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Author Topic: Experimental biplane  (Read 1583 times)

Offline Peter Germann

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Experimental biplane
« on: July 04, 2019, 06:39:40 AM »
Here is an experimental airplane I’ve recently built. The intention was to get tight corners and sufficient overhead line tension at low flying speed. This is what I have done:

•   To get tight corners: Low wing loading – high aspect ratio – narrow flaps – long elevator arm.

•   To increase line tension above 45°: Wing type Clark Y airfoiled fuselage.

Specifications
Wings: Span: 1’370 mm  (54 in)  MAC: 210 mm (8.3 in)   Aspect ratio: 6.5  Thickness: 18%  Flap: 22%
Area: 57.5 qdm (892 sq/in)  Loading: 31.7 Gr/qdm (10.4 oz/sq.ft)

Fuselage: Rectangular, inside flat, outside Clark Y.  Height: 200 mm (8in) Thickness:  100 mm (4 in) max.
Hingeline-to-hingeline: 550 mm (21.6 in)

Flight setup
•   Weight: 1’826 Gr (64.4 oz)
•   Prop: Fiala electric E3 pusher prop 13 x 6 in  10’450 RPM constant speed
•   Lines: 18 m (59 ft.) x 0.36 mm (0.015 in)
•   Tip weight 32 Gr. (1.12 oz)

Initial flight results
•   El. power in level flight: 556 Watt
•   Speed. 5.2 sec/lap
•   Inner wing approx. 15 cm (6 in) down
•   Line tension level: good
•   Line tension 45°: sufficient

When entering an inside round loop from level flight at 45° the airplane abruptedly banked inwards, entering an inverted spiral dive and crashing far inside of the circle. It was damaged beyond repair.

Questions remaining are:
•   Why did the airplane, despite more than sufficient tip weight and definitely no warps, bank-in so much in level flight?
•   How comes it that at time of landing, with the prop stopped, the wings were level?
•   With the very high part of the fuselage being less than (30 mm / 1.25 in) behind the large prop, could it be that the infamous spiral flow of the (CW rotating) prop was causing the airplane to bank-in?

Thanks for comments, Peter
Peter Germann

Offline Peter Germann

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Re: Experimental biplane
« Reply #1 on: July 04, 2019, 06:54:10 AM »
One more pic...
Peter Germann

Online Tim Wescott

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Re: Experimental biplane
« Reply #2 on: July 04, 2019, 11:07:36 AM »
Hey Peter:

In order for the airfoiled fuselage to be effective you need to taper it to a point at the end of the "main" fuselage -- in other words, make a stubby section of wing, then attach a boom to it.  Your fuselage looks airfoiled when you look straight down on it -- but it won't look airfoiled to the air.

To further increase the effectiveness of the airfoil, end the fuselage airfoil before or at the TE of the wing, so the wings act to cap the airfoil.  The wings will end up looking like tip plates to the air.

Better yet, just build a conventional looking biplane, but make full-chord interplane "struts".  The struts will form lifting surfaces, and they'll leave you free to design the fuselage any way you want.  For experimentation, put flaps on the trailing edges of the struts -- adjusting the angle of a flap will adjust the effective angle of attack of that whole strut, so you can experiment with different settings.

If you have more money than building time, there's probably an RC aerobatic ARF biplane that's close to what you want -- you'd just need to build the struts and install a control system.
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Online Brett Buck

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Re: Experimental biplane
« Reply #3 on: July 04, 2019, 11:10:48 AM »
Questions remaining are:
•   Why did the airplane, despite more than sufficient tip weight and definitely no warps, bank-in so much in level flight?
•   How comes it that at time of landing, with the prop stopped, the wings were level?
•   With the very high part of the fuselage being less than (30 mm / 1.25 in) behind the large prop, could it be that the infamous spiral flow of the (CW rotating) prop was causing the airplane to bank-in?

Thanks for comments, Peter

    It appears to be very likely to be unstable in yaw - as soon as it got a little disrupted, it decided to swap ends, at which point your best option is trying to outrun it. Ignore the wings, look at  the fuselage by itself in plan form- it's got a huge "wing" and a tiny "tail", it's almost certainly unstable.
 
      I think building things that are intentionally yawed is generally a big mistake, and doesn't really make for a net positive, but you will note how Aldrich did it, and compare that to yours  - the Nobler and similar is built like a very exaggerated arrow in yaw with huge "fin" at the rear. You have tons of area forward and a vestigal fin/rudder. Your are putting the fins on the nose of the arrow instead of the tail. It works as long as there is sufficient line tension to force it to fly nose-first, but as soon as the slightest disruption occurs , it wants to yaw violently around.

    It probably doesn't matter which way it diverges first - nose-in or nose-out - nose-in, it violently rolls, too, crash directly. Nose-out, it recovers the tension when it comes to the end of the lines, also wildly rolling, the reaction from the lines whips them forward first, then back, from which it can never recover, same effect.

