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Author Topic: Almost finished with it, my stunt Biplane  (Read 24893 times)

Online John Miller

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Almost finished with it, my stunt Biplane
« on: November 14, 2010, 08:50:49 PM »
I can't believe it, I'm almost finished with the 2 Bits Bipe plans. It's a design I've had on the burner since about 2004, but, I had to make some changes as I learned more about biplane design.

I wound up with a pair of relatively high aspect ratio wings, spaced about 1.3 chords apart, Though the individual wings are High A.R. the combined chords make it fall into the more normal A.R. we see in stunt ships. I'm depending on the individual high A.R.'s will give me the efficiencies these planforms are known for, and the combined lower A.R. will help to avoid the tendency to become unsettled in turbulence.

I also decided to make it into an extreme take apart to make it easier to travel to contests.

Attached is a pair of jpeg's of the plan sheets. Tomorrow, I'll finish up the pattern sheet. Can't wait to get started building this one.

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Offline Mark Scarborough

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Re: Almost finished with it, my stunt Biplane
« Reply #1 on: November 15, 2010, 12:14:06 AM »
Pretty awesome John, makes me wish I would have continued on with My bipe I showed ya , ,Guess your just gonna beat me to the punch this time,,
when are you going to start cutting wood on this one?
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Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #2 on: November 15, 2010, 08:27:30 AM »
John-

You've really outdone yourself on this one! That is just a great effort, AND it is the only CL bipe design I've ever seen that gets to the heart of advantages biplanes may have. You've apparently done your homework, something I doubt has been done before in CL. The high A/R wings are also necessary to get the numbers to work well. Your gap is impressive, and we all should remember that biplanes with that gap can be very efficient!

Bipes are sometimes shunned due to complexity, asymmetry, and parasite drag (struts, braces, wires,...), effects reduced with modern materials and probably not a problem in CL stunt anyway. Before your plane, most in CL seemed to ignore the fact that biplanes are otherwise more efficient than equivalent-area monoplanes. Since each wing needs only to supply half the required lift, their individual induced drags at large gaps are each only 1/4 of the total drag of the monoplane. With realistic gaps, the interference drag does not eat up the remaining advantage. Illan Kroo's Stanford group (and I in the lost posts in the SH crash), has computed that something like your bipe should still have an effieciency of 1.36 or better, meaning that your bipe should have a L/D ratio of 1.36 times that of an equivalent elliptical wing - barring lost efficiency due to lower Reynolds numbers.

Your plane is the first CL stunter I've seen that seems to employ these potential biplane advantages, and I'm really looking forward to hearing about your prototype's performance. I haven't read anything about the R/C pattern bipes, but they must work. My only concern is that your design performance may vary a bit more with size, due to Reynolds Number effects.

Anyway, Way to go!

SK

Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #3 on: November 15, 2010, 09:23:04 AM »
Thanks Mark and Serge

Serge, you nailed the items that concerned me the most with this project. I appreciate your input, it tells me that I addressed it to the best of my ability. I posted this in the design forum, as I wanted to collect design information on CL Bipes. The recent postings on making a better Flying Fool, were helpful.

Mark, I expect to finish up the pattern sheet today. As you know, the pattern sheet is used by the laser cutter to cut the parts. As soon as I can afford it, the parts will be cut, and I can commence with the build. I expect that sometime in December, I'll start construction in earnest.  H^^

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Offline wwwarbird

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Re: Almost finished with it, my stunt Biplane
« Reply #4 on: November 15, 2010, 10:39:36 AM »
 Looks great John, this will be interesting for sure. Looks like you've got everything covered.

 This is trivial I know, but the only thing I'd change (being my usual nit-picking self) is the outline of the wheel pants. I'd make them with a straight-ish raked L/E to better tie in with the profile of the fin/rudder. Just a little detail that stands out to me on the drawing, and would help tie things together.
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Offline sleepy gomez

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Re: Almost finished with it, my stunt Biplane
« Reply #5 on: November 15, 2010, 05:43:39 PM »
I love biplanes!  My Silk, on another recent post, seems to be about the same size but without flaps.  I would like your thoughts on why the use of wing stagger other than for appearance?  Also on a stunt bipe would you comment on having one wing higher off the center line? 

Offline Allan Perret

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Re: Almost finished with it, my stunt Biplane
« Reply #6 on: November 15, 2010, 07:06:03 PM »
What are the numbers on this one?  Span, area, engine size ?
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #7 on: November 15, 2010, 07:49:45 PM »
I love biplanes!  My Silk, on another recent post, seems to be about the same size but without flaps.  I would like your thoughts on why the use of wing stagger other than for appearance?  Also on a stunt bipe would you comment on having one wing higher off the center line?  

I love them too. I did an on line search to make sure that my answers to your questions are correct. I thought I would cut and paste the following.

http://findarticles.com/p/articles/mi_qa3819/is_199802/ai_n8799373/

It will get you to a great sight on Biplane design. A google search on "Biplane wing stagger" will yield an entire evenings worth of articles and explanations on the subject. I recommend this search whole heartedly.

My simple answer to your questions though would be the stagger, in this case positive, helps with lowering drag, and better lift during parts of the flight regimen.

The height difference of the lower wing vs the upper wing, allows the upper wing to fly in cleaner air, and should allow it to create slightly more lift.

