REFLECTOR:Fire Detection System

[Jim Agnew]

You will probably need more than 3 sensors, read on 

Jim

My system is very simple and consists of components
available from DigiKey (WWW.DIGIKEY.COM).  It is a POSSIBLE
FIRE WARNING SYSTEM!  System failure could produce a false
indication.  It should be used in conjunction with a
verification plan, in my case start a sharp turn left and
look for trailing smoke.

It consists of two sections, the indicator is a flashing
LED (with a built in flashing circuit) in series with a
current limiting resistor (limit is about 15ma) between the
plus 12V and the LED.  The detector section consists of 8
Thermal Cutoffs in series 

(See
http://www.maco.panasonic.co.jp/www-data/pdf/AFA0000/AFA0000CE1.pdf)

that are wired to ground and to the resistor-LED junction. 
This shorts circuits the current flow and keeps the LED
off.  The constant 10-15ma current flow is a closed circuit
alarm loop and breaking any part of the circuit results in
the LED flashing.  The Thermal Cutoffs are nothing more
than one time thermal fuses and must be replaced when
blown.  I can't remember, however, I used either 145 or
168C rated Thermal Cutoffs.

I looked at the possible liquid fire possibilities in the
cowling area and at high & low airflow conditions and
decided that there are two logical conditions.  At low
airflow the liquid would flow forward to the engine
bulkhead and burn there, at high flow it would flow toward
the prop and form a blowtorch like condition.  I then
decided to use 4 Thermal Cutoffs in the front and rear of
the cowling at the 2-4-8-10 O'Clock positions forward and
aft.  The forward ones are midway between the engine
bulkhead and the engine (basically in line with the mags)
and the rear are just in front of the rear cowl reinforcing
hump.  I used 22GA wire and soldered the Cutoffs in line
and covered the wire with one fine bid leaving the Thermal
Cutoffs exposed and glued in place with a little RTV.

I can draw you a schematic and parts list if you like. 
There is a more elegant solution using thermal snap
switches that are self resetting and look like tab mount
transistors, however they are about $15 each rather than 50
cents each.

*** NOTE *** without a fuel cutoff valve at the sump tank
any warning system is useless. You must be able to
positively cut of fuel flow to the engine since a
windmilling prop will just continue to pump fuel!!!
Additionally I don't have any fuel filters etc. (fuel
collecting devices) inside the cowling they are all inside
the plane.

Jim



--- Don19dw@aol.com wrote:
> Thanks to Mack Murphree, for the FAX info on his system.
> Mack's system is a 252degreeF heat sensor on the engine
> side of the firewall 
> with a relay, push to test button, green light for system
> OK and a red light 
> for WARNING.
> He has 2 separate systems with 2 separate sensors for
> redundancy. (Hope these 
> details are accurate) Mack says they determined max
> firewall temp(during 
> climb)is 165degreesF.  Mack, is your normally closed
> sensor a fuse or will it 
> reset??
> 
> My idea is one system with 3  thermal fuses in series,
> one located near fuel 
> servo,
> and one attached to each wing root aft nearest rear
> exhaust pipe. Cowlings 
> could be removed without  having to disconnect system 
> wires. At my Radio Shack 
> the lowest temp thermal fuse is 262degreesF, part # 8125.
> Has any one measured 
> max cowling temps at these 3 mentioned areas?  For any
> builders that have a 
> system installed, what temps are your fuses? 
> Inputs welcome. 
> Don White   N19DW    XL/RG
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=====
James F. Agnew
Jim_Agnew_2@Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying