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Flea style "Piojo Flying MiniBike"

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Sockmonkey

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It can be made to be that way, but then it means that the rear wing is always flying so far away from its maximum CL that you end up with a lot more wing area (and weight, and material, and cost) there than you'd have otherwise. The ideal situation has the main wing, under the worst-case situation, reaching just below its maximum CL capability when the canard lets go and stalls. That results in the smallest, lightest, cheapest design possible for the particular configuration. That's hard to do when part of the rear wing has flaps "up" and the rest has flaps "zero".

It's just far easier to resolve the stalling issues and keep the airplane as small and light as possible if the front wing contains the pitch control and the rear wing has roll controls only, if anything at all. All the Rutan designs, all the tandem wing designs, have done it this way.
That being the case, it's going to be simpler and more effective to have the fore wing handle both pitch and roll. Fewer parts and less twisting force on the fuselage as then both the fore and aft wings will be contributing to rolling the plane.
 

cluttonfred

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Sigh...we’ve had these tandem wing conversations so often that even I am getting tired of them. Rough guideline for all aircraft with two lifting surfaces (tandem or canard or monoplane with lifting tail) is front wing loading about 150% or more rear wing loading for reliable stall behavior. So yes, tandems are less efficient because the rear wing never works that hard, but there can be advantages in handling or packaging that make up for that.

So rear wing ailerons only make sense for canards (where most of the lift is at the rear regardless because of the relative areas), for the others it’s the tail trying to wag the dog. For tandems, front ailerons, rear pitch control makes a short-coupled conventional monoplane with a big tail and works fine (Autoplan).

Front ailerons and central elevator could work in theory as long as you make sure the wing tips keep flying when the center section stalls. I have sketched some tandem designs with fixed front and rear wing incidences and ailerons and an extended-chord central elevator on the front wing for that reason.

Front ailerons on a wing also changing incidence for pitch control seem like a bad idea, again because of stall behavior, because you don’t want just one side of the wing stalling first. If you want to use Mignet pitch control then just go full Mignet two-axis pitch and rudder control, use tricycle gear and land crabbed in a crosswind like an Ercoupe.
 
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Clutton FRED, we should find a way to make your explanation pop up whenever someone types the word "tandem". It is beautifully thought through and clearly put.
 

Sockmonkey

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As FRED says, going full Mignet is simplest. I'm saying that if you do go 3-axis on a flea type, your best bet is to have so the left and right fore wings can tilt both together, and in opposition like the spratt control wing instead of sticking ailerons and flaps on them.
 

Topaz

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... So rear wing ailerons only make sense for canards (where most of the lift is at the rear regardless because of the relative areas), for the others it’s the tail trying to wag the dog...
The Rutan Quickie, QAC Q2/Q200/Tri-Q, and Viking Dragonfly, would like a word. They all have rear-wing ailerons and forward-wing elevators and were very successful designs.

There is exactly zero distinction between a "canard" and a "tandem (wing)" design, except for nomenclature because people like different names for things. The aerodynamic principles are the same between the two types, and there is no "line of demarcation" where a certain amount of the MTOW on the front wing denotes a "tandem" design and less than that is a "canard." I'm sure some people have tried to come up with such a distinction, but it's arbitrary and artificial at best.
 

Sockmonkey

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There is exactly zero distinction between a "canard" and a "tandem (wing)" design, except for nomenclature because people like different names for things. The aerodynamic principles are the same between the two types, and there is no "line of demarcation" where a certain amount of the MTOW on the front wing denotes a "tandem" design and less than that is a "canard." I'm sure some people have tried to come up with such a distinction, but it's arbitrary and artificial at best.
Generally true, but the flea is a unique case because of the way the wings interact.
 

Topaz

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That being the case, it's going to be simpler and more effective to have the fore wing handle both pitch and roll. Fewer parts and less twisting force on the fuselage as then both the fore and aft wings will be contributing to rolling the plane.
Rutan tried that on either a VariEZE or LongEZ prototype at one point. While it may have been due to the small forward wing, the flight characteristics were... "interesting" in the "May you live in interesting times" sense. The idea was quickly abandoned.

