From: prang@ssc-vax.boeing.com (Ron Wanttaja)
Newsgroups: rec.aviation,sci.aeronautics
Subject: Simulator Flight Models (was Re: PC based simulators)
Date: 18 Aug 92 21:09:44 GMT
Organization: Boeing Defense and Space Group, Seattle WA

In article <1992Aug17.175303.10716@cactus.org>, rdd@cactus.org (Robert Dorsett) writes:
> 
> In the meantime, the "upper-end" simulators are starting to slug it out.  
> "Flight Simulator" has never been a training product: its "realism" is mainly 
> marketing pap for the masses.
> 
> ... detailed flight models are all but irrelevant in non-motion, 
> non-tactile-feedback-based interfaces.

Indeed.  It's interesting to note that the first "true" flight simulator,
the Link models C (Prewar civilian) and ANT-18 (WWII standard trainer), 
modeled their flight via a vacuum-operated analog computer.

Not to mention that the "flight characteristics" of these units bore no
relation to the actual motions of a real aircraft.  For instance, the
artificial horizon was implemented by a pendulum device... Yet the simulator
still had a conventional turn and bank indicator.  What that meant was that
if the simulator were banked, you couldn't center the ball!  Here's the
typical sequence of a qualified-pilot yet Link neophryte turning left in a 
model ANT-18:

1.  Stick goes hard left.
2.  Wing drops until the unit hit the stop.
3.  Nose starts swinging slowly RIGHT.
4.  Left rudder pressed.
5.  Unit starts swinging left, but left wing starts lifting
6.  Stick goes hard left again.
7.  Left swing slows, but continues.  At this point, the turn needle shows
    about a standard-rate left turn, with the ball in the lower left side.
8.  Harder left rudder to try center the ball.  Left wing starts to lift
    again.  Eases off on rudder.  
9.  Target heading reached.  Stick goes hard right to level the wings, and
    the rudder is neutralized.
10.  Turn stops.  Left wing remains down.
11.  Stick remains hard right.  Left wing remains down.
12.  Instructor picks up mike.  "Lower the nose to gain speed, then raise
     the wing with left rudder."
13.  Eventually the pilot does so... and wings return to level.

At first, I thought this inability to raise the wing was due to low vacuum
to the bellows.  Yet the under-nose of the fuselage had a rounded fitting
to allow this "dive-and-level-wings" techique, and another, similar Link I
flew had exactly the same characteristic.  When flying these aircraft, you
would make all turns by rudder, only using ailerons to counteract the
tendency for the rudder to pick up the wing in the direction you wanted to
turn.  In this way, you'd make picture-perfect turns using the
turn-and-bank... but the artifical horizon would show the wings
relentlessly level.

In my CAP days, where me and a batch of other cadets had taken on the
restoration of an old wartime Link, this worked to our advantage.  When we
got almost finished, some Senior members showed up to try it out... obviously
with the thought of taking it over from the "kids".  Eventually, every one
of them got stuck with a wing down, with the vacuum motor whining like mad
as they tried to level off.  "It's working just the way it's supposed to,"
we tell them.  They left red-faced, and we weren't bothered again.  But we 
still didn't know *Why* it worked that way.

Enlightenment came just a few years ago.  A PIREP on a Curtiss Jenny
reported EXACTLY the same characteristic... that an excessively dropped 
wing had to be picked up with rudder and extra speed.  Note that Ed Link
started working on the trainer in the late '20s and early 30's, and that
there was a good possibility he'd learned to fly in a Jenny.

Second, a re-reading of _Stick and Rudder_ found a passage describing how
some people (even pilots) erroneously thought airplanes flew.  This "wrong"
method exactly described the flight characteristics of the early Links.

Back on the subject of flight simulator models... I wrote a
commerically-marketed PC simulator in the early '80s (anyone remember IFR
FLIGHT SIMULATOR for the Commodore?).  Reviews by pilots endorsed the
realism of the flight model.  What they didn't know was that the model
merely consisted of two single-order equations.  The slowness of the
computer and the awkwardness of the Atari-stick interface hid the
simplicity of the model.

                                          Ron Wanttaja
					  prang@ssc-bee
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                                          prang@ssc-bee.boeing.com
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