of others have built Jack Sheeks Staggerwing and I'll bet they fly quite nicely. Lots out there to choose from.
Dan McEntee
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Generally, wing loading and drag.
Complexity of building and trimming must play a role too.
I've been watching John Miller's bipe development because bipes are cool, and recall that, for a moment, he had some issues because a bipe has more components to get aligned just right.
With abundant power available from modern engines and motors, drag might not be such a big issue: the engine keeps the speed up, no matter what, and the drag keeps the speed down on the downhills.
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Why doesn't one of the top flyers spend the time to get a biplane competitive.
Cuz I just got done with a 3 year build, but there is a biplane in my near future...
If the lightning is guaranteed I will let Brett fly it. >:D
Derek
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I don't recal the name of a European, Germany I believe, who has had a couple if fairly successful bipes in stunt. Some of you USA team members weigh in here with his name? He's well known, but the hair color here interferes with having the name right now.
Derek, let PW fly it, he's used to the zapping. (Lawrenceville nats, 1990)
I think Klaus Maikis built a couple of very beautifu Bipes that reportedly flew somewhat competitively.
Randy Cuberly
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I don't recal the name of a European, Germany I believe, who has had a couple if fairly successful bipes in stunt...
Claus Maikis.
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I remember that Nats Will. Maybe that is why he wanted me to hold off starting the Team Trials this year...
Derek
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Claus Maikis.
Here's his site: http://www.clacro.de/
Featuring his designs and all sorts of other C/L related stuff. Worth a visit, if you ask me.
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One of the original advantages of biplanes, 100 years ago, was structural. You might be able to "buy back" some of the drag, weight, and alignment penalty of a biplane by making the bracing functional: if you plan on having the full 4-wire bracing on each wing, with functioning turnbuckles, then you don't have to worry about the individual wings being torsionally rigid, or even capable of carrying half the aircraft without snapping in two -- you just need to make sure that when the wires are tensioned up to provide that strength, that they can't pull out of the wings.
Then install the light, misaligned, and floppy wings on the plane, and bring everything into alignment and stiffness with the flying wires.
(Note that there were a few biplanes that did the whole bracing thing with solid struts instead of wires -- you could do that to. The nice thing about it is that because the struts are solid you can count on them in compression, where wires will only carry tension. The not-nice thing about them is that they carry both physical and visual mass, and they are both a physical and visual drag on the airframe. But construction is easier, and it'll certainly distinguish you from all the other guys with biplanes).
By the way: don't hold your breath waiting for me to do this. I've got "Moitle" on my to-do list, but she's going to have no more external wing bracing -- and no less balsa in the wings -- than is specified on the plans.
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This is a beautiful biplane.
Charles
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Don Hutchinson has built several Stearman biplanes. I judged him flying one at a St. Louis contest. It flew quite well, but did not win first place. I have an Australian Wombat biplane with a Merco 29 diesel. I have only put two or three flights on it, but expect it will do a recognizable OTS pattern when I finish fooling with it.
Incidentally, there are semiscale warbirds which will fly at the top competitive level, and have done so. Paul Walker's B-17 is an extreme example.
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Incidentally, there are semiscale warbirds which will fly at the top competitive level, and have done so. Paul Walker's B-17 is an extreme example.
I know, and I was thinking of them when I made my comment. Every article I've read about them, however, makes it sound like you have to work at them harder, to get them to respond well and be light, etc., than with a flat mid wing. So my point was that if you developed a biplane to that point, and you were that good of a flyer -- then you could compete at any level.
And if you weren't that good at building and flying, then you'd at least have the satisfaction of flying a really kewl plane.
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Here is a photo of one of Don Hutchinson's 44" span Stearmans. It's a great performer!
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Don't go overboard - start with a 38 Special and an LA-46 or Brodak .40 for power. That combination can fly a decent pattern, I won Profile Stunt a few years back at a major NorCal contest with one. From there, design your own,or if classic is your scene-check out the "Biceps" stunter. Another good flying bipe with a ST-60 or similar. 8)
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I would like to pursue another biplane. In the right conditions I think they can come close to the best monoplanes. I also believe they will always give something up in wind and turbulence. Maybe I will be proven wrong....
The picture below is my first biplane test bed I built over the winter of 2010/2011. I started from scratch (not influenced by any other design) and it's not a bad start. If things work out I would like to fly it in profile stunt, but weight has been an issue with the airfoil I chose. I've made some changes to it since the pictures were taken, but the current airframe is the limiting factor. I need to dedicate some time to it this spring and make some solid decisions so I can cut some wood for V.2.
