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Todd,
Its normal to find some pressure
in your sump when the ambient temperature is that high even without the
engine running.. And you are correct - that passing throught the fuel
lines and fuel rails, the fuel does pick up heat. If your high pressure
pumps feeds are located on the bottom of your sump, you should have minimum
problems with percolation. Extend runs on the ground will cause heat to
build up in your fuel to some extent. Normally, the warm fuel is replaced
with cooler fuel from your wing/fuselage tanks, however, when the engine is
operated long enough and the fuel gets warm enough, you will
experience some heat build up in the fuel. That tends to be
aggrevated when the fuel flow is reduce to say idle. Then you do not have
as much of the cool fuel being drawn into the header tank as you are using
less of it out of your header tank.
With the location of your header tank, I
would be very surpised that you would see any signs of percolation while flying
for two reasons. 1. you will have less heat under the cowl as the
cooling air flows through and 2. the fuel consumption rate is much higher
than at idle so the fuel does not have as much time to pick up heat and return
to the sump and that fuel does not stay in the sump very long which also tends
to keep the heat build up down.
However, I think it wise you installed
a boost pump. I would attempt to make the situation as bad as you can on
the ground (within reason {:>)) to try to get it to percolation and then
confirm the operation of your boost pump will supress the percolation - before
flying. Once flying, I doubt you will have any problem. I have flow
in 100F weather with my sump tank on the forward of the firewall and once I have
climbed to altitude I turn off the boost pump. As you say, you would not
want to be dependent on the boost pump continiously while
flying.
Ed Anderson
----- Original Message -----
Sent: Sunday, July 27, 2003 3:51 PM
Subject: [FlyRotary] Fuel Header
tank
Due to my addition
of auxiliary fuel tanks, I installed a header tank to return fuel to. After
reading Ed's chronicles of his tank and exchanging a few e-mails with him, I
decided to build a tank large enough to provide a usable reserve and locate it
aft of the firewall. I had concluded that the larger fuel volume and cooler
location would preclude the percolation problems that Ed has
experienced.
Well it seems that most of the heat is picked up as the fuel runs
through the system, as I'm seeing some signs of heated fuel. It has been warm
here (30C+) and problems didn't start showing up until after extended taxi
runs (40min), but I'm trying to take care of worst case scenario. Turning on a
boost pump does help, but I don't want to have to run one continuously. I'm
hoping this will not be necessary while in-flight.
I did equip this header tank with a vent valve to purge the air out
before starting engine. This valve would normally never be open during
operation as I expected the tank to be under a vacuum as it draws fuel up from
wing tanks, but when I did crack it open, I was surprised to see it was under
pressure. If this continuous, I may add a press gauge to monitor this.
S. Todd Bartrim
Turbo 13B RV-9Endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm
"Whatever you vividly imagine, Ardently desire, Sincerely believe in,
Enthusiastically act upon, Must inevitably come to pass".
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