Tandem-wing LSA/microlight concept and poll

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Which tandem-wing configuration would interest you the most (pick one in each of four categories).

  • A1 - High wing forward, low wing aft (Flying Flea) OR

    Votes: 18 36.7%
  • A2 - Low wing forward, high wing aft (Quickie);

    Votes: 27 55.1%
  • B1 - Two-axis controls (no rudder pedals like an Ercoupe) OR

    Votes: 9 18.4%
  • B2 - Three-axis controls (with rudder pedals like a Cessna);

    Votes: 35 71.4%
  • C1 - Conventional (taildragger) gear OR

    Votes: 23 46.9%
  • C2 - Tricycle (nosewheel) gear;

    Votes: 21 42.9%
  • D1 - Tractor engine (engine and propeller at front) OR

    Votes: 33 67.3%
  • D2 - Pusher engine (engine and propeller at rear);

    Votes: 13 26.5%

  • Total voters
    49

Sockmonkey

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A rigid paraglider, huh? That would be fun until it hits you in the head on landing. It could make a great UAV, though, with the sensor slung below a freewing and isolated from most of the turbulence.

Whee! Your servo tab is a Junkers flap, and tip rudders makes it so short that you could stick it sideways in a shipping container.
 

Martin R.

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One interesting thing about a true freewing with a control tab on the wing is that it should be essentially stallproof just by limiting the deflection of the tab, especially if the tab is on the center section and the wing stalls there first (as you want it to in just about all cases).
Why do you want to limit the deflection of the tab? On a true freewing the tab deflects always in the opposite sense of the wing panel:

NASA CR 3135 / page 3 (pdf-page 12)
https://ntrs.nasa.gov/api/citations/19790013869/downloads/19790013869.pdf

hba_Spratt_01a.jpg

Of course, the axis of rotation must be located forward of the aerodynamic center of the wing panel. But if that’s the case, the wing panel reacts exactly like a weather vane and it’s impossible to reach an AoA who could provoke a stall.

On the contrary: NASA CR 3135 / page 4 (pdf-page 13)

hba_Spratt_02.jpg
 
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cluttonfred

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The lift coefficient comment is related to high-lift devices, not the performance of the airfoil itself. It certainly does seem possible for a tab to push a freewing to the point of stall, at least a momentary stall and nodding recovery like a plank flying wing, especially with the type of external forward or rear tabs on booms described in the article you are quoting.

My point was that you want to push the freewing to a point just short of stall. For example, the NACA 23112 popular on modern Mignet types can do a max CL of about 1.5 and you’d want to be able to maintain close to that, say 1.4 or 1.45, for landing.
 

Martin R.

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The lift coefficient comment is related to high-lift devices, not the performance of the airfoil itself. It certainly does seem possible for a tab to push a freewing to the point of stall, at least a momentary stall and nodding recovery like a plank flying wing, especially with the type of external forward or rear tabs on booms described in the article you are quoting.

My point was that you want to push the freewing to a point just short of stall. For example, the NACA 23112 popular on modern Mignet types can do a max CL of about 1.5 and you’d want to be able to maintain close to that, say 1.4 or 1.45, for landing.
Sorry!
In my previous posting I better should have shown the text in the NACA contractor report CR 3135 page 1 (pdf-page 10) https://ntrs.nasa.gov/api/citations/19790013869/downloads/19790013869.pdfhba_02.jpg
As I’m not aerodynamicist I just can repeat what I read in the different NACA “freewing”- reports that I studied (CR 3135 / CR 2046 / CR 2946 and CR 1523). And in none of these reports I found a remark or a calculation result that it's possible to push a freewing to a point "just short of stall". Rather the opposite!

BTW: the basis of the calculations in the above studies was the NACA 23012 airfoil which is not very different from the 23112 that you mention.
 

Hot Wings

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These 3 articles may help visualize, without much math, how the Spratt concept works.
Despite what was thought to be correct back when the referenced CR report was issued, high lift and stall proof in a Spratt like configuration are not mutually exclusive atributes.
For a math intensive description of the Spratt look up NASA CR 2946.
 

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cluttonfred

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I hear you, Martin. I don't doubt that's what the report says but it doesn't seem logical to me that a plank flying wing can and will stall but a freewing cannot. To me a freewing is essentially a plank flying wing with a load suspended on pivots and it is certainly possible for a trailing-edge control surface to stall a plank flying wing.

1622226738287.png 1622226768630.png

That would absolutely be the case with the external trimmers shown in the report.

freewing aft trimmer.png freewing forward trimmer.png

This may be a question of terminology since a momentary stall with immediate nodding recovery does not have the same potentially disastrous effects as a wing maintained in the stalled condition by a separate tail. I believe the report is saying that their particular freewing as configured would not stall, not that all freewings are unstallable, at least in the momentary sense.
 

Hot Wings

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but it doesn't seem logical to me that a plank flying wing can and will stall but a freewing cannot.
Two different dynamic critters.

