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Such planes have an engine-driven fuel pump. If the pump isn't strong enough, wouldn't suction feeding assist?

How does a modern fighter jet / aerobatic plane overcome this?

Also, any general information on inverted flight and its effects on fuel/oil/hydraulics is welcome.

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  • $\begingroup$ Possible duplicate $\endgroup$ – TomMcW Jun 25 '16 at 0:34
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    $\begingroup$ It overlaps some. I didn't VTC though. I don't VTC unless they are really exact dupes $\endgroup$ – TomMcW Jun 25 '16 at 0:38
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I find it hard to make a general answer since the fuel systems could be different between various aircraft, but, according to Jan Roskam's Aircraft Design series (I just had to return the book, but I think it's Part IV in the series) some model of the F/A-18 utilizes the following system which, in sustained inverted flight (or any sustained flight condition wherein the aircraft does not experience a positive load factor), would eventually starve the engine of fuel. Keep in mind too that fuel isn't the only factor for sustaining this flight condition: oil flow through the engine is typically gravity fed (reference for that is an engineer from Bell Helicopter who worked on the V-22 powerplant). Suction is used to scavenge the oil out of the engine/gearboxes/etc, but the flow of the oil up to a point where suction will pull it out is via gravity (or whatever acceleration the aircraft is experiencing). Hence, not only do we need to provide fuel, but we also need to provide for proper oil flow as well.

Anyhow, the F/A-18 fuel system. Again, I wish I had the image from the text to show you, but the basic idea is this: if you had a flop tube capable of scavenging from the entire fuel tank, it would engender more problems than it solved. Hence, inside of the primary fuel tank there's a second, smaller tank. This tank has a series of poppet valves that allow fuel to flow into the inset tank during coordinated (i.e., positive load factor) flight. When the aircraft is not experiencing a positive load factor, the pressure of the fuel on these valves holds them closed and the fuel stays within the inset tank. A short flop tube within this space provides fuel to the engines for the duration of the maneuver. However, as noted, this inset tank is only a portion of the larger main tank, so its capacity would be some fraction of the main tank, limiting the duration of the maneuver.

So, to answer your question (how does fuel starvation happen and why isn't it solved via a pump) it's a design problem that, per the printing of that text, could not be solved by adding capacity to the pump. There just simply wasn't fuel present at the fuel sump to be scavenged. The real design problem would lie in the design of flop tubes capable of scavenging from the entire tank without creating more design problems (an assumed point on my part). Examples, I think, would crop up if you decided to make a multiplicity of flop tubes (larger pumps required, more parts, more complex plumbing, etc.), or longer flop tubes (potential risk for damage to the tube head/tank with increased "flopping" inertia, larger pump required for greater head losses, etc.)...but I also don't know what the current standard is.

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Adding on to @Marius's very good answer:

Another thing to point out is - there simply isn't a need for a fighter jet to fly around inverted for long periods of time (outside of a Blue Angels show!). It's uncomfortable for the aircrew, and doesn't really do anything tactically. Therefore, it's not worth the extra expense/maintenance.

As a side note, the Blue Angels do in fact, have a fuel system that can sustain inverted flight indefinitely.

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    $\begingroup$ Do you know how their fuel system works? I'd love to see an example, if it can be distributed for a modern model of the aircraft. $\endgroup$ – Marius Jun 25 '16 at 2:32
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    $\begingroup$ I don't, unfortunately. I know that the Blues have an added inverted fuel tank, plus I've heard from guys about how the jet can fly inverted indefinitely. Thats about it. This Gizmodo article mentions some of the changes made to a normal F/A-18 Hornet to make it a Blue Angel. gizmodo.com/5847503/how-an-fa-18-hornet-becomes-a-blue-angel $\endgroup$ – Tyler Jun 25 '16 at 3:07

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