Jeff Spitzer wrote:

Bill Freeman wrote:

 Note

also that Cessna runs the fuel pump return fuel back to the header
 tank which has been speculated may cause fuel heating during
prolonged ground operations (low power, low fuel flow to engine,
high bypass flow, low airflow), but since the header is vented
back to the mains, any vapor generated is dealt with.  Also note
the check valve in the fuel return line to the header tank (Cessna
calls it a fuel accumulator tank).

Let's clarify the danger of hot fuel in the header tank.  Usually hot fuel
does not boil while just sitting in the header tank.  I don't believe there
is a common danger of excessive vapor buildup in a header tank.  The pump
will stop pumping before you get the fuel hot enough for this to happen.

The reason is because ALL pumps suck when they pump.  Even if you have
gravity head the fluid must accelerate into the pump to fill the pumping
chambers whatever form they take.  This acceleration causes a pressure drop
and requires adequate vapor pressure margin.  As the fuel gets hotter, more
vaporization (cavitation) takes place at the pump inlet and pump output
(flow) is gradually reduced.  

Eventually, as the fuel gets hotter OR YOU
CLIMBOUT!!!!! the ratio between vapor pressure and head reaches a critical
value where wholesale pumping loss occurs.  Prior to this, you probably
won't generate vapor in the header tank and you probably won't have a
problem idling.   The regulator will compensate for the loss of flow output
and the problem will be hidden until more flow is needed or the conditions
change as noted above.  This is an extremely dangerous situation if you
take off with the pump barely keeping up (regulator orifice closed) and
climbout rapidly.  

You WILL experience power loss.  Trust me on this one
guys, I've done a ton of experiments on fuel systems and fuel pump
cavitation.  Gasoline is a high vapor pressure compound.  Some food for
thought:  Your fuel pump is cavitating every time you fly.  Don't believe
me?  Do the experiment.  If there is no cavitation, the pump will put out
identical flow when the inlet pressure is reduced as long as the outlet
pressure is identically reduced.  IOW, if the pressure rise across the pump
is the same, the output should be the same.  If you try it you will find
that as soon as the inlet pressure is even slightly reduced the flow will
drop.  This is because of *increased* cavitation.  Also try some tests with
hotter fuel.  Same results.  

If you compare this to a water pump or oil
pump or something you will find that flow is only a function of pressure
rise until some significant inlet restriction is applied.  The nature of
cavitation is that it comes on rather slowly and then drops like a cliff.
Beware of the cliff.

Jeff Spitzer

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