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

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.

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 :- ))

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

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.

   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.

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

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.

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 .  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 .

   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.

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

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.

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.

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

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