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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)
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