From memory the specs are:
Span: 44"
Area: 630sq"
Weight: 50oz (with battery)
(http://i1213.photobucket.com/albums/cc465/ekimcl/115.jpg)
(http://i1213.photobucket.com/albums/cc465/ekimcl/112.jpg)
(http://i1213.photobucket.com/albums/cc465/ekimcl/005.jpg)
Mike
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For a stunt biplane why do most ,if not all, build in wing stagger? Is this monkey, see monkey do?
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Or Monkey See, Monkey Like.
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Mike P,
That is a very cool looking bipe design, looks like it's ready for Reno! I'll take mine in a nitro version though. ;D
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For a stunt biplane why do most ,if not all, build in wing stagger? Is this monkey, see monkey do?
From Wikipedia:
Overview
In a biplane aircraft, two wings are placed one above the other. Both provide part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere. For example, in a wing of aspect ratio 6, and a wing separation distance of one chord length, the biplane configuration can produce about 20 percent more lift than a single wing of the same planform.[1]
Stagger
Many biplanes were designed with the wings positioned directly "one-above-the-other," as was first done with the Wright's 1903 Flyer I. Moving one wing forward relative to the other can help increase lift and reduce drag, though it may distort the box girder effect of the wing and reduce the structural benefits of the biplane layout. Many biplanes have been designed with the upper wing positioned with its leading edge ahead of the that of the lower wing, in a "positive stagger" format. Less common have been biplanes with the lower wing's leading edge ahead of the upper wing, called "negative stagger". Examples of negative stagger include the Airco DH.5, Sopwith Dolphin, and the Beechcraft Staggerwing.
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Suitable today only for OTS, Howard Thombs won the '49 Nats with a little-known design, the "Di Doe" - a profile fuselage, 28" span bipe with 15-degree engine offset. (Thombs may have flown two at once, though I can't recall for certain.)
Twenty years ago I built one, powered with an O&R .29 (of all things!). While it was a lot of fun, it certainly couldn't have been considered competitive - though perhaps it was me rather than the design. None the less, it's a fun little design. Tom Dixon has the plan if anyone's interested.
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I have always thought that with a biplane you now have 4 wing tips to feed instead of the usual two (double the tip losses.)
And the trade off with something like the full size 'Pitts Special' was the greater ability to snap roll due to its shorter wing span giving less inertia to overcome.
But rolls are not something most control line stunt fliers want to contemplate in the standard schedule.
Centralization of mass is another slight problem and monoplanes have the advantage here - their wing is much closer to the CG.
Also a span length of line drag is well masked by the inboard wing in mono's.
The thing with a bipe is proving that all the above is inconsequential.
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Mike P,
That is a very cool looking bipe design, looks like it's ready for Reno! I'll take mine in a nitro version though. ;D
Thank you! A glow version wouldn't hurt my feelings. ;D
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I have always thought that with a biplane you now have 4 wing tips to feed instead of the usual two (double the tip losses.)
And the trade off with something like the full size 'Pitts Special' was the greater ability to snap roll due to its shorter wing span giving less inertia to overcome.
But rolls are not something most control line stunt fliers want to contemplate in the standard schedule.
That's why when ever I worked on it, I wound up with very high aspect ratios. Much like John's, actually.
Brett
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I have wanted to piddle with the R1 bipe Jimmy Doolittle flew...very slick!
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Guys-
'sorry to get into this so late, but I have to comment quickly due to club and life things that are getting in the way. First, I posted several detailed entries on biplanes, most of which were unceremoiously swept away by the big crash a couple or more years ago, but a search on biplanes will bring up interesting stuff on John's bipe and maybe some of the older stuff. Try SSWF too.
That having been said, there are a couple or more posts in this thread that are wrong and have gone unchallenged. Due to extremely short time here now, I'll try to just give the quickest version of what I have posted in the past.
First, the problems are wing spacing, structure, and Reynolds numbers - not drag. The large drag associated with full-sized bipes is parasite drag from interference, bracing and struts, the latter two not always required in models now (except for lead-out guide). Biplane tip losses are actually less than for what is called an "equivalent" monoplane, and ideally much less, if you work it right.
With the same lift as a monoplane, two equal biplane wings of equal span and half the monoplane area must each carry half the lift. If they have elliptical lift distributions, and most tapered wings come close enough to merit this argument, then each wing's induced drag is proportional to the square of it's lift. Therefore each biplane wing has 1/4 the induced drag of a monoplane whose span and wing area equal that of the biplane, IF there is no interference between the wings.
Dimonoplane = K(L)2
Dibipe = K(L/2)2 + K(L/2)2 = KL2/4 + KL2/4 = kL2/2
So ideally, the biplane is actually twice as efficient as a monoplane. Since the biplane wings cannot be placed infinitely far from each other, there will be interference losses that result in drag gain from the the 50%-induced drag ideal. It turns out that a realistic goal is a biplane with 37% more efficiency (137% the 'equivalent' monoplane efficiency). Some of this is eaten up in the rest of the compromizes, but you should be able to come out pretty close in a model that's large enough. FWIW, because of shock wave formation and angles, there actually were a whole family of supersonic biplane designs and studies in the early jet age.
