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Date: Mon, 20 Sep 2004 10:42:20 -0400
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From: "Jerry Fisher" <jeremyfisher@verizon.net>
Subject: RE: [LML] Re: HUDs
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Skip Slater asked, "Do you think BAE will ever be able to develop a small
enough, light enough
and affordable HUD for GA aircraft?"  I used to work for Marconi, now part
of BAE, on HUDs (including briefly the AA one); The simple answer is that I
don't think any of the large avionics companies will be interested.  Their
overhead rates are too high, and they would run a mile from the threat of
lawsuits.  However it could certainly be practical to create a HUD for our
experimental aircraft within certain constraints.

There are two main types of HUD, cursive and raster.  Most of the HUDs that
you see on fighters and airliners are cursive, which means that the electron
beam in the CRT draws continuous lines in the phosphor to produce symbology.
As a result the symbology is bright enough to be visible even in sunlight,
which is just as well, as no more than 30% of the light is reflected from
the combiner to the pilot. Night vison HUDs use raster imagery, from a FLIR
or camera, which is like your TV set.  The symbology may then be either
raster or cursive, depending on the depth of your pocket.  For obvious
reasons, raster is not nearly as bright, and cannot be viewed in sunlight
under normal circumstances.  The problem for GA aircraft is not only the
HUD, but also the symbol generator.

The problems can be summarized as:

1.  Symbol generation.  EFIS systems use raster, so it would be more
difficult and expensive to add a cursive symbol generator, as opposed to
using an output from the EFIS.
2.  Display generation.  CRTs are brighter, but they are expensive, and they
require a lot of space and power (high voltage at that).  The ideal is LCDs,
either reflective or transmissive, but they are not as bright, and they can
only show raster imagery and symbology.
3.  Optics.  Military and airline HUDs use fairly sophisticated (read,
expensive) optical systems to minimize aberrations.  We have to collimate
the imagery so that the pilot does not have to refocus his eyes to read the
symbology.
4. Space.  Conventional HUDs are fairly large, and you have to design the
instrument panel, coaming and often the canopy around them.  Only a flat
panel based system would be small enough to fit reasonably easily in the
average GA cockpit.

In my opinion, the best compromise for our use would be a flat transmissive
LCD panel, with a simple lens system, driven by an output from an EFIS.  To
make the symbology visible in sunlight, we would have to either use a dark
filter on the combiner, or a special frequency-specific coating on the
combiner, or both.  It can be done, but it would need someone like Chelton
or Blue Mountain Avionics to pick it up, to provide the symbol generation.
The BAEs of this world are unlikely to get involved.

Jerry Fisher