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|
<html>
<head>
<link rel="stylesheet" href="acmdoc-styles.css">
<title>ACM - Inside the program</title>
</head>
<body>
<h1>ACM - Inside the program</h1>
<p align=right>
by Riley Rainey
<br>updated by Umberto Salsi
</p>
<h2>Contents</h2>
<a href="acmdoc.html">Back to general contents</a>
<blockquote>
<a href="#geodeticcoordinatesandthewgs84ellipsoid">Geodetic coordinates and the WGS-84 ellipsoid</a><br>
<a href="#definingnewzones">Defining new zones</a><br>
<a href="#definingnewscenes">Defining new scenes</a><br>
<a href="#definingnewaircraft">Defining new aircraft</a><br>
<blockquote style="margin-top: 0; margin-bottom: 0;">
<a href="#frameandaerodynamics">Frame and aerodynamics</a><br>
<a href="#engine">Engine</a><br>
<a href="#landinggear">Landing gear</a><br>
<a href="#armament">Armament</a><br>
</blockquote>
<a href="#definingnewobjects">Defining new objects</a><br>
<a href="#ieee12781discompliance">IEEE 1278.1 (DIS) compliance</a><br>
<a href="#suggestedfurtherreading">Suggested further reading</a><br>
</blockquote>
<a name="geodeticcoordinatesandthewgs84ellipsoid"></a>
<h2>Geodetic coordinates and the WGS-84 ellipsoid</h2>
<p>
The WGS-84 defines a reference shape for the Earth which is very close to the real shape of our planet; the concepts of local vertical, local horizontal plane, altitude and longitude are all referred to this ideal shape named <i>geodetic ellipsoid</i>. The figure below illustrates the right-handed Cartesian reference centered on the geodetic ellipsoid:
</p>
<center style="margin: 2em;">
<img src="acmdoc-ellipsoid.svg">
<br><b>The WGS-84 geodetic ellipsoid. In this figure, P=[x,0,z].</b>
</center>
<p>
The surface of this ellipsoid are the solutions of the equation
x<sup>2</sup>/a<sup>2</sup> + y<sup>2</sup>/a<sup>2</sup> + z<sup>2</sup>/b<sup>2</sup> = 1 where "a" is the length of the major semi-axis (or equatorial semi-axis) and "b" is the length of the minor semi-axis (or polar semi-axis):
</p>
<blockquote>
a = 6378137.0000 m<br>
b = 6356752.3142 m
</blockquote>
<p>
These values, although very close to the real size of the Earth, are by definition of the WGS-84 model. Although very accurate measurements of the Earth are available and the actual shape of the Earth, named <i>geoid</i>, is known with very high precision, such an accuracy does not make much difference in most practical applications and is negligible. So in general any measurement of latitude, longitude and altitude, if not otherwise stated, is to be intended as <i>geodetic</i>, that is refereed to the WGS-84 ellipsoid.
</p>
<p>
The north pole is located at Cartesian coordinates [0,0,b] and the south pole is located at [0,0,-b]. The rightmost point [a,0,0] has latitude zero and longitude zero; the Greenwich meridian crosses this point joining the two poles.
</p>
<p>
For any point P=[x,y,z] in the Cartesian reference, a corresponding mapping to the geographic coordinates [lat,lon,alt] has to be defined. First, the <u>local vertical</u> is the line crossing P and perpendicular to the surface of the ellipsoid in its point A; the tangent plane to this point is the local horizontal plane at altitude zero. The local vertical line is the local plumb line. Note that the vertical line in this model, just like on our real planet, does not cross the center of the ellipsoid but intersects the x axis at some point B.
</p>
<p>
The <b>latitude</b> is defined as the angle between the local vertical and the xy plane. Calculating this latitude given the Cartesian coordinates is not simple; curious may look at the module <tt>src/dis/dis/earth</tt> in the sources of ACM. Latitudes range from 90S at the south pole, up to 90N at the north pole. The 0N latitude (or 0S if you prefer) is the equator.
</p>
<p>
The <b>longitude</b> is defined as the angle between the point P and the xz plane measured around the z axis: lon=atan(y/x). No surprises here. Longitudes range from 180 degrees west up to 180 degrees east; obviously the 180W meridian coincides with the 180E meridian. The 0E meridian (or 0W if you prefer) is named Greenwich meridian.
</p>
<p>
The <b>mean sea level</b> (MSL) is the surface of the ellipsoid; <b>altitudes</b> are measured as distance of the point P from the local horizon beneath it. All altitudes under ACM are intended referred to this MSL.
</p>
<p>
The <b>local terrain altitude</b> (measured vs. the MSL, of course) varies from point to point. ACM looks at the altitude of the nearest runway within 100 NM to determine the terrain altitude; if none found, it assumes zero. Negative altitudes are also possible.
</p>
<a name='definingnewzones'></a>
<h2>Defining new zones</h2>
<p>
A <b>zone</b> is a range of longitude and latitude bound to the <b>scenery</b> file where the items inside the zone are listed. Sceneries are dynamically loaded and un-loaded by the program as necessary as the plane moves around the globe.
</p>
<p>
As the program starts and initializes, it looks for a zones file named <tt>zones.txt</tt> normally located under the <tt>objects/</tt> directory. The zones files lists all the zones that are available to the program along with the range of geodetic coordinates the scene covers, for example:
</p>
<blockquote><pre>
# zones.txt file:
35N 40N 125W 120W zones/usa/sfrancisco.txt
30N 35N 120W 115W zones/usa/losangeles.txt
35N 40N 120W 115W zones/usa/lasvegaswest.txt
#35N 40N 115W 110W zones/usa/lasvegaseast.txt
30N 35N 115W 110W zones/usa/tucson.txt
30N 35N 100W 095W zones/usa/dallas.txt
40N 45N 075W 070W zones/usa/newyork.txt
35N 47N 006E 019E zones/europe/italy.txt
30N 35N 040E 045E zones/middleeast/iraq.txt
</pre></blockquote>
<p>
The Italy scenery file has been compiled by hand.
The Iraq scenery only contains some sample runways and it is otherwise incomplete.
All the USA sceneries have been generated automatically from the FAA data base using the the program available under <tt>tools/faaairports</tt>; further zones covering the USA territory can be added in the same way.
</p>
<p>
The image below summarizes the currently defines zones, but other might be added in future release and others can be added by the player itself:
</p>
<center style='margin: 2em;'>
<img src="acmdoc-zones.jpeg">
<br><b>Zones defined by the sample zones file contents above.</b>
</center>
<p>
As the simulated aircraft flies, any zone within 200 NM from the aircraft is loaded and made available to the simulation, so runways, radio stations and features therein defined enter the simulation: runways can be seen, radio stations can be tuned, etc. Zones left behind or too far away are un-loaded from memory.