   You could probably get it to "work" if you put a huge fin on the rear, but then you still wind up trying to crab a giant fuselage sideways through the air. With a gigantic enough engine, OK, you just put in more fuel. Better get a HUGE battery.

     Brett

p.s. you could easily do a crude check on the stability by using a rudimentary rocket science technique - the "cardboard cutout" method. Cut out the side view of the fuselage from cardboard, sufficiently stiff to stay flat. Then balance it on a sharp edge. This is the *very conservative" estimate of the center of pressure. If the CG is behind this CP, it will certainly be unstable. If it's close, it may or may not be unstable, because the "cardboard cutout" method results in a CP location that is further aft than you will actually care about, or, effectively, the CP at a 90 degree angle of attack. There are more sophisticated methods that can determine it for lower AoA but this should be sufficient, because I don't think it's going to be a close call.
« Last Edit: July 04, 2019, 11:29:15 AM by Brett Buck »

Offline Igor Burger

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Re: Experimental biplane
« Reply #4 on: July 04, 2019, 01:19:17 PM »
I think building things that are intentionally yawed is generally a big mistake

IMHO that is the only way how to make indoors on 5m long lines fly overhead at the same angular speed like large models. Centrifugal acceleration at such speed is much smaller than gravity, so it must "fly" on its fuselage side area which is aproximately the same like wing area.

So that could be answer for Peter, because he is trying to do exactly what indoors do - slow flight at usefull line tension. The solution is - microscopic negative yaw stability. How to reach it is well described by Brett and that is also how I did it, just with little difference - instead cardboard I used depron :- ))) That is how I got that huge nose and why R/C foamies do not perform well on lines. Everyone who tried it, will tell that they do not fall from sly at any at leas minimaly "positive" speed, so the solution exists.



And if I can comment that roll problems - biplanes for cl stunt are simply not the best way - lines have large stabilizing effect, if you make will half smaller, they also act half way. So I would make wing larger - you simply need larger indoor and problem solved :- ))

Online Tim Wescott

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Re: Experimental biplane
« Reply #5 on: July 04, 2019, 01:42:05 PM »
...   It probably doesn't matter which way it diverges first - nose-in or nose-out - nose-in, it violently rolls, too, crash directly. Nose-out, it recovers the tension when it comes to the end of the lines, also wildly rolling, the reaction from the lines whips them forward first, then back, from which it can never recover, same effect. ...

I've experienced this, with an original design with a too-small tail.  It wasn't divergent in yaw, but it was seriously underdamped.  Any maneuver that challenged the thing in yaw (i.e., squares) would result in the thing wiggling all over the sky for about a lap.

I'm pretty sure from the feel of it that by itself it was divergent, but any time it had line tension it would self-correct (think combat wing).  Adding about 40% to the rudder area fixed it right up, and the next plane in that series had a bigger fin from the get-go.
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Online Brett Buck

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Re: Experimental biplane
« Reply #6 on: July 04, 2019, 02:22:42 PM »
IMHO that is the only way how to make indoors on 5m long lines fly overhead at the same angular speed like large models. Centrifugal acceleration at such speed is much smaller than gravity, so it must "fly" on its fuselage side area which is aproximately the same like wing area.

So that could be answer for Peter, because he is trying to do exactly what indoors do - slow flight at usefull line tension. The solution is - microscopic negative yaw stability.

     Indoors, with no turbulence except that you create yourself, in an extreme condition, at a Reynolds number of a gnat. Not outdoors in 65 foot lines with the usual turbulence sources.

      BTW, as soon as you lose line tension, the lines don't stabilize it any more. They can keep it from diverging further nose-out, but as soon as it noses in (which is probably inevitable)  the lines will lose tension and  make it diverge much faster. The only hope this scheme has is to make it so unstable with so much offset that it never gets upset. Conceivable indoors, more-or-less inconceivable outdoors.

      So it works until something gets disrupted - which it will, eventually, outdoors in normal conditions - and then, *exactly what happened to Peter, happens*. Which tends to contradict the argument about how well it works.

    This is not a new idea, I have seen numerous experiments along these lines, with similar results, although none were as extreme as this one. You can still accomplish the (very dubious) goal, by merely adding fin area to make it stable, and sufficient rudder offset to get the desired angle of attack - then you can safely count on the line tension to keep if from nosing "up" (actually, out) and stalling, which is what would happen if you were flying freely. If it comes loose on the lines, the restraint goes away, and then it rapidly yaws out and restores it. There's no reason it has to be unstable to get the lift from the fuselage, and it's a lot less likely to go nuts and crash. In fact, it has to be sufficiently stable to overcome the destabilizing effect of the slack lines.

    I would add, I think I was probably wrong about the cardboard cutout method - it's conservative in the case of conventional rocket layouts - i.e. it gives you a CP estimate that is further forward than it really is, and you can remain sufficient stability with the CG further aft than you would think. In this case, however, it probably gives you a highly *unconservative* CP position, that is, it will still be unstable with the CG further forward. If you are using cardboard cutout to figure it, you are probably more unstable than you think.

