A moderately high AR plank flying wing - RC Groups
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Jan 09, 2017, 08:16 PM
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Carolynne's Avatar
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A moderately high AR plank flying wing


A little while back a number of things conspired to make me very interested in making a "high performance" plank type flying wing.

These things included:

My friend Tim starting to build and absolutely gorgeous scale flying wing called the FS 26. Another friend Klaus, showing up one day with a delightful 2.5m F5J Plank that flew so very well and had an airfoil with only the slightest whisper of reflex. Next there was this really nifty thread here that talked in very authoritative tones about getting the most from a high aspect ratio plank. Finally, my swept flying wing, a Taborca, keeps on being just so pleasant to fly.

So, with pencil in hand and little else to lose I started sketching the 'Calliope'. Some months later a carbon fuselage emerged. a few more months passed, and i started to wonder about control configurations and so I built a 'Little Plank' which has been both educational and fun. Since then, and with lots of hubris, work on the Calliope has continued, with this morning being the first time all of the big bits came together. I was so pleased with the way it looked that i thought that I would share:

Of course, the Calliope has yet to fly, but I am sure that at the current rate, it should certainly fly before the end of the decade

Carolyn
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Jan 10, 2017, 06:47 AM
Deniable plausibility
Shedofdread's Avatar
Well, if aesthetics are any predictor of success, you'll be fine! (hope I haven't jinxed things with that one... )
Jan 10, 2017, 10:12 AM
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Knoll53's Avatar
Cool project.

It is a rather advanced plank. The inclined rudder hinge line should be interesting. You will have plenty of elevator control with that big central control surface. At 2kg it just might move out. Can you tell us something about your airfoil choice (s)? ( I want to fly it! )

I'm guessing that this a dedicated slope plane. Should be fun. I hope your maiden flight is soon and goes as well as this one. Landing on sand is the best for test flying. It really takes the fear out of giving her a toss.

Kent
Jan 10, 2017, 01:57 PM
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Carolynne's Avatar
A few things were taken into account in designing the Calliope:

Amongst these, Klaus mentioned that his wing was very sensitive to elevator and this is why the elevator is restricted to the middle and this area is stretched towards the rear to give a bit more damping. Klaus also said ( and I saw) that his wing had an exceptionally good L/D, especially at speed and so really needed a drag device to get down comfortably. Klaus is trying spoilers in the wing, but the Calliope is going to have a split rudder.

I've used split rudders in the past, and so felt comfortable using it here. The fin and rudder was designed with this in mind. The sweep of the fin is to maximise the fin moment arm as I am worried about yaw damping. (By the way, Kent thank you for your video!It gives me hope that things will work out). The hinge line and disposition of area was chosen to give a pitch compensation for the usual nose down change when the split rudder is opened. I also really like the shape and aesthetics, and of course this is of huge importance to me in designing things.

The airfoil turned out to be an MH 22. The centre section was stretched to give the extra chord while not increasing the local spar depth. Tim ( who is an exceptional engineer and computer wizard ) ran the design and airfoils through both XFOIL and Profili and after a few iterations of tweaking areas of the airfoil, we found a combination the computer liked.

The Calliope will most likely get an electric motor and will be used in my rather large backyard and to cause uproar at the local F5J competitions.

As of this morning, The calliope has its tips installed ( Hoener) and i an going to start playing with the construction of the rudder halves. I am a bit concerned about how to keep the thin ( 4mm ) halves stiff and straight, so my first attempt will be to use a glass/ balsa composite. Good sticky fun!
Last edited by Carolynne; Jan 10, 2017 at 02:02 PM.
Jan 10, 2017, 02:54 PM
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Knoll53's Avatar
Quote:
Originally Posted by Carolynne
The hinge line and disposition of area was chosen to give a pitch compensation for the usual nose down change when the split rudder is opened.
Just from a drag point of view, regardless of hinge line, I would think that the rudder would induce a nose up when split rudder is applied. The rudder's center of drag is well above the wing's center of drag. Increase the rudder's drag and it will cause a rotation. In this case a nose up rotation.

