Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Tue, 11 May 2004 18:43:16 -0400
Message-ID: <redirect-3227429@logan.com>
From: Jeffrey Peterson <jbp@fire.phys.cmu.edu>
Subject: led tail strobe.
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII


LML:

I posted new photos of the LED tail strobe, now with the circuit 
completed, at 
http://w1.lancair.net/pix/led-strobe

i also posted the schematic.

the particular component choices give a 1 second period and 
50% duty cycle.  tinker with values to get your favorite pattern

i used monolithic ceramic capacitor because they are small, but
they do have a bit of variattion of C with temp.  might be 
better to use something more stable.

The circuit works quite well, but after some soul searching I have 
tenatively decided NOT to put the system on the LNC 360.

reasons:
-its heavy...235 grams. Installing the strobe will require more than 3 
times its mass added to the rudder counterweight. perhaps a kilogram 
total.   all this at the maximum rearward moment arm. yuck.

-it gets quite warm.  after aminute or two the temp is maybe 40C at the 
base of the LEDs with a fan pushing air past at about 10 MPH.  in still 
air it overheats. if i forgot to turn it off on landing,
I could possibly slag my rudder.

-the system satisfys the FAA Regs, but with no margin...I prefer a 
safety factor 2. 

-its a complicated system.  lots of machining. tight fits. odd angles
no finesse.

it just doesnt seem like a bulletproof system to me, so i may just chalk 
this one up to experience and develop plan B.

Plan B:   get a bunch of 5 mm  diameter superbright white leds and
mill the hemispherical ends off the plastic cases.  this will give them a 
lambertian pattern. no need to polish the ends, might even sandblast them
to make the pattern even broader.

glue these into holes drilled in the curved trailing 
edge of the wingtip.  i will need many more of these LEDS than with 
luxeons, but that will help distribute the thermal dissipation.

the weight might actually be lower this way, than with luxeons since the 
aluminum heat sink will no longer be needed.  and the weight will be 
near the center of lift, and will not need a counterweight.

-Jeff