     I would have to think about the assumptions in the more sophisticated methods to see if they are violated for this condition. In particular, the extremely low aspect ratio of the "wing" VS the moderate to high aspect ratio of the tail probably helps until the fin stalls, then it swaps ends even faster.

    Brett

Offline Igor Burger

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Re: Experimental biplane
« Reply #7 on: July 04, 2019, 02:47:05 PM »
BTW, as soon as you lose line tension, the lines don't stabilize it any more. They can keep it from diverging further nose-out, but as soon as it noses in (which is probably inevitable)  the lines will lose tension and  make it diverge much faster. The only hope this scheme has is to make it so unstable with so much offset that it never gets upset. Conceivable indoors, more-or-less inconceivable outdoors.


Yes, it is so much (I mean properly, not exesively) unstable that it never gets upset - if it loses tension, it will immediatelly turn out and gain it back - we flew it also outside and also in little wind (relative to its size - it will clearly not fly in 9m/s) and it flies very well also on upwind side (especially on upwind side - because wind acts as souce of power for them as they are yawed out) - never falls from lines.

Offline phil c

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Re: Experimental biplane
« Reply #8 on: July 04, 2019, 03:10:34 PM »
Peter- I expect there is a great deal of turbulence and interaction that caused the roll into the circle.  The fuselage has an horrendous shape for an airfoil.  The boxy front end will cause lots of turbulence in any attitude.

The turbulence could easily blank out the small rudder.  At 45° the airflow off the fuselage would be rolling outward from both top and bottom, tending to go between the wings.  This would increase airflow over the top of the bottom wing and increase the pressure under the top wing.  That could cause a left roll.

The airspeed significantly low for ~60ft. lines, 49mph. The wing chord is substantially shorter than monoplane stunters at that speed.  Lower Reynolds number would cause more drag on the inboard wing, aggravated by likely a longer inboard wing.  The prop rotation would also be tending to make air hit the top of the left wing, causing it to stall.

Just to note, the Bi-slob, which performs at similar speeds, has shorter wings separated by a full chord.  It also has something like 10° of engine offset.  Even so, when one stalls up around 45°  it will roll into the circle also.
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Online Brett Buck

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Re: Experimental biplane
« Reply #9 on: July 04, 2019, 05:03:18 PM »
Yes, it is so much (I mean properly, not exesively) unstable that it never gets upset - if it loses tension, it will immediatelly turn out and gain it back

   If it is unstable, it's just as prone to turning nose-in as nose-out, and the slack lines will greatly favor it continuing to turn in/nose right if it ever gets loose.  Particularly when the fin stalls and the forward fuse does not (because of the exceptionally low aspect ratio). Do your cardboard cutout with *just* the front end, as if the tiny high-aspect ratio fin has stalled - but you can easily see the answer just by looking.

   I think you want it to be stable but set to have a strong left loop ("up", right-hand-rotation about -Z), then you can have your "lift" but also have some hope of recovering.

   On the other hand, balsa trees are not on the endangered species list, far as I know, so having the same thing happen a few more times should prove the point well enough.

    Brett

Offline Igor Burger

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Re: Experimental biplane
« Reply #10 on: July 05, 2019, 02:41:19 AM »
   If it is unstable, it's just as prone to turning nose-in as nose-out

Maybe I did not write it well. "It" - means the fuselage itself is ustable, but system with lines is stable. Lines make classic PD controll system. Just like weigth on pring - weight itslef IS ustable - it will fall down. But attached to spring it became stable. And it is only question of mass inertia, controll force and damping if it oscilllates, convergates or not.



Yes that is true, model on lines will work only in some boundalies, but it is the same with weight on string also - too large deviation from setpoint will make model fall from lines - or sping to break or unhook. And it is exactly like with human heart. It feeds itself, if it stops, it will stop, while it works it can work, we have just believe it works, and it will work :- ))))))))))))) ... so while we have tight lines they keep themself tight ;- ))

You can see on video it can be made working - also with huge inputs from large prop of diameter equal 1/3 of span making yaw in every corner.   

And regarding stalling - I think it is not the case - nose will certainly stall earlier than tail - it has much larger side AoA because of circular path. That makes 2 things:
1/ excessively yawed fuselage will have CP back of hingepoint - so in extreme it will stabilize itself. That is why fuselage needs to be only little unstable. It will became stable in extreme as CP moves back. CP back of hingepoint will cause no chance for recovery in case of fall from lines because nose will turn to the center. That is clear problem.

2/ That circular flow around fuselage makes it yaw out also without lines at constant rate - not because nose is large and CP front of hingepoint, but because of circular flow. So it needs really only microscopical unstability.

I borrowed several thing from thin on Maxbee fuselage it is outdoor model, works perfectly. The only difference to this is, that the nose is only little smaller, so it does not have self stearing with weak lines, because there is enough centrifugal force, otherwise all is the same and works well. Result is model immune of turbullence.

