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Dart

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Hello all,

I've been thinking about this for a few years, and just upgraded my CAD package, so decided to try to crystallize some of my thoughts about an alternative wing configuration.

Eventually I'd like to try building this, though I'll likely RC model it first.

My design goals were to enable very low speed flight, water takeoff and landing, efficient flight (low fuel consumption per distance) and tight wingspan.

In general after a lot of research it seems like a very light single seat plane with around 30 hp motor, given a 120 sq ft wing area, might have a stall speed in the range of 30km/hr. As a trial power plant I'm looking at a powered parachute motor and prop, eventually would like to move to a larger motor inside and forward (to maintain CG) and a much larger prop using a belt drive speed reduction.

These are some of my thoughts. The largest question I'm facing right now feels like what should my wing area distribution be. You can see in this variety of sketches that I'm toying with a number of idea's. I'm posting all this so as to get some input on this question as well as general input.

While tis quite blue sky at this point, as you will see if you visit my wind turbine page, when I get enthusiastic about idea's, I do take them a long way.
Art Turbine

Here's a few sketches. I tried to save each as a multi view, but it seems that's another thing I haven't mastered yet....

Image 2015-05-01 at 8.02 PM.jpgImage 2015-05-01 at 8.04 PM.jpgImage 2015-05-01 at 8.09 PM.jpgImage 2015-05-01 at 8.11 PM.jpg

I'm looking forward to your input, and thanks for all the great info on your site! Very well moderated in what can be such a hot topic amongst enthusiasts!
P.S. my work sometimes makes it impossible for me to respond in a timely way. Please be patient if you are waiting for a response to a question.
 

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Tiger Tim

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To my untrained eye it looks very un-airplane-like. Could you explain the reasoning behind the various features of that shape?
 

delta

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I think you'd find out when you reach the RC stage that you'd need a couple of engines on the top wing to bring the cg far enough forward. You might even have to loose the rear engine.
 

gtae07

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The narrow join of the wings will present an interesting structural challenge. The aerodynamics will also be challenging, and I suspect you'll have a lot of induced (and other) drag issues. "Efficient" and "tight wingspan" don't usually play well together.
 

Tiger Tim

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Things that spring to mind when I look at it:
1: figuring out the weight and balance point of each component will tell you how much stagger you need. The centre of pressure for each wing (this is where we picture the lift happening) will fall roughly between 1/4 and 1/3 of the way between the leading and trailing edges. I think you'll want the whole airplane to balance around 1/4 of the way back between the centres of pressure of the upper and lower wings. If the pilot is fixed in the fuselage and the engine can only go in one place and the lower wing can only be at the back, then all that leaves is moving the upper wing to suit.
2: those curved wings sort of reminiscent of a parachute (to me) turned up in a lot of unsuccessful early (1900-1910) airplanes and haven't been seen since. I worry for you that there's a reason for that.
3: going from 30 hp in the tail to something much bigger in the nose doesn't sound like that kind of thing that happens on one airplane. Maybe not impossible, just awfully rarely seen.
4: it's going to be tricky to make any control surface work on a curved wing like that. The closest I can think of is how Vought handled the flaps through the bend in the Corsair's wing. Probably worth looking at.
5: my gut tells me that after all of the study and tweaking and refinement looking at airflow and optimizing and going back to the drawing board this thing will end up looking an awful lot like a single seat Synergy. That would actually be pretty cool and just might be the spiritual successor to the BD-5.

**DISCLAIMER: the above thoughts had no real analysis done and as such are only good enough to get you started on an RC model at best. You've designed something weird, expect to do a lot of study to make it work and even then expect the unexpected.
 

Dart

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Thank you all for your comments, especially to the Corsair control surface reference which I will look into. The posted images leave out a lot of the detail my thoughts contain, however drawing in some of the detail requires me to move forward on setting relative wing area.

My thinking has been inspired by a wide variety of aircraft.

In looking at the flying flea, which once the wing stagger was made appropriate, and in some cases, the rear wing made all flying (but pilot adjusted, unlike the forward wing), was a serviceable 18hp airplane with 20' wingspan, and combined wing area of 137sq ft.

