Paul,
Thank you for your reply.
I don't have a header tank. I haven't done any research as to why a high wing airplane would need one over say a low wing aircraft.
My previous airplane was a Cirrus and it had header tanks inboard of the fuel tanks- still in the wings though and not inside the fuselage.
As I recall the reason was to have a usable supply of fuel to use during maneuvering so as the fuel tank outlet would not become "un-ported"
I suspect that the same would apply for the high wing design. with low fuel levels and in designs that would "un-port" the tank outlet connection (during medium or steep bank turns or turbulence) it would be desirable to have a header tank with 10 or 15 minutes of uninterrupted fuel supply available.
However, don't think supply quantity is the problem.
I suspect that regardless of available supply, the fuel pumps will always have a negative pressure (suction) at the inlet. Fuel pressure (PSI) at the inlet is very low even in a high wing airplane where the gravity flow is a benefit. The pump obviously produces a high pressure (PSI) at the outlet. The result will always be a negative inlet (PSI) at the fuel pump. One way to decrease the inlet pressure at the inlet pump is to add another pump upstream in the system. The AUX pump. But the AUX pump encounters the low pressure at the inlet (suction). You are just passing the problem upstream. However, the variable of temperature can be an advantage since the temperature in the cabin is lower than the engine compartment. Thereby increasing vaporization margins. Not much but some. Also, once fuel is pressurized the vaporization margins increase, around 2.2 degrees F per PSI. Therefore a small pressurization of the system for fuel at the inlet of the main fuel pump would increase vaporization margins.
Again, there may be too many negative impacts to justify adding an AUX pump.
It is far simpler to lower under cowling and fuel temperatures as much as possible and switch to a lower RVP fuel.
Yes the fuel system should be as friction free as possible so that the negative pressure at the fuel pump inlet is as low as the design allows, but again, I suspect that the pumps are designed to run at some negative (PSI) number.
Regarding leaving both pumps on during all phases of flight, that is up to the operator. The only requirement that I know of is that in the Operators Manual-(and/or specific airframe Operators manual) requiring both be "ON" during takeoff and landing.
I don't leave both on partly because, depending on which fuel pump fails, I am not sure how I would know if one fails until I attempt the next "run up" before takeoff. I suppose that there would be a drop of 1 to 2 psi , but my engine monitoring is not set up to alert me unless the fuel pressure drops out of the normal range. My instruments would show the pressure but I don't traditionally look at the pressure only if it is in the green.