 H^^
« Last Edit: November 15, 2010, 08:18:20 PM by John Miller »
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #8 on: November 15, 2010, 07:52:25 PM »
What are the numbers on this one?  Span, area, engine size ?

Hi Allen, the span is 51.5 inches, area is 688 sq. inches, and the power is .60 and up. I'm showing a Stalker .61RE, but that would be the smallest I would consider. H^^
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Offline wwwarbird

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Re: Almost finished with it, my stunt Biplane
« Reply #9 on: November 16, 2010, 12:10:27 AM »
 Just curious, what's the wing chord from the L/E to the hinge line at the center of the wing?
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #10 on: November 16, 2010, 06:57:05 AM »
Just curious, what's the wing chord from the L/E to the hinge line at the center of the wing?

6.395" leading edge to flap hinge. 8" leading edge to flap trailing edge.  H^^
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Offline WhittleN

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Re: Almost finished with it, my stunt Biplane
« Reply #11 on: November 16, 2010, 09:31:27 AM »
John

Just amazing. It would do well with 'lectricity power.

Norm

Offline Randy Powell

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Re: Almost finished with it, my stunt Biplane
« Reply #12 on: November 16, 2010, 09:54:08 AM »
John,

The only issue I can see is it will need to be light. I'd do everything I could think of to insure that it comes in under 70oz. Be a trick, but I have faith.

Very cool design, my friend.
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Offline wwwarbird

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Re: Almost finished with it, my stunt Biplane
« Reply #13 on: November 16, 2010, 12:01:44 PM »
John,

The only issue I can see is it will need to be light. I'd do everything I could think of to insure that it comes in under 70oz. Be a trick, but I have faith.

Very cool design, my friend.

 Might get away with a tad more than 70, but I agree with Randy on that one. Not trying to get you to to re-engineer it here John, it looks awsome, but you might save a little weight by skipping the take apart tail and just having the wings be removeable. There is going to be a fair amount of hardware and a lot of structure in this design, it will be a major feat to keep it light. Double the hinges, horns, linkages, and paint on two wings etc. I would even consider just building it in one piece. 51.5" is good sized, but not overly big for transporting. Granted, it's going to be taller though.

 I'm no engineering wizard, and you've definitely covered a lot of the bases, but doesn't basic frontal area come into the drag equation as well?
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #14 on: November 16, 2010, 03:14:10 PM »
Thanks again guys for the helpful suggestions and encouragement.

I must tell you that I may n ot have gotten over the major hurdles without Norm Whittles helpful suggestions. The mans an incredible designer in my not so humble opinion. Norm will tell you "It's Old Math son, old math."

I agree that weight will be my biggest challenge. Especially as I'm flying at 4600 ft ASL to start with. 70 oz's  here will be a brick.

As it stands, I estimate the airframe, everything, but the covering, and finish, in the low to mid 50 oz range. It remains to be seen if I can stick to that projection.

Making it a one piece airplane might shave off about 3 to 4 oz's, but I really want to make it a take apart. I used much the same techniques on a Sheeks Stagger Wing and it came out OK.

Well, with the laser cut files, I'll find it easy to perhaps build 1 of each style.   VD~ H^^
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Offline proparc

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Re: Almost finished with it, my stunt Biplane
« Reply #15 on: November 16, 2010, 06:31:50 PM »
Brother Miller,

What you trying to do!!! That aspect ratio is off the charts. :o

Are you trying to compensate for the increased drag of a biplane? Is it something to do with the your high altitude? Or maybe, your just going to play it close to the vest on this one.
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Offline dave shirley jr

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Re: Almost finished with it, my stunt Biplane
« Reply #16 on: November 16, 2010, 06:49:26 PM »
looks good John
dont know if you remember seeing it but my dad and i colaborated on a bipe a couple of years ago. it flew fairly well but crashed after only about five flights due to what appeared to be a launching accident,(the rudder and fin got hit while being released and came loose after a few minutes of flying).
anyway I redrew the design with a few mods but havent yet built another one. i think i showed some promise but had a few problems that we were going to fix.
one was to make the flaps larger, which you have allready done, another was to reduce the weight ours was 69 ounces dry and definitelly flew like it was a little heavy allthough yours has a definitelly higher aspect ratio  i would try  to keep the weight way less than what ours weighed (we had a PA65 on pipe so power wasn't the problem).
the last thing we addressed was to use a thicker more blunt airfoil on the next one, yours is about what i put on the second version. i was worried about drag in verticals but it was a non issue and the thicker airfoil might have helped the hard corner issue
other than slowing down in hard corners quite a bit it wasnt too bad a flier.
one issue that wasn't really addressed on the second redraw was the dificulty we had reaching the needle valve and glow driver with all those wings in the way while the engine was running. (just something to be aware of, it was a little precarious reaching around the carbon three blader when it was running)
one thing i noted on your plans that we came up with on ours was the bellcrank to flap pushrod angle. we elected to run the pushrod from the bellcrank directly to the elevators and then another back forward to the flap horn. it allows a much better angle at the horn and less chance for unintended diferential.we figured the extra carbon fiber pushrod length was worth it.
 one last thing, the C/G on your plans looks way far forward of what i would have come up with ? just a thought.
anyway good luck with the bipe! ours looked great in the air unfortunaley it didn't last very long.
Dave jr.