The compromises though, as John and Brett discovered, are that the individual wings turn out to be pretty high in aspect ratio and thus short in chord to be comparable to a typical stunt wing of A/R = 5+. There may indeed be side issues, like increased captured side area to aggravate gust upsets, if you don't want to use lots of struts. That's the fun. Box wings (biplanes with closed wing tips) have a realistic theoretical top efficiency of 146%, which should allay some structural and lead-out guide concerns and hint a way to unconventional success. So there's a lot of room for thinking "outside the box."
Now one last thing. Yes, this is simple mathematics, which frightens some, but it is an example of where real things are otherwise hard to see. This "theory", however, is real-world valid. Last time I posted this, someone who should have known better indicated that he felt himself to be above physical laws. If you have doubts, look at 100 years of aerodynamical research by Prandtl, Munk, Whitcomb, Jones, and others and look at the craft that have been built using the same "theory" (like the Earth is not flat). This is not a matter of opinion. Do with it what you want, but let's not have silly discussions on things that form the basis of aeronautical theory and a century of aircraft progress.
You may or may not be able to overcome the compromises required by biplane theory or CL requirements, but it's fun. That should be reason enough to talk about or build them. Back to newsletter editing and the wifes new on-line career exposure.
SK
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Brothers, what exactly is driving the elevated aspect ratios in biplanes?
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Brothers, what exactly is driving the elevated aspect ratios in biplanes?
Roll stability.
Brett
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Back for a quick break from editing "stuff"...
Brothers, what exactly is driving the elevated aspect ratios in biplanes?
If you use usual aspect ratio wings in the biplane, the "equivalent" monoplane of equal area and span to which it is compared turns out to have a very low aspect ratio wing, say, 2.5, which is not very impressive in induced drag losses. That doesn't make the biplane's higher efficiency very impressive either, compared to a normal stunter. The bipe wings should be be developed in comparison to a good monoplane stunt wing, which requires their chords to be much smaller to keep the same span and area.
Then if we want to reduce biplane wing aspect ratios from, say, 10 to 7, the math then shows a much smaller effeciency advantage that can be eaten up by other compromises. That's why I think a bipe can be competitive, perhaps a bit better, but also perhaps a bit worse than monoplanes. It's not hard to compute the aspect ratio of a biplane which would give a theoretical efficiency equal to "equivalent monoplane", and that figure is pretty close to 7 (I may have done that in John's thread). The "box plane" configuration with higher efficiency than a normal bipe of comparable size and aspect ratio, seems a better approach, the wing tip connections serving as "struts" too. Otherwise, it seems to me that larger bipes, with greater Reynolds Numbers would be more successful, since they could have the higher aspect ratios and still avoid critical RN problems. Some recent R/C pattern contenders have been bipes.
FWIW: Flying models this month is concluding a series on the history of R/C pattern planes, which may address this and is relevant to the old thread on R/C conversions for stunt. 'seems that they are flying slower with much larger fuselages now and still evolving.
SK
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Roll stability.
Brett
Hey, what is wrong with a bit of torque roll after take off?
Looks kinda scale I think!
(Tongue in cheek Brett as I know what you mean here. :P)
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Could Serge or one of our other aerodynamical luminaries comment on (a) the maximum lift coefficient that one can expect on a bipe vs. a monoplane of similar area (Douglas Ames quoted Wikipedia as saying that a bipe can't generate as much lift as a monoplane, this doesn't seem to match things that I've learned elsewhere), (b) if any 'oddball' measures help, like cambering the top and bottom airfoils differently or rigging them so that the average camber lines aren't parallel, (c) how effective flaps are on a biplane, and (d) if you use flaps, if the flaps and the stagger interact in any weird and/or bad ways.
I think that a biplane that flew just as well as a monoplane would be worth while to build just for the coolness factor -- one that actually performed better by some significant measure would be better yet.
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I just cut and pasted what I thought was relevant from Wikipedia. As you might know it is a user edited site and who confirms the validity of the submitted text is anyones guess. One of the glaring flaws of the website.
The Wiki overview comparison states "- of the same planform", not wing area.
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I just cut and pasted what I thought was relevant from Wikipedia. As you might know it is a user edited site and who confirms the validity of the submitted text is anyones guess. One of the glaring flaws of the website.
The Wiki overview comparison states "- of the same planform", not wing area.
I wasn't trying to diss you in any way -- I just want to hear from someone I can trust more than Wikipedia.