Offline Peter Germann

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Re: Experimental biplane
« Reply #11 on: July 05, 2019, 02:47:01 AM »
Thank you, friends, for your quick and very interesting comments. I must confess that yaw stability was not looked at when I designed the bipe and it may well be that the lack of it has contributed to the crash.
However, what this does not explain is why the airplane flew very substantially banked-in in level flight.

Here are few additions/remarks:

Yes, Igor, after flying your Indoor Gee-Bee I was inspired to do same for F2B

As I was looking for very light wing loading for to allow sharp turns, a high aspect wing would have come to an impractical (for my car) span of more than 2.6 m (8 ˝ ft.). Other than a triplane, the bipe seemed to be the way to go.

The cardboard fuselage method was used to find the C.G of the fuse walls. If this c.g. is equal to the center of pressure then it is at approx. the same position as the c.g. of the airplane.

rgds, Peter
Peter Germann

Offline Wolfgang Nieuwkamp

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Re: Experimental biplane
« Reply #12 on: July 05, 2019, 07:02:20 AM »
Peter,

since the (pusher) propeller is quite close to the fuselage, the prop wash could have caused the banking. The attached picture illustrates what I mean.

It was taken from https://www.icas.org/ICAS_ARCHIVE/ICAS2004/PAPERS/065.PDF

Regards,

Wolfgang

Online Brett Buck

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Re: Experimental biplane
« Reply #13 on: July 05, 2019, 08:51:38 AM »
The cardboard fuselage method was used to find the C.G of the fuse walls. If this c.g. is equal to the center of pressure then it is at approx. the same position as the c.g. of the airplane.

    I think that this method underestimates how far forward the CP might be, meaning it is substantially more unstable than you think. Also, during an upset, the tail is likely to stall with a sharp "break" due to the moderate aspect ratio, whereas the forward fuselage/"wing" is going to just sort of softly fall off - meaning, for the most part, the tail will disappear from the equation. Redo it with just the forward fuselage, and it will shift the CP forward dramatically.  As soon as that happens, it might diverge in either direction. Nose-out and the line tension increases and prevents it from going further - at least to start with! Half a line whip period later, it's going to pull you nose-in, and then it's a matter of whether the instability is diverged so much to prevent it from nosing in.    If it noses in to begin with, the tension is reduced, the effects of the lines to hold it straight disappears completely, and all the lines do is drag it further nose in. In either case, it also rolls dramatically almost immediately. *Which is exactly what seems to have happened".


     If your airplane is banked towards you in level flight, you want to add tipweight until it stops. Nothing it getting "blanked" at these very low Reynolds numbers and even if it was, the "lost lift" is right at the wing root, where it can cause only minimal rolling torque. It's more likely that the reaction force of the fuselage wanting to straighten the flow off the (backwards) prop caused a roll torque. If that is the case (which seems to violate conservation of angular momentum), then if you get it level upright with tip weight, it will be rolled dramatically away from you in inverted flight, and roll violently during changes of direction. But you have to have an otherwise flyable airplane in order to get far enough to discover that, which you will not have unless you first make it safe to fly.

    As an aside, you can just build a hand-launched or RC glider that looks like your fuselage, and see what it takes to achieve stable trim settings. If RC, you can also see what happens when it gets upset at the CG you are going to use.  It will have a huge low-aspect ratio wing with a substantial camber, and a tiny vestigal tail - so it's not that hard to predict what will happen.

      Brett


Online Howard Rush

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Re: Experimental biplane
« Reply #14 on: July 05, 2019, 01:00:22 PM »
Peter's airplane has a pretty high aspect ratio vertical stabilizer aways aft of the CG, so I would guess that dCn/dβ is stable (until the tail stalls).  The fuselage camber might give it some yawing moment, though.

I suspect, as do Peter and Wolfgang, that swirly air from the prop caused the airplane to bank in in level flight.  One wouldn't want to initiate an inside loop from 45 degrees with that bank.  I recommend building another identical biplane with one of those Polish counterrotating prop mechanisms to test the yaw hypotheses.
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Online Brett Buck

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Re: Experimental biplane
« Reply #15 on: July 05, 2019, 03:30:17 PM »
Maybe I did not write it well. "It" - means the fuselage itself is ustable, but system with lines is stable. Lines make classic PD controll system. Just like weigth on pring - weight itslef IS ustable - it will fall down. But attached to spring it became stable.

   It's not a spring, it's not a simplex spring/mass/damper sustem and the analogy is not apt. If you model it as a spring and also only look at the restoring force resulting from line tension,  the spring has got a K that greatly increases as it moves one direction (as the line tension increases) and goes to, effectively, 0 in the other direction.  It won't do as you show, you will get very small displacements nose-out but very large displacements nose-in.     You can model it as a linear spring only under very narrow circumstances that are very easily and quite obviously violated.