Offline wwwarbird

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Re: Almost finished with it, my stunt Biplane
« Reply #17 on: November 16, 2010, 08:04:46 PM »
Thanks again guys for the helpful suggestions and encouragement.

I agree that weight will be my biggest challenge. Especially as I'm flying at 4600 ft ASL to start with. 70 oz's  here will be a brick.

 Just ship it here then, 1260 ft, and we'll fly it. ;D :##
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #18 on: November 19, 2010, 12:13:44 PM »

the last thing we addressed was to use a thicker more blunt airfoil on the next one, yours is about what i put on the second version. i was worried about drag in verticals but it was a non issue and the thicker airfoil might have helped the hard corner issue
other than slowing down in hard corners quite a bit it wasnt too bad a flier.
one issue that wasn't really addressed on the second redraw was the dificulty we had reaching the needle valve and glow driver with all those wings in the way while the engine was running. (just something to be aware of, it was a little precarious reaching around the carbon three blader when it was running)
one thing i noted on your plans that we came up with on ours was the bellcrank to flap pushrod angle. we elected to run the pushrod from the bellcrank directly to the elevators and then another back forward to the flap horn. it allows a much better angle at the horn and less chance for unintended diferential.we figured the extra carbon fiber pushrod length was worth it.
 one last thing, the C/G on your plans looks way far forward of what i would have come up with ? just a thought.
anyway good luck with the bipe! ours looked great in the air unfortunaley it didn't last very long.
Dave jr.


Hi Dave, did the one that slowed down in hard corners have the thin or the thicker wings?

It's always a bit dicey to get your fingers that close to the prop. I'm hoping that the nose is long enough to allow easy access to the needle.

I considered doing the same thing you did. I set up the controls that way on my Staggerwing bipe, and it worked well, but probably did add some extra weight. I spoke about my plans to Norm Whittle, whose opinion I respect, and the result of that conversations was to try and keep the controls simple.

I checked the angle at the horn, and found that it was essentially the same as on a normal mono-wing design, 9 degrees. Since it works fine on a mono-wing, I figure it'll also work fine in this case. It also made it simpler to remove the wings.

Interesting comment about the CG. I computed it based on the combined wing plan view, when observed from directly above.I found the MAC of the combined wings, then set the Cg at the 24% point on that MAC line. That's what I've shown. I have a belief that it's located pretty close to where it needs to be, though recent readings seem to indicate that it should be no further aft than 23% of MAC. Thank goodness that it's all adjustable.

I've sent the laser cut files out for quotes, hope to be soon starting constructiom.  H^^

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Offline Tom Niebuhr

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Re: Almost finished with it, my stunt Biplane
« Reply #19 on: November 19, 2010, 03:40:30 PM »
John,
What ever happened to the Kraut Twin?
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #20 on: November 19, 2010, 04:55:34 PM »
John,
What ever happened to the Kraut Twin?

It was showing great promise until I had a control system failure. The flap rod came undone from the horn. It pained me immensly watching it go in. I have the pieces, and it is repairable, but it'll be one of my projects for this building season.

Up until that point, I had gotten the engines sorted out. It was tracking very nice, upright and inverted. I had the turn rates pretty close, and was at the point where the next flight was going to be the first full pattern. Post mortem showed that I had possibly forgotten to tighten down the lock nut. I know I didn't secure it with loc tite. "Aint gonna happen" agin.
« Last Edit: November 19, 2010, 07:27:24 PM by John Miller »
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Online Brett Buck

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Re: Almost finished with it, my stunt Biplane
« Reply #21 on: November 20, 2010, 02:00:20 PM »
What you trying to do!!! That aspect ratio is off the charts. :o

Are you trying to compensate for the increased drag of a biplane? Is it something to do with the your high altitude?

     Whenever I worked on biplanes, I always came up with a very high aspect ratio, too. Solves several traditional problems with biplanes, including the reduced roll restoring torque, and the high induced drag. John's design is very close to what I have come up with, and I think it's a potential winner.

     I am skeptical that it can be built to the target weight but I don't necessarily think it needs to be

      Brett

Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #22 on: November 20, 2010, 04:59:07 PM »
Edit: In view of my next post, I've changed this edit. I should know that aspect ratio is only indirectly related to induced drag, since it is span loading that determines the actual amount. So I've rethought what follows here and submitted another post below.

I agree,

...and if the aspect ratio allows the plane to have the same lift and span as an elliptical monoplane wing, then with sufficient gap, the induced drag should be significantly lower than an elliptical monoplane wing's.  Of course, John's aspect ratio is not extreme enough to give that monoplane wing an aspect ratio of 5+, but I doubt that he will be penalized much or perhaps at all with additional induced drag.

For aspect ratios that would do so (like 10?) there would be some real gains with a bipe. Taking into account interference drag and assuming a gap/span (yes, span) ratio of 0.2, such a bipe theoretically will have 36% higher efficiency (ratio of induced drags) than that elliptical monoplane. I got this from Stanford's Illan Kroo (who analyzed wakes) and used the old Walter Diehl/Munk method to get the same result. With infinite gap, that theoretical efficiency goes to 200%. I've posted this in the past, but closing in the tips to form a box plane gives theoretical gains of around 45%.

