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

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

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  1. Sep 29, 2016 #1

    Thunderchook

    Thunderchook

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    *ducks to avoid the shower of abuse, empty beer bottles and tin cans*

    So, I've trawled through several threads and picked up one or two things about tandem wings, but I really don't see why they aren't more popular.

    I understand that if you don't get the centre of gravity correct, then they can be quite unstable.
    Henry Mignet seemed to (eventually) get it right, so clearly it can be done and, indeed, one of the Flea's claims to fame is its hands-off stability.

    I'm sure that Koen will chime in here, but aren't they technically more efficient?
    You've got two wings that are lifting up!
    The regular way involves one big wing doing all the work and one little upstart up the back making nasty comments and fighting against it!

    In the example of the Flying Flea, it seems to me that it's a biplane with one less drag-inducing aerofoil.
    Isn't that a plus?

    Is the stability issue for tandems that more insurmountable than a regular configuration such that it's not worth pursuing?

    Are there any other issues that make tandem wings less attractive?

    Thoughts?
     
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  2. Sep 29, 2016 #2

    BJC

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    Seems to me that their inverted spin characteristics could be very nasty.


    BJC
     
  3. Sep 29, 2016 #3

    Thunderchook

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    Is that inherent to tandems or canards?
     
  4. Sep 29, 2016 #4

    karoliina.t.salminen

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    There is one thing why tandem wings are not said to be popular: like in canard, the rear wing of tandem wing can not use flaps. This results in low Clmax for the rear wing and this leads to larger wing area than often desired and extra wetted area to carry and it may be also harder to store in a multi plane hangar (or T-hangar for those places who have the luxury of having them) because long wings are sticking out of both ends instead of just one in the center. If that is not a problem for the design, I don't think there are other reasons, except maybe that some people think tandem wing looks odd and the customers on aviation markets tend to want to have tranditional conventional things, and also many aircraft designs are not that sophisticated and adding tandem wing to the mess would have made it even bigger mess than it already was. Burt Rutan is the expert of tandem wings and for him they are easy. Yet he also is doing now conventional configurations, including in the SkiGull. The advantage of tandem wing is utilised in the Rutan Proteus where the center section (payload) is replaceable as it is neat that it is directly placed at the center of gravity. As it is apparent in the WhiteKnight 1/2, it could have been also done with the conventional configuration, and even Rutan himself did so in the later iterations. Biggest advantage in the Proteus-like configuration is that it is cool-looking. Would be very desirable airplane to own (the Proteus).
     
  5. Sep 29, 2016 #5

    TFF

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    Without being acclimated to either design, regular pilots don't like how they fly. They want conventional reactions when they fly. If they want to learn the quirks, which are not a big deal, they will ok. Stability when you are not expecting it can be disconcerting. Just jumping into one will make unconventional design cross with conventional instruction.
     
  6. Sep 29, 2016 #6

    cluttonfred

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    There are circumstances in which tandem wings make sense, but absolute efficiency is usually non one of the them. Simplified controls, stall resistance, shorter span, easier wing folding can all come into play. The flaps question is a red herring...you can use flaps, but you have to design for it, like anything else. The one thing that I have come to understand is pretty universal in tandem wings and canards is that the wing loading of the front surface must always be higher than the rear surface for safe handling characteristics. That means, for a given surface area, the tandem wing is always going to be a little less efficient since some of the surface area isn't working as hard as the rest.

    On the spinning, the jury is still out. Mignet disciple Emilien Croses designed the EC-6 Criquet, one of the most popular Mignet-type homebuilts before the emergence of the microlight category. Tested by French government pilots in an official research project, it was put through all the static and flight testing required for a full production certificate, except one. French regulations required that any aircraft considered for certification demonstrate spin recovery, but no one could make the Croses plane spin. There was even talk of hanging one from a swivel under a helicopter to MAKE it spin, but nothing every came of it.

    13.jpg

    He later designed the B-EC-7 Tous Terrains (All Terrain), which was considered for factory production as a light utility and medical evacuation aircraft since it could take a stretcher in the three-seat cabin, but again it could not be made to spin.

