Tandem wings (Yes, again.... Sorry!)

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Thunderchook, Sep 29, 2016.

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  1. Oct 10, 2016 #41

    Jay Kempf

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    Of course you couldn't load the back end past stall. The overall premise is that both ends stay flying. Some sort of control algorithm so that the lift and speed are managed within the flying range of both wing systems. Would be boring to fly but it would be a great cargo platform. The point was huge CG range.
     
    Last edited: Oct 10, 2016
  2. Oct 10, 2016 #42

    Sockmonkey

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    Not for correcting when the fuselage pitches up. There the front wing does the canard thing and drops the nose back to level.
    Negative AOA would be a fatal flaw though now that I think about it. The tail would keep lifting if you started to nose down which would result in the fuselage hanging vertically from the rear wing.

    Ah, I know the solution. Both fore and aft wings on a tandem can be freewings as long as they maintain the same pitch relative to each other. A pulley or linkage can do that.
    That does introduce some weird potential possibilities and problems though.
     
  3. Oct 10, 2016 #43

    Norman

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    Spin that aft wing and you've just invented the first nose sitter VTOL :gig:
     
  4. Oct 10, 2016 #44

    Norman

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    Seriously though if you linked up the wings to maintain decollage you might have a pitch stable airplane that would be able to maintain a level deck angle during high alpha flight but the CG range wouldn't be changed. However the huge H-stab already gives tandems an unusually large CG range. I don't see a real advantage unless you're trying to serve snacks during climb and descent.
     
  5. Oct 10, 2016 #45

    Joe Fisher

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    Cherokees have free wing tails.
     
  6. Oct 10, 2016 #46

    Norman

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    Not exactly. The Cherokee has a stabilator with an anti-servo tab. The anti-servo tab pushes the stabilator back toward neutral.
     
  7. Oct 10, 2016 #47

    cluttonfred

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    "All-moving" and "freewing" are not at all the same. Even if it's just the antiservo tab and the pilot's hand on the stick, the all-moving stab on a Cherokee or a Volksplane is not free to change incidence with changing airflow like a Spratt. Even a Mignet type is somewhere in between because the wing's change of incidence is somewhat restricted by the pilot's hand on the stick.
     
    Last edited: Oct 10, 2016
  8. Oct 10, 2016 #48

    Sockmonkey

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    I suppose you might do some kind of STOL with the engine in the nose since it would let you tilt the fuselage up at a steep angle without stalling the wings.
     
  9. Oct 10, 2016 #49

    Jay Kempf

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    I was envisioning more of a tandem wing B-52 where the fuselage always stays level and the wings move or are flapped to change CL drastically. If each wing is flying all the time and they have a huge range of CL and speed range then you have a huge range of lift each one can generate. Of course you could never allow the rear wing to stall.
     
  10. Oct 10, 2016 #50

    Sockmonkey

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    You could do both I think. Adjusting the link coordinating the wings lets you set the angle of the fuselage at the angle you would like, or lets you compensate for uneven load. You can still put flaps on a freewing as long as the input linkage has a swivel to allow for the pitching of the wing.
     
  11. Oct 10, 2016 #51

    Jay Kempf

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    Thinking about this more I think it has been done with either a blown flap setup or a double tilt wing with basically vectored thrust not all the way to VTOL but to augment lift at slow speeds. Still think it could work. If you think of most quad or hex-copters now they all can compensate for drastically changing loads when an motor fails or a loss of a prop. The control algorithm just adjusts the amount of nominal lift at each remaining motor and re-balances for the new CG location with regard to available lift and location from the CG.

    My thought was to have a main wing and canard at each end. With the wing on a pivot. That would allow mechanical control of stall of both wings. The canards could be quite small like a tail on a stick if the wing was fairly high aspect ratio. If you then control the pivot of each wing so that the fuselage stays level you have two identical airplanes lifting each end of the fuselage. Drive both the canards to stall and the whole thing mushes but level. Put the motors in nacelles on the wings so that they pivot with the wings and you get proportional augmented lift vectors. Program it all together to balance lift and I think you could have a drastically wacky load that would be stable. Take it the rest of the way and you have a VTOL setup I guess. Guess I am going back to the drone I already have designed.

    Anyway, just spit-balling but I have been noodling on tandem wings for a long time. A canard is one thing but a true tandem if it is possible I think has possibilities.
     
  12. Oct 10, 2016 #52

    Sockmonkey

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    Well, if it's computer controlled then you don't need the canards and the four wings can be individually actuated by servos for roll and pitch control. That also lets you do V-22 type VTOL if you use four engines. If you wanted your freewings to be self-adjusting without active systems then you would want a regular elevator behind each wing rather than canards since a canard setup like that requires additional structure to have the pivot point be between the wing and canard. Just sticking a little elevator on a little boom behind it would probably be less draggy since these elevators don't have to create downforce or lift, they just need to adjust the angle of the freewing.
     
