Tailless Aircraft - Reflex and other design issues

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Sockmonkey, Jan 10, 2019.

  1. Jan 10, 2019 #1

    Sockmonkey

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    Moderator Note: This topic was broken out of the "Motorcycle of the Air" thread, since it's OT for that material.

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    As long as we're being goofy, I may as well ask. Do Fauvel type tailless planes need less reflex than planks due to how the root chord is almost as long as the fuselage, and would you have them be reflexed along the whole span or just the roots?
     
    Last edited by a moderator: Jan 11, 2019
  2. Jan 10, 2019 #2

    blane.c

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    Re: Modern day "motorcycle of the air" aircraft class?

    Are you asking if you can hinge them at the fuselage and flap them using your arms?
     
  3. Jan 10, 2019 #3

    Sockmonkey

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    Re: Modern day "motorcycle of the air" aircraft class?

    No, I'm asking if you only need to reflex them at the point furthest aft.
     
  4. Jan 10, 2019 #4

    Victor Bravo

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    Re: Modern day "motorcycle of the air" aircraft class?

    I cannot say for sure but I do not believe that a longer root chord (Fauvel, Marske) would use any more or less reflex than a constant chord (Backstrom).

    The reason is that the reflex is proportional to the airfoil characteristics, not the chord. So if you have a 12 inch long chord airfoil whose pitching moment requires X degrees of reflex... and you scaled that airfoil up to 12 foot chord... the relationship between the pitching moment of the airfoil and the amount of reflex remains unchanged.

    Now, the smart guys like Norman and Topaz (and several others) can feel free to correct me if needed :)
     
  5. Jan 10, 2019 #5

    Topaz

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    Re: Modern day "motorcycle of the air" aircraft class?

    You only need to have enough reflex over enough span such that the total aircraft pitching moment is zero at the design airspeed. So, strictly speaking, you can have only part of the span reflexed, and the rest anything you want.

    But, as usual, the whole reality is more complex than that. If only part of the span is reflexed, that section has to be reflexed more than would be needed if the reflex were done full-span. And even more, since its positive pitching moment is "fighting" the non-reflexed span that presumably has a negative pitching moment. If only part of the span is reflexed, and you have a relatively modest engine (or no engine at all), what is the resulting distortion of the spanwise lift distribution and resulting increase in induced drag going to do to climb and glide operations (and every airplane is a glider when the engine quits)? You can try to balance a larger wing chord over the reflexed section with the reflex itself, resulting in a smooth lift distribution, but how much harder is that wing going to be to build, with the strange planform that results?

    Lots of issues to consider, although none of them are absolutely insurmountable. Just more work to do in the design stage.
     
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  6. Jan 10, 2019 #6

    jedi

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    Re: Modern day "motorcycle of the air" aircraft class?

    IMHO Yes. But recognize that the reflex works on the entire wing cord of the airfoil not just as an elevator at the trailing edge. Controlling the center of pressure on a long cord is more effective than it would be on a narrow cord.
     
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  7. Jan 10, 2019 #7

    Sockmonkey

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    Re: Modern day "motorcycle of the air" aircraft class?

    Thanks guys. See, I'm specifically thinking of wings like on the AP-10 and AP-12 that have a reverse delta shape that places the aft of the root chord nearly as far back as a conventional elevator would be.
     
  8. Jan 10, 2019 #8

    Topaz

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    Re: Modern day "motorcycle of the air" aircraft class?

    Everything that's been said still applies.
     
  9. Jan 10, 2019 #9

    Sockmonkey

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    Re: Modern day "motorcycle of the air" aircraft class?

    Okay, next question. Does the reflex also "recycle" the tip vortexes back into thrust the way washout on a Horten flying wing does, and how does it compare to having a standard tail in terms of drag?
     
  10. Jan 10, 2019 #10

    Topaz

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    Re: Modern day "motorcycle of the air" aircraft class?