</p>
<p>
The zones file is a simple text file organized by lines, each line representing a zone with 5 fields:
</p>
<ol>
<li>Minimum latitude covered.</li>
<li>Maximum latitude covered.</li>
<li>Minimum longitude covered.</li>
<li>Maximum longitude covered.</li>
<li>File name of the scenery as path relative to this file. Example:
"<tt>../../myscene.txt</tt>". The extension of the file does not
really matter. White spaces are not allowed inside the path.</li>
</ol>
<p>
Latitude and longitudes can be indicated in several ways, here are some
examples:
</p>
<blockquote><pre>
10N 10-20-30.400S 30.500S
10E 10-20-30.400W 30.500W
</pre></blockquote>
<p>
Fields must be separated by at least one white space. Apart from that, white
spaces are ignored. Empty lines and lines beginning with '#' are ignored.
</p>
<p>
Notes and restrictions
</p>
<ul>
<li>The program gives error is the latitude or longitude fields are
not valid or out of the range.</li>
<li>The program gives error if the file path of a scenery file cannot
be resolved. The existence and readability of each scenery file
is checked as the program starts; their content is checked only whenever
the scenery file is loaded.</li>
<li>The program gives error if any two zones overlap.</li>
<li>The right and upper edge of the range does not belong to the zone, so a new zone can safely be defined next to the other.</li>
<li>Items defined in the scenery of a zone must lie inside the range of
geographical coordinates of the zone. The points in the right and upper edges does not belong to the zone. The program gives a non-fatal error if an item lies outside the zone. For runways, the reference point is their center; for ILS, the reference point is the locator.</li>
</ul>
<p>
Note that for extended items, some of their points may lie outside the zone. For example, although the center of the runway must be in, one or both of its ends can be out; the locator of the ILS must be in, but the corresponding glide slope antenna can be out.
</p>
<p>
Note that the maximum latitude is 90N, the north pole; if a zone extends
up to this limit, its upper edge is a point. Due to the rules above, the north pole does not belong to the zone, and there is no way to put an
item at that exact location; this does not seem to be a severe
limitation, though.
</p>
<blockquote style="border-style: solid; padding: 0.5em">
<b>Don't try to fly over the poles!</b>
There is a know bug in the program that prevents to fly over the poles.
Near to these locations (below 1 NM), the computation of the aircraft's next step fails and the aircraft tilts unpredictably, mostly crashing -- look at the comments in the <tt>src/dis/dis/earth</tt> for more.
</blockquote>
<p>
There is no limit to the number of zones that can be defined. If each zone covers a 5x5 DEG range of latitude and longitude, considering only the range of latitudes [80S,80N], and since 70% of the Earth is covered by water, one may expect the maximum number of zone that might be needed to cover all the globe be about 692.
</p>
<a name=definingnewscenes></a>
<h2>Defining new scenes</h2>
<p>
The scene file defines the contents of a zone. It should be stored as a ASCII
text file. Every record occupies exactly one line, with leading and trailing
spaces ignored. Fields in a record are separated by one or more spaces or
tabulation characters. Empty lines and lines beginning with the character '#'
are ignored.
</p>
<h3>Team Locations</h3>
<p>
Friendly and Opposing DIS participants may have a resupply base where to get fuel, munitions a repairs. Servicing starts as the aircraft stops close enough to the specified location. Typically, these location will be positioned and oriented at the end of a runway at the player's air force base airport. Example:
</p>
<blockquote>
<pre>
# Friendly air force base:
TEAM1_LOC 32-58-18.798N 096-50-16.604W 0 0
# Opposing air force base:
TEAM2_LOC 33-58-00.000N 097-50-00.000W 0 0
</pre>
</blockquote>
Fields:
<blockquote>
0 Type: TEAM1_LOC, TEAM2_LOC<br>
1 Latitude<br>
2 Longitude<br>
3 Initial Altitude MSL (ft) (this value ignored)<br>
4 Initial Heading (degrees, true) (this value ignored)
</blockquote>
<h3>Ground Color</h3>
<p>
An X11-style <code>#<i>RRGGBB</i></code> color specification defining
the color of the ground. Red, green and blue color components are
hexadecimal value from 00 to FF. Example:
</p>
<blockquote>
<code>
GROUND_COLOR #305030
</code>
</blockquote>
Fields:
<blockquote>
0 Type: GROUND_COLOR<br>
1 Color
</blockquote>
<h3>Runway record</h3>
<p>
The record entry of the ACM scene describing a runway can take two forms,
depending on the available informations: the old one (RWY) and the new
one (RWY2). The RWY format requires the geographic coordinates of both
the ends of the runway. The RWY2 requires only the center of the runway
and its true heading. The syntax of the runway identifier (field no. 2)
must be of the form:
</p>
<blockquote>
<i>H</i>[<i>S</i>]<b>/</b><i>H</i>[<i>S</i>]
</blockquote>
<p>
where <i>H</i> is the heading (an integral number between 1 and 36)
and <i>S</i> is a capital letter ("L", "C" or "R"). The <i>S</i> part
of the identifier is optional.
A leading "0" in an identifier is allowed.
The runway identifiers can be specified in any order.
</p>
Example of the RWY record format:
<blockquote>
<code>
RWY ADS 15/33 644 7202 100 32-58-40.245N 096-50-25.820W 32-57-33.383N 096-49-56.608W
</code>
</blockquote>
Fields of the RWY record:
<blockquote>
0 Record Identifier: RWY<br>
1 Three/Four letter airport code<br>
2 Runway identifier<br>
3 Runway Altitude (ft, MSL)<br>
4 Runway Length (ft)<br>
5 Runway Width (ft)<br>
6 Near End Latitude<br>
7 Near End Longitude<br>
8 Reciprocal End Latitude<br>
9 Reciprocal End Longitude<br>
</blockquote>
Example of the RWY2 record format (same runway):
<blockquote>
<code>
RWY2 ADS 15/33 644 7202 100 32-58-06.81N 096-50-11.21W 160.0
</code>
</blockquote>
Fields of the RWY2 record:
<blockquote>
0 Record Identifier: RWY2<br>
1 Three/Four letter airport code<br>
2 Runway identifier<br>
3 Runway Altitude of the Center (ft, MSL)<br>
4 Runway Length (ft)<br>
5 Runway Width (ft)<br>
6 Latitude of the Center<br>
7 Longitude of the Center<br>
8 Runway geographical heading (DEG)
</blockquote>
<p>
The model of the terrain simulated by ACM is very simple and efficient:
the elevation of the nearest runway gives the local terrain altitude,
the distance being calculated relatively to the center of the runway.