  13. Oct 13, 2016 #53

    Thunderchook

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    Not intending to cross-thread here, but I noticed that a good number of the proposed designs in the "Modern-day Flying Motorocycle" thread were tandems and/or canards.

    It would seem that, for a small, simple, low aspect ration/short wingspan design (and hopefully uncomplicated runway requirements), tandem wings are a worthwhile idea.

    I do recall references to Tandem Wings/canards having certain inefficiencies and annoying overheads (e.g. higher landing speed and therefore distance.)
    I'm not sure I can understand how this can be, given (for example) the Flying Flea who have as lower landing speeds (and therefore distances) as any conventional wing aircraft.
    Can anyone here demonstrate the numbers on this?
     
    Last edited: Oct 13, 2016
  14. Oct 14, 2016 #54

    Sockmonkey

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    I don't have the numbers, but I can describe why the different types of tandem have different qualities.
    With the canard types that most of us are probably familiar with, (Rutan) the nose is angled up by elevons pushing the main rear wing down. That costs lift and extends your takeoff run.
    With the hemiptere, the larger main wing is in the front. It achieves pitch stability in the same way as a canard, but pitch control can be conventional elevators on the aft wing so you don't need to spoil your main source of lift to pitch up. Additionally, you can use flaps on all four wings at the same time to climb while level.
    For the flea, The front wing is your main lifting surface, but it's downwash creates lift for the rear wing, so you can increase lift on both at the same time without having to pitch up.
    This explanation is really really crude, and I suggest reading the articles other people have linked to on the subject, but that's the bare bones of it.
     
  15. Oct 14, 2016 #55

    Swampyankee

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    On canards, pitch control is provided by the forward surface; stability constraints demand it have a greater lift curve slope than the wing, and stall first. Usually, general aviation canards don't have trailing edge flaps on the wing because they force the canard to be lightly enough loaded so they can stall the wing. The canard also puts downwash on part of the wing, requiring specific tailoring to re-optimise the lift distribution.
     
  16. Sep 6, 2019 #56

    Speedboat100

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    In Lightning Bug ( Brian Austein ) the elevator is in the rear wing. Canard is sorta like a trim element. Am I correct ?
     
  17. Sep 7, 2019 #57

    wsimpso1

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    Sorry I did not see this one sooner... Hmmm, the issues this brings up:

    First, your tail/elevator on each lifting machine is serving as an all flying servo tab to drive the wing portion to the right angle of attack;

    Making both the front and the back lifting machine free in the pitch axis is hardly neccessary, you could fix one and make the other rigid;

    Simultaneous control of both lifting machines could be tough to manage without augmented stability. You could deliberately slow one down and make manual control easier;

    I am assuming a single degree of freedom connection of one lifting machine to the fuselage so each individual airplane can be pitched to null out pitching moment on the whole thing, but can not move in yaw or roll freely;

    Are we assuming we have dispensed with natural stability? If we have, lots of stuff is possible. If we have not, I suspect that we must connect one either rigidly or with a slow acting pitch adjustment while the other can pivot in pitch. Somebody who is interested in the answer more than I am would have to set up the lift and pitching moment derivatives and see if it can be made stable over a wider range of CG than in more common configurations.

    Have fun...

    Billski
     
  18. Sep 7, 2019 #58

    wsimpso1

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    No they don't. They have a stabilator whose angle of incidence is set by the control column position.
     
  19. Sep 7, 2019 #59

    jedi

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    Original post included here for reference.

    There is one issue here that has not been mentioned and that is that the tandem wing is a biplane and has the typical biplane disadvantages. However, it does not have the biplane advantage of the Warren truss structure.

    What are the disadvantages of the biplane?

    1. Low aerodynamic aspect ratio combined with high structural aspect ratio wings.
    2. Biplane wing interference.
    3. Construction high parts count. Four wings and lots or ribs and fittings.
    4. Poor visibility. Everywhere you look a wing gets in the way.

    What advantage do you propose to offset the negatives?

    1. Improved stability.
    2. Wide CG range.

    I see a roughly two to one advantage with the monoplane and specialized tail or canard.

    The question is really more of where do you draw the line between tandem wing, canard and conventional tail aircraft. Oh, and lets include in the flying wing which has the tail and canard both hidden in the wing structure.

    If the tandem wing definition requires identical wings this is a severe design constraint as the fore and aft wings do not operate in identical environments nor do they have identical functions. Take these variables into consideration and you end up with the many varied designs found to meet the widely varying design criteria.
     

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