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The SB-13 swept wing tailless beauty was developed in the early 1990s as a small recreational glider with relatively large upright winglets. The design proved very spin prone.

It occurred to me these vertical "winglets", set upright, would generate a yawing "lift" component in side slip. Since these are set behind the CG, the plane would spin INTO the slip (as the change in relative wind increased their AOA).

This phenomena seems to have similarities to excessive spiral instability in a conventional wing/tail design. Would reducing the size or shape of the winglets help cure this tendency? Or lengthening the nose a bit?

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  • $\begingroup$ 'It occurred to me these vertical "winglets", set upright, would generate a yawing "lift" component in side slip.' - That's exactly what an ordinary horizontal stabilizer does, isn't it? $\endgroup$
    – Tanner Swett
    Sep 10, 2019 at 12:29
  • $\begingroup$ RE: winglets, they look like vertical stabilizers (complete with rudders) to me. And being behind the CG, why would you expect them to act any differently from a normal VTP? $\endgroup$
    – AEhere supports Monica
    Sep 10, 2019 at 12:43
  • $\begingroup$ Yes, indeed. And forces need to be properly balanced. $\endgroup$
    – Robert DiGiovanni
    Sep 10, 2019 at 13:15

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Probably not. Spins usually start from a stall, not a side slip, and yaw stability generally improves spin resistance and recovery.

For model flying wings, removing the tip fins usually makes spins far more likely and much harder to recover from. The usual explanation is that the end plate effect delays the stall at the wing tip.

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  • $\begingroup$ Yaw stability, absolutely. So maybe instead a large high aspect winglet we want more rudder authority. Good point of view from the model side. Washout or slats are prescribed, as well as "endplates". But once spin has started, side area imbalance is contributing to the spin. In other words, without strong opposite rudder, the Vstabilizer actually keeps the spin going. $\endgroup$
    – Robert DiGiovanni
    Sep 10, 2019 at 13:26
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One reason for spin issues with tailless designs is that a spin is a stalled condition, and there's no secondary surface that can be forced out of the stall (or prevented from stalling entirely) as is the case with canards and conventional layouts.

Standard spin recovery in most conventional aircraft is to apply forward stick and use rudder to stop the spin -- but with the entire wing stalled, on a tailless design there will be little or now pitch-down authority. The combination of sweep and large tip fins probably contributes by giving enough continued pro-spin yaw force that rudder (if even present, I'm not directly familiar with the SB-13) can't stop the spin.

So, yes, most likely the tip fins contribute to making spin recovery harder, as compared to, say, all-moving tip rudders, or a coventionally located rudder with conventional fin/rudder proportions -- but they're far from the whole story, as the tailless design itself also contributes.

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