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Date: Sat, 06 Jan 2007 11:02:28 -0500
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From: "Hamid A. Wasti" <hwasti@starband.net>
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Subject: Re: [LML] Heated pitot and static
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peter.sokolowski@gmx.net wrote:
> 3. In case the heat is to much, I am thinking about putting them in serial, reducing the heat by ca. half - is the glass capable with this at least? - I will fly "only" VFR and install the probes for "in case".
>   
If you put the two devices in series, the current through the pair would 
be half the current of a single device (twice the resistance, same 
voltage).  Each device will see half the voltage it used to see, since 
the voltage is being divided between the two devices.  Half the voltage 
multiplied by half the current means a quarter of the power, not half.  
If you are willing to live with a quarter of the power, that should 
work.  However, I should caution you that putting devices in series like 
this works only for purely resistive devices like heaters and bulbs.  It 
will not work for active electronics like radios, strobes, trim etc.  It 
is also important to point out that when you put things in series, you 
reduce your reliability at the system level because a failure in one 
device will cause the other device to stop working as well.

That said, I would join several other posters in questioning the basic 
premise for the need for a heated static port.

Changing topics and pulling out my soap box, a point that I find 
disturbing in the post is the statement: install the probes for "in case".

What exactly does "in case" mean?  You either need something or you 
don't.  If you don't need it, do not install it.  If you need it, then 
you need to do a good enough job to make sure that it will be there when 
you do decide to use it and will perform as expected.  The notion that I 
really do not need to have it so I will do some half baked job that may 
or may not work is crazy.    Not only does that give you a false sense 
of security, it may lead to a real decrease in safety if you start 
counting on it, which is inevitable.  If it is going to be on your 
airplane and is for anything other than creature comfort, treat it like 
it is the one thing that stands between you and a certain death and 
design and install it with the due diligence.

Regards,

Hamid