    7587L.jpg

    He then designed the EC-9 Para-Cargo, with the goal of certification as a low-cost plane for skydiving. The government inspector said that it would be dangerous for the parachutists because of the struts...forward of the jump door! So it was never approved for testing.

    22895d1362742712-cargo-plane-approach-flying-car-aa.jpg

    Sigh.
     
    Last edited: Sep 29, 2016
  7. Sep 29, 2016 #7

    TFF

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    Im not saying bad flying just odd flying. A Pou with yaw control and limited down elevator to someone who expects a Cessna response will be put off. Acclimation and acceptance and you have a happy pilot. Canard; don't stall close to the ground and you will be taking off and landing faster than normal platform. Acclimation and acceptance and you are a happy pilot. Fly a Pou, a EZ, a Pancake and you are not in Kansas any more. As Foghorn Leghorn would say,"That's a Joke."
     
  8. Sep 29, 2016 #8

    bmcj

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    Tandem/canards can be flapped to raise the Cl-max. Both the Rutan Grizzly and Rutan ATTT were designed with tandem flaps.

    +1 on Karoliina's comment... the Proteus is very cool looking!
     
  9. Sep 29, 2016 #9

    rbrochey

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  10. Sep 29, 2016 #10

    delta

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    .
    I'm exploring joined tandems mainly for engine placement and strength for an amphibious hull. I don't know if they would be stable in all attitudes but who knows. The quickies I have are supposed to be unstable in negative g situations. I'd prefer to be able to do aerobatics and not be worried about an upset recovery in the wake of a 757 with whatever I'm in.
    The df in the middle might be considered a tandem wing, even though the wings are on the same plane and real close together. The outer panels join the two wings and would be removable for limited road use.
     

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  11. Sep 29, 2016 #11

    Swampyankee

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    Canards can have flaps on the main wing, but the design problem is much worse, as flaps increase the pitching moment of the airfoil (the airfoil is being aft loaded, which increases the nose-down pitching moment). If the canard has too much pitch authority, it can stall the main wing, which is not good.
    http://docs.desktop.aero/appliedaero/configuration/canardProCon.html
     
  12. Sep 29, 2016 #12

    Himat

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    Canards and tandem wing aeroplanes, some considerations:

    For stability and safe handling in pitch, at least two criteria must be met when designing a canard/tandem wing:
    1. The front wing/canard must stall first when running out of lift and cause a pitch down.
    2. The rear wing must increase lift slower than the front wing as speed increases and make the airplane pitch up.
    If the first point is not fulfilled the airplane will be susceptible to violent pitch up and continue flight with the airplane reversed. Of some reason several canard prototypes have had names that can be pronounced to sound like “ass sender”.

    If the second point is not fulfilled, the plane can be susceptible to a high sped “tuck” pitching down and increasing the speed on it s own if disturbed.

    Now, depending on how the two criteria are met, the design might not do so if flown inverted.(Ass sender again. This time from inverted flight.)

    The two criteria are not set in stone, modern canard jet fighters do instead have a computer stabilizing the airplane.

    If canards or tandem wing airplanes are more efficient than an airplane of conventional layout?
    That depend on by what metric the efficiency is measured. In pure aerodynamic efficiency like best lift/drag ratio the conventional layout win. That is as long as the airplane is not for some reason span limited. The load/empty weight efficiency might be better for a tandem wing than a conventional layout, very much depending on what other constraints there are on the design.

    As others have said, both canard and tandem wing airplanes can have flaps to reduce take off and landing speed. Actually one design constraint is then the same as with a conventional layout, there must be enough “tail” area to compensate for the increased pitching moment.
     
  13. Sep 29, 2016 #13

    wsimpso1

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    Reading all of the descriptions above, I feel a need to summarize (and help us overcome the erroneous assumptions in some of the comments above).

    If you have gyroscopes and an autopilot and fast control surface actuators all coupled up to augment stability, things change a little, but let's talk like we are not in that realm.

    Also, I will talk in terms of canard and wing. It does not matter if the canard is tiny or close to the size of the wing, same stuff applies...

    Stability - All airplanes that are naturally stable in pitch must have:

    Sum of all vertical forces equal to zero;
    Sum of all pitching moments equal to zero;
    Sum of first derivative of all pitching moment with respect to deck angle is a negative number.

