Effects of Airpockets

by EZEEMINHAS in General Tech Discussion 7 years ago

Can anyone one enlighten me as to how an aircraft with low wings suffer fuel starvation (while low on fuel) when it is trapped in an air pocket (a severe down draft). Can the above situation result in engine flame out ?
"A question put up by my student".

Re: Effects of Airpockets

by Bill Hertzel » 7 years ago

Bill Hertzel

13 1.95k 7.96k

One scenario is if you are drawing fuel from only one tank and find yourself uncoordinated in a slip to that side.
All the fuel will slosh to the outboard portion of the tank leaving the fuel pickup starved.
This has a higher likelihood of causing a total flameout.
You will need to hold the slip for10 seconds or more.
Restarting may take a while as the fuel lines and the carb bowls will need to refill.
This could be a bad scene if you are low on fuel, The other tank is dry and you are trying to slip it in for a heavy crosswind landing.
This is why header tanks were invented.

More commonly... the fuel pickup in the fuel tank is near the bottom of the tank and usually located near the inboard end of the tank.
Under extreme turbulent conditions, the fuel will float towards the top of the tank at the same time the pilot floats out of his seat.
This will leave the fuel pickup without any fuel to pick up.
This rarely lasts longer than a second or two.

Assuming a carburated engine. The fuel in the carb bowl does the same thing.
The initial indication will be the engine loosing power while it is weightless.
As soon as gravity returns the engine once again regains its fuel flow and power.
A momentary power loss is common, A Full Flameout is not likely.

The air pocket in the fuel line may take a little time to reach the carbs.
Once it does, the air will quickly blow through the carb bowl vents giving the carbs an extra bit of fuel vapor it was not expecting.
This may produce a slight misfire for a half second about 5 seconds after you thought everything had recovered.
~ ~ ~

You can demonstrate this by intentionally going weightless.
Assuming a fixed Wing aircraft.... Don't try this in a Rotorcraft!

Snug up the seat belts.
From level flight, initiate a slightly aggressive dive to build up an extra 10-20 knots.
Then begin a moderately aggressive climb with about 30° nose-up. That's a lot!
Once you lose the extra speed you gained in the dive, smoothly start pushing the nose over.
Keep pushing (Hard) until everything in the cabin that isn't tied down (including you!) starts to head for the ceiling.
The engine will lose power at this time and you will lose 30-40 knots from what you had at the bottom.

Hold this stick/yolk position until the airspeed returns to your level flight speed.
Do not get carried away with trying to extend the zero G time.
Watch your airspeed. You will only get 3-5 seconds but it is a real rush!
You will be considerably nose-down.

Now, Pull out of the dive. Expect to pull about 2Gs in the pullout. The Engine comes back to life on its own.
You will need to make up for the Zero G time with an equal amount of high G time.
Be prepared to reduce throttle if the airspeed builds too fast.

Rotax permits up to 5 seconds under zero/negative G conditions.
The negative Gs do not stress the airframe.
The 2g pull out should be well within the limits of anything with an airworthiness certificate.
If you have never done this, first get a demonstration of it with someone who has.

This is how the astronauts train in the "Vomit-Comet".

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Effects of Airpockets

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