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I was reading Why do many GA aircraft not have a "Both" fuel selector? and voretaq7's excellent answer got me to thinking.

He cites the regulations noting that the plane must only draw fuel from one tank or have a method for preventing air from entering the fuel lines. However all his examples assume gravity in a somewhat constant direction.

What do you do when this is not the case? For example in a barrel roll do you set the fuel selector for the wing going up first? Does it depend on the plane? Do planes in air shows have special fuel systems?

How does this regulation apply to aerobatic aircraft, and how is the regulation satisfied for such aircraft?

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    $\begingroup$ As long as it's a positive G maneuver, it isn't a problem. Barrel rolls aren't necessarily negative G maneuvers, in fact when done correctly they are +1G through the entire roll. Planes that can fly negative G maneuvers usually have an inverted fuel and oil system. $\endgroup$
    – Ron Beyer
    Commented Dec 17, 2016 at 18:31
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    $\begingroup$ Related questions on fuel systems for acrobatics, inverted fuel systems in fighter jets and why inverted flight may still starve the engine of fuel. $\endgroup$
    – fooot
    Commented Dec 19, 2016 at 21:22
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    $\begingroup$ I've read those questions, but they do not address the air bubble regulation. $\endgroup$
    – hildred
    Commented Dec 19, 2016 at 22:18

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A typical GA airplane will draw fuel from at or near the bottom of the tank.

An aerobatic airplane will have a hose from the engine to the middle of the tank, then on the inside of the tank there will be another flexible hose with a weight on the end. This inside hose is called a "flop tube".

When the plane is in upright flight, the "clunk weight" on the bottom of the "flop tube" (these are the real terms for them, I'm not making this up!) will rest on the bottom of the tank and draw fuel from there.

When the plane is inverted, the weight will move (along with the fuel) to what was formerly the top of the tank.

This ensures that as long as the tank has a reasonable amount of fuel in it, the clunk weight will be submerged most of the time. There are occasions (usually rolls while going straight up or down) where the weight (and end of the flop tube) is momentarily exposed to air in the tank. The engine will "hiccup", but there's usually enough inertia in the propeller and engine parts to restart the engine by itself when fuel flow is restored a fraction of a second later.

There's an additional component - a special header tank - that helps by confining the flop tube to a very small tank that's 100% full of fuel 99% of the time.

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  • $\begingroup$ ok, so the fuel draws fine. What about preventing air? $\endgroup$
    – hildred
    Commented Dec 19, 2016 at 20:33
  • $\begingroup$ I've edited the answer. Details on how aerobatic header tanks work should probably go into a different question, lest this answer become too long. $\endgroup$ Commented Dec 19, 2016 at 21:09

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