    Satisfy these, and the airplane is stable. This is done by getting the CG forward of the Neutral Point, and applies to all airplanes that do not have augmented stability with an autopilot and fast control surface actuators. How to do this with tandem wings (or with canards, same problem)? Higher unit loading on the canard than on the main wing. The Wrights figured this out and were doing it at Huffman Prairie.

    So, once the plane is stable in pitch, you also have to make sure that the canard stalls at lower alpha than the wing to prevent deep stall. Hmmm, since the canard is higher loading than the wing, this tends to take care of itself unless you make awful choices in the foil for the canard. But to get the airplane to low speed for takeoff and landing, the tendency is to make the elevators on the canard full span slotted flaps. Min speed is set when the canard with its deflected slotted flap at stall, and the wing is at lower Cl. So, compared to a conventional bird with a flapped wing, either you carry quite a bit more wing to get to the same landing speed or you must accept higher landing speeds. Go three surface, with an aft tail (Rutan Catbird and Piaggio Avianti). It can work but usually compromises some other features.

    All of these solutions require a lot of engineering effort to make them work well and have the coupling between surfaces be reliable and have failure modes that are safe for when something does break, which reduces its advantages in the mean while.

    On to negative g flight. Everything still works OK, until you get to low speed behaviour. Cambered wings can have awful negative g stall behaviour. For instance, look at Mike Melville's negative g snap rolls while on one of his astronaut flights. Get to negative alpha stall, well, it can be bad. Usually, the elevator will not have enough up travel on the canard to get to stall, so I do not know how you could get to an inverted spin, but maybe you could... If you used symmetric airfoils and elevators, you could build a good negative g airplane, but accepting higher landing speeds would be needed.

    Thing is, with a big part of your drag coming from your wing, you want it no bigger than you need for your mission. The little tail out back, runs at lower Cl and drag to stabilize it, and with flaps on the wing, it makes a good flexible package. To meet the same mission and landing speed, most canards ever built have more drag than the conventional airplane would have. There are some exceptions, based upon missions. Proteus is one example of an airplane that carries big payloads in the middle of the airplane and usually needs the wings away from the payload. It works.

    If you want to have fun with canards and tandem wings, have fun, just remember that unless you discover some new things and figure out how to incorporate them, you will have the same limitations that the past inventors had.

    Billski
     
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  14. Oct 1, 2016 #14

    Sockmonkey

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    I think part of the reason for the predominance of the classic airplane shape is that it seems to be the easiest to improve incrementally. By that I mean that it's not as complicated to change one detail at a time to see if there's any improvement. With other designs you generally have to have everything worked out beforehand. It's not a big problem nowadays with all the data, computer sims, and ease of making test models available. Back in the day however, it could mean wasting a ton of development money or getting the test pilot killed.
     
  15. Oct 1, 2016 #15

    cheapracer

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    What are they doing, incrementally improving them 1 millimeter (0.040") at a time? They sure don't look much different than 100 years ago :gig:
     
  16. Oct 1, 2016 #16

    BJC

    BJC

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    1916 state of the art:
    image.jpeg

    2016 state of the art:

    image.jpeg


    BJC
     
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  17. Oct 1, 2016 #17

    Swampyankee

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    Compare the performance of a 777 with that of a DC-3: faster, longer-lived (many aircraft like 747s are still in service with tens of thousands more hours than any DC-3), better L:D, lower costs per seat-mile.
     
  18. Oct 2, 2016 #18

    AdrianS

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    It took me a moment to work out what L:D meant!
     
  19. Oct 3, 2016 #19

    wsimpso1

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    Thread drift alert - A friend of mine remarked three decades ago that if the Sopwith Camel was a front line fighter as long as the F-4 Phantom was, it would have seen use in WWII. We could say the same thing about the P-51, the F-15 and Vietnam.

    The funny thing is a century later, most airplanes are still configured pretty much the same way - Wing ahead of a tail that is usually pulling down, landing gear of three wheels. Cars too, usually have the engine in front, sitting on four tires, steering the front ones. Yeah, materials are different, and propulsion has changed some too, but good basic configuration was worked out early.

    Billski
     
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  20. Oct 3, 2016 #20

    cluttonfred

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    I still haven't figured it out.
     

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