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Thread: Tailless Aircraft - Reflex and other design issues

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    Tailless Aircraft - Reflex and other design issues

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

    ----------------

    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 Topaz; January 10th, 2019 at 07:09 PM.

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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?

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

    Quote Originally Posted by blane.c View Post
    Are you asking if you can hinge them at the fuselage and flap them using your arms?
    No, I'm asking if you only need to reflex them at the point furthest aft.

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

    Quote Originally Posted by Sockmonkey View Post
    No, I'm asking if you only need to reflex them at the point furthest aft.
    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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    Re: Modern day "motorcycle of the air" aircraft class?

    Quote Originally Posted by Sockmonkey View Post
    No, I'm asking if you only need to reflex them at the point furthest aft.
    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.
    The most elegant theory can never change reality but even a mediocre theory can predict reality most of the time.

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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.

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

    Quote Originally Posted by Sockmonkey View Post
    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.
    Everything that's been said still applies.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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

    Quote Originally Posted by Topaz View Post
    Everything that's been said still applies.
    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?

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

    Quote Originally Posted by Sockmonkey View Post
    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?
    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.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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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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    Re: Tailless Aircraft - Reflex and other design issues

    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.

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    Re: Tailless Aircraft - Reflex and other design issues

    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...=rep1&type=pdf

    Swept flying wings can have airfoils much closer to neutral pitching moment than planks can.

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

    Quote Originally Posted by Topaz View Post
    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.
    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.

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

    Quote Originally Posted by Sockmonkey View Post
    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.
    You are hereby banished to having only a moderately-good single-malt tonight!

    Quote Originally Posted by Sockmonkey View Post
    Is the greater induced drag mostly a result of the wing itself having less lift per unit area than a standard wing?
    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.

    Click image for larger version. 

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    Quote Originally Posted by Sockmonkey View Post
    I'd say going tailless for homebuilts is more for compactness and a cheap way to make the thing strong and light.
    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.
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry David Thoreau

    Design Project: Conceptual Design of an "Inexpensive" Single-Seat Motorglider
    Discussion Thread for the Project: Discussion: Conceptual Design of an "Inexpensive" Single-Seat Motorglider

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