In my nascent understanding of airplane design, drag is a combination of factors (the relative proportions of which vary with speed), but simply, for low speed aircraft, including firstly and dominantly, frontal area of barn are you pushing. Next viscous drag, how much total surface does you barn have. Lastly but not least, induced drag from wingtip vortices.

What a bit of playing around on the NASA foilsim software shows me is that in low speed flight, simple slightly cambered plates approach the L/D of airfoils. (while this data isn't authoritative, it's useful in getting a big picture of what works) Other reading tells me that flat plates can be improved quite a bit by the additions of a well faired in rounded leading edge. A structural issue is that flat plates are not stiff, and it's likely heavier to build a stiff enough flat plate, than it would be to build a two surface foil. However if that flap plate is distorted slightly into a surface with 3D camber, it will gain significant stiffness. If the leading edge is used as structural tube, then the cambered plate behind it may be able to be fit into the frontal area of the structural tube. In sailboat design where I live a new type of hull construction has become popular. It's been popularized variously as "fold boats" "origami boats" and probably other names as well. It's being used to create very durable 24'-60'ft sailboat hulls, without, or with very minimal internal frames. The idea being that if you take a flat sheet that's anything but stiff, and cut the appropriate darts, pull the edges together, then weld, you can create very stiff almost 3D curves (really complex conic's but approaching the stiffness of 3D curves).

Another set of airplanes that are inspiring to me are the flying flapjack Vought XF5U - Wikipedia, the free encyclopedia and the Custer Channel Wing. Both airplanes that use propulsion flows to reduce induced drag by either ingesting the tip vortex, or straightening it with prop wash. The case of the Channel wing is very interesting, and Custers son, who was present for it's development and a test pilot has sent me video's of it that are not publicly available. These, as well as the public ones show that it was very capable of STOL. People were able to run beside the plane as it lifted off. The exact aerodynamic mechanism I'm not sure is fully understood, however Custer's thinking was (paraphrased poorly sorry)"it's the speed of the flow over the foil, not the speed of the foil through the flow". This technique, having propulsion flows forcing air over foils seems to have been accepted as the A-10, and many others including Honda's new jet, make use of it.

In my design, by swinging a very large prop over a set of short wings, I'm hoping to see some of these benefits. More with my second engine choice than my first.

In the engine arrangement I've been unclear, or mis understood. My thinking is that a 30hp power parachute motor weighing about 40kg's or so, is a trial solution that may be all I ever want. It's weight would be right forward of the prop, and a long way from CG, necessitating the pilot to be well forward of CG. If I were to switch to inboard power a belt drive speed reduction would be used to drive a much larger prop, and the prop centre would be moved up. The inboard engine would be much closer to CG, placed to maintain approximate CG. In the design as drawn I'm using a 54" prop circle (by recollection), but by going to an inboard motor and belt speed reduction, and moving up the centre of the prop, a 72" prop circle could be used with around 6" inches of clearance from the wing at the closest point.

In the long run my goal would be to make this a hybrid electric/gas plane. My thinking is that an electric motor could be mounted inboard, still using the belt drive reduction to keep the motor weight down. The weight could be replaced for CG stability with a pair of the Honda 3kW generators, and the resulting 8hp could keep the plane aloft for a long time, with a smaller battery bank for takeoff. People seem to report that the flying flea, which would have a much larger frontal area can fly with around 8hp. The pair of Gen's could also be replaced with batteries, if desired.

One thing that seems difficult to anticipate is interference drag. It seems clear that some planes like the flea boarder on positive interference drag, but it seems that only CFD or wind tunnel analysis and the real world can optimize for this. The gentlemen who did the studies on the multibladed wingtips seemed to show that tight spacing worked well. This was also true in the early tightly placed flying fleas, with the aft wing in some unfortunate circumstances producing much more lift than desirable (nose down).

In my drawings one thing I haven't shown is that the whole upper wing would "fly", just as in a flea. The narrow joining point to the lower wing should be at about 1/3 aft allowing it to pivot. Above the pilot would be a tube frame, with a pivot in line with the centre of lift, hanging the airplane from the wing. This ideal would be a little like having a flying flapjack above you, able to be adjusted to high angles of attack, while the hull stays closer to level.

