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Inverted flight as a part of manouveres, dogfights, acrobatics is fairly common but my question is:

Are any aircraft capable of sustained inverted flight without losing altitude? e.g. Could you fly 10 mins inverted at the same flight level?

To clarify: I'm not wondering about engines, fuel systems, air-frame strength etc. Fundamentally there's no reason why those systems cannot be engineered to be robust against inverted flight.

What I'm curious about is specifically the aerofoil itself.

i.e. As far as I can see, you'd need to balance the gravity vector with lift.

Ergo, are aircraft wings generally capable of providing enough compensating lift in the "wrong" direction to balance gravity? Are all / most fighters / arerobatic certified craft capable of this?

We'd need an aerofoil that can provide a lift vector pointed in the +z direction normally but changable to a large vector pointing in the -z direction in response to a flight surface deflection?

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    $\begingroup$ Just as a clarification, actually being "inverted" doesn't matter, the only thing relevant is if you experience negative G. Staying inverted at a constant altitude would mean sustaining -1 G in the airplanes frame of reference. $\endgroup$
    – falstro
    Jan 25, 2016 at 9:04
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    $\begingroup$ I forget who and what plane, but there is a female pilot in the US, who flies 767s from the left hand seat for a living, who has the world record for inverted flight at 600+ miles. Remember, it's angle of attack that matters. Most of my flying has been in aircraft with symmetrical wings, albeit helicopters :) $\endgroup$
    – Simon
    Jan 25, 2016 at 10:40
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    $\begingroup$ Joann Osterud ? $\endgroup$
    – curious_cat
    Jan 25, 2016 at 11:08
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    $\begingroup$ If you can make it go fast enough, you can fly anything inverted... $\endgroup$
    – Jon Story
    Jan 25, 2016 at 16:01
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    $\begingroup$ Would you believe inverted takeoffs and landings? youtube.com/watch?v=1ApSDpnA2k0 $\endgroup$
    – Fred Larson
    Jan 25, 2016 at 17:00

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Yes they are! Aerobatic aircraft can have symmetrical wing to improve inverted performance. So with these aircraft there is no any problem at all. Other agile aircraft, gliders can fly inverted as well. Of course is far from optimal but possible. Don't forget that lift depends of angle of attack as well. So if you fly inverted angle of attack and drag will be bigger to compensate for a "wrong" wing profile.

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    $\begingroup$ Ah yes. I did indeed ignore the angle of attack. $\endgroup$
    – curious_cat
    Jan 25, 2016 at 8:26
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    $\begingroup$ To be fair, gliders can't maintain altitude unless in an updraft. Inverted or otherwise. Gliders are better at flying inverted though, because their engine wont quit. ;) $\endgroup$
    – falstro
    Jan 25, 2016 at 8:59
  • $\begingroup$ @falstro their pilots might, though (literally and figuratively) $\endgroup$
    – John Dvorak
    Jan 25, 2016 at 17:37
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    $\begingroup$ @curious_cat: Never forget the humble paper airplane. The flat plate airfoil of a paper airplane also generate lift. Airfoil is not about lift. It is about efficiency. If you don't care how much fuel you burn and if you have plenty of excess horsepower feel free to fly with an upside-down airfoil. $\endgroup$
    – slebetman
    Jan 25, 2016 at 18:15
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    $\begingroup$ I'm far from an expert, but my understanding is that angle of attack isn't just another factor, it's the factor. The shape of an airfoil is just a way of providing a positive angle of attack in a way that reduces parasitic drag. The fact that it's round in the front, curved on the top, pointy on the back basically means that if you drew a line through the "average" of the cross-section, you'd find that it tilts a bit down -- that is, it provides an angle of attack. $\endgroup$
    – yshavit
    Jan 25, 2016 at 19:29
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Most aircraft use a cambered airfoil. Such an airfoil only gives you a higher stall speed, otherwise it will just cope fine with prolonged inverted flight. The wing's twist will most likely increase the induced drag since the circulation distribution over span is designed for upright flight, and maybe the airfoil will operate outside of its laminar bucket (if its polar has one), so viscous drag will be higher.

If the wing uses a symmetric airfoil and has no washout, like in some aerobatic aircraft, the aircraft will perform identically in both attitudes.

