In article ,
"Kyle Boatright" wrote:

"anaconda" wrote in message
...
I'm building an RV, and am considering using mogas, or at least having the
option of using it. Problem is that mogas has a higher vapor pressure
(among
other anomalies), and this in turn can cause vapor lock. Routing of the
fuel
lines is quite important, as well as the proper cooling of the gascolator
and insulating the firewall forward lines.

Even with all the above, risk of vapor lock still exists, especially when
the bird has been heat soaking on a ramp during a flying intermission.

Anyone ever consider the use of a Peltier effect device (like the ones
used
in those solid state coolers, or CPU coolers) to cool down the fuel as it
passes through the cockpit (lowest ambient temperature... lower than FWF
anyway).

Haven't done the BTU math yet and was wondering if anybody else is going
through this same exercise.

This device could be used when the risk of vapor locking is high (high
altitude, temperature etc.)

Comments?

Comments? Certainly...

I built and now fly a 160 hp carburated RV-6. I always run 100 LL, and have
starting problems I attribute to vapor lock on hot days when I do a medium
length stay on the ground - say 20 to 45 minutes after flying for a while.
I think the starting problem would be much worse using mogas. I've never
had a problem with vapor lock once I got the plane started.

I've read the same thing you have about mogas - it typically has higher
vapor pressure, which is compounded if you happen to get some winter
formulation mogas with higher (winter) vapor pressure during the warm
months. Given my experience and flying patterns (I typically cruise at
10,000' or more during the warm months), I can't see me putting mogas in my
airplane during the summer months. Too many opportunities for trouble.
However, if I lived in a cool climate, or didn't plan on flying high, I
might use it in the winter.

If I was going to use mogas, I would consider a blast tube on the mechanical
fuel pump, plus I'd do everything possible to minimize heat build up inside
the cowling. Additionally, I would follow the EAA mogas STC's plan of
avoiding sharp bends in fuel lines. Beyond that, I might add a recirc line
or purge valve to purge the fuel lines firewall forward before a hot start.


The above precautions are just good design/building practice and should
be a part of the plane in the first place. Add a boost pump aft of the
firewall, to help keep a positive head on the engine-driven pump, too.

IMHO, the purge valve might ad more problems than it solves, in case of
valve failure.

As a side note, the Varga has the most cockamamie (and certificated)
fuel plumbing under the cowl that I have ever seen.


You should be able to find a pretty good write up on mogas STC's using
google. There is some good information out there, particularly at the EAA
site, IIRC.

KB

--
Remove _'s from email address to talk to me.