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In order for a conventional horizontal tail plane to generate a nose pitch down effect in a stall condition, it must increase its angle of attack even more (by use of elevator or pitching up more). A front mounted canard would pitch DOWN into the airstream, increasing chances of remaining unstalled. A canard would also be in relatively undisturbed airflow.

The old fashioned solution is to mount a lower aspect, wedge , or delta shaped horizontal horizontal tail that clearly allows the main wing to stall first. Newer designs, especially in airliners, feature reduced size Hstabs of similar planform to the wings, relying on computers to maintain pitch control.

Would a downward pitching canard be a better solution to avoid deep stall or stalls in general?

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  • $\begingroup$ Tail stall at positive AOA generally is not a concern. Tail stall is a concern for down load, in zero-G pushover, for example. $\endgroup$ – JZYL Sep 7 '19 at 18:51
  • $\begingroup$ @Jimmy and is not a "zero G pushover" requiring tail UPload. See what can happen in deep stall? $\endgroup$ – Robert DiGiovanni Sep 7 '19 at 19:53
  • $\begingroup$ It's the force reversal following the push over. There were several Twin Otter crashes caused by pilots doing a zoom climb following takeoff followed by a hard push over that put the airplane semi ballistic. The pitch rotation got high enough that the local AOA at the horizontal tail exceeded the stall angle, and the tail more or less completely quit lifting down when downforce was called for. Being only at about 100 ft at this point, the airplanes pitched straight into the ground. $\endgroup$ – John K Sep 7 '19 at 20:21
  • $\begingroup$ @RobertDiGiovanni are you proceeding from the notion that the horiz tail lifts in steady state flight and needs to be able to keep lifting after the main wing stalls to ensure a pitch over? Conventional airplanes pitch over at the stall because the tail's downforce can no longer overcome the main wing's pitching moment at the stall because of the sudden aft CP shift and down the nose goes. Airplanes with regular tails, like the Ercoupe, were made "stall proof" simply by limiting elevator authority to limit downforce near the stall. The tail simply couldn't push down hard enough. $\endgroup$ – John K Sep 7 '19 at 20:42
  • $\begingroup$ @John K I am proceeding from the notion that large swept wing aircraft can deep stall, and post stall, the computer won't know which way to pitch to unstall due to altered airstream near the tail. With a canard there would be no confusion with pilot or computer, pitching down (much like the foreplanes of a submarine) would lower the nose. $\endgroup$ – Robert DiGiovanni Sep 7 '19 at 22:03
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A couple of confusion here. For starters, unless we are dealing with T-Tails, there is no deep stall for a tailed airplane. On the other hand, a canard airplane has the potential to enter deep stall.

Next, for a transport category aircraft, tail stalling in the positive incidence is generally not a concern. Tail stalling in the down load, however, is; for example, zero-G pushover in icing condition is a real issue to consider during design. In a well designed aircraft, the tail would have a higher sweep and lower aspect ratio than the wing so that the wing stalls earlier than the tail.

(Zero-G pushover may stall the tail because of the pitch rate build-up. Nose-down pitch rate equals negative flow incidence on the tail. Once the tail stalls, you lose the ability to pull.)

It could be an issue, however, if you have a T-Tail. For a swept wing in cruise configuration, the pitching moment would likely reverse near stall. At that point, the elevator would still have good effectiveness, but depending on how aggressive the pitch up is, there may not be enough time to lower the AOA before the aircraft enters deep stall.

Once in the deep stall, the authority on the tail as a whole decreases dramatically. Even with full nose-down stabilizer and nose-down elevator, you may not have enough authority to break the stall. In that situation, without an anti-stall chute, it's game over. The point is, you don't want to ever get there. Preventative measures include stick pusher and envelope protection functions.

Would installing a canard help? Sure, more authority always helps. But does it warrant the complexity and weight increase when other solutions exist? Probably not.

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  • $\begingroup$ good answer. But pitching moment of tail, by design, reverses from down to up in ALL cases where AOA increases beyond a preset limit (trim). (It also reverses when elevator is used (obviously) or when CG is too far back to begin with). Yes, a larger tail, as another writer pointed out, is another solution, as Tupelov implemented after the Trident deep stall crash. My query was inspired by the B ONE. $\endgroup$ – Robert DiGiovanni Sep 8 '19 at 5:34
  • $\begingroup$ Also, the canard would be moveable, used in conjunction with a conventional tail. $\endgroup$ – Robert DiGiovanni Sep 8 '19 at 5:39
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    $\begingroup$ @RobertDiGiovanni Well your AOA is increasing because you are commanding it to increase. While untrimmed tail load gets increasingly more positive with increasing AOA, if you are commanding the nose-up, then pitch-rate aside, it is in trimmed condition. Have you seen incidences where a conventional, non T-Tail aircraft has tail-stalled pitching up? $\endgroup$ – JZYL Sep 8 '19 at 6:03
  • $\begingroup$ The early F-86 Sabre jets did, and they solved with slats! It's all about pitch authority, as you say. The Sabres supersonic tail forget lessons from 40 years earlier, when low speed maneuverability ruled. Many gliders have T tails (their high location saves a tiny bit of drag by pitching nose up without the need to deflect a lower tail). Slats also are in slightly more predictable airflow, better for dumb computers and pilots like me. $\endgroup$ – Robert DiGiovanni Sep 8 '19 at 13:06
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    $\begingroup$ @RobertDiGiovanni Sources please. I don't know what effect "loss of lift at the swept wing tips" you are referring to. You made a bunch of claims; some of which are misplaced, others you haven't substantiated. $\endgroup$ – JZYL Sep 8 '19 at 19:18

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