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Tim-
I'm not feeling so "luminous" right now, but will try to look back into some of this, when I can come up for air. I'm really buried now. However, I do have a couple old texts, in particular one by NACA's Diehl, that date to the 1930's, deal with biplanes, and are sophisticated enough to give good approximations with the tools of the time. I did an analysis like that once, and my post might still be in the SSWF archives. If I can find time I'll try to refresh myself on this by looking there or in some old NACA and new Stanford reports. I hope I'll be able to do that, but unfortunately, it can't happen untill after this weekend.
For now, I'll just say that sometimes the question might not always be the right one. I always wanted to concentrate on lift and wondered at first why they only talked about induced drag. The problem is that aspect ratios, spans, interference between wings, etc. affect things, but all these wings can produce the requisite lift - until they stall. The most efficient wings or systems do it with the least induced drag. That's the biplane's potential advantage, given that its wings have sufficient separation and are of high enough aspect ratio to be comparable or superior to a decent equivalent monoplane. You may get your lift efficiently with a well designed bipe or wing with multiple tips, but it's quite possible that enough power can push even an inefficient monoplane to a greater maximum lift. The lift curve slope too is modified slightly by these considerations. An extreme example would be an elliptical wing of aspect ratio 1.5; it can get a whopping maximum lift without flaps, but with a lot of drag at an a.o.a. of around 45 degrees - admitedly because of tip vortex interference. The research deals with induced drag.
Decalage on biplane wings, stagger, and asymmetry are more complicated and perhaps not for stunt CL models. Some concern upright cruise conditions, I suppose, and they might have values attuned to certain speeds. I honestly don't remember the specifics of what I read on that. The effects of flaps on biplane wings is covered in the NACA literature, probably available on line.
Edit: OK I checked my files - You can find most of these on the net:
General Biplane Theory - Munk, NACA TR 151 (1922)
Stagger - NACA TN 70.
Comprehensive Historical Synopsis of gap, stagger, etc. effects: Zyskowski, "Incorporating Biplane Wing Theory into a Large Subsonic All-Cargo Transport", AIAA Techfest XX (7/93), NASA 1/94 (Report No. not recorded)
Biplane Winglets, NASA TM 86350 (1/85)
Flap Deflection Effects on Lift Coeff. - NASA TM 75059 (1976)
"Design and Analysis of Optimally-Loaded Lifting Systems" - Kroo, Stanford (update of AIAA paper 84-2507 (1984) (Gotten from Stanford site)
"Non-Planar Wing Concepts for Increased Aircraft Efficiency" - Kroo, Stanford
Multiple wing forms and combinations - DeYoung NASA CR 3357 (1980)
End of Edit
Wikipedia pages are often very good, but sometimes in technical matters you run into some credentialed person who can't see the forest for the trees, but is willing to state opinion as fact. The internet is a source of both information and misinformation.
Perhaps someone else will chime in?
Meanwhile, the easiest place to look for such answers would be on line, but at dependable sites. You might look up Diehl's NACA work there. The NASA/NACA Tech Report Server site, or whatever they're calling it this week, is valuable, and early NACA reports like Max Munk's on general biplane theory are probably available with some others there. Good modern analyses can be had online from Stanford's Illan Kroo, who has taken a great interest in what he calls non-planar wing forms, where he also considers biplanes.
'gotta run.
SK
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I wonder if you could build a biplane light enough to be as good at stunt as the monoplanes we're flying.
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I wonder if you could build a biplane light enough to be as good at stunt as the monoplanes we're flying.
I think that could be an advantage - you could put much less structure in the wings and count on bracing it to keep it together. That's why they did it back in the day. They couldn't build a monoplane sturdy enough and light enough. All the parasitic drag tends to obscure the induced drag issue. As long as you can have a piped 75 or 90, drag and lift certainly aren't issues.
Based on other people's experiments over the decades, the only problem was the lack of roll stability/restoring force which led to difficult trim issues. Raising the aspect ratio and getting rid of 40-year-old trim ideas would appear to solve that problem. The drag issue goes away with the unlimited power. So it is my considered opinion that we now have what we need to make a biplane that is "close enough".
Brett
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I keep looking for a picture that I have of one that Steve Buso designed years ago.....
Johnny Duncan had it almost finished...
I'll look harder...
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Based on other people's experiments over the decades, the only problem was the lack of roll stability/restoring force which led to difficult trim issues. Raising the aspect ratio and getting rid of 40-year-old trim ideas would appear to solve that problem. The drag issue goes away with the unlimited power. So it is my considered opinion that we now have what we need to make a biplane that is "close enough".
Presumably you'd also want to put the leadout guide at the end of the wings, rather than just tacking it onto a "conventionally located" interplane strut.
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I think that could be an advantage - you could put much less structure in the wings and count on bracing it to keep it together. That's why they did it back in the day. They couldn't build a monoplane sturdy enough and light enough. All the parasitic drag tends to obscure the induced drag issue. As long as you can have a piped 75 or 90, drag and lift certainly aren't issues.
