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Date: Sun, 27 Jan 2013 20:45:41 -0500
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Subject: Re: [LML] Also, does the Primer work?
From: Paul Bricker <pbricker@att.net>
Thread-Topic: [LML] Also, does the Primer work?
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George,

Questions I also had while building my ES.

First, there is a bypass circuit on the engine driven fuel pump. That's
how the electrical pump can get fuel to the engine when the engine driven
pump fails.

The priming works by just forcing fuel through the spider and injectors.
It pools in the intake until it runs out the overflow ports (and on the
hanger floor, tarmack, nose wheel, etc.)

I had a document on the flow rate/pressure for the Dukes pump in both Low
and High modes, but I can't find it now. As I remember in the low pressure
mode it would provide more fuel then I needed for cruise LOP, but not max
power at sea level. Since this was to allow we to get somewhere to land I
considered it sufficient. I ran an IO-550N. If you're running TSIO you may
need to plan on reduced power.

I originally wired my AC with the LOW-OFF-HIGH switch, and went to OFF-LOW
later with a mom PRIME button. Too much fuel forced into the 550 can make
it stop running, so a momentary HIGH seemed safer to me. From what I've
read one failure mode of the altitude compensating fuel pump can be very
dangerous. There are threads on this in the archives.

In the ES at least HPAT is (was?) training to take-off with LOW boost ON.
It always made sense to me. I don't know about other Lancairs.

Paul Bricker

On 1/27/13 10:30 AM, "George Wehrung" <gw5@me.com> wrote:

>As a continuation to my previous post, referencing the build manual
>chapters 14 and 23:
>
>Chapter 23: wiring:  denotes a Single Pole Double Throw rocker switch for
>the LOW-OFF-HIGH positions of the fuel boost pump.
>
>Chapter 14: Firewall Forward, depicting the fuel flow does not show a
>separate routing for the fuel primer circuit.  So how does the primer
>function work?  Is it merely pushing a higher volume of fuel through the
>circuit?  I know it's necessary for hot starts to quickly circulate the
>cooler fuel to the manifold but does it somehow bypass the gascolator and
>the engine driven fuel pump?  This would make sense if it did as some
>aircraft call for pushing in the primer and possibly holding it in to
>keep the engine running in the event of an engine driven fuel pump
>failure.
>
>So then why do I only have a low selection of the boost pump?  Should I
>replace my boost pump switch with a three-position LOW-OFF-HIGH switch.
>Should I have the boost pump on LOW for takeoff, descent and landing?
>
>Thanks again
>
>George
>
>
>
>
>--
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