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Date: Fri, 04 Mar 2005 10:01:41 -0500
From: Ernest Christley <echristl@cisco.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: fuel cutoff valve necessary?
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rijakits wrote:

> Reaching?
>
>     ----- Original Message -----
>     *From:* Russell Duffy <mailto:13brv3@bellsouth.net>
>     *To:* Rotary motors in aircraft <mailto:flyrotary@lancaironline.net>
>     *Sent:* Thursday, March 03, 2005 9:53 PM
>     *Subject:* [FlyRotary] Re: fuel cutoff valve necessary?
>
>     When your electric system starts to act up, you might not be able
>     to shut down your pumps! You still can stop your engine by cutting
>     the fuel! 
>      
>     You're reaching a bit here.  A fuel pump is an on/off device. 
>     It's not like we have a bank of CPU's running the pumps :-)  Not
>     at all!! I fly quite a few hours every month in a machine that has
>     an electric clutch engagement. It happens quite often that
>     something in that installation goes dead (microswitches in the
>     tension adjustment system) and the system stays "on" or "off" -
>     point is, it can keep a circuit open/closed - eg. your pumpswitch
>     brakes and the pump stays on via the relay (if so installed) or
>     what if the master solenoid "freezes" - happens also quite a lot.
>
Now we're comparing the reliability of a 5A-NC electrical switch to a 
fuel valve and several connections which I have to fabricate.  I trust 
the valve.  There is a great chance of a goof-up in a one-off fabrication.

>     But keeping it to a part that is not really a shut off valve, like
>     the fuel pump, no way! What if the pump shortens out/gets
>     hot/leaks/etc. I want to be able to cut the fuel at the source! 
>

A fuel pump draws less than 5amps and is a two wire device.  No need for 
relays or such.  Just a large, solid interrupter switch.  Heck, even an 
exposed piece of the hot wire that you can yank in an emergency would be 
sufficient.  If the pump shorts out, cut the wire to it.  Our pumps are 
cooled by the fuel going through them, and we have no way of knowing 
that they're getting hot.  Leaks?  Each connection and additional device 
dramatically increases your chance of a leak.  That's why I'm proposing 
that leaving the shut-off valve on the ground can be safer.

>     What if your fuel valve connections leak, or the shaft of the
>     valve?  Then you're sucking air, which is a much more ominous
>     problem.  If it is just used as a emergency shut off, there should
>     be no leaks as you hardly ever move it.
>     My point is, I want a system that I can shut down no matter if I
>     have electric power or not or if it is shortening out somewhere.
>     Have a look at the shut offs Robinson uses, never saw one leaking.
>     Though the tank drains start to leak frequently, especially when
>     they get rather dated....
>

The lesson I've learned from a lot of wizened souls is that in an 
airplane EVERYTHING moves.  Adding unnecessary complication provides for 
more moving parts which translates directly to a higher chance of 
failure.  The safest systems are usually the simplest.  I believe the 
robustness of a the rotary is a direct result of its simplicity.

Every connection introduces a chance for a goof-up.  Comparing our 
installations to Robinson or any other production system is pointless 
without accepting the caveats that we are not in production.  It is 
assumed that they have extensive engineering and testing resources on 
multiple installation, and the expensive tools to make every 
installation exactly the same.  We, for the most part, are winging it on 
one-off installations in our garages.  It's an apples and orange 
installation that must be taken with a large grain of salt, and don't 
curse your neighbor until you've flown a pattern in his plane.

Ernest (I'm sorry.  I'll stop now)