John's gap/span is about 1/6 = 16.7%, and aspect ratios seem to be around 7, but I still think that his model's induced drag will compare well with a comparable monoplane stunter. I doubt that I'll have time to check on it, but I'm seure that John has come pretty close to the best aspect-ratio/structure compromise. The Reynolds Numbers are the only real worries that I can see. Really, it should work well. I think it's a pretty fine effort!
SK
« Last Edit: November 22, 2010, 09:29:37 AM by Serge_Krauss »

Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #23 on: November 22, 2010, 08:45:58 AM »
As posted on SSWF, in the light of a new day, I've changed my mind and think that John's biplane has about the same induced drag as an elliptical lift distributed monoplane with the same area and an aspect ratio of 5 . Induced drag depends only on lift, span, and gap. John's span is shorter than a monoplane stunter's of that wing area would normally be. So biplane advantages just compensate.

Here's my reasoning, so you can check and see whether I've made any false assumptions:

For some span "b" and lift "L", Induced drag Di = L2/(pi q b2) = kL2 for a given span.

If a biplane's wings have equal span and area and an infinite gap, so that there is no interference drag, and each contributes half the lift "L", then the biplane's induced drag is...

k(L/2)2 + k(L/2)2 = kL2/4 + kL2/4 = kL2/2,

or half the induced drag (kL2) of a monoplane with the same span and elliptical lift distribution.

Move the wings closer together, so that the gap is 20% of the span,, and this is raised to about 73.5%, or an efficiency 136% of the monoplane's, where efficiency is defined as the ratio of the induced drags for the same lift.

John's wing has only a 51.5" span, while an elliptical monoplane of aspect ratio 5 and the same area (688 in2) would have a span of (688 x 5)1/2 = 58.65". If John's biplane gap were 20% of it's span, the ratio of induced drags for the same lift would be...

.735 x (58.65/51.5)2 = .954.

So, with that gap, John's plane would have 95.4% of the monoplane's induced drag for the same lift. Since John's bipe has a slightly smaller gap, this percent increases, but they seem quite comparable. Diehl's graphs of Munk's span factor indicate that it changes from 1.16 to 1.136 as gap diminishes from 20% to 16%. As an approximation (1.16/1.136)2 x .954 = .995. So maybe john's plane is 99.5% as efficient as an elliptical monoplane? You'd have to read the old literature, but the span factor is what Munk multiplied a biplane span by to get the equivalent monoplane span.

Of course all this is based on elliptical lift distribution, but John's wings look very efficient to me. Any false assumptions or mistakes here? What say?

SK


« Last Edit: November 22, 2010, 09:51:27 AM by Serge_Krauss »

Offline wwwarbird

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Re: Almost finished with it, my stunt Biplane
« Reply #24 on: November 23, 2010, 11:22:17 PM »
 Huh? :##
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Offline PerttiMe

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Re: Almost finished with it, my stunt Biplane
« Reply #25 on: November 24, 2010, 12:52:33 AM »
Bipes are cool.

More work to build and set up, I believe, but cool  CLP**
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Offline Chris Wilson

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Re: Almost finished with it, my stunt Biplane
« Reply #26 on: November 24, 2010, 03:22:38 PM »
Where the heck would the aerodynamic centre of a biplane be?

If its between the wings then I just can't see it turning as sharply as a model that has the AC actually on the wing.

And would not the stagger give an inbuilt pitch effect that has a bias that going to be a real biatch to trim out?

Just thinking out aloud here!
« Last Edit: November 24, 2010, 03:40:08 PM by Chris Wilson »
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Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #27 on: November 24, 2010, 05:10:56 PM »
Chris-

I think the turning thing may "turn out" not to be so bad, considering the narrower wings, although the quickness in initiating turns might suffer. The other things - stagger and decalage - are only used with specific goals that may not be relevant here. For non-aerobatic planes, asymmetrical configurations and rigging must give better efficiency. Obviously that would not hold for flying inverted too. So equal wings with zero stagger might be best for symmetrical performance. I just posted a bunch of links on SSWF that will answer those last concerns. I'll have to read further myself, but I did see in passing that decalage was used by some to get a plane to cruise on one wing, with the other more or less "feathered". That would be for range and economy. 'haven't read what other reasons, other than induced aoa's, there might be, but these things all affect one another.

SK

Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #28 on: November 24, 2010, 06:01:58 PM »
Chris, I'm more visual than mathematical. I'll leave it to Serge and others to answer your question in detail.

What I will do is answer based on my visual abilities.

Your attached drawing looks pretty darned close to what I came up with, with the exception that the two wings are different in chord, and therefore area. That alone may cause some biasing. If they are also different in span, as well, there may be even more biasing.

I truly do not believe the sharpness of the turn will be that different from a mono wing. with the way I set mine up.

Let me walk you through the way I visualize the wings in flight.

While the wings have positive stagger, they are of the same span and area. The individual wings have a high A.R., about 7.2-1, but with the stagger, they should act together during the turns as if they are a mono wing at about 5-1 A.R., with one major possible difference.