The real limit to prolonged inverted operation is the fuel system and, on some engines, the lubrication. If both are designed for prolonged inverted flight, only the fuel level and the physical condition of the pilot will limit the duration of inverted flight. It is much more comfortable when your body weight is supported by a comfy seat than by two narrow straps running over your shoulders.

The higher stall speed will translate into a higher speed for the minimum power and minimum drag optima, so an aircraft which could keep its altitude in regular flight might have too little power for sustained inverted flight. Also, the trim range of the elevator might be too small to trim the stick forces. In inverted flight the sideslip-induced rolling moment will have the wrong sign, so sideslipping will make you roll into the wind. But if you tolerate all this and engine power is sufficient (which should be true for all fighters since WW II), flying inverted at the same altitude is no particular problem.

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  • $\begingroup$ Any idea what aircraft Milo Burcham & Joann Osterud used to set their inverted flight records? 4+ hours sounds incredible. $\endgroup$
    – curious_cat
    Jan 25, 2016 at 8:24
  • $\begingroup$ When flying negative G it is much more comfortabe to pull the straps over your legs very tight and leave your shoulder straps quite loose. $\endgroup$
    – Wirewrap
    Jan 25, 2016 at 8:37
  • $\begingroup$ @Wirewrap: True, but not for tall pilots. My head would rest on the inside of the canopy if I loosen the shoulder straps. $\endgroup$
    – Peter Kämpf
    Jan 25, 2016 at 9:49
  • $\begingroup$ Areobatics is one of the few walks of life where it pays off to be a short skinny guy. $\endgroup$
    – Wirewrap
    Jan 25, 2016 at 15:08
  • $\begingroup$ The problems that you describe are for aircraft which are not designed for inverted flight. If I'm not mistaken, some aerobatic aircraft have fuel and oil systems designed with pickups on both sides of the tanks just so that they can stay inverted without an issue. The symmetrical wings also eliminate the problems cause by the wing twist which you describe. $\endgroup$
    – Lnafziger
    Jan 31, 2016 at 23:11
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I realize you explicitly stated:

To clarify: I'm not wondering about engines, fuel systems, air-frame strength etc. Fundamentally there's no reason why those systems cannot be engineered to be robust against inverted flight.

But I'm going to answer for part of this because it does seem to be the limiting factor of your question.

The main problem with inverted flight is not aerodynamic but propulsive. Most aircraft fuel tanks require gravity to feed fuel to the engine. Military and high performance aircraft typically include some mechanisms for maintaining fuel flow during negative G maneuvers like inverted flight. However, there are usually very strict time restrictions.

For example, the B-1B bomber possesses only a single fuel tank able to operate under negative G maneuvers. During normal flight fuel is pumped from gravity fed tanks into this special fuel tank. Then during the negative G maneuvers only fuel from this special tank is available for continued engine operation.

What this means in practice is that aircraft performing any negative G maneuver starts a count-down clock. When the clock reaches 0 its engine(s) die.

Which is a very bad thing during normally powered flight.

The amount of time any particular aircraft has is dependent upon its particular fuel tank design.

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  • $\begingroup$ Glad you answered! I didn't mean to make it answer-blocking. :) $\endgroup$
    – curious_cat
    Jan 25, 2016 at 18:04
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When at the top of a loop, you would normally still have positive g. The issue here is to fly inverted sustainably at -1 g. With a symmetric wing (most fighters and some aerobatic planes) this would be easier as it won't try to create lift towards the earth in that instance

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Yes. Modern high performance military and civilian aerobatic aircraft have dedicated inverted fuel and oil systems allowing them to operate in the negative G regimes of their flight envelope without airframe or engine damage.

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    $\begingroup$ Something came to mind. Most oil-systems of jet-engines are not capable of sustaining long inverted flight. From the several fixed-wing aircraft I've flown (including jets) the longest limit I've seen (also the most common limit) of sustained inverted flight was 30 seconds. So, the fuel and oil systems would have to be specifically designed for very long periods of inverted flight (or fly electric). Still, I fail to find a practical reason why one would fly inverted for longer periods, even for UCAVs). $\endgroup$
    – Chris V
    Nov 28, 2017 at 18:32

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