Nevertheless, this behavior has a strange implication: between two runways
at different altitudes there is a <i>step line</i> where the local
altitude suddenly change. Crossing this step line taxing on the ground,
the aircraft would crash its landing gear.
</p>
<p>
In the figure below the terrain to the left side of the step line has
elevation 500 feet, while the terrain in the other side has elevation
510 feet. Unfortunately in this case we have a problem, since the step
line crosses both the runways.
</p>
<pre>
</pre>
<center>
<img src="acmdoc_html-runway_step.png">
<p>
<b>The problem of the step line crossing the runways.</b>
</center>
<pre>
</pre>
<p>
The only practical solution to this problem is to keep all the runways
of a given airport at the same exact altitude.
</p>
<h3>Navaid Record</h3>
<p>
A "NAV" record may be a VOR, a DME or a NDB. The TACAN stations of the
ACM world behave simply as VOR/DME stations.
VOR and ILS stations frequency ranges from 108.00 MHz up to 117.95 MHz.
</p>
<p>
OMARKER, MMARKER and IMARKER are just like NDB stations but with limited
range (20 NM rather than 100 NM). NDB station frequency ranges from 200
KHz up to 529 KHz. This latter upper limit is a rather arbitrary value
set in ACM, but most of the stations do not exceed 415 KHz.
</p>
Example:
<blockquote>
<pre>
NAV TTT VOR/DME 32-52-08.98N 097-02-25.81W 540 113.10 -
NAV FWH TACAN 32-46-17.46N 097-26-22.07W 663 108.7 024X
NAV RBD NDB 32-40-36.98N 096-52-15.91W 670 287 -
</pre>
</blockquote>
Fields:
<blockquote>
0 Record Identifier: NAV<br>
1 Up to four letters identifier<br>
2 NAVAID Type: VOR, DME, VOR/DME, VORTAC (=VOR+DME), TACAN (=VOR+DME), NDB,
OMARKER (=NDB), MMARKER (=NDB), IMARKER (=NDB)<br>
3 Transmitter Latitude<br>
4 Transmitter Longitude<br>
5 Transmitter Altitude (ft, MSL)<br>
6 VHF Frequency (MHz for VOR/DME/ILS stations and KHz for NDB)<br>
7 TACAN channel number, or '-' if not available. Ignored by ACM anyway.
</blockquote>
<h3>ILS Record</h3>
<p>
An ILS (instrumental landing system) is a LOCALIZER antenna (emitting the signal
giving the right heading toward the runway) that may include also a glide slope
antenna (GS, giving the deviation from the glide plane) and a DME antenna.
</p>
<pre>
</pre>
<center>
<img src="acmdoc_html-ils.png">
<p>
<b>Typical ILS arrangement around the runway.</b>
</center>
<pre>
</pre>
<p>
In the real world, each antenna has its latitude, longitude and elevation,
but under ACM the simplified ILS record does not allow to specify the
location of the DME antenna and provides only one elevation field whose value is
shared among the LOCALIZER, the GS and the DME antenna; the distance displayed
by ACM actually is the distance from the LOCALIZER.
The recommended workaround is to indicate the elevation of the GS antenna to allow
precise landing at the expected distance from the threshold;
if not available, the elevation of the DME antenna should be indicated for closer
match between displayed distance and the expected value;
finally, the elevation of the LOCALIZER antenna can be indicated.
Limitations and recommendations are detailed below.
</p>
<p>
<b>The LOCALIZER antenna</b> is normally located near the reciprocal end of the runway
and the signal is normally aligned with the centerline of the runway.
The exceptions are the LDA and SDF antennas which may deviate from the centerline
of the runway or even be completely unrelated with any runway at all.
</p>
<p>
<b>The GS antenna,</b> when available, is normally located near the side of the runway threshold.
The elevation of this antenna is critical in order to land at the specified
spot on the runway, so its value should be indicated when available.
</p>
<p>
<b>The DME antenna,</b> when available, is always assumed to be
coincident with the LOCALIZER antenna; there is no way to specify another
location, so the distance displayed in the program may differ from the
official navigation charts.
</p>
<pre>
</pre>
<center>
<table border='1' width='70%' cellpadding='5' cellspacing='0'>
<tr>
<th>Type</th>
<th>Description</th>
</tr>
<tr>
<td valign='top'>
LOCALIZER<br>
LDA<br>
SDF
</td>
<td valign='top'>
LOCALIZER antenna only available. LDA and SDF are handled just like
a LOCALIZER, the only difference is that the chart generator program
is advised a corresponding runway end may not be available for this ILS
and so no error should be emitted. The elevation field should be set
with the elevation of the locator antenna.
</td>
</tr>
<tr>
<td valign='top'>
ILS<br>
LOC/GS
</td>
<td valign='top'>
LOCALIZER and GS antennas both available. The elevation field should
specify the elevation of the GS antenna, when available.
</td>
</tr>
<tr>
<td valign='top'>
LOC/DME<br>
LDA/DME<br>
SDF/DME
</td>
<td valign='top'>
LOCALIZER and DME antennas both available. LDA and SDF are handled just like
a LOCALIZER, the only difference is that the chart generator program
is advised a corresponding runway end may not be available for this ILS
and so no error should be emitted. The elevation field should
specify the elevation of the DME antenna, when available, or the
elevation of the LOCALIZER otherwise.
</td>
</tr>
<tr>
<td valign='top'>
ILS/DME
</td>
<td valign='top'>
LOCALIZER, GS and DME antennas available. The elevation field should
specify the elevation of the GS antenna when available, or the
elevation of the DME or of the LOCALIZER (in this order of preference).