       And this is only to first approximation, effectively, you have at least 2 spring/mass/damper systems in series - you are not modeling line whip. This might be negligible with featherweight kevlar/Spectra/whatever thread you use for indoor models, and the damping may be high, but in a outdoor model scale it is heavy, lightly damped, and as the advantages of operating as the cosine of the yaw angle instead of line tension, which offers restoring force as the sine function. Figure it as a relatively heavy additional spring/mass/damper system, operating directly fore/aft on the inboard wingtip. Also with a non-linear K - goes up nose-out and down (and to zero) nose-in.

   Even this more complex system ignores the root cause - the aerodyamics in yaw make it want to diverge from either position. It only works even for a little while because as the yaw diverges aerodynamically (nose-out), the K of the spring you model goes up. It noses out, which only increases the aerodynamic tendency to go further, and the only thing that stops it is the fact that the line tension goes up along with the law of sines. It reaches a conditionally stable equilibrium. It would go *much further* if the line tension did not go up.

    I would also point out that increasing the line tension *was the original purpose of the exercise*. So, the entire goal is to make the K go up as it noses out - which also makes the K go *down* as you nose in.

      That's swell, as long as it diverges nose-out and stays that way. Inevitably, it will get upset nose-in, and then the K of your supposed spring goes way down, finally to zero as the airplane stops flying in a circle, now you still have the aerodynamics driving it further inboard, and the line drag alone also driving it inboard.

     The dynamic system you are modeling considers only diverging in one direction and staying here, which does provide the conditional stability you think you are modeling.    The real situation is strongly non-linear, and subject to upsets in use which makes your spring disappear completely. What I would expect is it would work for a while until the first sufficiently large disturbance comes along, then wildy diverge nose-in, roll violently due to side-slip, and crash. Which, I feel compelled to point it, *is what appears to have happened*. Very similarly to *all the other people to have tried the same idea for the past 50-60 years*.

   You don't need it to be aerodynamically unstable in yaw for your idea to "work", you can make it stable with a positive restoring force, then, while it is still non-linear, with other forcing functions you have not modeled, it's not subject to rapid divergence as soon as your conditions are exceeded. Aldrich did it in 1951.

    Brett


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Re: Experimental biplane
« Reply #16 on: July 05, 2019, 10:44:44 PM »
Well , this has gone off at a rip roaring pace .

First Glance , my thinking is the tad pole fuselage was the worry , + insufficent vert. stab.  were there reasons for that .

Throwing a few thoughts about , this F L O H or whatever it is , say ELONGATED would not be disimilar ,



back in 73 , had a Mercury Tigermoth , that won F/F Jnr. Scale at the N.Z. Nats . Primarilly as there was no other entrants. It being a late addition there .
A Stuka won C/L , cant recall who was responsable there . The Big club points war was of some relevance . And Id thought the Scouts were into bribery .

ANYWAY , as it was so bleedy hopeless , I threw in a OS Max 10 , a bellcrank  etc , and briefly flew it control line .
One memorable day at a demo , with a bit of steady breeze , tried a loop , a lap inverted ( No worries at all ) Engine cut , landed dead up wind ,
blew around 1/2 lap ripping along tail level till it stopped , dead downwind .

Suggesting the layout wasnt totally hopeless . Fairly sensitive / resposive , stable & manouvreable . AND ive got a kit on the shelf NOW , and TWO OS 10s !

SO ,

Contemplating BI PLANES over the decades , theres a few ideas formulated , as follows . First I think , for your slow & tight corner endevours ,

Something like the early Fairey torpedo bombers . add more bays & lengths , keep it in proportion . wire Bracing so as not to ornithopiate .



The Long and type 184 SHORT of it .  :-\

The Big Issue being INTERFEARANCE on Bi Planes . Wing Chord Vs GAP , about 1.2 x the Chord considered necesary , with scrawny thin wings .

Which brings us to Theory TWO .

Throw a monoplane though the bacon slicer . Horizontally at Chord Line . Say one of Casales In Line Ships . stick some lolipop sticks between the top & bottom . As in six or eight inch gap . Mirror Image . So it IS symetrical - in a certain sense . Tho obviously airfoils FLAT inner face .

No . We wouldnt expect aerodynamic marvells with this . but it would be intresting in comparison to its original .
Would think the first stage in development would be to do symetrical airfoils ./ wings . of THAT thickness . i.e. 1/2 of monoplane wing .

FREE .

While we're at it . The *darm TAILPLANE . a similar treatment there . ONE WOULD THINK , if we're thinking BI PLANE , the advantages would be ENHANCED
with a Bi Plane TAILPLANE ASSEMBLY .

This accentuates the Bi Plane Bothers . VOTICIES and Interfearnce Drag . ( Intersection of aerodynamic / airflow - forces )

FOUR the keep it simple & dont reinvent the wheel trip .


Something like a Sopwith Tabloid , Bristol Scout D , Sopwith Pup . they ALLREADY WORK . Keep Changes Minimal . THIN symetrical airfoil .

ASSUME 1/2 thickness of monoplane . As in look at it as 1/2 the total area . Start with a new sheet of paper , F2B Biplane . Prototype .
                                                                                                                                                              ----------------------------
Or TOUGH like a SNIPE or DOLPHIN .