A big issue here could be visibility, but having the pilot far forward should help with that, as well an overall issue with the design could be entry/exit. I'll see what I can do to post some new drawings that might make some of this easier to visualize.

A question I have about wing area calculations. In my simple physics view, wing area is simply the total area of the wing, regardless of anhedral or dihedral. The idea being that while a wing with significant angle will produce the same lift as a wing that is flat (0% anhedral or dihedral), however that lift vector may not be vertical, but inclined perpendicular to the wing. As long as both wings match, the outward or inward lifting moments balance, and therefore should effectively convert to vertical lift. In most aircraft this probably isn't a big issue, however in this design it may be, I'd be interested in hearing thoughts about this. FYI, right now with a wingspan of about 10ft, and an overall length of about 18ft, I'm well over 120sq ft of area (including hull as I'm hoping that it's act as a lifting body.

Thanks once again for having such a great forum to blue sky this design, and for all your thoughtful comments.

Best Wishes,
Drew
 

Dart

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Another question for the unusual aircraft configuration guru's is

Given that wickedpedia tells me

The PrandtlPlane configuration derives from the application to aeronautical engineering of the "best wing system" concept by Ludwig Prandtl, who in 1924 demonstrated that a box-wing system, under proper conditions, provides the minimum induced drag for given lift and wingspan.[17]

So in theory at least induced drag can be reduced by a joined wing.


but why do the Prandtl type designs, including the wonderful synergy design, have the upper wing aft? At high angles of attack it seems like the front, and lower wing, would "shade" or occlude flow from the aft and upper wing. Wouldn't an idealized situation have the aft wing low and the upper wing forward?
 

Tiger Tim

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but why do the Prandtl type designs, including the wonderful synergy design, have the upper wing aft? At high angles of attack it seems like the front, and lower wing, would "shade" or occlude flow from the aft and upper wing. Wouldn't an idealized situation have the aft wing low and the upper wing forward?
Maybe they found peak efficiency happened with the tip plates swept back, maybe they're hoping for a stall-proof effect like a canard has, maybe it just looks cooler or maybe something else.
 

StarJar

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Much of your wing is non horizontal. Those areas will produce load and drag, but no lift.
This may make it tough to compete with other aircraft.

The Synergy's rear surface produces anti-lift, so it is not a wing.
 

Dart

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Thanks for the comment StarJar,

My understanding, which could be incorrect, is

that a wing moving through the air even when not horizontal still creates lift. This lift however isn't vertical, but perpendicular to the wing. An airplane with two symmetric wings with high dihedral (or anhedral), combine the two non vertical lifting forces into a vertical lifting force. IMG_5583.jpg

Is this correct do you think? It's a bit counter intuitive, however for a while at least, and maybe even presently the record winning paper airplane designs were both annular wings. One of which is actually quite close to my geometry (upper wing forward).

F8O50MAGPLJ1B7Y.LARGE.jpg
 

Dart

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My work as a designer has shown me that in general many things are made the way they are not because it works better that way, or is easier to make, but simply because it's easiest to draw. Historically drawing complex geometries in 2D paper drawings was a very difficult task. So even if a complex geometry had benefits, if plans were too difficult to create, designers would redesign to make it simpler to draw. On top of that designers would of course be trying to make things simple to build as well as simple to draw. These two factors, as much as anything else as far as I can see, explain the predilection among aircraft designers for fairly flat wings. It simply makes makes the most sense using conventional rib and spar geometry. In general designers are trying to give the best bang for the buck. If making something 10% more efficient costs 10x more up front, it might not be worth doing.

If you consider alternatives to rib and spar construction, this might all be changeable.


A good example is the well discussed faucet plane

facetmobile.com

This plane in a comparative analysis done by NASA (available for download at the link above), was found to have a lower cost to construct, and much higher load factor than all the conventional designs it was compared to. By recollection it had effectively 2x the load capacity of the compared Cessna, with a smaller engine and fuel consumption.

In a way what I'm trying to do is create a facet mobile like plane, with a slot, combined with suction over both surfaces (ala channel wing).