</td>
</tr>
</table>
<b>Types of ILS record.</b>
</center>
<pre>
</pre>
Example:
<blockquote>
<code>
ILS 30 ILS IAIW 108.9 34-18-47.161N 097-01-38.280W 34-17-59.533N
097-00-35.018W 729.2 4.92 315.5 3.00
</code>
</blockquote>
Fields:
<blockquote>
0 Record Identifier: 'ILS'<br>
1 Runway Identifier:<br>
2 ILS Type: see table above<br>
3 Four letter Identifier<br>
4 VHF Frequency (MHz)<br>
5 Localizer Transmitter Latitude<br>
6 Localizer Transmitter Longitude<br>
7 Glide Slope Transmitter Latitude ('-' for Localizer-only approaches)<br>
8 Glide Slope Transmitter Longitude ('-' for Localizer-only
approaches)<br>
9 Glide slope antenna elevation when available, or localizer antenna elevation
(ft, MSL)<br>
10 Localizer beam width (DEG)<br>
11 Localizer geographic bearing (DEG)<br>
12 Glide slope angle (DEG) ('-' for Localizer-only
approaches)
</blockquote>
<h3>Features</h3>
Use feature records to place objects in an ACM scene. Example:
<blockquote>
<code>FEATURE features/tower.obv 32-58-04.800N 096-50-16.800W 644 0
</code>
</blockquote>
Fields:
<blockquote>
0 Record identifier: FEATURE<br>
1 Object filename, either absolute or relative to the scene file<br>
2 Latitude<br>
3 Longitude<br>
4 Altitude (ft, MSL)<br>
5 Orientation angle (degrees, true)
</blockquote>
<a name=definingnewaircraft></a>
<h2>Defining new aircraft</h2>
<p>
The ACM program loads all the aircraft models defined in the <tt>aircraft.txt</tt> file; this file is searched in all the specified object directories. Lines beginning with <tt>#</tt> are ignored by the program and can be used to add comments.
</p>
<p>
The inventory file may contain several include and aircraft records. The include record has syntax
</p>
<blockquote>
<tt>include "some/other/file"</tt>
</blockquote>
<p>
and allows to include the content of other inventory files in a more structured way. The path can be relative to the current inventory file. The aircraft record is described below in detail.
</p>
<p>
The stout-of-heart may be interested in creating new aircraft types. Some
of this information must be generated by hand, but I did create a program
to help me generate aircraft objects: GEDIT. GEDIT is a rudimentary
Motif program that allows you to create 3-dimensional objects. It is
available in the directory <code>gedit</code> of the distributed package.
</p>
<p>
The syntax of the aircraft record is more articulated. The program does not complain about missing numeric values,
and they are assumed to be zero.
</p>
<blockquote><pre>
# Aircraft Inventory for ACM-6
# See ACM-Bibliography for further information about the sources of this
# information.
# MiG-29 Fulcrum
# MiG-29M information was derived from two sources: [Spick87] and [AirI Aug92].
aircraft "MiG-29" {
Description "Mikoyan-Guryevich MiG-29M Fulcrum"
Object "mig29.obj"
</pre></blockquote>
<p>
The visual description of an ACM aircraft is stored separately in
V-library format and specified in the <tt>Object</tt> field. The name of the file may include a relative path which is resolved against the directory of the current file. This file describes a set of polygons that roughly
approximates the shape of the actual aircraft. I used GEDIT to create
most of the ACM aircraft objects. If not available, you may leave this parameter commented-out, but the UFO object will be displayed instead...
</p>
<a name="frameandaerodynamics"><h3>Frame and aerodynamics</h3></a>
<blockquote><pre>
WingArea 400.0 # (wingS) Wing surface area (ft<sup>2</sup>)
WingHalfSpan 18.87 # (wings) Wing half-span (ft)
WingHeight 0.0 # (wingh) Height of the wing aerodynamic center
# above the CM (ft)
Chord 9.61 # (c) Mean geometric chord of wing (ft)
AspectRatio 3.56 # (aspectRatio) aspect ratio
EmptyWeight 22500.0 # (emptyWeight) Empty mass (lb)
</pre></blockquote>
<p>
These values are usually quoted with an aircraft's performance figures.
The <code>WingHeight</code> is used to estimate the ground effect, and
its value is positive for wings above the center of mass.
</p>
<blockquote><pre>
# Max. wing load (lb):
MaxLoadZPositive 273000 # 10*(EmptyWeight+50%*MaxFuel) (+10 g)
MaxLoadZNegative 137000 # 5*(EmptyWeight+50%*MaxFuel) (-5 g)
</pre></blockquote>
<p>
The maximum vertical load, either positive climbing (<code>MaxLoadZPositive</code>)
or negative diving (<code>MaxLoadZNegative</code>). If the weight the wings have
to sustain is greater than that, the structure suddenly breaks and the
aircraft gets destroyed.
</p>
<p>
For <b>civil aircraft</b> you may guess these values as 2.5*MTOW and
1.0*MTOW. <b>Military aircraft,</b> and fighters expecially, are reported to
sustain up to some number N of positive "g" and some other number Q of
negative "g". If not indicated, you should suppose at least 60% of the fuel
contributing to the mass:
</p>
<blockquote>
MaxLoadZPositive = N * (EmptyWeight + 60%*MaxFuel)<br>
MaxLoadZNegative = Q * (EmptyWeight + 60%*MaxFuel)
</blockquote>
<p>
where typical values for N range from 7 to 10, and Q=3.
</p>
<pre>
</pre>
<center>
<img src="acmdoc_html-reference-frame.png">
<p>
<b>The reference frame of the aircraft, with main inertial axes<br>
and center of mass (CM) indicated.</b>
</p>
</center>
<pre>
</pre>
<blockquote><pre>
# (I(x,y,z)) Moments of inertia (lb ft<sup>2</sup>)
Ixx 10000
Iyy 75000
Izz 80000
</pre></blockquote>
<p>
These are hard values to simple guess. I have grabbed values where they
were available and simply guessed sometimes, too. Moments of inertia
in an actual aircraft are not constant. Most notably, as an aircraft
burns fuel, it's weight distribution, center-of-gravity, and moments of
inertia change. In ACM, however, moments of inertia and CM do not change.
</p>
<blockquote><pre>
# (cFlap) Lift due to flaps (yields Clift of 1.0 at max extension)
CFlap 0.64279
BetaStall 15.0 # (deg) Stall angle for rudder
CFlapDrag 0.0467 # (cFlapDrag) Drag due to Flaps
CGearDrag 0.03 # (cGearDrag) Drag due to Gear
CSpeedBrake 0.03 # (cSpeedBrake) Drag due to Speed Brake
# at 90 degrees
# (CDb) Drag Characteristic equation [Wave+body]
# independent variable is mach number
CDb { # (CDb) Drag Characteristic equation [Wave+body]
# independent variable is mach number
0.0, 0.020,
0.2, 0.020,
0.95, 0.015,
1.05, 0.045,
2.00, 0.030,
10.0, 0.030
}
</pre></blockquote>
<p>
In ACM, wave/body drag coefficient is a function of the airspeed expressed
as a Mach number. Less drag makes for a faster aircraft. This function
generally has a maximum near Mach 1.0. You probably should not change
this table unless you really know what you're doing.
<br>
For the CFlap coefficient see below.