    The answer is "no", but then the Horten wings don't do that, either. The Horten BSLD twist and lift distribution is such that the resultant of all the forces on the outer parts of the wing panels is angled slightly forward, giving a slight "thrust", which equates to a drag reduction at some flight conditions, and not at others. Al Bower's work covers this as well, and in much more detail than did the Hortens. In fact, Dr. Bowers is zeroing in on being able to say with some confidence that the Horten's BSLD is somewhat better if one is trying to minimize sink rate, while Prandtl's 1933 BSLD is somewhat better if one is trying to maximize L/D.

    How any tailless aircraft compares to an "equivalent" tailed one in terms of "drag" is entirely a case-by-case affair. There is no general rule or function that can be applied. The only generality that I could venture is that, all else somehow being equal, a tailless design is likely to have a somewhat reduced parasite drag compared to an "equivalent" tailed one, and (unless the former has exactly neutral static stability) somewhat increased induced drag under most flight conditions compared to the "same" design with a conventional horizontal tail. So whether or not a given tailless design sees any benefit compared to a tailed one depends entirely on the specifics of the designs in question, and their design missions. Again, no general "rule" applies.

    Based on what's been achieved so far, any drag benefit a sportplane-class airplane might see from being tailless is going to be small and, compared to things like cooling drag or the drag from the fuselage-wing interaction, relatively inconsequential. For airplanes such as we generally discuss both here in the "Motorcycles of the Air" thread and HBA in general, you go tailless because you don't want (or want to build) an aft fuselage and horizontal tail, not for any kind of performance benefit.
     
  11. Jan 10, 2019 #11

    Aesquire

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    Re: Modern day "motorcycle of the air" aircraft class?

    Hang gliders are mostly tailless because of unique take off and landing requirements.

    If you tried to launch a Cub off a cliff at a jogging pace, you'd get a tail strike, abrupt pitch down, and might not clear terrain before flying speed is reached. Landings are full stall, zero ground speed, ( ideally ) and your Cub would impact tail first, then whip the cockpit into the ground.

    The compromises to make foot launch practical affect every aspect of the designs.
     
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  12. Jan 11, 2019 #12

    plncraze

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    Don Mitchell wrote that he liked tailless because it was simpler and lighter. Thats in an old Sailplane Homebuilder magazine. Marske thought it was safer. He mentioned that in his yellow book.
     
  13. Jan 11, 2019 #13

    lr27

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    I think on a Fauvel, it might make sense to use an airfoil with a more positive pitching moment in the center and something closer to neutral, though still positive, at the tips. Even more so, perhaps, on the AP-10, though at low aspect ratio I expect the aerodynamics are a bit more complex. The wider the chord is, the more powerful the pitching moment is. I think it's better to talk about pitching moment than about "reflex", since reflex isn't always obvious.
    http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.520.8139&rep=rep1&type=pdf

    Swept flying wings can have airfoils much closer to neutral pitching moment than planks can.
     
  14. Jan 11, 2019 #14

    Sockmonkey

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    Re: Modern day "motorcycle of the air" aircraft class?

    Yes, as soon as I typed that I was was kicking myself for not being specific enough about what sort of plane and performance I'd be looking for. Rookie mistake. Bad Sockmonkey.

    Is the greater induced drag mostly a result of the wing itself having less lift per unit area than a standard wing?

    I'd say going tailless for homebuilts is more for compactness and a cheap way to make the thing strong and light.
     
  15. Jan 11, 2019 #15

    Topaz

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    Re: Modern day "motorcycle of the air" aircraft class?