On a plank the Cg is fixed with respect to the AC. The Spratt Cg gets to float around. Also, there are (IMHO) different 'flavors' of the Spratt formula depending on how the control is configured. You can choose springs, control tabs or a combination of both to adjust the AOA and therefore the speed.

If using the spring method. like the 1968 Spratt patent, then the wing - can - stall if the spring is too stiff or travel limited.

If you really want your brain to hurt look up the papers on combining a Freewing/Spratt/Zuck with a canard.
 

Sockmonkey

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A freewing doesn't have to deal with the inertia of rotating the fuselage so it can respond near-instantly to stop itself from stalling.
 

Martin R.

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These 3 articles may help visualize, without much math, how the Spratt concept works.
Despite what was thought to be correct back when the referenced CR report was issued, high lift and stall proof in a Spratt like configuration are not mutually exclusive atributes.
For a math intensive description of the Spratt look up NASA CR 2946.
Thank you Hot Wings that you emphasized the word "like" in your posting. I see that the same way. But since I learned that the Mignet Cordouan* is no longer a Flea because he as ailerons, I try to be as restrictive as possible in this forum ;)

For example I ask myself if this mechanism in a so called "Spratt Controlwing Flying Boat"** is really a freewing in the sence of the NASA studies or if the expression "controlwing" perhaps doesn't mean the same as "freewing".

hba_01.jpg


I also agree at 100% with your comment:
If you really want your brain to hurt look up the papers on combining a Freewing/Spratt/Zuck with a canard.
That’s why my firm conviction is, that mixing the Spratt/freewing/controlwing-system logic with tandem and/or flying flea system logic is very problematic and leads to false interpretations. And as the title of this thread begins with "Tandem-wing" ......... ;)

* The planes - Mignet Aviation
** https://www.homebuiltairplanes.com/forums/attachments/02spratt-jul1974-1-pdf.111081/
 
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Hot Wings

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Thank you Hot Wings that you emphasized the word "like" in your posting.
I'd rather not get into a taxonomic like debate about what is and isn't a "Spratt" like some want to do with what is and isn't a 'Flying Wing"
To me any wing with a pivot at or near the aerodynamic center (order) is a Spratt/Zuck and the rest of the aircraft that is grafted to that pivot then determines the family/genus.

And, much like the current debate over elliptical vs Prandtl D lift distribution it depends on what variable(s) in this mess you decide to keep constant - if any.

It does use up a lot of bandwidth discussing the interesting options............not all of it wasted.
 

Bigshu

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Umm, while I would agree that there is continuum from conventional monoplanes with lifting tails (big in front, little in back), tandems (wings of roughly equal size), and canards (little in front, back in back) and some designs can be hard to categorize, I would definitely call the Lockspeiser a canard. That does underscore how much lift you give up if you design a canard according to the 3:2 front:rear wing loading rule of thumb with identical front and rear airfoils and chords. For a proper canard I would be tempted to break that rule by using other methods to keep the main wing flying (or force the canard to stall first).

So, if you did make the wings closer to equal in size, is there no rule of thumb on the wing loading? I'm really intriqued by the LDA-01, but I'd like to fiddle a little bit...can't resist. It checks a lot of boxes for what I'm looking to accomplish.
 

cluttonfred

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I think that the relative wing loading bit applies regardless of relative wing area *if* the airfoil and chords are similar. I think the problem they many designers have had over the years, even Rutan, is that airfoil behavior can be unpredictable so apples and oranges combination can bite you if the theory doesn’t hold. Apples and apples remains a pretty safe bet.
 

Bigshu

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I think that the relative wing loading bit applies regardless of relative wing area *if* the airfoil and chords are similar. I think the problem they many designers have had over the years, even Rutan, is that airfoil behavior can be unpredictable so apples and oranges combination can bite you if the theory doesn’t hold. Apples and apples remains a pretty safe bet.
Looking at Sockmonkey's designs with the delta in back, regular wing/canard in front, definitely gets into apples and oranges territory!
 

cluttonfred

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I was not saying that a Payen or Delanne or Sockmonkey apples and oranges approach couldn't work, only that 3:2 wing loading rule of thumb would not necessarily apply.

Looking at Sockmonkey's designs with the delta in back, regular wing/canard in front, definitely gets into apples and oranges territory!
 

Sockmonkey

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I was not saying that a Payen or Delanne or Sockmonkey apples and oranges approach couldn't work, only that 3:2 wing loading rule of thumb would not necessarily apply.
I would still use the 3:2 wing loading rule just in case. Plus, a delta should have a low area loading anyhow so the L/D ratio doesn't suffer.
 

cluttonfred

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I think that delta pusher layout could work with a larger canard for a light single seater perhaps with a two-stroke paramotor engine so the pilot dominates weight and balance considerations. Place the rear wheels forward of the most rearward allowable CG position and make the pilot's seat adjustable. If the nose-gear rocks up and it sits on the tail then your seat (and the CG) is too far back. That said, I don't think it really needs the canard if you have elevons on the delta. Just keep it simple.

This aircraft takes Sockmonkey’s Delta rear wing to the extreme. Maybe a larger canard would make it more ‘tandem’
 
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