</p>
<blockquote><pre>
CLift { # (CLift) Lift-slope curve (vs alpha)
-3.0, 0.0,
-1.05, 0.0,
-0.576, -2.199,
-0.556, -2.23,
-0.524, -2.199,
0.524, 2.199,
0.556, 2.23,
0.576, 2.199,
1.05, 0.0,
2.0, 0.0
}
</pre></blockquote>
<p>
In ACM, the coefficient of lift is purely a function of angle of attack (alpha).
The CLift table gives the coefficient of lift for some values
of the AoA "alpha". In the example above, the maximum coefficient of lift is
2.23 at 0.556 RAD = 32 DEG of AoA. Above this angle, the lift drops abruptly
and the plane stalls. The coefficient of lift also depends on the current
flap deflection angle according to the function:
</p>
<blockquote>
C<sub>L</sub> = CLift(<i>alpha</i>) + sin(<i>flaps</i>) * CFlap
</blockquote>
<p>
being CLift(<i>alpha</i>) the interpolated value given by the <tt>CLift</tt>
table above.
The total lift force is given by the equation:
</p>
<blockquote>
<i>L</i> = 1/2 r WingArea V<sup>2</sup> C<sub>L</sub>
</blockquote>
<p>
where r is the air density (r=1.225 Kg/m<sup>3</sup> at sea level), V is the
airspeed and C<sub>L</sub> is the coefficient of lift.
</p>
<blockquote><pre>
CnBeta { # (CnBeta) Yaw moment due to sideslip equation
0, -0.08125,
0.523540, -0.078125,
0.785340, -0.0609375,
1.047198, 0.125,
1.58, 0.0
}
</pre></blockquote>
<p>
In ACM negative yaw due to sideslip means that the aircraft is
"weathercock stable", that is, the aircraft tends to yaw in a way that
reduces the sideslip component.
</p>
<blockquote><pre>
ClBeta { # (ClBeta) Roll moment due to sideslip equation
0, -0.0125,
0.43633, -0.015,
0.78540, 0.125,
1.58, 0.0,
3.142, 0.125,
}
</pre></blockquote>
<p>
A negative value means that the aircraft will roll in the opposite direction
of the sideslip component, tending to make the aircraft more stable.
</p>
<blockquote><pre>
CDBOrigin 0 # (CDBOrigin, CDBFactor) Drag due to sideslip
CDBFactor 0.5
CDBPhase 0 # (CDBPhase) sideslip drag phase (deg)
</pre></blockquote>
CDB values define the body drag added by sideslip on the aircraft.
<blockquote><pre>
CYBeta -0.85 # (CYbeta) Side-force from side-slip [dCY/dBeta]
</pre></blockquote>
<p>
CYBeta is the "lift" due to sideslip. This is the factor that allows
to roll 90 degrees from level, kick the rudder a lot, and still stay
level. Not all simulators take this into account.
</p>
<blockquote><pre>
EffElevator 0.60 # (effElevator) Elevator effectiveness
CmAlpha -0.30 # (cmSlope) CmAlpha curve slope
EffRudder 0.35 # (effRudder) Rudder effectiveness
MaxRudder 20 # (maxRudder) max Rudder (deg)
MaxAileron 20 # (maxAileron) max Aileron (deg)
MaxFlap 20 # (maxFlap) max flap setting (deg)
# (flapRate) flap extension rate (about 2 secs to fully extend flaps)
FlapRate 10 # (deg/s)
MaxSpeedBrake 80 # (maxSpeedBrake) max Speed Brake setting (deg)
# (speedBrakeRate) rate of speed brake extension (2 secs to full ext)
SpeedBrakeRate 40 # (deg/s)
# (speedBrakeIncr) speed Brake increment per keystroke
SpeedBrakeIncr 80 # (deg)
</pre></blockquote>
<p>
These values determine the characteristics of the aircraft's speed
brakes (speed brakes are used in the air, wheel brakes are used on the
ground). MaxSpeedBrake determines the maximum extension of the speed
brake panels. SpeedBrakeIncr determines how far one press of the <b>s</b>
key will increase the speed brake extension. It does not need to be an
even increment of MaxSpeedBrake. SpeedBrakeRate determines how fast
the brakes will deploy or retract.
</p>
<blockquote><pre>
Clda 0.048 # (Clda) roll moment from aileron offset
Cldr 0.004 # (Cldr) roll moment from rudder offset
Clp -0.27 # (Clp) roll damping
Cmq -8.0 # (Cmq) pitch damping factor
Cnr -2.0 # (Cnr) yaw damping factor
</pre></blockquote>
<p>
These factors characterize some of the roll, pitch and yaw characteristics
of the aircraft. They are in the NACA form. Damping factors determine how
quickly an aircraft returns to a steady state after some change in control
input. Larger negative damping factors make for a more stable aircraft.
</p>
<blockquote><pre>
# Speed limits at MTOW. Leave undefined or set to 0 if unknown.
MTOW 32250.0 # maximum takeoff weight (lb)
#Vs0 42.0 # stall speed, full flaps (kt)
#Vs1 50.0 # stall speed, no flaps (kt)
#Vfe 100.0 # max speed with flaps extended (kt)
#Vno 145.0 # normal operation speed (kt)
#Vne 164.0 # never exceed speed (kt)
</pre></blockquote>
<p>
These parameters allow the program to set or calculate several speed limits (IAS).
These speed limits are used to draw the colored arcs on the air speed
indicator. Every arc is displayed only if its start and end are known, and
it is now drawn otherwise.
These speed limits are also considered to simulate structural failures
when exceeded, as detailed below.
Leave commented-out or set to 0 the parameters that are unknown: the program
will try to guess the missing values or simply will left them undefined
if this guess is not possible.
</p>
<p>
<b>MTOW</b> is the maximum take off weight (lb).
</p>
<p>
<b>Vs0</b> is the stall speed at MTOW, full flaps, at sea level.
It is indicated by the beginning of the white arc in the anemometer.
If undefined or set to zero, the program tries to calculate this value
with the following formula:
</p>
<blockquote>
Vs0 = sqrt( 2 MTOW earth_g / (r WingArea C<sub>L</sub>) )
</blockquote>
where:
<blockquote>
earth_g is the gravity acceleration at sea level.<br>
r is the air density at sea level.<br>
C<sub>L</sub> is the lift coefficient calculated with:<br>
C<sub>L</sub> = maxCLift + CFlap sin(MaxFlap)<br>
being maxCLift the maximum coefficient of lift as given by the CLift curve.
</blockquote>
<p>
<b>Vs1</b> is the stall speed at MTOW, without flaps, at sea level.
It is indicated by the end of the short white arc in the anemometer.
