Message-ID: <390086A4.6FDD2102@regandesigns.com>
Date: Fri, 21 Apr 2000 11:49:40 -0500
From: Brent Regan <brent@regandesigns.com>
To: "Rumburg, William" <william.rumburg@cdicorp.com>
CC: Lancair List <lancair.list@olsusa.com>
Subject: Re: Magnetos
References: <FDD045B6B83FD311A9880090273053E20B6713@CDIM-PTS-MAIL>
Mime-Version: 1.0

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"Rumburg, William" wrote:

>         OK...I have to speak up....
>            Magnetos are not invincible, they can and do fail. In fact, they
> fail about once every thousand hours on average. What saves you is that
> there are two of them, with no common mode, so the chance of two failing at
> one time is 1/1000 x 1/1000 or 1/1,000,000.

Sounds like pretty good odds to me, Bill.

>
>            I have dual electronic ignition, each with it's own timing
> mechanisim plugged into a magneto drive. They are as completely independent
> as magnetos, except for a common voltage source. A small, dedicated battery
> eliminates failure due to a single voltage source. It's charge is maintained
> by connecting it to the charging system through a diode. The electronic
> ignitions are then switched to the dedicated backup battery with a simple
> Emergency Power switch, whose operation is checked before flight.

If I were you I would put in two switches and two backup batteries to eliminate
single point failure. If your emergency battery is the same chemistry as your
primary then the 0.6 volt drop across the diode is preventing the emergency
battery from charging fully. May I suggest you do a ground endurance test on the
emergency system to be sure that you have at least 20 minutes of standby power.

>
>            Electronic ignition's hotter spark ignites the flamefront more
> positively than the weaker magneto spark. It sets to 0 degrees for starting
> and senses both RPM and manifold pressure to adjust across the wide range of
> optimal spark advance for aviation conditions.

Another myth about SSI. If you light a stick of dynamite with two matches
instead of one does it make twice the boom? It takes 100 milli-Jouls of energy
at 28KV to ignight an air fuel mixture at near stochiometric. I have tested this
on an engine dyno. More than that and you don't get any more horsepower. Much
more than that and all you are doing is eroding the plugs and creating
additional EMI. You will need more if your mixture is way off (like during a
throttle back glide on final). The typical magneto produces 150-180mJ.

>            Magnetos are dinosaur relics of the 1930's.

As are the engines, light bulbs, threaded fasteners, the wheel.....Just because
something has been around for a while doesn't make it obsolete. Just because
something is "solid state" doesn't make it better.

If I told you that there were 1100 horsepower diesel engines that were used on
airplanes to cross the Atlantic, on regularly scheduled flights, in the '30 you
may think me a fool. If I went on to tell you that they had twice the fuel
economy and weighed less per horsepower than "modern engines" and had 500 hour
TBOs you might think me a liar.

The engine is the Junkers Jumo 208 and everything I said is true.

>
>            Given all this, you could just as well sit around and worry about
> a Martian Spaceship catching you in a tractor beam.

The Martians are no threat, it is those damn Romulans you have to watch out for
;)

> A retired Lear Jet pilot
> once told me that everything isn't redundant "God only gave you one heart".

True enough, but even the human heart has two ignition systems. There is a
backup natural pacemaker in the heart that takes over if the primary one is
damaged.

You don't "need" all this redundancy, but you will sleep better.

Regards
Brent Regan

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