The facet mobile is a very interesting fabric over tube construction. I'm wondering about forming it from distorted sheets. The "orgami boat way"

22382d1213274767-realistic-scantilings-blaster-006.jpg

Home - Origamiboats: The Art of Frameless Steel Boatbuilding

In these designs a true monoquoce shell with a complex curving surface (self stiffening to increase buckling resistance) is formed from two flat sheets with a simple set of "keyhole" sheets.

If I'm willing to use more than two sheets I should be able to create a facet mobile like plane, with minimal internal framing?

Once again, thanks for all the comments.
 

WonderousMountain

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Biplane Wings with the upper wing aft of the lower wings increase the airs ability to stay attached to the lower wings trailing edge, helping in high angles and drag. That said, it's a pretty subtle type of aerodynamics.

If you're planing on using complex curves, it's not the drawing that holds creators back, it's the manufacturing IE - cost.

If your plane can be flattened here and there, it's easier to do it. If you are working on something very clever, then flat may not work at all. Curved is not a great problem anymore, but will always be more of a problem than strait.

One thing I have noted, continuous curves are better than joints where a buildup and load transfer must be drawn up, calculated and fit to all the details, shipped or custom carved, and glued or bolted.

Your wingtip joints look disastrous. You need to choose one, sturdy tips, or unconnected. We're not into sci-fi just yet.

LuPi
 

Topaz

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Couple of thoughts.

The center of gravity is fairly high, and the wing-root "sponsons" are relatively close to the centerline, even compared to the short wingspan. Unless the water surface is glass-smooth, side-to-side stability on the water is going to be poor. It's going to roll, in the boat sense. Probably a lot.

If I'm reading your drawings correctly, the pitch control surfaces are on the trailing edge of the lower wing. Which is dragging along the water during takeoff. A nose-up control input will lift that trailing edge up, resulting in a curved surface being pulled along the water. The potential for a strong Coanda effect exists, which might "pin" the aircraft to the water and make takeoff challenging.

Third point, realized while I was writing the last. Those pitch control surfaces, contacting the water as they do, are going to see quite large impact loads from the water. I don't see any way to keep those from being transmitted back along the control system to the pilot, and causing the entire pitch control chain to be structurally sized by the impact loads. Meaning very probably very heavy.

Just some random thoughts. I appreciate your unconventional approach to the design.

EDIT: I just read one of your posts in more detail. Seems the pitch control is via pivoting the entire upper wing. If I'm reading that correctly, then that obviates both of the last two points above. Sorry to have missed it.
 

Himat

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Another question for the unusual aircraft configuration guru's is

Given that wickedpedia tells me

The PrandtlPlane configuration derives from the application to aeronautical engineering of the "best wing system" concept by Ludwig Prandtl, who in 1924 demonstrated that a box-wing system, under proper conditions, provides the minimum induced drag for given lift and wingspan.[17]

So in theory at least induced drag can be reduced by a joined wing.

but why do the Prandtl type designs, including the wonderful synergy design, have the upper wing aft? At high angles of attack it seems like the front, and lower wing, would "shade" or occlude flow from the aft and upper wing. Wouldn't an idealized situation have the aft wing low and the upper wing forward?
A good question, more questions?:)

I can't remember to have read any discussion of the finer points of this question. John may or may not say something about the choice for Synergy. As for others, practical issues or other issues may have taken the lead. There have been built joined wing aircraft with the front wing forward and I do think someone had patented the high front, lower rear joined wing, at least for UAV use. One "other" issue could then have been patent rights.
 

Dart

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To all, thanks for your comments

either the idea will be improved by them, or my understanding of things will be Win/Win regardless

Wondrous Mountain,

I guess what you are describing is like a reversed set of Junkers flaps? That makes sense to me. In my thinking the high forward arrangement would provide a similar benefit but perhaps a larger one. This was shown to be the case in the tragic crashes of the early higher powered flying flea's. Apparently at higher hp's than was tested by the designer, in a fast decent the flow accelerated downward by the upper wing attached to the lower rearward wing increasing the lifting moment of the rear wing and a dive from which recovery was often impossible. This was addressed by increasing the stagger, and in some cases I believe by making the rear wing AoA pilot adjustable as well.

