Aft delta tandems and the people who love them

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Sockmonkey, Jul 22, 2019.

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  1. Jul 22, 2019 #1

    Sockmonkey

    Sockmonkey

    Sockmonkey

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    So, my latest batch of ideas involve exploring the possible advantages of tandems where the aft wing is a delta.


    Seaplane
    [​IMG]
    [​IMG]
    Cargo/passenger plane
    [​IMG]
    General fun flyer
    [​IMG]
    Utility cargo/passenger/bush plane
    [​IMG]
    Small fun flyer
    Will the directional stability imparted by the delta be enough to not need a vertical rudder?
    [​IMG]
    Small utility plane
    Note the larger aft section under the delta that clamshells ope so you can keep crap back there.
    The larger fuselage side area means proper rudders will likely be needed.
    [​IMG]
     
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  2. Jul 23, 2019 #2

    pictsidhe

    pictsidhe

    pictsidhe

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    I like the last one. The high fuselage at the back will be stabilising. You could stick a rudder on the back of the fuselage if the taper is shallow enough to avoid separation nasties.
     
  3. Jul 23, 2019 #3

    Sockmonkey

    Sockmonkey

    Sockmonkey

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    That's what I would do if not for the aft door for loading crap.
    [​IMG]
    This version does look pretty nifty though.
    Nice thing is that the fore wing would stall way before it gets high enough to create interference for the rear one. Fore wing span is six meters BTW.
     
    Last edited: Jul 23, 2019
  4. Jul 24, 2019 #4

    pictsidhe

    pictsidhe

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    Proportion the front rectangular wing and rear delta right and you could get sorta close to an elliptical lift distribution. It is the lift distribution of the entire aircraft that determines induced drag.

    rear cargo door? Open the tail sideways. You'll be quite limited as to the weight that you can put back there, though.
     
    Last edited: Jul 24, 2019
  5. Jul 24, 2019 #5

    jedi

    jedi

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    I would like to see a list of proposed advantages of the aft delta tandems. What are the design goals for this configuration study?

    First consideration is advantages and disadvantages of tandem wing configurations in general.

    In reality most conventional and unconventional aircraft are tandem wing. A tailed aircraft is a tandem wing as is the canard aircraft. When does a conventional design become a tandem wing? A flying wing is not a tandem wing but one could argue that the sweep of a non-plank flying wing is a form of a tandem wing where the tandem wing not only hides the tail it puts it in a spanwise arrangement to eliminate the fuselage.

    I propose that the tandem wing design intends to build on the canard principal that both wings contribute to the positive lift component. The tandem wing has the rear wing large enough to have a positive lift at low speed and all CG loading conditions. This requires that the design avoids a condition that the aft wing not stall before the fore wing and still have a positive lift at forward CG loading. This configuration favors the aft delta wing in that delta wings do not exhibit the sharp stall break of straight wings.The aft wing can not have a sharp lift discontinuity or stall charistic.

    Furthermore the tandem wing arrangement addresses the plank flying wing issues of low pitching moment of inertia and limited CG range.

    I proposed a design goal of optimized low speed performance and minimized dangers of low speed stall and low speed loss of control issues.
     
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  6. Jul 24, 2019 #6

    Sockmonkey

    Sockmonkey

    Sockmonkey

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    You nailed it.

    For the plane in image seven, I was thinking of this one Australian guy who does various maintenance and tech support jobs that require him to fly out to remote locations. He spends a lot of time waiting for the chopper or plane to take him there, so an LSA that's stable, has a simple robust structure, and a short takeoff run would be just the thing for him.

    One other thing I'd like to explore is if the directional stability imparted by a delta is enough to not need vertical rudders, so that elevons on the straight fore wing providing two-axis control would be enough.
    That would not only eliminate the structure needed for the rudder, but also shorten and simplify the control runs.
    As you can see with that last orange one, the placement of the wing relative to the pilot means the elevons can be actuated by short pushrods plugged right into the base of the control stick.
    Additionally, the high-mounted delta provides enough effective dihedral that we can get away with using a low straight wing, built around a single six meter spar, which also doubles as the mounting for the main gear, further simplifying the construction.

    In essence, I'm looking for that elegant simplicity where one part made just right can do multiple things.
    Full span control surfaces on the straight fore wing of a tandem can function as elevators, ailerons, and flaps. If they can clamshell open, to act as drag rudders, they're doing the job of every basic control surface.

    Likewise, the aft delta is doing all the stability jobs while also providing lift.

    Image four is based on the Rutan Quickie, using a rear delta with significant dihedral turning it into a lifting V-tail. The fore wing there is mounted mid-fuselage for less drag as that one is meant to be sportier.
     
  7. Jul 25, 2019 #7

    jedi

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    Sockmonkey,

    You mention, "One other thing I'd like to explore is if the directional stability imparted by a delta is enough to not need vertical rudders, .....".

    The sweep of the delta provides a dihedral effect creating a yaw roll couple. The "vertical rudder" AKA vertical stabilizer creates a yaw dampening effect. Excessive dihedral with minimal vertical stabilizer causes dutch roll.

    I do not know that the rear delta will eliminate the need for a vertical rudder. In addition the rear fuselage vertical surface typically has a thick boundary layer making it a poor location for a rudder. The BD-5 is a good example. I listened in on a discussion between Molt Taylor and Ed Lesher when Jim Bede first displayed his non flying BD-5 at KOSH. Molt explained how the BD-5 would needed a vertical stabilizer for a tail. Next year, after it had flown, it had one.
     
  8. Jul 25, 2019 #8

    Sockmonkey

    Sockmonkey

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    Well, I'm thinking that the straight low wing creates an anhedral effect so the effective dihedral of a high mounted flat delta wouldn't be excessive. Now, the yaw-roll coupling is part of what creates the dampening effect, but the air pressure on a strongly swept leading edge also creates a little directional drag making the wings want to align with the airflow.

    One question that needs answering is if the delta wing being aft of the planes CG and overall COP means the directional stability it imparts is improved, lessened, or the same.

    On the plane in image four, the delta does have excessive dihedral, to the point that it's going to act like a vertical rudder in the manner of a V-tail. Again, the low straight front wing creates an anhedral effect to counter the dihedral, while not canceling out the vertical rudder effect.
     
  9. Jul 26, 2019 #9

    jedi

    jedi

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    Reference post #8 above.

    A lot of good questions. Simple flat plate wings on profile models will give some answers. Below are only thoughts and opinions.

    The dihedral of model 4 will give a V tail effect as you mention but with an adverse roll/yaw effect. The flat (no diehedral) delta will not. Again I refer to Molt Taylor. An inverted V tail (if used as a control surface) can give the vertical surface you desire with a favorable roll/yaw characteristic.

    There is a lot of where and how much to be answered and a sketchup drawing can only do so much but it is good to ask. Keep the thread active with your ideas. That is what is great about HBA.com.
     
  10. Jul 26, 2019 #10

    Sockmonkey

    Sockmonkey

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    If the V-tail is able to prevent the yaw, you won't get the roll. Plus the low straight wing is giving an anhedral effect, so I think it should balance out. Need to get off my butt and do a test model.
     

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