Fuel Flow/Pressure Anomalies

[skippydiesel]

While I quite enjoy & learn from many of the  "thread drifts" that have come to typify this Forum, I am concerned about the tone of some of the respondents and the lack of any relationship to matters aircraft.

 

SO! I thought I might repost this interesting (to me) matter, that is homebuilt aircraft in its origin & intent;

 

"My Sonex/912ULS has an interesting fuel anomaly;

To give you some ides where the matter may arise from, I should tell you that my Sonex is fitted with 2 x 30L wing tanks and a 40L in fuselage "header" tank. I call it a "header" because it has no external fuel point, being "fed" from the two wing tanks and the fuel return.

I can select from any of the three tanks but prefer to use just the "header" .

The plumbing to to achieve, the above, is quite convoluted.

I have an electric fuel transfer pump and a "boost" / auxiliary. The latter being used for all higher risk situations i.e take-off/landing, low level operations.

For the most part fuel pressure is between 4-5+psi.

The anomaly;

In Cruise, my Dynon will occasionally (it seems to be quite random) give me a low fuel pressure warning - Pressure may be indicating a low as 1 psi. 

If I am high, I will just monitor the situaton and pressure will slowly recover, without any intervention on my part.

If low, I will turn on my Boost pump for an immediate increase to 5 psi - Turn off the pump, pressure remans between 4-5psi.

At no time has the engine ever hesitated.

The above is puzzling enough - what could be happening ?

BUT

There is another weird symptom - When I switch on the Boost pump, in response to low pressure, my fuel flow rate reduces by up to 5L/hr.

My Red Cube (x2)/Dynon fuel flow indication has, despite my best efforts to calibrate accurately, never been much good - usually over indicating fuel flow by a significant margin.  I think it likly, the indication, with the Boost pump on, is reflective of the actual flow rate in Cruise.

The Fuel Used/Remaining is acurate.

To rule out engine heat causing fuel vaporisation, I have double insulated the, in cowling, fuel supply to the mechanical pump. The fuel distribution lines from the pump are all coved with orange fire sleeve.

 

My best guess:

Due to the convoluted fuel supply system, low pressure is being generated somewhere, causing the fuel to "cavitate". The cavitation encourages bubbles to form. The bubbles effect the fuel pressure sensor - low pressure warning!

Why:

•    The above only seem to happen occasionally, I can not explain.
•    Would fuel flow indication, reduce with the Boost pump on?
•    Does my fuel flow, without the Boost pump, indicate high flow?

 

Anyone got any other suggestions?"

 

😈