Need a little help, any ideas?

[roger lee]

I need a little help. Someone just called me and said that when he pushes the nose over the fuel starves a little and the engine coughs with no change in the throttle setting (4700 rpm). When he pulls back and flys flat and level or climbs it's just fine. Any ideas. I'm stumped on this one. It doesn't happen on normal descent. We aren't talking about a big negative G push over, but more than normal. I haven't had this happen on other Rotax 912ULS engines.

[drseti]

I would suspect the fuel tank output ports and vents, rather than the engine. Sounds as though the fuel flow is being interrupted or disrupted. Is the customer able to monitor indicated fuel pressure while duplicating the anomaly?

[roger lee]

Hi Paul,

He has a FlyDat monitor and it doesn't have fuel pressure. I agree that would help. The other thing is by pushing the nose over it may be pushing the floats in the carb up which would shut off the fuel.

[Jack Tyler]

Roger, your description makes it sound like there's an immediate, almost simultaneous interruption in engine performance with the pushover. IOW not much lag between the control input, change in attitude, and the engine faltering. If that's the case, I doubt the fuel tank/supply line/vent portion of the fuel system is at fault. Instead, it sounds like the problem lies with the carburetor(s). Good luck on the hunt.

[FastEddieB]

I think this behavior is typical of any carburetor with a float bowl and floats.

Both my Citabrias would quit instantly when going to negative g's. Citabrias and Decathalons need fuel injection* and other mods to the oil system for sustained inverted flight.

My working mental image has the floats floating up and causing the needle and seat to close, stoppingbthevflow of fuel. The fuel already in the bowl also begins to float, possibly exposing the fuel pickup to air.

I don't think anything happening at the tank would cause this behavior. If you've ever turned the fuel off with the engine running, you know it takes a finite amount of time for the engine to use the fuel already in the lines and fuel bowls.


*I think there's something called a "pressure carburetor" which addresses this issue, though I'm ignorant of how they work. Maybe I should do something about that!

[FastEddieB]

OK, I've been enlightened.

From Wikipedia...

Most aircraft of the 1920s and 1930s had a float-type carburetor. The float operates a valve which keeps the fuel level in the carburetor consistent despite varying demands. However, since the float is dependent on gravity to function, a float carburetor will fail to flow any fuel if the aircraft is flying under negative-G conditions. This is not a problem for civil aircraft which normally fly upright, but it presents a problem for aerobatic aircraft which fly upside-down or otherwise be subject to negative G, especially military fighters. If an airplane equipped with a float-type carburetor is flown under zero-G or negative-G conditions for more than a few seconds, the engine runs out of fuel, and it stops running. The problem was keenly felt by the RAF during the first years of the war, as the Rolls-Royce Merlin equipped Hurricanes and the Spitfires suffered this effect, unlike the direct fuel injection engines of their German counterparts. The problem was solved by installing a flow-restricted orifice that opened only when flying inverted or under negative-G conditions (the R.A.E. restrictor was known as "Miss Shilling's orifice"), but this was only a stopgap solution.
The pressure carburetor solves the problem by taking gravity out of the system as it operates on pressure alone. For this reason, the pressure carburetor will operate reliably in any flight attitude. The fact that a pressure carburetor operates on the principle of fuel under positive pressure makes it a form of fuel injection.