If undefined or set to zero, the program calculates this value with
the following formula:
</p>
<blockquote>
Vs1 = sqrt( 2 MTOW earth_g / (r WingArea maxCLift) )
</blockquote>
<p>
<b>Vfe</b> is the maximum speed with flaps extended.
It is indicated by the end of the longer white arc in the anemometer.
Exceeding this speed with flaps deployed, also partially, causes a flaps
failure that makes them unusable.
If undefined or set to 0, the Vfe is unlimited.
</p>
<p>
<b>Vno</b> is the normal operation speed, that is the maximum speed in
turbolent air conditions.
It is indicated by the beginning of the yellow arc in the anemometer.
Flying above this speed in turbulent air, a vertical wind gust of 30 ft/s
would cause a vertical positive or negative acceleration above the
maximum structural load capacity of the wings.
If undefined or set to 0, the Vno is estimated with the following formula:
</p>
<blockquote>
Vno = 2 earth_g (MaxLoadZNegative + EmptyWeight) / (r WingArea * b * MAX_GUST)
</blockquote>
where:
<blockquote>
b = 6.302 is the slope of the CLift curve in its linear range; this value was
actually calculated for the C-172RG but it is also very close to that of the
other aircraft models, so it is assumed as constant.<br>
MAX_GUST = 30 ft/s is the conventional maximum vertical wind gust intensity
assumed as reference to calculate Vno.
</blockquote>
<p>
<b>Vne</b> is the speed that must never be exceeded.
It is indicated by a short red line.
Flying above this speed limit the aircraft may suffer a catastrophic structural
failure.
If undefined or set to zero, this limit does not apply.
</p>
<blockquote><pre>
# (viewPoint) pilot's viewing location
ViewPoint { 14.75, 0, -5.375 }
</pre></blockquote>
<p>
This is the location of the pilot's eye view outside of the cockpit.
</p>
<blockquote><pre>
# (tailExtent) as we rotate, this part may drag
TailExtent { -18.165, 0.0, 1.191 }
</pre></blockquote>
<p>
This defines where the aft-most lower part of the aircraft is
located. This is the part of the aircraft that may scrape the ground as
we rotate. This value is not currently used. Instead we simply limit
up-pitch on the ground to 20 degrees.
</p>
<a name="engine"><h3>Engine</h3></a>
<blockquote><pre>
EngineType "GenericJetEngine"
MaxFuel 9750 # (maxFuel) maximum internal fuel (lb)
# engine lag factor (how fast does it respond to throttle changes
EngineLag -3.0
# Engine data based on updated RD-33K engines cited in [AirI Aug89].
# (maxThrust) max static thrust, military power (lbf)
MaxThrust 27000
# (maxABThrust) max static thrust, afterburner on (lbf)
MaxABThrust 44000
HasThrustReverser 0 # 0=no (default), 1=yes
</pre></blockquote>
<p>
This section describes the engine and its performances. One among these
types of engine can be chosen:
</p>
<dl>
<dt><tt>EngineType "NoEngine"</tt></dt>
<dd>Basically, a glider. The remaining engines parameters are then
ignored. This is the default.</dd>
<dt><tt>EngineType "GenericPistonEngine"</tt></dt>
<dd>Propeller driven aircraft with pistons engine. Its
thrust is proportional to the current RPM and to the factor
(ρ<sub>h</sub>/ρ<sub>0</sub>)<sup>1.6</sup>, being
ρ<sub>h</sub> the air density at the current altitude and
ρ<sub>0</sub> the air density at sea level.</dd>
<dt><tt>EngineType "GenericJetEngine"</tt></dt>
<dd>Jet engine. Its thrust is proportional to the square of the current RPM
and to the ratio between air density at the given altitude and air density
at sea level.</dd>
<dt><tt>EngineType "GenericRocketEngine"</tt></dt>
<dd>Rocket engine. Its thrust is proportional to the indicated fuel pump
RPM. Thrust does not depend on altitude, so this aircraft can fly up to
the edge of the space before loosing directional control.</dd>
</dl>
<p>
Maximum static military and afterburner thrust values can usually be
found in documents that describe aircraft performance.
Setting <code>MaxABThrust</code> to the same value of <code>MaxThrust</code>
disable the AB for those aircraft that lack this device.
For the conversions, remember that 1 pound = 1 lb g = 4.448 N = 0.4536 Kgf.
</p>
<p>
The <code>HasThrustReverser</code> parameter tells if the aircraft has a thrust
reverse device. Set to 1 if available or 0 if not. If this parameter is missing
the default is "no". With thrust reverse deployed and engine at the max power,
the resulting reverse thrust is calculated as 50% of the max forward thrust.
</p>
<blockquote><pre>
Thrust { # (Thrust) Mach Number vs. thrust
0, 1,
1.7, 1.5,
2.0, 0.84,
5, 0.5
}
ABThrust { # (ABThrust) afterburner thrust table
0, 1,
0.5, 1,
1, 1.21,
1.7, 1.7,
5, 1.64
}
</pre></blockquote>
<p>
The Thrust and ABThrust corrects the static thrust value for the
aircraft's Mach number. Unless you have hard information about this,
leave these fields alone. The <code>ABThrust</code> table is ignored when
the AB device is disabled.
</p>
<blockquote><pre>
# (spFuelConsump) specific fuel consump(lb fuel/lb T x hr)
SpFuelConsump 0.68
# (spABFuelConsump) AB specific fuel consump(lb fuel/lb T x hr)
SpABFuelConsump 2.55
</pre></blockquote>
<p>
These fields define the specific fuel consumption (fuel consumed per
pound of thrust per hour).
</p>
<a name="landinggear"><h3>Landing gear</h3></a>
<blockquote><pre>
# Landing Gear control parameters
GearRate 30 # (gearRate) gear extension rate from 0 to 90 DEG (DEG/s)
MuStatic 0.08 # (muStatic) static coeff of friction no-brakes
MuKinetic 0.05 # (muKinetic) moving coeff of friction no-brakes
MuBStatic 0.7 # (muBStatic) static brakes-on
MuBKinetic 0.6 # (muBKinetic) moving brakes-on
</pre></blockquote>
<p>
These four values characterize the amount of friction that the wheels
generate when in contact with the ground. If F is the force (usually a weight)
that load a wheel, then F*MuStatic is the friction force when the wheel is
at rest; F*MuKinetic when the wheel is rolling. MuBStatic and MuBKinetic
have the same meaning with the only difference that the brakes are on.
The maximum theoretical value of any of these is 1.0.
</p>
<p>
<b>NOTE 1.</b> In our landing gear, only the wheels of the main gear have
brakes; the nose wheel can't brake.