Presuming one wants a full three-axis control tandem-wing airplane, and not a Pou du Ciel type (for the benefit of the purists, yes, they are distinct, because the latter is explicitly two-axis control), it seems to generate the best flight characteristics and the least-complicated airplane if you put the pitch control (elevators or all-moving forward wing) on the front wing, and the roll control (ailerons) on the rear wing. If that's still "too much build" for your tastes, then the Pou du Ciel two-axis formula is about as simple as you can get for a tandem-wing airplane. A Spratt Controlwing type may actually have a slightly lower parts count, but there are compromises with that design, too. The simplest heavier-than-air flying machine of any type is a powered parachute.

Choose your poison, somewhere along the spectrum.
 

Topaz

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Generally true, but the flea is a unique case because of the way the wings interact.
No, that's absolutely false. The wing and canard on a Dragonfly, Q2, or Quickie interact just as much. The all-moving forward wing on a Pou du Ciel type creates a broader range of the same kind of interactions, but the flow patterns and math is the same in all tandem-wing cases. The downwash field from the front wing affects the flow field over the rear wing, and the upwash field ahead of the rear wing affects the flow field on the front wing. Period. That's it. All the effects described for "tandems" and "Pou du Ciel" aircraft are special cases of that fundamental interaction.

The "canard" designs have the same interactions, with the sole difference that the canard's influence on the rear wing is limited to slightly less than the canard's span for downwash effects, and that there's upwash effects on the rear wing from the circulation pattern around the canard's tips. But even that is yet another special case of the same overall effects.
 

Sockmonkey

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Moving the fore wing on a pou changes the lift on the aft wing, so increasing the lift on one side of the fore wing with either an aileron or spratt type tilt will increase the lift on the aft wing on that side. That means both fore and aft wings are contributing to the roll force. That's why I'm so insistent on the spratt type fore wing for 3-axis control. It also means you can go back to two-axis with the spratt wing handling pitch and roll by replacing the rudder with a fixed fin.
 

Topaz

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Moving the fore wing on a pou changes the lift on the aft wing, so increasing the lift on one side of the fore wing with either an aileron or spratt type tilt will increase the lift on the aft wing on that side. That means both fore and aft wings are contributing to the roll force. That's why I'm so insistent on the spratt type fore wing for 3-axis control. It also means you can go back to two-axis with the spratt wing handling pitch and roll by replacing the rudder with a fixed fin.
Try it. Get back to us. What you're going to find is that, no, increasing the angle of attack on the front wing increases the strength of the downwash field impinging on the rear wing, decreasing the lift of the latter. On a Pou or any "conventional" tandem-wing aircraft, it's not enough to generate a pitch-reversal, because the downwash angle change at the rear wing is relatively small compared to the increase in lift force on the front wing.

The Spratt Controlwing concept has not only the wing being all-moving for pitch control, but also all-moving (differentially) for roll control. The "tail" on a Controlwing is mostly to keep the fuselage properly in-trail with the rest of the assemblage. That's arguably simpler, mechanically, than putting ailerons on a forward wing that's all-moving for pitch control only.

I've never seen a tandem-wing design with a full Spratt Controlwing-style forward wing, all-moving in both pitch and roll control. The closest I've seen is Rutan's attempt to put elevons on the canard of an EZ-type canard. That effort failed because of poor control qualities. If you'd like to give it a try, more power to you. Personally, at that point, I see very little difference in the amount of build time or parts count compared to a simple three-axis "conventional" airplane. The only real build-time advantage I see with a genuine Pou du Ceil or Spratt Controlwing is that the wing panels are possible to build in one piece, and don't need a conventional carry-through box and manufacturing join in the fuselage. But, when you think about it, neither does a strut-braced conventional airplane.
 
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cluttonfred

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Sigh, thanks for the compliments and here we go again... I’ll be brief.