I'm trying to understand what you see as the problem in the wingtip junctions? In my understanding an elliptical or tapered to point wingtip is considered optimal for reducing vortex induced drag. The theory being that vortex's are created by higher pressure below the wing escaping upwards around the tip into the lower pressure air, creating the vortex.

In what I'm trying to do, my upper wing has a large amount of sweep, this sweep would in general encourage circulation flow and therefore could lead to increased vortex losses, however my hope is that flow would wrap around the joint, emerging on the upper surface of the lower wing, and as it interacts with the flow on that wing, increasing the overall velocity, and due to the very strong reverse sweep on the outer portion of the lower wing, contribute to lift on that wing, rather than being allowed to swirl into a vortex as is normal the case.

Total conjecture, no evidence that this will take place!

In my thinking, the non-planar wings allow me to bring the tips close together, allowing a minimal junction (that also houses the pivot for the upper wing). This relative to box wing type designs, should minimize non lift frontal area. I'm not at sure about this. I am perhaps very unfortunately a Sci Fi fan..... Much Sci Fi is now fiction. I recall the first computer I bought. PC XT, pre 286, 40mb hard drive! I recall mocking the Sci Fi "tablet" computers.... An early job had me doing some training and servicing of Gov computers. My reaction to Windows, "it's a fad! no one needs their computer to do more than one thing at a time (I have 10 tabs open in my browser and 6 different software packages running right now.....)) I force myself to eat crow about many of these sorts of things so that I stay open to new idea's, mine and others, and to not be to attached to my own opinions about my ideas.


Topaz,

Your comment pre PS is actually what I see as one of the biggest issues to remain, not just pitch control, but control in general. Yes, I do intend to be able to use the forward upper wing in pitch control, but given the complex flows that may result from this design, I assume that I will need to have some control on both for and aft wings to trim effectively and efficiently. In landing, moving surfaces on the aft wing impacting water, and attached to pilot controls could possibly break wrists or ankles. Non-0ptimal..... I also as you can see am lacking a rudder, and am not sure what to do about that. In one of my drawings in the first post you can see an inverted V tail aft of the prop. I don't particularly like that, so omitted it. I've got some other idea's, but haven't settled on anything yet. One possibility is a sort of reverse Junkers set of control surfaces on the rear wing. Another is to house the prop in a partial duct (bottom half only), and have vertical control surfaces on the upper rear portions of that.

In regards to the water takeoffs and a Coanda effect keeping me pined to the water, yes, this is quite possible I think. I'm hoping that air forced under the wing by forward motion will minimize that. This seems to have been done effectively in some WIG craft.

Not present in these drawings are my idea's to make this an amphib. What I've been thinking is a single fat low pressure wheel on a trailing link air bladder suspension right behind the driver with a tail wheel assembly far aft. For lateral stability I was thinking that largely vertical, but slightly forward leaning pieces of piano wire could jut down, with small foil sectioned ski's on the ends. These "ski's" would be freely mounted at their C/L or just ahead, and would help keep the thing upright when stopped on land. On water when stationary they would have little effect. When moving though they might act as hydrofoils, helping to lift off the water. More discussion of this will require a drawing.

Himat,

I'd sure like to see a general discussion of the dynamics of staggered wings and joined wings. If you see any, please point me towards them. In regard to IP, everything I'm writing about, I'm doing so with intent that it be public domain. I've got a patent on my turbine, and am in negotiations to commercialize the design. If I can work that out then the rest of my career as a designer will aim toward open source most likely.

I am aware of one UAV system using a joined wing system, forward high, aft low. While they did apply for a patent, I think that if the challenged my design as infringing that I can show prior art which would invalidate their patent. Patents are often granted, and often fail. Just a tool in the business box.

Thanks once again to you all.

In specific if anyone has any suggestions around vertical control surfaces, control surfaces on the aft wings, or anything else, I'll read whatever you contribute very carefully and I'll keep you all up to date on updates to the design. I may not get another chance this weekend though. (it is such fun, designing, that I may not be able to keep away :)..
 