    You are hereby banished to having only a moderately-good single-malt tonight! :gig:

    No, it comes down to a fundamental issue in tailless aircraft, resulting from the fact that their pitch-control surfaces are located on the wing. If the airplane is flying steadily at its design speed, the airplane should be self-trimmed by its design (reflex and/or sweep-twist) and the pitch control surface is exactly in trail, undeflected, on the trailing edge of the wing. Angle of attack remains constant. Everything is rosy, and no different than any other airplane. Now, at any steady airspeed above or below the design speed, and presuming the airplane has positive static stability, the pitch control surface will be deflected some amount so that the airplane is trimmed at that airspeed, with the angle of attack neither increasing or decreasing. With the pitch control surface deflected, that portion of the span is generating either more or less lift than the rest of the wing, and the shape of the spanwise lift distribution now has a big bump or valley in it. Whether you like one of the BSLD's or an elliptical distribution, if it's not a smooth curve of the chosen shape, you're paying a penalty in increased induced drag. No way around that. The faster or slower the airplane is flown from the design speed, the more the pitch control surface must be deflected to maintain trim, and the bigger the distortion in the lift distribution, resulting in a bigger induced drag penalty.

    Statically-stable tailless aircraft therefore have an inherent induced drag penalty at any airspeed other than their design speed. A neutrally-stable tailless aircraft can be trimmed at any airspeed with the pitch control surface in trail (no penalty), but would require an artificial stability-augmentation system to be flyable by virtually any pilot - and since the artificial stability system would be "creating" stability by deflecting the pitch-control surface to adjust for minor excursions from the desired angle of attack, there would still be a small induced drag penalty whenever it was in action.

    This inherent penalty is something you balance against the things you gain from going tailless. For an airplane where induced drag is not a general concern - one with a high power-to-weight ratio such as a high-powered homebuilt or a jet fighter - then the penalty can be ignored. For a homebuilt with a relatively small motor, or especially for a sailplane, the penalty would be much more significant, potentially having a big effect on climbing and gliding performance.

    The Genesis sailplane had a separate pitch-control surface (its little T-tail) away from the wing for this reason. Virtually all the stability comes from the wing, and that little tail is just there as a pitch control. As such, it doesn't suffer from the usual tailless-aircraft induced drag penalty, but isn't a "pure" tailless aircraft, either, which matters to some people. I think it was an excellent solution, myself, and the airplane had comparable performance to conventional gliders of the same class.

    4e9be8d383fba7f7cb557a52a585a053.jpg

    Agreed completely. Smaller, lighter, less-expensive airplane, and smaller, lighter, less-expensive trailer. Genesis gets 80% of this benefit with no performance penalty, which is nice.
     
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  16. Jan 11, 2019 #16

    lr27

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    A plank flying wing won't necessarily have a significant induced drag penalty. If you increase the chord where the elevator is, then the increased chord compensates for the effect of the elevator movement, at least if you have the c.g. in a certain position. This is covered in Nickel and Wolfahrt's Tailless Aircraft in Theory and Practice. You can see what I think is an example of this on the Marske Pioneer series sailplanes.

    I suppose this might help with swept flying wings too, though I think some of them have closer to a bell shaped lift distribution than an elliptical one. It's been argued, especially by Al Bowers, that some version of the bell shaped lift distribution will give you less induced drag per lb of structural weight. Span will be larger, of course, but wing root bending loads may be smaller. More on that here:
    https://www.youtube.com/watch?v=bCwtcDNB15E
    I'm pretty sure that the argument applies to straight wings as well as to swept ones, but in the latter cause the bell shaped lift distribution may give you your pitch stability.

    I'm guessing that the twin fins on some of the Fauvel flying wings help with the induced drag too, sort of like inboard winglets. But I could be wrong.

    From what I've read, for low aspect ratio designs, lift distributions may be less critical, and the AP-10 certainly has a low aspect ratio. I suspect the AP-10 would perform a bit better with a thicker root airfoil that would swallow up a lot of the fuselage. If you make the aspect ratio low enough, or the pilot prone enough, you can swallow up all of the fuselage, or almost all. For examples, see the Facetmobile, the V-173, or some of the Horten and Northrop designs at Nurflugel.com.
     
  17. Jan 11, 2019 #17

    Sockmonkey

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    Re: Modern day "motorcycle of the air" aircraft class?

