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I know that forward swept wings are unstable and all, and that's one of the reasons they aren't vastly used in planes in general.

However, I would like to know more about other types of wing configurations that were attempted or suggested in the past that would be as unstable as forward swept wings.

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    $\begingroup$ What kind of instability are you referring to? Many gliders have a slight forward sweep and they're usually pretty stable. $\endgroup$
    – Sanchises
    Jun 3, 2022 at 17:22
  • $\begingroup$ Google "tower jumpers". Oops, silly me. In the post September 11 2001 world that brings up something completely different from what I had in mind. Ok, google, "medieval tower jumpers". Oops, silly me, for some reason that brings up inflatable "bounce houses" for children. I give up! $\endgroup$
    – quiet flyer
    Jun 3, 2022 at 17:25
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    $\begingroup$ They aren't necessarily unstable, except for maybe reduced yaw stability and some degree of backwards roll/yaw couple effect with extreme forward sweep (which may be good for dutch roll behaviour anyway). In the end, as long as the Neutral Point/CG relationship is good, you should be stable as far as that goes. One of the biggest technical problems with radical forward sweep is a much higher torsional stiffness requirement than a normal swept wing, because bending increases tip incidence (wash-in from bending) instead of reducing it as with aft sweep and you really don't want that. $\endgroup$
    – John K
    Jun 3, 2022 at 22:13
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    $\begingroup$ I have a feeling the AD-1 may fit the bill. $\endgroup$
    – Ron Beyer
    Jun 4, 2022 at 3:01
  • $\begingroup$ @quietflyer - Would BASE jumping work for your search? Although I’m not seeing any unstable wing configurations… $\endgroup$
    – Jim
    Jun 4, 2022 at 6:54

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Stability is more a feature of the whole aircraft, not the wing. But I guess you are asking about wings that create a destabilizing pitch or yaw moment.

For yaw, the dominant factor is going to be sweep: forward sweep is destabilizing and aft sweep is stabilizing.

For pitch, any cambered airfoil creates a nose-down pitching moment which causes the Center of Pressure to travel aft as angle-of-attack is decreased, and causes the Center of Pressure to travel forward as angle-of-attack is increased. This is destabilizing. For aft-swept wings, washout creates a stabilizing effect in pitch, and negative washout (never actually used) would create a destabilizing effect in pitch. For forward-swept wings, washout would create a destabilizing effect in pitch.

We shouldn't forget about roll stability. In the roll axis, dihedral and sweep are stabilizing (tend to make the aircraft roll toward wings-level), and anhedral and forward sweep are destabilizing (tend to make the aircraft roll toward a steeper bank angle). Anhedral is very often used to offset the dihedral-like effects created by other aspects of an aircraft's geometry (e.g. a tall vertical fin that creates a lot of surface area above the CG), but in and of itself it could be called intrinsically destabilizing.1

Since we are talking about several different kinds of stability here, and since in any given aspect, the stabilizing or destabilizing effects of the wing geometry may be offset by other aspects of the aircraft's geometry, it's kind of hard to evaluate any given wing shape to say whether it is "as unstable as forward swept wings".

If there were a specific reason to use a forward-swept wing on an airplane, the resulting destabilizing yaw and roll characteristics could be offset by other aspects of the aircraft's design, such as a large, tall vertical fin, high wing placement, dihedral, etc. Arguably more troublesome are the structural characteristics of a forward-swept wing-- the wingtips tend to twist toward a higher angle-of-attack under heavy lift load, which further increases the lift load and twisting moment, and the bending moment on the wing spar(s). A forward-swept wing must be engineered to have a great deal of torsional stiffness to avoid problems in this area.

So in summary:

I know that forward swept wings are unstable and all, and that's one of the reasons they aren't vastly used in planes in general.

It may be "one of the reasons", but it's actually a rather minor one. The main issue is structural-- the torsion issue.

However, I would like to know more about other types of wing configurations that were attempted or suggested in the past that would be as unstable as forward swept wings

A wing with significant anhedral and little or no sweep would seem to fit your description, at least if roll stability is a parameter of interest, and we are looking at the wing alone, in isolation from the rest of the aircraft. One example of such a configuration would be the wing used on the Lockheed F-104 Starfighter. Another good candidate would be the wing on the McDonnell Douglas AV-8 Harrier II-- it has some sweep, but appears to have slightly more anhedral than the wing on the F-104, and certainly has more span, giving the anhedral geometry more leverage with which to generate a destabilizing roll torque.

Footnotes:

  1. In the interest of brevity, we'll avoid opening the whole "can of worms" relating to complex forms of dynamic instability that can actually be exacerbated by too much roll or yaw (directional) stability, such as "Dutch Roll" oscillations (exacerbated by too much "dihedral effect", whether from sweep, actual dihedral, high wing placement, or other aspects of an aircraft's geometry), and spiral instability (exacerbated by too much yaw (directional) stability.)
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  • $\begingroup$ Adding slight forward sweep to a slow flying glider minimizes "torsional" issues (and potential flutter), and, when fuselages were "boxy" with more surface area, provided a useful tool to reduce excessive yaw and roll stability. $\endgroup$
    – Robert DiGiovanni
    Jun 4, 2022 at 19:35
  • $\begingroup$ @RobertDiGiovanni -- re "provided a useful tool to reduce excessive yaw and roll stability"-- I dunno, I sense a new ASE question in the making here-- as far as yaw stability goes, it's very easy to shrink the fixed portion of the vertical fin down to virtually nothing-- the Scheibe Bergefalke went pretty far in this direction, and while still perfectly flyable, I wouldn't describe the resulting flight characteristics as ideal. Nose wanders off to one side (sideslip), and if you attempt to hold heading with ailerons only (no rudder input), there appears to be virtually zero tendency (ctd) $\endgroup$
    – quiet flyer
    Jun 4, 2022 at 19:48
  • $\begingroup$ (ctd) Nose wanders off to one side (sideslip), and if you attempt to hold heading with ailerons only (no rudder input), there appears to be virtually zero tendency for the nose to swing back into alignment with the actual flight path / airflow, the glider just keeps flying along happily with the yaw string streaming crazy sideways -- -- thanks in large part to adverse yaw from deflected ailerons-- (wings aren't level and aren't seeming to want to come back to level, due to roll torque created by interaction between sideways airflow and dihedral wing geometry (plus high wing placement))-- $\endgroup$
    – quiet flyer
    Jun 4, 2022 at 19:54
  • $\begingroup$ Even the much-loved Ka-6 has some tendencies along these lines $\endgroup$
    – quiet flyer
    Jun 4, 2022 at 19:54
  • $\begingroup$ Well, can accredit that to the large "schnozz", and less dihedral. Would agree that forward sweep would not be necessary with other options available. $\endgroup$
    – Robert DiGiovanni
    Jun 4, 2022 at 19:55
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Ziv Nedivi and Yehoar Gal flew an F-15 with only the left wing still attached. It was not as unstable as you might think. A maximally asymmetric wing will have the most torque on the fuselage. That will cause a pronounced spinning tendency.

I suppose you could make the situation worse by removing one wing from an aircraft with a lower wing loading and higher aspect ratio. Maybe one of these gliders with a wing removed?

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