REFLECTOR: Fuel pump cooling

[Bob Jackson (Jax Tech)]

Scott,

We also run almost exclusively LOP with our (custom and manual wastegate)
turbo IO-550N, so we should have a lot of the same parts as you do.

However, I haven't seen the problem you describe.  Our engine is always
perfectly willing to settle into solid LOP settings and operation after
long, hot climbouts.  We're using dual Lightspeed electronic ignitions, and
I know that facilitates trouble-free LOP greatly.  Our engine is neither
picky about very low LOP fuel flows, or very high LOP MAPs.  And adding
extra MAP really seems to help improve already good LOP cooling.  I
typically use 31-32" MAPs at 2500 rpm and 15-17 gph, and on the other end,
26-27" MAP at 2200 rpm and 12-13 gph for my LOP cruise settings.  The
'oversquare' MAPs really seem to help cooling without affecting power, which
seems mostly determined by the fuel flow.  

The two related issues we have seen, however, WRT to the fuel pump may
provide some help.  First, starting at 18,000 ft and above we always turn
our Boost Pump on LOW -- without it, the engine fuel pump seems unable to
provide steady fuel flows and surging and instability can result.

The other time we have trouble with the fuel pump is on Hot Starts.  When
the engine is really heat soaked, even once you get it started and
apparently running smoothly, a half minute or up to a full minute later, all
of a sudden, the engine will try to quit, and can only be 'saved' by a
temporary slug of High Boost.  Apparently this is being caused by 'latent'
air pockets that are still trapped in the hot engine fuel pump that require
High Boost to help clear them out.

Later, maybe we should start a thread again on Hot Start techniques for the
big bore Continentals.  It's been almost a year now since the Reflector last
visited that topic and I think I've finally decided on about the only Hot
Start procedure that really works consistently for me when the engine is
really heat soaked -- but let's save that for a later and different thread
discussion!

Bob 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Scott Derrick
Sent: Tuesday, August 28, 2012 9:21 PM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Fuel pump cooling

I've had this happen a few times this summer on flights longer than 2.5 
-3 hours.  Ran all winter with no problem so I think its heat related.

I typically run LOP, better mileage, lower temps,less pollution.

I can easily setup LOP when reaching my cruise altitude. I rarely have 
to re-adjust it after getting in the desired setting.

When its really hot, say 15C+ at 11500, after a couple hours the mixture 
seems to wander just a little bit and I have to periodically re-adjust 
to get it back.  We are talking less than 0.1 GPH change.

Once I get the engine really heat soaked after a few hours of 
flight(when the LOP starts to be work to maintain) and then stop for 
gas, take off climb to altitude, now its really hard to get it to run 
LOP?  It over  leans, then under leans.  If I then run it for awhile ROP 
and let things cool down from the climb, I can get it to run LOP but I 
still have that intermittent wandering mixture. Let the engine cool down 
over night, next morning LOP is easy again with a engine starting from 
cold.  I've had a few departures after it sat for a couple hours after 
landing and it leans easy then too. Only those hot fast turn arounds 
exacerbate the issue.

I noticed the T210 that uses this same engine has a cooling duct running 
to the fuel pup which is shrouded.  I don't have this. I'm thinking 
maybe it takes summer temps and a couple+ hours to get the fuel pump 
really hot enough to, I don't know, loose it mind?

The turbo setup on this version TIO520 is very sophisticated.  Once you 
set the throttle you can climb, descend and change prop rpm and the MAP 
doesn't move at all. Very easy to fly.  My fuel pump is from that 
original turbo setup.

Lycomings use an entirely different fuel pump so I don't think their 
setup would apply.

Any body else see this problem thats running LOP? Is your fuel pump 
cooled?  This is a 1986 vintage 520, with I think a similar fuel 
pump(carbon vane) is used on later models.

Scott

-- 
What the superior man seeks is in himself; what the small man seeks is 
in others.
Confucius