In general, how do aircraft handle differently in inverted flight versus in upright flight?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.