The top wing should be flying in cleaner air during level flight than the lower wing, possibly carrying a significant percentage of the total weight. The lower wing may have some interference from airflow from the forward fuselage, and circular flow from off the prop. Possibly 25%, or slightly more of the lower wing, may be partially blanked by this flow. In level flight, the plane should be stable both upright, and inverted, with possibly a slight percentage better stability when inverted.

I see this because inverted, the top wing, (now the lower), is flying in cleaner air, and there should be slightly less percentage of fuselage/prop blanking by the now top,(lower), wing. This relationship, may alter the force arrangements, making the flying plane seem slightly nose heavy, depending on the planes attitude. I believe that careful design, can help to minimise these effects in level flight.

Now, during the turn, and we're talking about the corners here. It appears to me that during a hard inside turn, the lower wing may see slightly less blanking. The upper wing should be now carrying more of the weight, and making the arrangement appear to be slightly more tail heavy during the turn. More elevator authority may help the turn considerably, but as soon as the design is brought back to level flight, the apparent CG should now appear to move, back to the original force configuration.

Does the same thing happen during the outside turn?

Visualization leads me to believe it does.

So, in my estimation, there are positive expectations that can result from a proper Biplane platform. The problem appears to be figuring out the way to get the LD under control without getting the induced drag too high. I believe that it's possible, and I hope my design is a step in the right direction.

Having said all the above, I realise that since I'm more visual in my application, the proper application of the mathematics may show me that, either I'm correct, totally wrong, or somewhere in between. I trust that people like Serge, and others, will help guide us with either verifying, or pointing out, the errors, or correct assumptions, those of us who are more visual make.

Once the plane is built and flown of course, a lot of questions should be answered. H^^



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Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #29 on: November 25, 2010, 12:00:45 AM »
John-

I think that biplanes - especially - are extremely complicated, so that math at my level is more likely to indicate what is practical, and if the input is "good", some relative effects and proportions. I didn't do any real engineering. Mine is, I hope, a reasonable estimate. For instance, I cannot tell from what I read how much twist or other aerodynamic variation is assumed in the simple biplane performance cited and how that relates to CL models with symmetrical sections that become cambered with pitch (flaps). These results I cited were said to be "optimized", probably regarding lift distribution for the particular configurations, although when I did my own version, I did not optimize anything intentionally past the elliptical lift distribution asumptions - 'don't know how different that might be for bipes with interference. It may be that all our models are hampered the same by our aerobatics-imposed symmetry, so that changes due to multiple wings are in the same proportion as those cited. Maybe not. Someone with CFD and a sophisticated computer model looks at things from a different, minute perspective, but what I showed were simple assumptions and simple computations - mostly proportions.

So let's not expect anything like perfect analysis. Despite what one member expressed - succinctly - here yesterday, I've provided no more than a good Junior H.S. analysis with first month Algebra notation, based on one accepted generalized idealization and two other derived, handed down principles. I'm hoping that it is indeed a good approximation of what to expect, but...? Remember, math is just a short hand for the common sense thinking that must go into design, given some aeronautical knowledge - and not "just enough to get in trouble." It's my own aid to visualization. I'm just hoping that what I "see" hasn't been the result of overlooking or oversimplifying!

Results are enough for me to think that you're on the right track and should have a successful plane, but possibly, among other things, it may need greater size. Perhaps it will exceed all expectations. I'm just vicariously along for the ride and enjoying your adventure and cheering from the gallery. It's from your vision, and you're the man!

SK

Offline Mark Scarborough

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Re: Almost finished with it, my stunt Biplane
« Reply #30 on: November 25, 2010, 11:00:41 PM »
theory is all well and good, its even fun to hash out ideas and try them.
John and I talked biplanes at the regionals what, three years ago? LOVED the conversation, I to am a biplane fan, and beleive that one can be built that will perform a very suitable pattern. Will it ultimatly be better than an Impact, Patternmaster, any of the other current designs, maybe , maybe not,, it remains to be seen . As John says,, a lot of questions will be at least INITIALLY answered, and further answered as flights build up.
The bottom line is, its a model, no one is risking life and limb to verify the outcome, and John is having fun doing it. I am QUITE certain this model is at least as capable when trimmed as John is of flying it,, it will be cool, and fun to watch this one progress..
Get on it John,, Hey, did the  pilot in the twin survive, maybe he would like a ride in this one too?
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #31 on: November 26, 2010, 04:37:09 PM »
Yep Mike, the pilot survived, but the Tuetonic Ace is totally commited to the ride he's in. So, I'll be coming up with a new reqriut for the Bipe. <= H^^
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Online Brett Buck

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Re: Almost finished with it, my stunt Biplane
« Reply #32 on: November 26, 2010, 09:29:53 PM »
John-

I think that biplanes - especially - are extremely complicated, so that math at my level is more likely to indicate what is practical, and if the input is "good", some relative effects and proportions. I didn't do any real engineering. Mine is, I hope, a reasonable estimate. For instance, I cannot tell from what I read how much twist or other aerodynamic variation is assumed in the simple biplane performance cited and how that relates to CL models with symmetrical sections that become cambered with pitch (flaps). These results I cited were said to be "optimized"  <<snip>>

   More generally, we really have no idea what parameters we need to optimize in order to make the airplane fly better - because we haven't ever determined in any engineering sense, what "better" really is. When things like changing the tipweight by 2 grams makes more difference than changing the aspect ratio from 5 to 6, I think it's going to be pretty tough to work it out in enough detail to make more than educated engineering guesses as to what we want. That's why "cut and try", guided by some fundamental principles used to do the analysis of the results, is about as far as we have ever gotten. Or probably ever will.

