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I'm really concerned for some of these fuel
designs. The fuel bleed has nothing to do with vapor lock. Virtually no effect
at all.
The bleed only helps if you blew your plumbing from
tank to pump....and don't want to fix the mistake. If you messed up your
plumbing you DON'T have a self priming fuel pump. Then the bleed solves the
problem. It lowers the pressure in the fuel rail and allows the pump to move air
into the fuel rails. All fuel pumps are really poor at compressing air.
Impossible to compress air to 35 psi.
Let's say you decided to place your fuel pump
ABOVE your fuel tanks. In this case, the pump inlet can't get wet. It's not self
priming as a result. The fuel bleed is your only solution for priming your fuel
pumps.
How to prove if I need fuel bleed? Drain all fuel
from tank and lines. Add 1 gallon to tank. Turn fuel selector to "on" and time
how long it takes for fuel to exit rails. If 7 seconds pass without fuel being
pumped, you know you blew your plumbing from tank to pump.
What's Vapor lock? When you put a pan of water on
the stove, those bubbles coming from the bottom of pan are O2 coming out of
solution. That's the Vapor. Air leaving the liquid. This always happens at the
lowest pressure point in the system. Almost always at fuel pump inlet because
the pump has to reduce pressure to flow fuel. If you had clear fuel line at pump
inlet, you could slowly pinch the line and watch bubbles form. It's really
cool...and educational.
Buy a Hodges fuel vapor tester. Measure vapor point
of your fuel. Now measure pressure of fuel line next to pump inlet. The
difference in those two pressures is your vapor lock safety margin. That's it!
That's the definition of vapor lock.
So vapor lock is very simple....except a bunch of
things affect those pressures. Temperature, type of fuel, type of fuel filter.
What's the best fuel design to prevent vapor lock?
Copy any auto mfg method. They all use the same method because it's SOOO
effective. OEM fuel designs minimize every REAL risk we have in our aircraft.
They use self cleaning filters that have around 10 or 20 times more surface area
than stupid in line filters. They minimize pressure drop at pump inlet by using
large coarse filters there. Submerged pumps. Every car mfg uses the same system.
Do you know how rare that is? They do it because the wet design with self
cleaning filters eliminates all historical failures.
I recently converted my sys to automotive style.
Details at bottom of page:
You can prove you have a safe system long before
you ever fly. You can build a fuel sys way way safer than traditional aircraft
types.
-al wick
----- Original Message -----
Sent: Tuesday, August 16, 2011 6:53
AM
Subject: [FlyRotary] Re: Dennis Haverlah
Fuel System...or any others, for that matter.
A logical cure for a common
problem. Well done.
Lynn E. Hanover
BTW, what
made me decide to do this was some reading I was doing on the forums.
The chamber in the base that holds the pressurized fuel also has an 1/8"NPT
outlet meant for a fuel pressure gauge. I was considering using this
for the pressure bleed connection. Some of the hot-rodders were
complaining about how the Aeromotive regulators weren't holding pressure
after the fuel pumps were shut down. They traced their woes to poor
machining of the steel ball's seat on the base's post.
Their spending
time and energy to clean up the machining to make a perfect seal, so I went
out and destroyed some beautiful machine work to make a terrible seal.
Go figure.
-----Original
Message----- From: Ernest Christley <echristley@att.net> To:
Rotary motors in aircraft <flyrotary@lancaironline.net> Sent: Sun,
Aug 14, 2011 6:38 pm Subject: [FlyRotary] Re: Dennis Haverlah Fuel
System...or any others, for that matter.
On 08/15/2011 01:27
AM, Ernest Christley wrote:
I removed the 4 screws, wrapped up the base in
some shop rags, leaving just the top of the post exposed, then used a
cut-off wheel in my Dremel to make a deep scratch in the steel ball's seat
on the top of the post.
I put it all back together, tested that the
pumps made 60psi, regulated it back down to 50, then watched the pressure
bleed off in about 5 seconds after I switched the pumps off. Then I
went in the house to recover from the gas fumes.
This method doesn't
add any weight, connections or extra hose, but is 100% effective at
relieving the fuel line pressure on shutdown.
BTW, what
made me decide to do this was some reading I was doing on the forums.
The chamber in the base that holds the pressurized fuel also has an 1/8"NPT
outlet meant for a fuel pressure gauge. I was considering using this for
the pressure bleed connection. Some of the hot-rodders were complaining
about how the Aeromotive regulators weren't holding pressure after the fuel
pumps were shut down. They traced their woes to poor machining of the
steel ball's seat on the base's post. Their spending time and energy to
clean up the machining to make a perfect seal, so I went out and destroyed
some beautiful machine work to make a terrible seal. Go
figure.
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