</p>
<p>
<b>NOTE 2.</b> The MuKinetic and MuBKinetic are probably misnomers as
these coefficients are more commonly known as "MuRolling" instead. Proper
"kinetic" coefficients are involved when the wheel skids over the runway,
either longitudinally (blocked brakes) or laterally. In ACM brakes
never block, but still wheels can skid laterally over the runway. The
coefficient involved in the skidding behavior is currently hard-coded
in the <code>gear.c</code> module. Moreover, the gear breaks when the
lateral force applied to the wheel exceeds the empty weight
of the aircraft. This limit too is hard-coded into the source program.
</p>
<blockquote><pre>
MaxNWDef 72 # (maxNWDef) maximum nosewheel deflection (deg)
</pre></blockquote>
<p>
The <code>MaxNWDef</code> is the maximum deflection angle of the nose wheel. If this figure is available from the characteristics of the airplane, the figure and the formulas below allows to calculate the minimum turning radius.
</p>
<pre>
</pre>
<center>
<img src="acmdoc_html-steer.png">
<p>
<b>Minimum turn radius of turn on the ground.</b>
</p>
</center>
<pre>
</pre>
<p>
If Q is the distance between the contact point with ground of the nose wheel and the middle point between the contact points of the main landing gear, then this latter middle point draws a circle or radius:
</p>
<blockquote>
r = Q / tan(MaxNWDef)
</blockquote>
<p>
while the contact point of the nose wheel draws a circle of radius:
</p>
<blockquote>
R = Q / sin(MaxNWDef)
</blockquote>
<p>
Sometimes the maximum deflection of the nose wheel is not reported, but a <b>minimum turning radius</b> is provided instead. Normally this is defined as the radius of the smallest cylinder inside which the aircraft can perform a full 360 degrees turn. From the figure above we see the this value could be estimated as the turn radius of the center of the main landing gear (r) plus the half-wings length:
</p>
<blockquote>
MinRadius = WingsLength/2 + r
</blockquote>
<p>from which:</p>
<blockquote>
MaxNWDef = atan( Q / (MinRadius - WingsLength/2) )
</blockquote>
<p>
Several "minimum turning radius" could be indicated for a given aircraft referring to:
<br> 1. the pilot using the pedals only;
<br> 2. the pilot using both pedals and differential brakes;
<br> 3. the pilot using the steering crank handle (where available);
<br> 4. the ground crew with the nose wheel hooked-up.
<br>Obviously, we are interested to the case 1 only.
</p>
<blockquote><pre>
# (rm) location of right main gear attachments {x,y,z} (ft)
Rm { -2.2, 4, 1.702 }
# (rn) location of nose gear attachments {x,y,z} (ft)
Rn { 9.395000, 0, 1.807 }
Km 18220.7 # main oleo spring factor (lbf/ft)
Kn 4278.34 # nose oleo spring factor (lbf/ft)
Dm 10000 # main oleo damping factor (lbf s/ft)
Dn 80 # nose oleo damping factor (lbf s/ft)
Gm 1.5 # main strut length with tire (ft)
Gn 1.5 # nose strut length with tire (ft)
CmMax 1.0 # (cmMax) main max oleo extension distance (ft)
CnMax 1.5 # (cnMax) nose max oleo extension distance (ft)
</pre></blockquote>
<pre>
</pre>
<center>
<img src="acmdoc_html-landing-gear.png">
<p>
<b>Parameters of the landing gear.</b>
</p>
</center>
<pre>
</pre>
<p>
The parameters <code>Rn</code> and <code>Rm</code> give the positions of the
nose and main gear attachments, while <code>Gn</code> and <code>Gm</code>
are their total length including the tire but excluding the oleo extension
<code>CnMax</code> and <code>CmMax</code>. As a first approximation of these
parameters, you can take these values (all the distances are measured in feet,
1 ft = 0.3048 m):
</p>
<blockquote>
<dl>
<dt><code>Rn.x</code></dt>
<dd>
Attachment point of the nose landing gear strut, x coordinate (ft). Positive for
tricycle landing gear, negative for bicycle landing gear.
</dd>
<dt><code>Rn.y</code></dt>
<dd>Always zero.
</dd>
<dt><code>Rn.z</code></dt>
<dd>
Attachment point of the nose landing gear strut, z coordinate (ft).
</dd>
<dt><code>CnMax</code></dt>
<dd>
The oleo + spring part of the nose gear extends from zero up to this length (ft).
</dd>
<dt><code>Gn</code></dt>
<dd>
Length of the rigid part of the nose landing gear strut (ft). The total
length of the nose gear strut including tire is then this value plus
the current extension of the oleo part.
</dd>
<dt><code>Rm.x</code></dt>
<dd>
Attachment point of the right main landing gear strut, x coordinate (ft).
Negative for tricycle landing gear, positive for bicycle landing gear.
</dd>
<dt><code>Rm.y</code></dt>
<dd>
Attachment point of the right main landing gear strut, y coordinate (ft).
Always positive; the left main gear it is assumed to be at -Rm.y but with
the same Rm.x and Rm.z components of the right main gear.
</dd>
<dt><code>Rm.z</code></dt>
<dd>
Attachment point of the right main landing gear, z coordinate (ft).
</dd>
<dt><code>CmMax</code></dt>
<dd>
The oleo + spring part of each main gear strut extends from zero up to
this length (ft). About half foot for light planes, and up to 1.5 feet
for larger ones.
</dd>
<dt><code>Gm</code></dt>
<dd>
Length of the rigid part of the each main landing gear strut (ft). The total
length of each main gear strut including tire is then this value plus
the current extension of the oleo part.
</dd>
</dl>
</blockquote>
<p>
Every wheel of the landing gear is attached to the aircraft through a spring
(K) and a damper (D). The parameters referring to the main landing gear are the
<u>sum for the left and right gear</u>, so the actual parameter for each wheel strut
is half the value reported in the inventory (see the picture above).
</p>
<p>
The landing gear parameters are difficult to estimate. We will give some
simple rule to set good starting values based on reasonable assumptions.
</p>
<blockquote>
<dl>
<dt><code>Km</code></dt>
<dd>
Total spring factor for the left+right main landing gear (lbf/ft),
then each spring is half this value.
The main landing gear must sustain most of the weight of the aircraft,
so a reasonable rule is to set this spring (left + right) so that
the strut be half compressed with the aircraft in its empty weight:
<p>
Km = 2 * EmptyWeight / CmMax
<p>
For example, if the empty weight is 10000 lbf and the strut maximum extension
is 1.5 ft, we have Km = 13300 lbf/ft.
</dd>
<dt><code>Kn</code></dt>
<dd>
Nose spring factor (lbf/ft).