The wings on my bipe are roughly 315sq" and weigh about 2.75oz to 3oz each (the numbers are from memory so they may not be exact). They certainly would not stay together without the bracing! Of course that sounds light but remember there is no bellcrank or bellcrank mount, tip weight box, leadout guide, flap control horn, pushrod, hinges or flaps.... I didn't even glass the center section(s). I don't think any significant amount of weight could be removed beyond what I achieved.
That being said I'm not sure you come out lighter in the end. Everything that is not in the wing is added to the fuse and then you have to include the weight of the struts and account for twice the glue joints, if not more. I don't see a weight advantage when building a bipe. I don't see any advantage anywhere. At least none that would win over a monoplane.
Mike
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I have been thinking about one for along time. I have one drawn up in CAD as well. Looks VERY similar to Mike's. I stole aesthetics off of the RC guys and their cool stuff from a couple of years ago. There is an enormous electric one with counter rotating 16" props!
I bounced a few ideas off of Al Rabe at a contest a couple of years ago and he said to think about reducing the wing thickness significantly since you have two of them. No need to build more than what you need right? Use the same amount of reduction in your double tapper as normal. It made sense what he said. At that time I was also noticing the RC guys were going with way way thinner airfoils as well and still getting all the lift needed and more.
For roll stability you are going to have to wing span. My design came in with 56" or 58" wing span IIRC. Total area was pushing 800 sqrs measured with the CAD program. Keeping the weight down was going to be a challenge for sure. I figured less than 70oz and it would a workable weight with that much area.
My tail section design is huge at 25% of total area, I don’t know if this is the correct approach in that respect? Any ideas?
Steve had an Ultimate Bipe RC plane converted for a few years. It was a solid flyer and lots of fun. It’s a profile plane and he had a VF40 on pipe mounted on it. Pretty Killer with loads of tension. One thing he learned with that one was position of the line slider. It must be inline with the BC or it will fly wing up or down once it’s in the air. It doesn’t work the same way as does when the lines exit the slider concerning YAW with the BC way behind and it still flys straight forward, or however you have it adjusted. He moved his slider down from the center of the wing inline with the BC and it straightened out nicely.
All movement on my design stopped as of a couple of years ago due to some crashes....monoplanes became the focus. With the stable full again this might be something to revisit.
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One thing he learned with that one was position of the line slider. It must be inline with the BC or it will fly wing up or down once it’s in the air. It doesn’t work the same way as does when the lines exit the slider concerning YAW with the BC way behind and it still flys straight forward, or however you have it adjusted.
Yes, it does.
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Yes, it does.
You mean yes it does you agree with me or you dont agree with me.
If you move the exit of the LO guide verticaly it changes roll angle of the wing. Steve started with his smack in the middle between the top and bottom wing and it looked very much exactly like Mike's does in the photo from behind. But the design Ultimate Bipe does have the top wing pretty high off the fuse so the slider in the middle had inline with the top of the fuse above the thrust line. The plane flew with a very large amount outboard wing rolled upwards in upright flight.
He then went back and moved the slider way down inline with the BC, incidentally this would also be inline with the center of the fuse. And it flew level from there. Maybe we were mistaken in thinking it was the line straightening out and it was actually putting the lines more inline with the center of the fuse to give it the correct roll angle.
Help me out.....I am always needing to learn more.
Also what is the correct spacing between top and bottom wing??
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He then went back and moved the slider way down inline with the BC, incidentally this would also be inline with the center of the fuse. And it flew level from there. Maybe we were mistaken in thinking it was the line straightening out and it was actually putting the lines more inline with the center of the fuse to give it the correct roll angle.
Flexible leadouts will pivot at the slider, so what's important for trim is the leadout guide location. Vertically, you want it located at the vertical center of gravity of the plane. Fore and aft, it needs to be somewhat behind the center of gravity because (a) line drag tends to pull the lines back, so you have to compensate for that, and (b) there's some trimming to be done with leadout location.
There's probably merit in not having too sharp a bend at the leadout guide. I'm sure that folks will argue about this, but if you consider that if your leadouts are spaced more closely than the bellcrank span you'll have some bend anyway -- I'd aim to have the bellcrank within 1/2 an inch or 1 inch of the 'perfect' vertical location (for a 3" or 4" bellcrank), and I wouldn't sweat things within that.
Help me out.....I am always needing to learn more.
Aren't we all.
Also what is the correct spacing between top and bottom wing??
Debatable. The farther apart they are, the better things are from first-cut aerodynamic theory. But the farther apart they are, the heavier the structure, and the more drag on the interplane struts (which, presumably, we left out of our first-cut aerodynamic analysis).