StarJar

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Joined wings' original advantage was the weight savings of the wings, also acting as struts. With the curves and pivots, hopefully your other features will outweigh the loss of the original joined wing benefits.
 

Dart

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Hi StarJar,

Apparently I can 't keep away.

In conventional spar-rib-skin construction complex curving surfaces probably does increase weight, because it likely necessitates extra ribs, extra fasteners, and curving structural members like spars represent extra complexity and extra parts that would likely lead to extra weight. As well deeply formed parts general need to be made from thicker materials, representing extra weight.

In an "orgami" type construction, like in the sailboat construction link I posted, weight gain vs conventional framed boats is neutral. I think that the possibility exists for weight savings, as complex curves are self supporting, making them resist buckling loads more readily. A good example is an egg. A design which takes this to an extreme. If you had a flat sheet of egg shell, it would be very weak in all directions. If you take that same sheet and form it with corrugations, it will still be weak in one direction, but in the other direction it will now have a stiffness proportional to the height of the corrugations. Take the same amount of calcium material formed as an egg, and it will support loads orders of magnitude greater than the sheet. Curves can make things much stiffer and resistant to buckling, and with proper design, those advantages should in theory reduce weight.

If you look at one side of the sailboat hull, and use a lot of imagination, the stern would be centerline, and the bow my upper wingtip. The hard chine wouldn't be there, and the overall curve would be reduced. In the sailboat hull there is one "keyhole". In what I'm thinking I will likely have 2, and perhaps a dart. The drawing software I'm using should be able to fairly accurately plot out the cuts I need to make to a sheet to create the resulting form.

The front 1/3rd of the foil would be double skin, the last 2/3 should be single skin. I might rivet in some ribs for extra stiffness. Just like in a traction kite, the upper skin should form a sort of catenary curve, and so load's should stiffen it, holding it in the correct shape. I do have some concerns about what that might do in inverted flight? I don't intend to fly it upside down, but still it would be good to know that you could.

I'm not sure that it's a give that I've lost the advantages of joining the wings or that curving surfaces will add weight, but we will have to weight and see.....

The pivots could be a problem in additional weight. Especially because one of my thoughts is to try different for/aft positions for the upper wing. To make this possible I was thinking that I might make a link between the wings, that would have 2 pivots, and be adjustable in length (ground adjustable, not in flight). Hopefully I can show that in my next drawings as I'm sure that it's hard to understand what I'm saying without seeing my arm waving and sketches.

Thanks,

Drew
 

Himat

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With joined/non planar wings you can have a more efficient wing without increasing the wing span. If this is used to make a more compact airplane, less wingspan, or more efficient airplane, keeping the wingspan, is up to the designer.

Some comment on the design:

The plane may rock when afloat, but roll stability will probably by ok as the design is very much a "wing float" design. But this not only make placement of aerodynamic control surfaces on the lower wing difficult as Topaz says, the design pretty much prohibit this.



To be able to take of in the shortest possible distance the wing must be able to reach the angle of attack corresponding to the highest permissible lift coefficient. There are two possibilities, either the wing must be rigged close to this this angle as the airplane sit on the undercarriage or the airplane must be able to rotate for takeoff. The hull, step and afterbody angle do not look like it will fulfill either of these requirements.



A practical consideration how do the occupants get in and out of the plane when docking?
If it is not possible to get in and out of the plane when docked the usefulness is severely diminished. The plane must then either be beached or driven up a ramp.


Last, have you read the various threads on the Fly Nano?
https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/14041-photos-flynano-unveiling-event-helsinki-verkkokauppa-com.html


Edit:
Bottom line, I do like your design and I do think that the idea can be developed into a useable airplane. There are some quirks, like the ingress/egress when docked, but you have overcome one of the major difficulties with a high front/low aft wing, to make it look good.
 
Last edited:

WonderousMountain

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I was concerned that the lift (wing movement) from the top and bottom wing will create a shear load at the tip junction. You can leave a ~2 inch gap and avoid the whole issue, or a full joint like the nano shown above. What you have shown may work, but would likely bend a lot. Something to think about.

LuPi
 
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