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Semitailless1 swept-wing aircraft without active stability enhancement2 generally3 have extremely poor handling characteristics, most notably a tendency towards violent pitch oscillations at high subsonic mach numbers, and yaw and roll oscillations at mach numbers slightly beyond this. As a result, while the swept-wing semitailless layout was briefly quite popular for transonic and supersonic aircraft in the late 1940s and early 1950s (witness, among others, the DH 108, X-4, and F7U), it was quickly abandoned, only making a comeback much (much) later, with the advent of computerised active-stability-enhancement systems.

In the meantime, supersonic aircraft near-universally switched over to delta-winged layouts, which worked just as well for supersonic flight and had none of the nasty quirks of semitailless-swept-winged layouts. This despite the two layouts being extremely similar; indeed, a semitailless swept-wing aircraft can be seen as a delta-winged aircraft with part of the inboard trailing edge cropped out:

what a difference that small triangular section of wing makes

Why do two such similar layouts have such different handling characteristics?

1: Aircraft that do not have a separate horizontal tail, but do have a vertical tail.

2: A system where a computer constantly monitors what the aircraft is doing and continually makes tiny flight-control inputs to keep it doing what it's supposed to.

3: Although not always.

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The function of a "tail" in this case is pitch stability. Therefore, all stable aircraft must have "tails". Aerodynamically, the rear portion of the delta is its tail. Removing it should have predictable consequences, even in the subsonic realm.

Transonically, with shock waves dancing on the wings and a moving center of pressure, pitch stability is critical. This is also true in slow flight, where a shifting center of pressure can overwhelm weaker pitch stability.

Subsonically, the swept tailless design can be helped by "washing out" (lowering the AOA of) the wing tips or by adding slats, which helps control movement of the center of pressure at higher angles of attack. As Dunne ingeniously realized over a century ago, this design made for a stable aircraft. Indeed, a swept wing aircraft with insufficient tail area could be helped with stronger slats! (If the wing tips can handle the extra load).

Transonic and supersonic flight is another issue. The notion of "taillessness" was a blind alley in aviation design, a mistake not to be repeated.

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  • $\begingroup$ That still leaves the rear portions of the outboard wings, though, which should be able to provide pitch stability on their own. $\endgroup$
    – Vikki
    Aug 29, 2020 at 23:36
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    $\begingroup$ They do, but if the (swept) tips stall first, stability is lost. Notice on (even modern) supersonic aircraft, the vertical (usually 2) and horizontal stabilizers are oversized. $\endgroup$
    – Robert DiGiovanni
    Aug 29, 2020 at 23:45
  • $\begingroup$ The Me 163 actually had fixed wing tip slots and a relatively unswept low aspect wing. It is a excellent glider. $\endgroup$
    – Robert DiGiovanni
    Aug 30, 2020 at 0:01
  • $\begingroup$ And to make it so that the wing roots stall before the tips, aircraft with rearward-swept wings need significant washout (decrease in angle of incidence going from the root to the tip), which makes the wingtips fly at a lower attack angle than the wing roots... which (as the wingtips are located considerably aft of the wing roots, such that the washout ends up also expressing itself as a considerable degree of decalage), conveniently enough, also takes care of pitch stability! $\endgroup$
    – Vikki
    Aug 30, 2020 at 20:37
  • $\begingroup$ @Sean but very draggy. Retractable slats are much better. $\endgroup$
    – Robert DiGiovanni
    Aug 30, 2020 at 20:49
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The pitch oscillations come from the transsonic air coming off the main wing and interacting with the elevators/Stabilators. If you look at the variants of the X-1 you will see that the one that broke the sound barrier has its elevators further above the main wings than the previous models

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  • $\begingroup$ With a semitailless aircraft, the elevators are located on the main wings. A semitailless aircraft, by definition, has no separate horizontal tail. $\endgroup$
    – Vikki
    Aug 31, 2020 at 2:21

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