Why no competitive biplanes in stunt

[Tim Wescott]

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.

[Serge_Krauss]

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 

[Howard Rush]

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.

[Tim Wescott]

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?

[Dennis Moritz]

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.

[Howard Rush]

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).

[Randy Powell]

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.

[Serge_Krauss]

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

[Doug Moon]

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.

[Tim Wescott]

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.

[Mark Scarborough]

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)

[Robby Hunt]

http://www.clacro.de/Seite_falco.htm

[Tim Wescott]

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...

[Tim Wescott]

http://www.clacro.de/Seite_falco.htm

Ooh, ahh.  (I like it).

[Douglas Ames]

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...

[Howard Rush]

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.

[Howard Rush]

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. 

[John Leidle]

   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

[Tim Wescott]

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.

[Howard Rush]

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.

[Howard Rush]

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.

[Douglas Ames]

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

[Mike Palko]

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

[Howard Rush]

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.

[Steve Helmick]

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!  Steve

[Douglas Ames]

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  dang it, more drag. Ok, I'll put my pen down.