FIVE , something about symetrical . Like you have . or the Floh , Habelstdab or however you pronuncite it , Or the BRISTOL FIGHTER .

To me its the ideal set up , if time consuming , unless all of C/F .  S?P Not That Tecnical really . a few cardboard / jigs for assembly / alignment .



https://www.stickandtissue.com/cgi-bin/yabb2/YaBB.pl?num=1517688583/17

Plus  , like the  Sopwith Triplane , theres a larger tailplane assembly ASCALE  , adequate for our purposes .



« Last Edit: July 05, 2019, 11:11:24 PM by Matt Spencer »

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Re: Experimental biplane
« Reply #17 on: July 05, 2019, 11:25:32 PM »
Having Flown my profile 54 inch plus length meteor twin , the ' plank ' tagental to lines set up was severly evident .

Your Forward Area isnt compensated aft in a proportionate manner , as in the feathers are on the FRONT of your arrow !

A moulded / round edged ( top and bottom blocks ) would have less ' hard edges ' to catch the prop wash .

Further to general Bi Plane ramblings ,

The Short Horn / Flyer / super elementry stuff , through the simple ' streamlined ' ones



To something a bit challengeing . Assembled in a jig and the strutting dropped through . such as .  :P



though it might be said some people dont know where to stop .  :-\

Theres tapes / u tube , of a Venitian Blind - multi plane , and a twin engined quadroplane , that does the schedule quite well , as a prototype .

Offline Igor Burger

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Re: Experimental biplane
« Reply #18 on: July 06, 2019, 09:39:41 AM »
   It's not a spring, it's not a simplex spring/mass/damper sustem and the analogy is not apt. If you model it as a spring and also only look at the restoring force resulting from line tension,  the spring has got a K that greatly increases as it moves one direction (as the line tension increases) and goes to, effectively, 0 in the other direction.

It keeps nose out force also when nose is yawed IN. That is the point. Reason is curved air passing around. If fuselage is tangent to the circle, air hits nose still from left side and rudder from right side, so it makes force yawing still out. Plus there is motor out offset, so also in case when it completely goes off the lines. it will still turn right. But it never happes, because it is so effective, that it keeps lines on at also very difficult conditions.

Well whatever theory we invent here, it will not change fact that it simply works :- )))

Here is vide showing it with motor set very slow and flying outside in wind which stops it in figures. In some figures came stronger gust so it completaly stopped model and it continued only when wind stopped. If rudder was larger (positive stability or CP aft of hingepoint) wind will simply turn fuselage with nose to the center and it will simply fall down. Yaw is very stable, model is not hinging on lines and model is controllable also at 0 speed. Just to test it, I did several figures on up-wind side. Also no problems. Wind on video is comming to the objective of camera. Comments of people in background comments that, unfortunately it is slovak. The guy is surprised that it flights on upwind side, while he will never go it with large model in the same donditions.


Offline Air Ministry .

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Re: Experimental biplane
« Reply #19 on: July 06, 2019, 09:26:22 PM »
Just posted this n Olde Time . Think he didnt do to bad a job . Meaning to put it here for years ,

From about 1954 . theres a straight wing inverted engine DeH 60G moth , unswept T M basically .


Talking to a guy with a F F Catalina ! today , intrestingly said his H P 400 trimmed out good , after dropping the Counter Rotateing airscrews ,
said the thing wasnt really any bother , tho electric . Thoughts of the Vimy & suchlike .

setting everything blocked / aligned on the surface table , then dropping all the struts in & glueing .
Think the thin winged airfoil jobs would benifit from wire bracing , .015 solid wire ? ?
F F scale types put a U or V of thin tube through under the struts etc , in the wing .
The bracing wire feeds through , so a bit like a pulley , progressively tensiong , the super glue at the tubes .
Would take enourmous load if done securely , leaving the structure itself light , tho maybe 1/2 x 1/8 vertical spars on rib center .

Just a few thoughts for people looking to do a twin wing thing .  H^^

my Mercury Tiger Moth was similar , but Std F F Kit , with ' stock ' elevator ( scale ) split . about 50 / 50 TP / Elev . Clark Y wings .






« Last Edit: July 06, 2019, 09:49:16 PM by Matt Spencer »

Offline Peter Germann

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Re: Experimental biplane
« Reply #20 on: July 07, 2019, 03:38:52 AM »
Well whatever theory we invent here, it will not change fact that it simply works :- )))

Thank you, Igor.
The Gee-Bee upwind video supports my aussumption that the very refined concept of our today's F2B airplanes may possibly leave room for further improvement. Despite my "flying fusealge- biplane" experiment has failed, I look forward to you and others contributing, and publishing,  to further developments.

rgds. Peter
Peter Germann

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Re: Experimental biplane
« Reply #21 on: July 11, 2019, 08:43:50 PM »
One Wonders about this thing , I had a overweight plank wing semi scale HURRICANE , 52 in , 2 Kilo . After I lengthened the rear fuse , it actually flew quite well , precisely even . Tho at the time I was tempted to throw on another wing , to halve the loading .
Id just use a ply pylon on the centerline ( fuse. ) and a outer wing strut Ea Side . Non Scale . Pitts like .