The inclined hinge line will give rudder deflection a downward force, thus causing a nose up.

If the split rudder is effective at reducing speed, then maybe nose up would be useful, but it could be unwanted as well. If you are already flying at minimum sink, reducing speed and lift will cause a stall. Add to that a nose up input and you might get snap roll. That is if you are really lucking. Seeing this big plane snap roll would be really exciting. Currently I have only one plane that snap rolls.

The current split rudder arrangement, with it's assumed nose up behavior, could be coupled to spoilers on top of the wing. These two could work very well together. I would think that spoilers alone would cause a nose down. Couple the two and I'm thinking automatic landing.

Light glass over balsa should be good for the slender split rudders and will give you a durable razor sharp trailing edge.


Kent
Jan 10, 2017, 04:39 PM
B for Bruce
BMatthews's Avatar
Quote:
.... I am a bit concerned about how to keep the thin ( 4mm ) halves stiff and straight, so my first attempt will be to use a glass/ balsa composite. Good sticky fun!
From that I am going to assume that the total section thickness at the hinge line is closer to 8mm.

With that in mind I'd suggest you make the halves "hollow" and interleave the stiffening ribs. That is use solid pieces at the front and a skin for the outside. But then make the ribs as deep as you can without touching the opposite skin and stagger the ribs so they don't try to take over the same space when closed.

That should give you a pretty decently stiff half rudder.

For what you are doing I'd go with either 1/32 plywood for the skins or perhaps go with epoxiglass laid up on a sheet of glass or if the halves have some curve to the shape in a suitable dished mold to lay up the skins. Along the trailing edges I'd likely lay in a bit of carbon tow just a little forward of the actual trailing edges to aid in stiffening things as well but so that the glass still to the rear allows for a narrow trailing edge.

I think if it were me I'd have looked at the idea of using a slightly thicker airfoil at least down at the root so that the control horns could be located inside the fin as well. But with only 8mm total to work with that would be tough You'd want something more like 12mm. Still, even if the horns do need to be external this sounds like an interesting idea.
Jan 10, 2017, 07:02 PM
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Knoll53's Avatar
Just for fun, I compared my Plank 368 airfoil with the MH22. I'm thinking that your plane will be a little faster.................
I hope that your crew can manage a video for the maiden flight.

Kent
Jan 10, 2017, 09:17 PM
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Carolynne's Avatar
One of these days I should really learn to procrastinate a bit more. If I had dawdled a bit this morning I would have read your neat ideas before launching into my construction of the rudder halves. So, bright and early this morning I've laminated 2 layers of 2mm balsa with the grain crossing at 70 degrees ( about) and laid up an outer skin of 1 layer of 1.5 oz glass cloth and a veil of 0.75 oz cloth on top of the balsa and squeezed the lot in a press. When this all hardens I'll remove about a cm of balsa along the trailing edge and 3mm along the rest of the perimeter. then I'll taper the balsa to 0 thickest at its trailing edge and then lay up an internal close out of a layer or two of 0.75 oz cloth. Anyway, if my approach doesn't work I can always start again!

The TE of the fin is 11mm wide. Because the rudder halves will be tape hinged to the outside surface of the fin, the gap between the rudder halves is to allow them to move over each other when they work together as a rudder. As for the pushrod arrangement, in reality I don't think that the drag from exposed pushrods and stuff would be huge or even noticeable in the grand scheme of things. Nevertheless, I do like to keep things neat. So, because the wing will be gently filleted into the fuselage, I shall arrange for the trailing edge of the fillet to extend onto the rudder and shall use this to conceal the rudder linkages. In a similar anal retentive vein, there will be no external elevator horns with a fuselage mounted servo driving both elevators through a Dodgson coupler to account for the hinge geometry. The aileron pushrods will receive similarly picky treatment when I get to them.

