Me and my brother are at a loss, we understand the basics of how the upside down thing works, but how does the engine get fuel when the tank is turned around, is it a vacoom and how would that work or is the intake from the tank in the back of the it, so the fuel is forced into the engine? Thanks.
In: Engineering
Fuel tanks on the F-15 have flapper valves on their baffles. There is still a limitation on how long the aircraft can stay inverted before fuel starvation. I believe it’s 7-8 seconds at most. Reference T.O. 1F-15E-1.
Blue angles have modified fuel tanks to extend inverted flight.
IIRC, the F-16 has an additional boost pump on top of the fuselage fuel tank. All other tanks feed that one, so it should be full unless you’re low on fuel. The limiting factor on the Viper is the oil pump.
The F-15 has baffles to keep some fuel at the sump, but can’t stay negative g for more than a few seconds.
In addition to everything said Some planes have a time limit they shall not exceed when flying upside down at 0G or negative G. The Canadian F18 has a 10seconds limit iirc or you start having oil supply issues in the engines. But if you pull Gs that’s fine.
Fuel tanks on the F-15 have flapper valves on their baffles. There is still a limitation on how long the aircraft can stay inverted before fuel starvation. I believe it’s 7-8 seconds at most. Reference T.O. 1F-15E-1.
Blue angles have modified fuel tanks to extend inverted flight.
It’s a similar problem in orbit on a spacecraft — liquid fuel just floating in the middle of a tank. Some spacecraft will have an “ullage motor” that basically thrusts enough to get the fuel to float to the “bottom” end of the tank where the fuel intakes are. When the engine is running, the thrust of the engine keeps the fuel by the intake. Typically there is a fuel pump and/or the tank is pressurized with inert gas.
The very simplified answer is that they’re fuel injected. Older fighter planes that used carburetors couldn’t fly upside down too long before stalling the engine but modern aircraft, and really most modern engines of any type, are fuel injected, meaning that the fuel is sprayed from a pressurized system that isn’t affected by orientation.
The F-18 can only fly inverted for a limited amount of time, something like 30 seconds as the fuel already in the feed lines will keep the engines going for long enough to get the plane righted. The Blue Angels have to modify their aircraft by adding an extra fuel pump to the top of the tank to maintain fuel feed during long inverted maneuvers.
The fuel is almost in a bag (some jets). As the fuel is used the bag collapses. Think bagged drinks and a straw.
One thing to keep in mind is that the issue isn’t necessarily flying upside down, it’s negative Gs.
Many upside-down aerobatic maneuvers are positive G maneuvers. For example, barrel rolls and loops are positive G maneuvers even though the aircraft is upside down during part of the maneuver. An aileron roll is an example of a negative G maneuver. Of course, at the end of a half loop or half aileron roll, the aircraft would remain upside down until the next maneuver.
Many aircraft rated for negative G flight still have limitations on how long they can stay inverted. This could be a fuel supply issue, an engine oil supply issue, or another similar fluid supply problem.
As for how aircraft are able to sustain inverted flight, they will have been designed with some sort of pressurization at some point in the fuel supply system to ensure supply for a limited time.
Lots of partially correct answers here along with lots of wrong answers.
Most fighter jets don’t have a floating fuel pickup tube. Their max fuel consumption is way too fast for a floating fuel pickup tube to work, it would just collapse. They have feed tanks that are always filled from other fuel tanks throughout the aircraft. When the only fuel remaining in your tanks is just your feed tank, you better have the runway in sight unless you want to run your emergency checklist for fuel starvation.
Some fighters like the A-10 and F-15 have fuel foam lining their tanks; it’s open cell foam that looks like a sponge with very large holes in it that keeps the fuel from sloshing around too much while others have baffles in them to do the same thing.
Every aircraft, (fighter, aerobatic, passenger) doesn’t like pulling negative Gs, a full [aileron roll is a 1-g maneuver](https://www.youtube.com/watch?v=K7PPfm3-e9o), even while the aircraft is upside down, the fuel, the pilot and the aircraft are experiencing 1-g. The fuel is still at the “bottom” of the fuel tank. Only if the pilot stops at the top of the roll will gravity pull the fuel toward earth. This will lead to fuel starvation in most non-aerobatic aircraft.
Finally, every aircraft I’ve worked on will have an extensive checklist which includes replacing the engine if 0gs or less was experienced for over 1 minute.
Source: 20 year aircraft maintenance tech.
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