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Inverted flight is flight with a roll angle between 90° and 270° (although it classically referred specifically to flight with a roll angle of 180° - i.e., upside-down). In what ways, in general, does aircraft handling in inverted flight differ from aircraft handling in upright (non-inverted) flight?

Partially inspired by reading about the crash of Alaska Airlines Flight 261, including how the pilots apparently tried (albeit unsuccessfully) to recover their airplane from its final dive by rolling it inverted.

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    $\begingroup$ You may want to limit your question to a specific airframe (MD-83 perhaps based on your example) or perhaps a general group of aircraft. This question is fairly broad and some aircraft are designed to work well inverted while others may not be making a broad answer tough to pin down. $\endgroup$ – Dave Oct 10 '18 at 2:35
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    $\begingroup$ It would certainly matter whether the aircraft is negatively-loaded or positively-loaded. Either would be possible. $\endgroup$ – quiet flyer Oct 10 '18 at 2:36
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One factor in any type of flying is airspeed. 250 knots indicated would be an EF 5 tornado. Most objects that are not attached to earth will fly in any configuration, upright, inverted, even on their sides. The original X1 that broke the sound barrier had (by today's standards) amazingly glider-like wings. Reducing them allowed for even higher speeds.

Inverted flight limitations and concerns are mainly pilot training, fuel flow, trim, and objects falling to the ceiling. Notice that weight forward and elevator "up" will cause a rapid descent when inverted, which is corrected by trimming elevator "down" (is the new up).

The aircraft, retrimmed, will fly, although less efficiently. A fully symmetrical airfoil (common on aerobatic aircraft) minimizes these differences. Stall speeds will vary with aircraft in inverted flight, but can be avoided. The aircraft, even Flight 261, would have a workable flight envelope.

Sadly, in the case of Flight 261, they were dealing with a jammed and then completely detached horizontal stabilizer (allegedly poor design and improperly maintained) and simply had no chance once it broke free. Had the plane been in proper working order, I would not doubt those 2 experienced pilots, with permission, could have flown inverted quite easily.

One would expect less roll stability from the anhedral, but more from the fuselage and tail (weight) being below the wing. Shame they could not save it.

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    $\begingroup$ As an aside, re roll stability, wing sweep contributes a stabilizing effect in negatively-loaded flight as well as in positively-loaded flight; it's only at 0 G's that this effect vanishes. That could be incorporated into the last paragraph of this answer. $\endgroup$ – quiet flyer Oct 11 '18 at 20:51
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Typically, unless the CG is quite far aft (as is often the case with aircraft designed for aerobatics), lots of forward stick or yoke will be needed to keep the nose high enough for sustained inverted flight. With inadequate forward yoke or stick input, the airspeed may increase very quickly as the flight path curves earthward and altitude is rapidly lost.

This orange radio-controlled model glider doesn't even have ailerons, and it has a flat-bottomed airfoil, yet it may be flown in sustained inverted flight. (Roll control is challenging-- inverted, the ample dihedral is very destabilizing.) With the right technique, nearly any aircraft would likely be controllable in sustained inverted flight. Getting there safely from upright flight, and getting back safely to upright flight afterwards, may be another matter, especially if the roll rate is sluggish.

Obviously, whether or not the engines will receive adequate fuel and lubrication to keep running during sustained negative-G flight, and what will happen to the flight controls if they do not, will vary from one aircraft to another.

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    $\begingroup$ It can't be too hard in an MD-88. Denzel Washington did it while under the influence ;-) $\endgroup$ – TomMcW Oct 10 '18 at 21:11
  • $\begingroup$ This answer could be improved by adding a comment about adverse yaw as per aviation.stackexchange.com/questions/32865/… I'll not do that right at this moment-- I see now that the "reversal" of adverse yaw has also been mentioned in another answer $\endgroup$ – quiet flyer Oct 17 '18 at 1:28
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Another aspect, not mentioned so far is that, because you effectively hold "bottom half" of the stick now (relative to the direction of aerodynamic forces), it is not only pitch but also roll control, which is in a sense inverted. But pedals and rudder stays the same, so in order to make coordinated turn you need to move the stick in opposite direction then the pedal you push on.

Or, in other words, you need to roll your head outside the turn, not inside, when flying inverted.

Depending on how your brain works when flying an airplane, this can be quite a difference for some.

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    $\begingroup$ Aileron and rudder remain "same side" coordinated no matter what orientation. Inverted had my knees shaking at first, but gets much better with practice. Forget about sky/ground reference, fly the plane where you want it to go. Simulator or R/C experience will help build confidence. Trust the physics (and trim that stick back to center). $\endgroup$ – Robert DiGiovanni Oct 13 '18 at 16:51
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    $\begingroup$ Correct me if I am wrong (thinking right and left in various reference frames can get odd), but I believe the control is inverted in a sense. What I mean: if you want to turn right while inverted (let's say 'right' from pilot's perspective), you need to push with your right foot, but move the stick towards your left arm, right? Doing the right turn in non-inverted flight, you push right foot and move stick to the right. (I agree that it can feel completely normal and natural, if you think more what each control surface actually does, but the motion of your arm and legs is different.) $\endgroup$ – Martin Oct 13 '18 at 18:48
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    $\begingroup$ Martin, I wracked my brain a bit on this on too. "same side" means rudder deflection is into turn, which must be delicate with negative wing loading. Let's turn right inverted pilots perspective. Plane W.R.T. ground turns left. Setting up the bank really confused me. We would roll our heads away from the turn (left stick) and apply a bit more push on the stick and a bit more power to maintain lift. The "inside" would be the right rudder pedal. Our eyes, outside low, might have a tough time seeing what was ahead, but your inputs would work. $\endgroup$ – Robert DiGiovanni Oct 13 '18 at 21:12
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Most airplanes have asymmetric airfoils optimized for non-inverted flight. Among other things, this means that stall characteristics could be much different (presumably worse) for an airplane in inverted flight. I can't find it now, but I recently saw a video of two similar biplanes flying straight and level, one flying inverted directly above the other. The upper one suddenly dropped, striking the other, and I believe both crashed. Although I can't prove this to be the case (and I haven't heard the results of an official investigation), it seems plausible to me that the pilot of the upper airplane was unaware of the possibility of a significant increase in stall speed associated with inverted flight.

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    $\begingroup$ While you may be right that the increased stall speed contributed to the accident you mention, I find it hard to believe that stunt pilots would not have been painfully aware of that fact. Note: I presume they were stunt pilots well versed in their craft - most people don't put themselves in that kind of danger without knowing what they are doing (yes, I know there are idiots out there, I'm assuming these weren't among them). $\endgroup$ – FreeMan Oct 11 '18 at 16:12
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    $\begingroup$ I don't presume anything about their qualifications, and there was no way to judge from the video whether they were experienced stunt pilots. Whatever their qualifications were, they were evidently insufficient to forestall a fatal -- and likely avoidable -- mishap. $\endgroup$ – Grant Petty Oct 12 '18 at 4:15

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