—I know that aircraft with two longitudinally-separate lifting surfaces follow the same principles, hence my comment on the 150% wing loading guideline.

—I don’t know how to do a table in my phone, so just imagine AREA F>R F=R F<R across the top and POSITION F/R F-R R/F down the side with an example in each box. Broad generalizations about downwash don’t take into account all the combinations.

—My comments on ailerons were deliberately oversimplified and there are borderline cases like the Quickie family that can go either way but the principle remains valid.

—I have am nervous about differential wing incidence for roll control because of the possibility for nasty stall behavior, though I have note heard that Daniel Dalby’s designs have any problems in practice.
 

Victor Bravo

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At the risk of igniting a forest fire, I will suggest that the Piojo should be designed and built in such a way as to allow for different methods of control to be used.

Make the !)#$*(#& fuselage out material that is torsionally stiff enough to allow for rear-wing ailerons or elevons. Give the rear wing structure the torsional stiffness to allow using it for roll control.

If Fritz does this, then the people who want a 2 axis Pou can have it, and the people who want 3 axis control from rear wing ailerons can do that.
 

Topaz

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... I don’t know how to do a table in my phone, so just imagine AREA F>R F=R F<R across the top and POSITION F/R F-R R/F down the side with an example in each box. Broad generalizations about downwash don’t take into account all the combinations. ...
Then I'd appreciate seeing the math that makes a Pou some kind of fundamentally different aircraft, from a basic stability and control standpoint, from a Quickie or other tandem-wing airplane, be it a VariEZE or a Q-200. Because the air doesn't see any difference between them, and that's all that matters. The interactions between front and rear wing, downwash and upwash fields, induced angle of attack, and on and on, are exactly - exactly - the same for all of them. The shorter-span-canard VariEZE introduces an additional factor of the circulation on the canard tip inducing an upwash field on the outer rear wingspan, but that effect is actually at-play on all tandem-wing designs, even a Pou. On equal-span tandem designs, the induced upwash from the front wing is just outboard of the tips of the rear wing, and not much of a factor. But it's there, and the math is fundamentally the same.
 

cluttonfred

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For example, the Mauboussin M.40 Hémiptère quite a different beast than a Mignet HM.14 Pou-du-Ciel.

9EA6F52C-EA8F-4957-9C70-6629B5E63AE2.png

That said, Topaz, I think you’re arguing with yourself. While the principles and methods may be the same, the results and solutions may vary.
 

cluttonfred

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Just that downwash issues are clearly different depending on the vertical offset of the front and rear wings as well as depending on their relative area, so it's not really enough to say they're all the same, there needs to be some nuance. I agree with you that the fundamental principles and analysis are the same for any aircraft with two lifting surfaces, just that each combination will have some idiosyncrasies.

The very early Pou-du-Ciel was an example because the close vertical and horizontal proximity (even overlap) combined with a bad airfoils and cable/bungee control system made for some unexpected consequences not well understood at the time. To my knowledge, no one had worked out those issues before because it was a novel configuration. Once vertical and horizontal spacing and rigid controls (push-pull) were adopted, the Mignet tandem formula returned to known territory, so to speak.
 

Topaz

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Just that downwash issues are clearly different depending on the vertical offset of the front and rear wings as well as depending on their relative area...
Different quantitatively. The same qualitatively. The math is the same in all cases. The equal sign in an equation means more than, "the answer." It means things are the same.
 

Tiger Tim

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At the risk of igniting a forest fire, I will suggest that the Piojo should be designed and built in such a way as to allow for different methods of control to be used.
I might suggest that it just get designed and built, period. On the surface it’s an ultra-cheap ultralight so nobody is going to follow the plans anyways.

EDIT: I just re-read that and holy cow it’s cynical. I’m still not convinced it’s wrong, though...
 

Topaz

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I might suggest that it just get designed and built, period. On the surface it’s an ultra-cheap ultralight so nobody is going to follow the plans anyways.

EDIT: I just re-read that and holy cow it’s cynical. I’m still not convinced it’s wrong, though...
You're not.
 
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