From what I've read, you hit diminishing returns when the wings are about one wing chord apart -- and if you look at established practice for all but the oldest planes, the wing spacing seems to range from about 3/4 of a chord to 1-1/4. I'd probably go with 1:1, because I'm a simpleton.
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Just a quick check-in-
As Tim suggested, Howard was disagreeing; it's the c.g. that determines leadout exit location, and it's the leadout exit location that determines the best bellcrank position - to avoid sharp bends in the lines.
The further apart the wings, the greater the efficiency, but also the greater the distance between objects causing parasite drag, making it more difficult to maintain smooth flight (recovery from upsets). The 137% "ideal" efficiency of a bipe is a figure for about that 1:1 gap to chord with not too radical departures from the conventional. It is important to know though that the theoretical (elliptical theory, lifting line) derivations actually address the gap to span ratio, which sounds intuitively wrong, but really has to do, I suspect, with tip vortex interraction (I needto re-educate myself there).
What I think we are missing here, unless we just like certain biplane aesthetics/style, is the idea of "box wings", which do not have to look like boxes. Connections between wing tips, closing the box, serve as struts and anchors for leadout guides. With that higher 146% practical theoretical efficiency, these wings can lift more for the same drag or lose less energy in turns, depending on their loadings. Likewise, they'd tolerate lower aspect ratios than ordinary biplanes at the higher efficiencies. Shaped correctly and built of composites, they might be light enough, even with greater material for the end connections or plates. These are joined-wing bipes in a class of their own. Some interesting tip patents are out there too. The DeYoung reference I listed is really interesting in this connection, and I did get it from the internet: NASA CR 3357 (12/80). Kroo addresses these forms too.
SK
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You mean yes it does you agree with me or you dont agree with me.
You got the right result, but then made an incorrect conclusion. Not counting subtleties such as leadout stiffness and wing warps, the airplane CG will be in line with the direction the lines pull on the leadout guide. Where the bellcrank is doesn't matter.
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What I think we are missing here, unless we just like certain biplane aesthetics/style, is the idea of "box wings", which do not have to look like boxes. Connections between wing tips, closing the box, serve as struts and anchors for leadout guides. With that higher 146% practical theoretical efficiency, these wings can lift more for the same drag or lose less energy in turns, depending on their loadings. Likewise, they'd tolerate lower aspect ratios than ordinary biplanes at the higher efficiencies. Shaped correctly and built of composites, they might be light enough, even with greater material for the end connections or plates. These are joined-wing bipes in a class of their own. Some interesting tip patents are out there too. The DeYoung reference I listed is really interesting in this connection, and I did get it from the internet: NASA CR 3357 (12/80). Kroo addresses these forms too.
Howard referenced a Boeing white paper that pointed out that winglets only increase efficiency if the lift distribution on the wing and winglet is correct.
In the context of a stunt plane, where the coefficient of lift is changing constantly and drastically from -big to zero to +big, will a box wing with fixed end plates be able to realize the efficiency gain? Or would one theoretically need to be constantly adjusting the angle of attack of the plates, or worse, portions thereof?
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There's a guy who shows up at the NATs (I think) or Brodak, flies in Advanced. He flies planes with at least four wings and two engines. Ugly. Ugly. Ugly. White. Palko and Banjok have witnessed these oddities (travesties) fly competitive Patterns in class. Also starts his engines with a car battery pulled out of his car and carried by his wife to the starting line. Did anyone else see this. At Muncie. Or Brodak. Or was the shock too much, inflicting situational amnesia. Palko's bipe does a decent pattern. He just hates to practice. Most of the time, when flying conventional planes, he doesn't NEED to practice.
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Here's how I would approach the problem. First, what's the objective? If it's to make a sport biplane that looks cool, just do it. You could sell a lot of plans or kits. A nontrivial objective would be to come up with a configuration that would get as high a stunt score as a good stunt monoplane. Here are some things that affect stunt score that are functions of number of wings:
1. vertical gust response
2. lateral gust response (mentioned by Brett and Doug)
3. lateral sideslip response
4. induced drag
5. max load factor
6. weight to carry up hill and down dale
7. cool looks
The first six of those affect how accurately you can fly maneuvers. I would look at these in more detail, because one can. If it looks like accuracy of steering the airplane doesn't suffer much relative to coolness, it may be worth trying a biplane.
1. Vertical gust response is proportional to lift curve slope / wing loading. Lift curve slope increases with aspect ratio. The lower the lift curve slope / wing loading, the more virtuous the stunter.
2. Lateral gust response is a big deal in how accurately you can fly stunt-- more so in some planes than others. I'll amplify this in a later article. The airplane's propensity to get upset is approximately proportional to how far out the spanwise aerodynamic center on each wing is and to the wing lift curve slope. Its propensity to straighten out in roll is proportional to line tension and how far inside the circle the leadout guide is from the CG. How virtuous the airplane is in lateral gust response is inversely proportional to the ratio of propensity to get upset to propensity to straighten out.
