Jett .30

Jett .30
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Jett .30 - 10/24/2002 4:38:36 AM

Airwurthy

Posts: 104
Joined: 3/21/2002
From: Toledo, OH, USA
Status: offline

Last weekend I had my Jett .30 quit a couple of times in flight, the tank was about 1/4 full. Both times it quit from full throttle but after a dead stick landing and pulling the fuel line, it would fire back up. This tells me there was still some fuel left in the crank, so why did it quit? Different plug did the same thing. It was a very low ceiling that day and it was rain/misting but my webra .50 never missed a beat.
Thanks,
Skipp

_____________________________

Have we crashed here before or are we crashing here again for the first time?

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quitting engine - 10/25/2002 8:13:00 PM

bob27s

Posts: 4819
Joined: 4/9/2002
From: Cleveland, OH, USA
Status: offline

Hi, thanks for writing...

I think I can explain this one easily.

The fuel is likely foaming a bit, and the clunk is bouncing around. This will cause the engine to misbehave or quit.

To protect the engine, do not plan on flying the tank dry. Dub Jett recommends never flying below 1/3 fuel. Any conventional tank will start mixing air with the fuel as the tank runs down. On a smaller tank (say 6oz or so), a 1/4 tank of fuel will expose the clunk pick-up to air. This will first cause the engine to begin to lean out from small air bubbles or foam being drawn into the fuel line(possibly damaging the engine), then when a big bubble is picked up the engine will quit.

Making this more complex, the Jett engines in general turn higher rpms than others on the market. The higher RPM and resulting vibration frequency can cause fuel foaming where the situation had not existed perviously with another engine installed. Great care must be taken with regard to fuel tank installation.

In your particular case, I believe it is likely a combination of the two situation outlined above.

When you land, yes there will be some fuel in the tank. There will be residual fuel in the engine as well. The foaming will settle back into liquid fuel when the engine (freq source) stops.

This was a common problem with pylon racing with the development of Nelson/Jett/Edmunds class engines. We had to set the engine rich at launch to avoid the engine going lean and blowing a plug as the fuel ran low. In this case, the solution was to use 'bubble-free' fuel tanks, such as the bubble-jett and tetra tanks. With these, fuel is isolated from the air, and no foam or bubbles are possible. Thus, the engine can draw fuel to the very last drop without any change in performance.

Solution in your case:

1) ensure the fuel tank is wrapped in foam and is not touching any airframe structure. This will help prevent fuel foaming.

2) plan the flight to avoid running the tank dry. This will also extend the life of your engine by avoiding a lean condition as the tank runs dry.

I hope this has been helpful!

BTW... if you need any first-hand assistance, I do not live all that far away.

Bob Brassell

(in reply to Airwurthy)

Post #: 2

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