   What doesn't seem to work so well is "cut and try" guided by fencepost wisdom and a vivid imagination, which is sadly about where most people are, and almost everybody was before the  mid-late 70's. Most of the stunt folklore about design and analysis has been and continues to be utter and complete nonsense. By dint of perseverance there were a lot of good results, and some insights, but it was definitely the hard way. That's why it's still important to try to get to the fundamentals and argue out the reasoning, even if it don't lead you straight to the final answer.

     Brett

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Re: Almost finished with it, my stunt Biplane
« Reply #33 on: November 26, 2010, 09:33:31 PM »
And would not the stagger give an inbuilt pitch effect that has a bias that going to be a real biatch to trim out?

     I don't think a monoplane stunter is particularly "symmetrical", either, so I think a bit of experimenting ought to get you an equivalent result.

    Brett

Offline Howard Rush

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Re: Almost finished with it, my stunt Biplane
« Reply #34 on: November 27, 2010, 11:39:23 AM »
Another cool project, John.  My main caution would be the flap interconnect pushrod.  Aft weight on a control surface can cause flutter, and aft weight on a flap could reduce maneuvering stability, the reduction of which may be what Ted calls "excess lift".  Although it would be some bother to do, I'd put the flap control horn arms to which the pushrods attach forward of the hinge line.

You could take advantage of the biplane configuration and use wire bracing.  Then you wouldn't need the zigzag ribs or surface spars. 
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Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #35 on: November 27, 2010, 04:35:23 PM »
Another cool project, John.  My main caution would be the flap interconnect pushrod.  Aft weight on a control surface can cause flutter, and aft weight on a flap could reduce maneuvering stability, the reduction of which may be what Ted calls "excess lift".  Although it would be some bother to do, I'd put the flap control horn arms to which the pushrods attach forward of the hinge line.

You could take advantage of the biplane configuration and use wire bracing.  Then you wouldn't need the zigzag ribs or surface spars. 

Good point Howard. Flutter would not be good. I'll see what I can do. Might wind up driving from a single flap rod, right behing the cabane strut.

Thanks

John H^^
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Online Brett Buck

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Re: Almost finished with it, my stunt Biplane
« Reply #36 on: November 27, 2010, 10:02:21 PM »

You could take advantage of the biplane configuration and use wire bracing.  Then you wouldn't need the zigzag ribs or surface spars. 

    I like it, - you could tweak it by changing the tension on the wires. And Orville Wright is no longer around to sue you for patent infringement, either.

    Brett

Offline Chris Wilson

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Re: Almost finished with it, my stunt Biplane
« Reply #37 on: December 01, 2010, 04:30:22 PM »
     I don't think a monoplane stunter is particularly "symmetrical", either, so I think a bit of experimenting ought to get you an equivalent result.

    Brett

I know what you are getting at here Brett but I am looking at my Freebird plans as we speak and have just read the Yatsenko web site concerning the following - thrust line, symmetrical wing and tailplane are all inline and a side mounted engine.

The undercarriage drag on the bottom of the fuselage is offset by the canopy and rudder on the top and the only way to make this even more symmetrical would be to mount a second undercarriage on the upper half then do away with the canopy and rudder altogether - but looks come into the equation too.

So the blanket statement of monoplanes are not particularly symmetrical seems to be coming from models that are not inline or intentionally asymmetrical by choice, not from monoplanes in general. Combat wings come to mind here as the epitome of symmetry.

Here wing stagger with biplane wings is the deliberate choosing of asymmetry by choice and most probably done for looks but I still would consider that it adds in a trimming factor that does not need to be there. If you flew the finished model and was happy with the trim then would you think that turning inside as compared to outside would be the same if the stagger was reversed? (Just using an analogy you once told me about wing tip shape and the best way of comparing their efficiencies is to reverse the shape and then compare differences.)

I would think not, and it shows that the stagger needs to be factored into the equation somewhere.

Now I admit that I am no expert but I would expect that the aerodynamic center shift with a staggered wing would move differently depending on pitch up as compared to pitch down and it would need trimming out to equalize. And I would expect this to be be the most apparent during square corners due to to the momentary blanketing of one wing.

Thanks mate.
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Offline Serge_Krauss

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Re: Almost finished with it, my stunt Biplane
« Reply #38 on: December 01, 2010, 06:08:50 PM »
Remember that there are other than geometrical effects that make an otherwise symmetrical model really asymmetrical. For instance, prop wash, torque, and for pitch especially, the ever-present gyroscopic precession, which pitches the plane up in ccw flight, unless the prop rotation is reversed. Balancing upper structure vs. landing gear for drag and c.g. purposes is not exact either. SO...you have to trim and design around these things. I'd suppose that stagger is just another part of the overall sum of things for which one must trim and adjust anyway. Still, I agree that no stagger would be best.