    So the Genesis manages to have minimal induced drag in a broad speed range?
    Does a reflexed airfoil cause the COL to move back as speed increases? If so, could a properly designed trailing edge use the faster airflow to compensate for that? I'm assuming that still give the drag penalty but at least it wouldn't require trim adjustment.
    So, thought experiment. Take a piper cub and stick an Arup or Fauvel type wing on it. (would probably have to be the straight leading edge type) How does this affect it's performance? Structurally it would be stronger and cleaner, so maybe it would compensate for the penalties enough to be worthwhile.

    I have kind of a thing for designs where the wing roots are fat enough to tuck the engines and everything else inside.
     
  18. Jan 11, 2019 #18

    lr27

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    I don't know about the Genesis. It doesn't look like it has that feature. Maybe that tiny stab takes care of the trim??? It's supposed to perform very well, so the induced drag can't be too bad.

    COL isn't the right way to think about it. An airfoil with a negative pitching moment, such as a 4415, may still be trying to pitch down when the lift is zero. Where is the COL then? Anyway, an airfoil with a positive pitching moment, which many "reflexed" airfoils are, tries to pitch up, and if you go faster, it will try to pitch up more strongly.

    If you put a Fauvel wing on a Cub, with the AC (I think that would be at 25 percent MAC) at the same place as the old wing was, you'll need less up elevator or more down elevator. Maximum lift, drag, etc. would depend on the details of each wing and I don't think you can make a general statement. If you ditch the struts, that will probably reduce the drag significantly, but you'd better make sure the top of the fuselage can handle the loads. I would guess that, if the wings were of the same area, the Cub wing might stall at a somewhat lower speed, but without details it's hard to be sure. There are airfoils with positive pitching moments which also stall at higher lift coefficients than the Cub's USA 35B. Or, at least, Xfoil thinks so. It seems possible that reduced trim drag might make up for the slightly increased wing drag you might expect at some speeds. If the new wing were longer, it would probably have less induced drag, but that would be true for a conventional wing as well.

    You can probably find airfoils which will perform ok even if 20 percent thick, though they might have a little more drag. Not as much as junk hanging out in the wind, though.
     
  19. Jan 11, 2019 #19

    Topaz

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    Re: Modern day "motorcycle of the air" aircraft class?

    I've seen this claim in Tailless Aircraft... as well and, for the life of me, I can't see how it works at anything other than a particular lift coefficient. Perhaps I'm missing something.

    Compared to an otherwise identical "pure tailless" design, yes.

    As lr27 noted, Center of Lift isn't really used in aircraft design. At any rate, addressing your real question, recall that the pilot wants to be able to "trim" the airplane (aircraft design meaning, in that total aircraft pitching moments are zero) at any arbitrary airspeed. Essentially we're talking about pitch control or airspeed control, not zeroing out the loads transmitted to the stick (which latter is "trim", in the pilot meaning). So long as the aircraft has positive static stability, trimming at any airspeed other than the design airspeed will require a pitch-control surface deflection.

    I'm not sure you can make any such generalizations. Comparing the two options would be a design study, and that sort of thing is done all the time when a company is designing an aircraft to a particular requirement. You run the numbers on both, and see which does a better job of satisfying the requirements set. "Stronger" begs the question of whether the airplane was "strong enough" to meet the requirements in the first place (a pretty safe "yes" for an existing design such as a Cub) so making it "stronger" doesn't gain us anything. "Cleaner" is harder to quantify. Yes, the new low-AR version has no struts and protrusions than the original Cub design, so a lower drag-per-square-foot, as it were, but then it's probably got a lot more wetted area - more area where that drag value is applied. The end result could be that the low-AR variation has more parasite drag than the original. That would have to be determined "by the numbers."

    Nothing wrong with that. If such a design meets your needs and requirements in an airplane (one of which could simply be "looks cool!"), then that's the design to choose and build.
     
  20. Jan 11, 2019 #20

    henryk

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    "This is covered in Nickel and Wolfahrt's Tailless Aircraft in Theory and Practice."

    -in this work I was finde interesting statemant="only BKB1-A tailles glider is TUMBLE
    resistant "...

    https://www.youtube.com/watch?v=ZW7oeSm7zZ0
     

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