Contrary to most drivle , I think the theory was - If you filled the top wing with fuel , you could get to Malta . Non - Stop . At Night .

Online PJ Rowland

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Re: Experimental biplane
« Reply #22 on: July 13, 2019, 02:23:28 AM »
Interesting ideas..

Ive just recently finished some great testing with Igors Active unit inside my Biplane.
The great thing about electric is it allows you to assess a range of functions within the airframe.

I thought i knew alot about bipe technology before I did this. Now im into my third version, and getting better each time.


If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.” - Bruce Lee.

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Offline Peter Germann

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Re: Experimental biplane
« Reply #23 on: July 19, 2019, 08:29:30 AM »
...However, what this does not explain is why the airplane flew very substantially banked-in in level flight....
I’ve made a foam duplicate of the forward fuselage section of the bipe to measure torque generated by spiral flow.
When blown at by the flight prop at 10’000 RPM torque measured is 60 Gr x 250 mm or  0.147 Nm.
rgds. Peter

24.07.16 : Update
As per Wolfgang Nieuwkamp’s advice a pair of wing stubs has been added.
At the same RPM force measured is now 145 Gr x 285 mm or  0.4 Nm


« Last Edit: July 24, 2019, 07:49:33 AM by Peter Germann »
Peter Germann

Online Brett Buck

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Re: Experimental biplane
« Reply #24 on: July 22, 2019, 09:49:43 PM »
I’ve made a foam duplicate of the forward fuselage section of the bipe to measure torque generated by spiral flow.
When blown at by the flight prop at 10’000 RPM torque measured is 60 Gr x 250 mm or  0.147 Nm.
rgds. Peter

      This is about 1.3 in-lb. For a nominal 6-lb line tension, that means the airplane would have to  roll enough to move the wingtip about 0.21" to balance it (less than 1/4"). For a wingspan of 48", this is around 3 degrees, which is detectable but not outlandish.All of this ignores the reaction torque of the motor itself, which is much larger in the opposite direction, but also present in all other cases, too. So I don't think this is the issue.

   Of course, the wings are much larger and have much larger effect on flow straightening than even your large fuselage, and are present in everyone else's airplane, too. It is a simple matter to estimate the torque of generating this much HP, figure around .7 HP, you know the RPM, back out the torque. It should be *much larger" than the torque from flow straightening.

    Brett

p.s. if I did my sums right, the motor torque is about 4.4 in-lb, and will be in the opposite direction. You can measure your level flight power consumption, I would guess it is around 600 watts (.82 HP with some losses from the motor (heating it and the speed controller up). 

   Actually, if you measure the flow-straightening torque of the entire model, you could probably figure out if *any* spiral flow persists after it leaves the airplane. Just eyeballing it, it's probably going to be pretty close to *zero* net torque. Meaning there is nearly *no* net torque applied to the airplane from these sources.

Offline Peter Germann

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Re: Experimental biplane
« Reply #25 on: July 23, 2019, 03:27:55 AM »
      This is about 1.3 in-lb. For a nominal 6-lb line tension, that means the airplane would have to  roll enough to move the wingtip about 0.21" to balance it (less than 1/4"). For a wingspan of 48", this is around 3 degrees, which is detectable but not outlandish.All of this ignores the reaction torque of the motor itself, which is much larger in the opposite direction, but also present in all other cases, too. So I don't think this is the issue.

Thank you, Brett, for bringing up the "line tension roll moment" which I did so far (!) not consider at all.

I will come back with more "spiral" numbers generated by directing the prop airstream of my rig at one my airplanes. This will give me an idea of the level flight "net" torque as in level flight at 9000 RPM my motor-in power is 480 Watt  (700 W peak in manoeuvres). Assuming 25% losses this comes to 360 shaft W or a compensating torque of 0.38 Nm (3.4 lb.in)  http://wentec.com/unipower/calculators/power_torque.asp


Peter Germann

Online Brett Buck

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Re: Experimental biplane
« Reply #26 on: July 23, 2019, 09:25:08 AM »
Thank you, Brett, for bringing up the "line tension roll moment" which I did so far (!) not consider at all.

I will come back with more "spiral" numbers generated by directing the prop airstream of my rig at one my airplanes. This will give me an idea of the level flight "net" torque as in level flight at 9000 RPM my motor-in power is 480 Watt  (700 W peak in manoeuvres). Assuming 25% losses this comes to 360 shaft W or a compensating torque of 0.38 Nm (3.4 lb.in)  http://wentec.com/unipower/calculators/power_torque.asp

   I think your losses are less than you are assuming but that's in the right range, it's certainly within a factor of two. Point is, this is not a particularly powerful effect, and is unlikely to be causing any sort of wild roll angles in steady state. In any case, while you have a pretty big fuselage with more impact on it than usual, you also have two wings, with about the same area as everyone else, so the difference in the "flow straightening" torque is not drastically different, certainly not enough different to cause serious trim issues in level flight.