SK


Online Brett Buck

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Re: Almost finished with it, my stunt Biplane
« Reply #39 on: December 02, 2010, 09:25:27 PM »
I know what you are getting at here Brett but I am looking at my Freebird plans as we speak and have just read the Yatsenko web site concerning the following - thrust line, symmetrical wing and tailplane are all inline and a side mounted engine.

The undercarriage drag on the bottom of the fuselage is offset by the canopy and rudder on the top and the only way to make this even more symmetrical would be to mount a second undercarriage on the upper half then do away with the canopy and rudder altogether - but looks come into the equation too.

So the blanket statement of monoplanes are not particularly symmetrical seems to be coming from models that are not inline or intentionally asymmetrical by choice, not from monoplanes in general. Combat wings come to mind here as the epitome of symmetry.


     Do you have contra-rotating props?

     Brett

Offline Chris Wilson

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Re: Almost finished with it, my stunt Biplane
« Reply #40 on: December 12, 2010, 09:43:35 PM »

     Do you have contra-rotating props?

     Brett

Touché! Oh and you might want to add gravity to that list as it is very asymmetrical.
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Online Brett Buck

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Re: Almost finished with it, my stunt Biplane
« Reply #41 on: December 13, 2010, 05:46:49 PM »
Touché! Oh and you might want to add gravity to that list as it is very asymmetrical.

   It's not just a matter of verbal jousting. The almost universal need for down elevator, downthrust, or positive stab incidence to acheive proper tracking is, I think, the result of a nose-up torque arising from precession.

    Brett

Offline Chris Wilson

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Re: Almost finished with it, my stunt Biplane
« Reply #42 on: December 14, 2010, 04:26:28 PM »
   It's not just a matter of verbal jousting. The almost universal need for down elevator, downthrust, or positive stab incidence to acheive proper tracking is, I think, the result of a nose-up torque arising from precession.

    Brett

Interestingly a friend who flies electric uses a clockwise prop and reports that the opposite trim is needed.

I have launched his model (Firecracker) and witnessed that it pulls very strongly out of the circle at the moment that its released.

Thanks.
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Offline c.maikis

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Re: Almost finished with it, my stunt Biplane
« Reply #43 on: January 10, 2011, 11:22:48 AM »
Sorry, I’m a little late ( I don’t get to this forum very often ). I’ve always been a big biplane lover and I have built quite a lot. Most of them were more or less fun models without much demand for high aerobatic performance. In 88 I decided to build one which was intended for contest flying, so it was designed with a more serious approach. I tried to include all features which I thought necessary . While I still tend to think that monoplanes are superior, my goal was to find out how well can a biplane fly.
I’ve included a picture here. Sorry, the quality of the plan is very bad, but I don’t know how to make a better copy ( it’s a reduced size copy of the full size plan, scanned in and file size reduced ).
I was very impressed with the performance of the “Duetto”. I even won some contests with it; but I will freely admit that probably the judges just loved my biplane and gave me some free points. However there were also some nice placings among very strong competition which say something for my bipe. Looking back I guess that it has cost me some three places in a contest, on average.
Reading this thread has shown me that I have used pretty much the same design concept as John Miller did (regarding equal wings, stagger, aspect ratio,  gap ). I surely don’ have  the expert knowledge that Mrs. Krauss and Buck have , so I had to use what I had (knew).
Duetto had a span of 48 inch, 630 sq in, aspect ratio roughly 5,8 ,a little over 58 oz (okay – with a decent but not super finish) and used a Super Tiger 46. My experience with this biplane: it’s great fun to fly a bipe (in almost all cases you’ll be the only one with a bipe  -  in FAI wars, at that) and you will not make a fool of yourself. I got the impression that Duetto performed fine in cold weather but had problems when flying in high heat. Then the last corner of the triangle and the hourglass just were not corners.
If I had to do it again I think I would noticeably increase airfoil thickness, slightly increase gap, and maybe increase flap width.
There’s one thing I would never disregard: the optical appeal of a biplane, which almost dictates a very pleasing even if somewhat nostalgic shape ( forgive me).
Now  -  since I’m always interested in biplane theory and aerodynamics, I have a question. There’s one statement in one of John Miller’s comments. He says:

“I see this because inverted, the top wing, (now the lower), is flying in cleaner air, and there should be slightly less percentage of fuselage/prop blanking by the now top,(lower), wing.”
I do not understand this, especially the last sentence. John, can you or anybody else please explain this theory to me. Thanks for the answer(s).
Regards, Claus Maikis
« Last Edit: January 13, 2011, 12:10:10 PM by c.maikis »

Online John Miller

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Re: Almost finished with it, my stunt Biplane
« Reply #44 on: January 10, 2011, 05:43:04 PM »
Greeting Claus, thanks for your comments, and reviewing your experiences with Biplanes in competition.

I have to agree with you that they are fun, and may garner some "Wow" points, but your experiences with comps. that included some very good pilots, seems to show that designs like your Duetto, can be serious enough to consider,

Now, to your question about my thoughts about performance upright and inverted. First My thoughts come from visualizing what is happening rather than pure mathematics. So a warning, I may be all messed up.

The statement in it's entirety is included. I believe we should look at it this way to explain my thinking.