3. Lateral sideslip response is approximately proportional to the airplane's rolling moment due to sideslip / how far inside the circle the leadout guide is from the CG. Do a search on "rolling moment due to sideslip" here or on SSW to get the lowdown. How virtuous the airplane is in lateral sideslip response is how close to zero it is.
4. Induced drag for a stunter in a given maneuver is proportional to mass^2 / (span^2 * e). Those are the parameters that might differ between monoplanes and biplanes, anyhow. e is a fudge factor that's probably greater than 1 for a biplane and about .8 for our monoplanes. Serge has given it a lot of effort. I would reckon that a stunt plane should have about the same amount of induced drag as current stunt planes have. More would cause them to slow down in maneuvers. Less would cause them to tend to speed up in the wind. Brett may have more to say about this. The optimal induced drag for electrics may be less than that for glow engines because they can regulate speed better and because their fuel weighs more.
5. Load factor is lift / mass. Max load factor is how tight you can turn. The greater the max load factor the more virtuous the stunter, although a better figure of merit is max load factor before the airplane does perverted things. Look for Igor to address this in a coming article. Load factor is something that junior stunt plane designers have fixated on. Try flying stunt with an F2D plane on a blustery day and see what score you get.
6. Apart from the mass effects above, airplane weight makes the airplane slow down going uphill and speed up going downhill. It also reduces line tension in overhead eights and at the top of the hourglass, both directly and from the speed reduction of the airplane having gone uphill. These effects make it hard to fly accurate stunt. Mass is not a virtue here.
If a biplane comes out better on all this stuff, it may be worth a try. More likely, you'd trade some parameters to get an optimal combination of the the above stuff; that's how the pros do it. If that combination compares well to that for a monoplane, a biplane may be worth a try. Just offhand, it looks like stunt biplanes should stay indoors unless you decouple wingspan from the spanwise distance of the leadout guide to the CG (hint, hint, wink).
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Howard,
I think you should design and build a competitive bi-plane. I would love to see it ... is 6 or 8 years. But that's only if you start today.
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In the context of a stunt plane, where the coefficient of lift is changing constantly and drastically from -big to zero to +big, will a box wing with fixed end plates be able to realize the efficiency gain? Or would one theoretically need to be constantly adjusting the angle of attack of the plates, or worse, portions thereof?
I don't think they have to be constantly adjusted for our purposes (symmetrical behavior), although for upright cruise, I'm sure they're optimized. One thing I should have mentioned is that Kroo and associates probably figured out the efficiency,'e', with more modern methods, but once I came up with the same biplane figure using Diehl's theory and his adjustments for gap, A/R, and Reynolds number. So I trust it within it's limits of "optimally loaded," implying that there's twist or something else that doesn't give the same results inverted. So, since the box plane doesn't have elliptical distribution without changes, its generic efficiency may be less than shown. So here's the diagram, as well scanned as I can make it.
SK
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You got the right result, but then made an incorrect conclusion. Not counting subtleties such as leadout stiffness and wing warps, the airplane CG will be in line with the direction the lines pull on the leadout guide. Where the bellcrank is doesn't matter.
Yep, looking back on it it was just adjusting the LOs to match the needed placement for proper wing attitude and it had little to do with BC placement. It just looked that was you could see it on a profile.
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If a biplane comes out better on all this stuff, it may be worth a try. More likely, you'd trade some parameters to get an optimal combination of the the above stuff; that's how the pros do it. If that combination compares well to that for a monoplane, a biplane may be worth a try. Just offhand, it looks like stunt biplanes should stay indoors unless you decouple wingspan from the spanwise distance of the leadout guide to the CG (hint, hint, wink).
This is a bit extra-cynical, but I think that most of the aerodynamic advances in CLPA have been through trial and error (often catalyzed by hard work), with the guys who succeed marked as "genius" and their theories paid attention to, and the failures marked as "eccentrics" and their theories ignored. I don't think many -- if any -- folks have been able to just start with a blank sheet of paper and a brain full of theory and laid out a competitive stunter on the first cut.
So I think that one would have to put some serious trial and error work into a stunt biplane before one could even be sure that it didn't show significant promise. If 100 guys started fooling around with biplanes and pooling their results, then you might see serious progress.
It's a pity that John had his accident: he certainly seemed to be taking an approach that allowed for a lot of trials.