Previous experience with this layout of split rudder showed that the disposition of area was more of a contributor to trim change than the hinge line sweep. I found that a tall rudder would pitch gently nose up and one with equal area above and below the vertical CG would pitch down. Because of the way Calliope's rudder is laid out, with the broad chord at the base and very narrow tip, the amount of area above the vertical CG won't be too much more than that below and so I don't expect a huge nose up trim change. But we will see! As they say, data always beats theory.

Carolyn
Jan 11, 2017, 02:30 AM
B for Bruce
BMatthews's Avatar
I'm not surprised at that sort of pitch response from the area. After all drag of that magnitude is going to be up on par with the amount of positive pitching moment that provides the wing with it's normal stability.

Did you ever see that little odd looking model that used to show up in the Solarbo adverts in Aeromodeler? It was a flying wing with a swept forward fin and a small drag plate on the top of the fin. The fin provided not only yaw stability but also acted as a vertical fence to provide roll stability similar to dihedral. And the drag plate gave the flat wing a positive pitching moment that was related to flying speed.

I built one years ago for giggles. I found that it was able to restore itself to stable upright flight even when tossed up into really odd positions.

The model looked something like this sketch.

And while I didn't think to mention it above I have to commend you on a truly gorgeous looking design. You do have an eye for sleek and balanced while incorporating the wider center in a very eye catching manner. OK, enough sucking up... .
Jan 11, 2017, 09:55 PM
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Carolynne's Avatar
I really enjoy the sculptural side of this sport, both in the creation and flying side of things. I love the fact that you can design and create a beautiful fluid shape that can gently coerce the air into providing lift. So, Thank you very much for your comments and I hope that you enjoy my attempts at this art.

As for the Solarbo model, no I haven't seen it before but it certainly does illustrate the point!

Carolyn
Jan 17, 2017, 11:28 AM
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EdSoars's Avatar
Sailboats and sailplanes ARE sculpted by airflow! Which is why beautiful sailplanes/boats perform best!

I wonder if the little drag square could be engineered to create the same pitching moment-versus-drag increase as would a stronger reflex in the airfoil... room for research.

ed
Jan 24, 2017, 11:36 AM
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Knoll53's Avatar
Do you have any construction photos for us Carolyn?
Would like to see how the split rudder came out.


Kent
Jan 24, 2017, 10:47 PM
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Carolynne's Avatar
Hi Kent,

I shall post some more photos when I get back to my workshop in a couple of days ( It's a holiday long weekend here and I'm going flying with my very favourite bunch of boys!)

Things have progressed somewhat with the split rudder halves now glassed inside and out. the wing joiners have been glassed into the fuselage, fin and wing roots fitted, and I have made a start on filleting everything, though its early days yet. The rudder halves have turned out quite stiff and not too heavy ( no, I haven't weighed them!). If it turns out that they need to be lighter, I have this vague idea of routing out sections of the insides of the rudder halves in an array of hexagons ( because it is structurally efficient and will look neat). Of course, irrespective of what the final outside colour scheme is going to be, the insides of the rudder will be bright red so that opening them will show a 'stoplight' to anybody following.

With the aeroplane fully assembled with all of the available bits, the CG comes out only about 60mm behind the most forward designed position. So, by the time I get three servos ( 2 rudder halves and an elevator servo), the receiver, batteries and maybe a little electric motor system packed into the nose, I think that the CG won't be too far off and maybe there won't be much need for hexagons inside the rudder.

Carolyn
Last edited by Carolynne; Jan 24, 2017 at 10:50 PM. Reason: poor grammar on my part
Jan 25, 2017, 11:13 AM
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Knoll53's Avatar
Yes I would certainly fly it before chopping up the structure with lighting holes (unless of course that is your THING). Who knows, you may like flying it heavier than it's lightest AUW off of the building board. Recently I was surprised at how much fun a high wing loading was to fly at the slopes.
Jan 25, 2017, 08:12 PM
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Knoll53's Avatar
I noticed that you have the aileron offset towards the root.

Have you considered the same size aileron offset towards the tip. Beside providing more roll authority, such a layout would leave a nice gap between the aileron and elevator for a spoiler should you choose to experiment with one later after test flying.


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