      As a general rule, the only real stabilizing effect in roll comes from the lines, and that is pretty powerful - but just like yaw, it's not at all symmetrical, nor does it act like a static force. Even in steady-state flight, it goes up dramatically if you roll out, and down dramatically when rolled in. That's why you can tolerate a large amount of excess tipweight without disastrous effects, but only a small error in the direction of "not enough". Same with the yaw angle, the further nose-out it gets, the "stiffer" this restoring force gets, , but even a slight amount nose-in the weaker it gets. So a given error nose-in or rolled-in is much more dramatic than nose-out or rolled-out.

    This is why Igor's "simulation" of it as a damped oscillator above it not correct-  in his example, the restoring force is modeled more-or-less as a linear, fixed, spring constant "K", whereas in real life, even the simplified model "K" is asymmetrical, even if you only consider one mode in one dimension. It's good to start to visualize (you *always* want to progress from a simple model you can understand to more complex models).  A far bigger problem is that it's not just a single spring/mass/damper system, it's at least 2 spring/mass/damper systems in each of two dimensions, even to first approximation. One mode is the sort of effect of line tension to stabilize it discussed above - in both roll and yaw - and the other is the whipping of the lines in both dimensions. Even using just the first mode of the lines (a single loop from handle to airplane swinging back and forth) it quickly become vastly more complex to model, but is clearly not something that can be ignored, in fact, it can be the dominant effect in the system.

   This also disregards the aerodynamic stability or lack thereof in yaw. To simplify this already complex system,  it *is* probably OK to figure that roll, by itself, is neutrally-stable That's why I think it's so likely to go wrong when you try to count on it to stabilize an aerodynamically unstable yaw system, even if it is only weakly unstable (which is what the goal appears to be). As above, this works OK as long as it diverges nose-out (which you can probably ensure at launch - but *don't run your prop the conventional direction* because that is helping for the first few feet where it wants to roll and yaw wildly to the right, which more-or-less ensures it will diverge in the desired direction) and never gets upset enough to cross the line into aerodynamically diverging in at you, but once it does, the line tension goes away, the lines are whipping - and the first direction it goes in both directions is nose in and roll in - and you are in big trouble. As you found.

    This sort of reasoning still comes up quite short in trying to describe the motion in a practical case, this assumes a single upset to an otherwise static system. In real life, the corners come along in times comparable to the line whip periods, causing a series of upsets to the system that is already perturbed. Adding to the issue is the fact that as the line tension changes, the control input changes, since for a given hand motion, the control deflection changes as the line tension changes, too. That's why even tiny roll/yaw trim errors tend to cause hops and other evil coming out of corners.

   Brett

Online Brett Buck

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Biplane roll
« Reply #27 on: July 23, 2019, 08:00:34 PM »
As an aside, while I don't think this is the issue, biplanes *do* tend to be more prone to issues with roll stability. It's not at all tricky, for a given power level and weight, the wingspan tend to be substantially smaller. So, for a given line tension, the roll stability is reduced, just because the restoring torque = T*span/2*sin(phi), with a smaller span a give torque requires more roll angle.

   That's why all my (so far, hypothetical) biplane designs, and those of others, have generally had much higher aspect ratios than a typical monoplane. Also at least partly why rudimentary biplane stunt models like the Flying Fool and many others (with a few exceptions) have tended to flop around and fly, generally, "sloppy".

     Peter's airplane looks a lot like a "Moitle" aside from the aft fuselage, by the way.

     Brett

Offline Peter Germann

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Re: Experimental biplane
« Reply #28 on: July 26, 2019, 03:30:55 AM »
From the findings from this exchange (Thank you Brett and Wolfgang) it seems to be safe to state that the biplane crash was caused by the yaw instability of the airplane due to the the very high forward part of the fuselage and the lack of sufficiently stabilizing rudder area. The small span has furthermore reduced the roll restoring torque when (foolishly) trying to enter a loop from level flight at 45° line elevation angle. The resulting nose-in yaw has then induced half roll to the inside and a total loss of control.

For the time being I do consider the experiment of building a high fuselage generating sufficient lift overhead and thus allowing lower flying speed as failed.

In order to quantify spiralflow forces, further bench experimenting was done and I have published first results under “Stunt design” and “Spiral Flow Force Measure”

Thanks for commenting

Peter Germann

Offline Avaiojet

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Re: Experimental biplane
« Reply #29 on: July 26, 2019, 11:16:42 AM »
Here's my attempt at an Experimental EAA Biplane.

Electric, pusher prop, a real pusher prop, plenty of vertical lifting areas, and, in spite of the looks, the thing is light. unfortunately I'll need about three ounces up front to balance the thing.

There could be less weight added if I can find a heavy nose wheel. Still adding weight but the nose wheel is way out in front.



I'm hoping to finish the model this winter and fly it in the spring.

There is a parking lot here that is as smooth as silk.
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