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Tim,
while your premise is likely true,, I think that there is a LOT of theory when one sits down to design a bird,, I know that in our conversations,, (John and I) about biplanes,, we did a lot of theory discussion, this at lest allows you to start making semi valid scratches on that sheet of blank paper,, That said, I do agree that to optimize it, most certainly several iterations are likely to be created,,
However, I also believe, that aside from the top echelon, a person can make a reasonable attempt,, and if all parameters are investigated, the result should be a good solid platform with which to wage stunt wars ,, I know there are lots of conventional designs that are close,, and with judicious trimming become solid performers, or so is my theory,,
( and by stating this, it almost garauntees that in the near future, I will be enlightened as to my folly,, or so has been my history)
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http://www.clacro.de/Seite_falco.htm
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Tim,
while your premise is likely true,, I think that there is a LOT of theory when one sits down to design a bird
I guess that my real point is that theory should give you a good starting point -- but from that point on you have to cut & try. And more power to those who do, and succeed!
It gets back to my comment in the motors thread, that 90% of successful engineering is shamelessly copying someone else's success. That gets a lot harder when there aren't a lot of successes to copy.
However, I also believe, that aside from the top echelon, a person can make a reasonable attempt,, and if all parameters are investigated, the result should be a good solid platform with which to wage stunt wars.
Oh c'mon. We both know what you'd say if I started a thread on a biplane Skyray.
Hey... That could look pretty good, come to think of it...
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http://www.clacro.de/Seite_falco.htm
Ooh, ahh. (I like it).
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Would this work to solve the leadout issue?
You'd need an adjustable guide in each wing. Match the ratio of the upper & lower 90 deg. bellcranks with a 1 pc. bellcrank, translating bellcrank could be 1:1.
I dunno...
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Would this work to solve the leadout issue?
The leadout guide would need to be beyond the wingtip. Think of an F2A model superimposed on a biplane.
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This is a bit extra-cynical, but I think that most of the aerodynamic advances in CLPA have been through trial and error (often catalyzed by hard work), with the guys who succeed marked as "genius" and their theories paid attention to, and the failures marked as "eccentrics" and their theories ignored.
Yup. Lots of the stunt "theory" I see is misunderstood, misapplied, or wrong, so it probably does as much harm as good.
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When Paul won the Worlds in 1992 he brought home a tape on the contest I think made by the Japanese. I also think that tape had a Russian pilot with a biplane pretty sure it was that tape & contest .
John
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Would this work to solve the leadout issue?
I think it would just serve to couple roll motion into elevator motion -- and I think that would be bad.
Some sort of an active leadout guide that was free to move up or down a bit and actuate ailerons as a consequence would be interesting. I suspect that it would introduce more problems than it solved.
I think Howard wants you to make a leadout guide that consists of a CF arrow shaft that sticks a foot out from the wing and is rigidly mounted at its root. It should be great on bad landings, getting the thing into and out of the car, and during minor traffic mishaps.
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I think it would just serve to couple roll motion into elevator motion -- and I think that would be bad.
It would give a rolling moment proportional to control surface hinge moment. There might be a way of using this as Smart Tip Weight, but I don't even know what sign it should have.
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This is a bit extra-cynical, but I think that most of the aerodynamic advances in CLPA have been through trial and error (often catalyzed by hard work), with the guys who succeed marked as "genius" and their theories paid attention to, and the failures marked as "eccentrics" and their theories ignored.
If a guy is eccentric enough, he can succeed, prove it to everybody, and still be dismissed as an eccentric. One such guy lives just south of you.
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I think it would just serve to couple roll motion into elevator motion -- and I think that would be bad.
Some sort of an active leadout guide that was free to move up or down a bit and actuate ailerons as a consequence would be interesting. I suspect that it would introduce more problems than it solved.
I think Howard wants you to make a leadout guide that consists of a CF arrow shaft that sticks a foot out from the wing and is rigidly mounted at its root. It should be great on bad landings, getting the thing into and out of the car, and during minor traffic mishaps.
Didn't think about roll coupling, duh! Might be fun with a Bi-slob, lol
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The leadout guide would need to be beyond the wingtip. Think of an F2A model superimposed on a biplane.
I have been wanting to experiment with that. I have a feeling once the leadout guide surpasses the wing tip it will no longer improve roll stability. The wingspan will become the limitation. My thinking is that they are one in the same and you need both span and leadout position, not one or the other.
Mike
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Didn't think about roll coupling, duh!
Golly, Doug. You shouldn't have said that. Now if it does something good, you can't claim to have invented it intentionally.
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It would give a rolling moment proportional to control surface hinge moment. There might be a way of using this as Smart Tip Weight, but I don't even know what sign it should have.
Burma Shave. You're welcome! H^^ Steve
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Golly, Doug. You shouldn't have said that. Now if it does something good, you can't claim to have invented it intentionally.
Wait a min., what happens when you do multiple loops - your lines will look like a bowtie HB~> dang it, more drag. Ok, I'll put my pen down.