"The top wing should be flying in cleaner air during level flight than the lower wing, possibly carrying a significant percentage of the total weight. The lower wing may have some interference from airflow from the forward fuselage, and circular flow from off the prop. Possibly 25%, or slightly more of the lower wing, may be partially blanked by this flow. In level flight, the plane should be stable both upright, and inverted, with possibly a slight percentage better stability when inverted.

I see this because inverted, the top wing, (now the lower), is flying in cleaner air, and there should be slightly less percentage of fuselage/prop blanking by the now top,(lower), wing. This relationship, may alter the force arrangements, making the flying plane seem slightly nose heavy, depending on the planes attitude. I believe that careful design, can help to minimise these effects in level flight."

 

Upright, I see the top wing flying in clean air, but the lower wing has disturbed air from the prop, and front of the plane disturbing a section of the lower wings area. I believe that this makes the top wing more efficient than the lower wing. The plane, with more efficiency from the top wing, may be more stable in level upright from the "parasol effect" with the top wing.

Inverted, I still see what was the lower wing being affected by pretty much the same forces. Some vectors may change, but essentially the lower wing seems to be interfered with. The top wing, now on the bottom, may have some interference, but it will still should be in relativly clear air. It appears to me that a higher percentage of the available lift will be coming from the wing less interfered with. Normally, it appears that the CL will apparently move forward, based on the percentages of actual lift provided by the two wings. This should affect the stability in flight. I feel that the plane will be slightly more stable inverted, but I could be wrong, and it would become slightly less stable.

It's one of the parameters I hope to explore once the plane is built and flown.

Again, thanks for your interest and comments Claus
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Offline Tom Niebuhr

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Re: Almost finished with it, my stunt Biplane
« Reply #45 on: January 11, 2011, 03:59:57 PM »
John,
I think that bipes have a "wow" impression for most airplane lovers. There have been some very good bipes. The best that I have personally seen is Don Hutchinson's "Stearman". In the right hands it is a competitive airplane.

I have always been intrigued with Claus's "Duetto", but I have never seen a "Duetto" fly. I suspect that it is a very good airplane.

Don Yearout's classic legal "BiCeps" is interesting, but might not be as competitive, but I don't really know, so that is not an educated statement.

I recall seeing a profile bipe at the original old Pelham Bay field in NY. It was flown by a young man that had two hearing aids. Maybe someone recalls who he was. The airplane had high aspect ratio wings. He flew it fast but it flew very well to my very young and untrained eye. I was impressed by the biplane flying well as opposed to to the totally sport "Flying Fool' type.

Claus,

It is interesting that you would thicken the airfoil. The "BiSlob' has a comparatively thick airfoil, so I guess we all have much to learn about what really works well. Do you think that the stagger on the "Duetto" has any adverse effect" ?

John,
You showed this drawing in November. so can we assume that it will be in the air in a few weeks? We are looking forward to hearing and seeing more.

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Offline Tim Wescott

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Re: Almost finished with it, my stunt Biplane
« Reply #46 on: January 11, 2011, 08:56:58 PM »
John,
You showed this drawing in November. so can we assume that it will be in the air in a few weeks? We are looking forward to hearing and seeing more.
And build pictures, please?
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Re: Almost finished with it, my stunt Biplane
« Reply #47 on: January 12, 2011, 08:52:50 AM »
Tom and Tim, I'm still assembling the materials. I've a couple of good quotes for the laser cutting, and the wood. I'm busy trying to earn the extra bucks it's going to take to start the project.

I love most of what has happened since I retired, it's just getting used to no money. ~^
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Offline Chris Wilson

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Re: Almost finished with it, my stunt Biplane
« Reply #48 on: January 12, 2011, 06:40:32 PM »
Then the last corner of the triangle and the hourglass just were not corners.
If I had to do it again I think I would noticeably increase airfoil thickness, slightly increase gap, and maybe increase flap width

I take all this to mean that you ran out of lift at the critical points in the schedule and so are looking for ways to regain it.

Just wondering if an increase airfoil thickness on just the lower wing might do the trick here.
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Offline c.maikis

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Re: Almost finished with it, my stunt Biplane
« Reply #49 on: January 13, 2011, 01:36:48 PM »
That's right, Chris. When doing that last 60 degree corner in high heat my biplane just sank down a frightening amount of "height" if controlled in the usual fashion.
Tom, the reason I built that particular airfoil:
A biplane has a shorter span; thus the distance CG to leadouts is shorter than on a monoplane. From this fact plus my fear that the biplane is more sensitive to gusts (shape and size of struts ), I intended to fly the bipe a little faster than normal to compensate for this sensitivity around the roll axis. With this increased speed I thought that a not so fat airfoil might provide enough lift. Also the thinner wing would reduce drag, which was my main concern. For the same reason I used very narrow flaps.
Also, if you increase aspect ratio (Duetto has about 5,8) your wing automatically gets thinner (smaller chord). So the aspect ratio on the Duetto is a compromise between AR and wing rigidity ( = wing thickness = airfoil).
It seems I have gone too far. The airfoil is about 15% thick (including flaps, of course), and this is much less than what we use on a modern monoplane.  That's why I would change these two features (airfoil and flaps).
Chris, your idea of increasing thickness on one wing only sounds funny. Maybe some experts should take a look at this experiment. For this old lazy guy here this is not a suitable solution : I fabricate ALL ribs for both wings in one stack !!

Regards,  Claus


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