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I believe a twin engine aircraft, with 2-axis control (i.e. Rudder and Elevator), under normal circumstances is flyable. But is it safe at all? If one of the engine dies, will the rudder alone be sufficient to counter the consequent roll/yaw?

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  • $\begingroup$ Define safe. This would be a non-starter for Part 23 and 25. $\endgroup$ – JZYL Nov 16 '19 at 18:47
  • $\begingroup$ @Jimmy I mean "save" literally. What other definition do you need? $\endgroup$ – dammy999 Nov 16 '19 at 19:23
  • $\begingroup$ I don't think that you can have rudder/elevator only control for a twin in an engine-out scenario. The problem just isn't flying straight, you need to be able to get somewhere too. I don't think you have enough authority to both counter roll/yaw and be able to navigate, especially IFR (spatial disorientation in an extended slip, you need more authority to get out of a bad situation). The only situation where this may work is with in-line twins or twin-jets that have both engines mounted close together. $\endgroup$ – Ron Beyer Nov 16 '19 at 19:27
  • $\begingroup$ @dammy999 If your aircraft does not meet the minimum controllability requirement mandated by Part 23 (14CFR for example), do you feel safe? If so, what is your min threshold? $\endgroup$ – JZYL Nov 16 '19 at 19:41
  • $\begingroup$ It sounds like you're thinking through a hypothetical exercise for amusement (or for a bet with another pilot) where you lost aileron control in a twin and were trying to maneuver on rudder alone, then one of the engines quit. Is that about it? $\endgroup$ – John K Nov 16 '19 at 19:53
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For an ultralight with rudder only roll control, which achieves lateral control with a lot of dihedral to create a really strong roll/yaw couple, the issue would be how much reserve rudder authority is there once enough has been applied to counteract asymmetric thrust. That would determine how much roll rate you could still achieve trying to yaw toward the dead engine. If the engines are reasonably close together, it would probably be flyable but you would have a lot less turning authority into the dead engine than away from it. In the end, you would have to go try it to see what happens, or get someone to do it with an RC model version.

I think your problem is more to do with simply single engine performance. If one quits you are now at 50% power. Using two engines doubles the chance of engine failure, and if you can't climb decently on a single engine, you're still in a glider after a failure, just one with a flatter glide. IIRC, the Lazair twin was just able to maintain level flight on one engine at lower altitudes, and it was pretty clean as ultralights go. The Bloop being a lot draggier, you would probably need more total HP that it uses as a single, to have decent reserve power when one fails.

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