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Not to beat a dead horse here, but I still have some concerns/problems
with the fuel delivery issue. The Eggenfelder Subaru group has had
a second forced landing (controlled crash) due to what is believed to be
vapor lock. They have identified a couple of problems that I think
may cross over to rotary installations and would like someone smarter
than myself to comment.
First, it appears that at least one of the incidents resulted from the
fuel pumps/gas collator/lines getting hot enough to cause vapor lock at
the inlet of the pump(s). The a/c was running mogas at the time of
the engine stoppage. One solution appears to be to shield and cool
the pumps and related plumbing with fresh air.
Once this happened, it led to the second shortcoming (I have experienced
this with my 20B installation during ground runs). Once the system
had vapor downstream of the pump(s), the efi pump couldn't force it past
the pressure regulator. This can be the result of either vapor lock
or running a tank dry. Their recommendation here is to install a
bleed orifice (approx. .020") around the pressure regulator.
Now, the Subie guys are contemplating a re-design of their fuel
systems. I'm wondering now if I should make any changes to my fuel
system (2 wing tanks, 6-port Andair valve, dual EFI pumps mounted on
f/w)? Comments would be welcome, especially from those that have
been flying for a while.
Mark S.
At 07:42 AM 5/26/2004 -0500, you wrote:
Tracy,
Do you have any pictures or can you sketch a diagram of your
system. How many high pressure pumps do you have in your
system. As I understand it, you have a facet pump and one or two
high pressure efi pumps. What will you change (if anything) for
your RV-8?
Perry -- How is your sump supplied... gravity?
Mark
At 05:20 AM 5/26/2004 -0700, you wrote:
Very good to bring this up Mark. I
think this is not an issue with the fuel pumps inside a sump tank, the
arrangement I have. If the sump tank runs dry, switching to the other
tank refills it. The sump tank must have it's own vent line.
Tracy Crook wrote:
Good point Mark. This sounds
like another good argument for the arrangement that I use.
Always draw & return to the same tank. Refill from the other
tank when necessary using a low pressure facet fuel pump. This
eliminates the need to ever run the tank in use low and risk running dry
(unless you are really OUT of gas). No fuel selector needed.
Simple plumbing. Only requires a single return port.
Eliminates that nervous moment when you wonder "Should I switch
tanks now or use that last gallon of fuel in the tank & risk running
dry".
Tracy
----- Original Message -----
From: Mark Steitle
Sent: Tuesday, May 25, 2004 4:12 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Fuel Pump Design
Fuel system design hasn't come up in quite a while. But since a
large portion of accidents are fuel related, I thought this would be
worthy of discussion.
I have experienced a failure mode during ground runs on my Lancair ES
with 20B that seems to be a problem with other types of installations as
well. I recently read a first flight report by a professional test
pilot, David Allen, that almost had to set a Lancair ES, N711RG, down on
I-70 due to the inability to restart the engine after running a tank
dry. (See
www.geocities.com/daveandjj
for the full story.) This was a certified fuel-injected engine installation. Also, the Subaru guys (I have been lurking on the Eggenfelder Subie site today) had a similar situation, resulting in some glider time and a dead-stick landing.
The problem is that after exhausting the fuel in one tank an airlock forms between the fuel pump(s) and the fuel pressure regulator that the efi pump cannot overcome due to lack of fuel in the supply line. This is what I have experienced with my installation on ground runs. I can't get the efi pumps to re-prime unless I momentarily break a line loose between the pump outlet and the pressure regulator. As soon as I do that the pump will re-prime and all is well.
For the record, I have two of Tracy's efi pumps, with two efi filters, hooked in parallel. They are mounted low on the fire wall. Upstream is a gascolator and an Andair 6-port selector valve. Downstream of the pumps/filters is a map sensing pressure regulator. There is a -6AN going to the fuel rail, and a -4 return line back to the Andair valve/tank.
I was curious if anyone else has had this happen and what they did to resolve the issue. I have come up with two possible solutions.
1. Tee off the pump output and put an electric solenoid valve in the line and tee the outlet of the solenoid to the fuel return line, past the regulator. Push-button switch on panel would activate the solenoid. If needed, a second or two of activation should relieve the air-lock and allow the pumps to re-prime.
2. Install a bleed line around the pressure regulator with a small metering orifice (.020-.030") that would bleed off any air that might get trapped. (This appears to be the solution the Subaru group is focusing on). This would be a full time bypass. This seems to be the simpler solution.
3. Install a low pressure, high volume fuel pump in one, or both, of the wing tanks. Procedure would be to always keep reserve fuel in this tank.
Any comments from seasoned flyers would be welcomed.
Mark S.
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