The nose spring must sustain the static weight that depends on its distance
from the CM. Moreover, on bicycle landing gears the nose must sustain also the
braking. Here to we will assume the oleo damper be half compressed while the
aircraft is braking at its empty weight. The resulting quite complex
formula is:
<p>
Kn = 2 * EmptyWeight * (-Rm.x / (Rn.x - Rm.x) + MuBKinetic * (Rm.z + Gm + CmMax)/Rn.x) / CnMax
<p>
Note that the distances that appear in this formula have their sign,
so usually Rm.x is negative while Rn.x is positive. For bicycle landing
gear we can neglect the braking effect by assuming MuBKinetic=0 in the
formula above, but consider that these aircraft usually have an higher
static pitch for which this simple formula is not valid anymore.
</dd>
<dt><code>Dm</code></dt>
<dd>
Total damping factor for the left + right landing gear strut (lbf s/ft).
The principal role of the damping factor in the main gear is to adsorb
harder landings at high descending speed, when the spring cannot help
because is still not compressed at all. 600 fpm is usually the maximum
allowed vertical speed for which aircraft are tested. That means that
the force produced by the damper when the wheels of the main gear hits
the ground is F=Dm*600 fpm. Since in ACM there is an hard-coded
vertical limit to the main gear equal to 1.5 times the empty weight for
each leg of the main landing gear, the following formula gives a safe
value for Dm:
<p>
Dm <= 0.2 * EmptyWeight
<p>
For example, if the empty weight is 10000 lbf, then the damping factor should not
exceed 2000 lbf s/ft.
</dd>
<dt><code>Dn</code></dt>
<dd>
Damping factor for the nose landing gear strut (lbf s/ft).
A good starting value might be Dm calculated above.
</dd>
</dl>
</blockquote>
<p>
The program GEDIT can be used to determine correct landing gear
locations. I then use the program <code>tools/balance.c</code> to generate
the appropriate values for the spring factors for each new aircraft type that
I create. Note that neither GEDIT nor <code>balance.c</code> currently
calculate oleo damping factors.
</p>
<p>
Choosing the right values for the landing gear is difficult, and the program
may terminate abruptly with one of the following messages related to the
landing gear:
</p>
<dl>
<dt><b>Nose/left/right gear smash.</b></dt>
<dd>An excessive vertical load has fully compressed the suspension
transferring that load directly to the aircraft frame and causing
a permanent, fatal damage. Possible fixes: increase the spring factor;
increase damping factor.</dd>
<dt><b>Nose/left/right gear collapsed under too hight vertical load.</b></dt>
<dd>The landing gear transferred an excessive vertical load to the aircraft's
frame causing a permanent, fatal damage. The maximum vertical load the
each leg of the landing gear can sustain is hard-coded in the source to be
the empty weight for the nose, and 1.5 times the empty weight for each
of the left and right legs. Possible fixes: decrease the spring factor;
decrease the damping factor.</dd>
<dt><b>Nose/left/right gear broken under too high lateral force.</b></dt>
<dd>The wheel transferred an excessive torque due to a lateral force.
This is typically caused by an excessive landing pitch, or excessive bank
angle, or by steering at high speed. The maximum lateral force each wheel
can transfer to the frame is hard-coded in the source to be the empty
weight. Fix: touchdown should always be performed at low vertical speed
with zero bank angle and reasonable low pitch; do not brake while
steering.</dd>
</dl>
<a name="armament"><h3>Armament</h3></a><br>
<blockquote><pre>
StructurePoints 15 # (structurePts) maximum structural damage
</pre></blockquote>
<p>
This value characterizes how much damage can be absorbed by the aircraft
before it starts falling to the ground (drones simply explode). A detailed description
of the meaning of this field is available as a comment to the
<tt>objects/munition-map.txt</tt> file, where all the recognized warheads are
listed along with their effectiveness.
</p>
<blockquote><pre>
# Radar data based on N-019 Pulse Doppler radar cited in [AirI Aug89].
RadarOutput 15000 # (radarOutput) radar output (watts)
RadarTRange 38 # (radarTRange) tracking radar range (NM)
RadarDRange 55 # (radarDRange) detection radar range (NM)
TEWSThreshold 0 # Radar Warning Receiver threshold (watts)
</pre></blockquote>
<p>
If you have any information about the radar capabilities of the aircraft,
here's the place for them. The detection range is the maximum range that
a target can be seen on radar. No attempt is made to take into account
the radar cross section of the target. The tracking range is the range
required to get a lock onto the target.
</p>
<blockquote><pre>
# Weapons
WeaponCount 9
</pre></blockquote>
<p>
Number of weapon hard-points. The following fields are ignored if this value is
set to zero, so they can be omitted if the aircraft does not carry weapons at
all.
</p>
<blockquote><pre>
HardPoint0 { 7.0, -4.0, 0.0 }
HardPoint1 { 0.357, 15.6, 0.0 }
HardPoint2 { 0.357, -15.6, 0.0 }
HardPoint3 { 1.5, 9.0, 2.0 }
HardPoint4 { 1.5, -9.0, 2.0 }
HardPoint5 { 1.5, 8.0, 1.5 }
HardPoint6 { 1.5, -8.0, 1.5 }
HardPoint7 { 1.5, 10.0, 1.5 }
HardPoint8 { 1.5, -10.0, 1.5 }
</pre></blockquote>
<p>
These are the XYZ locations relative to the aircraft CM of each weapon. By
defining these, each missile or cannon fires from its appropriate location
on the aircraft.
</p>
<blockquote><pre>
WeaponStation 0 "M61A1" 500 0 0
WeaponStation 1 "AIM-9M" 0 0 0
WeaponStation 2 "AIM-9M" 0 0 0
WeaponStation 3 "AIM-9M" 0 0 0
WeaponStation 4 "AIM-9M" 0 0 0
WeaponStation 5 "AIM-9M" 0 0 0
WeaponStation 6 "AIM-9M" 0 0 0
WeaponStation 7 "AIM-120" 0 0 0
WeaponStation 8 "AIM-120" 0 0 0
}
</pre></blockquote>
<p>
The <code>WeaponStation</code> directive defines the type of weapon located at
each hard point. The first argument is the number of the hard point, that
ranges from 0 up to <code>WeaponCount</code>−1. The second argument is
the name of the weapon. Then three integer numbers follow whose meaning depends
on the specific weapon. The table below summarizes by examples all the
available weapons:
</p>
<table align=center border=1 cellpadding=5 cellspacing=0 width="70%">
<tr>
<th>Example</th>
<th>Notes</th>
</tr>
<tr>
<td valign=top align=left>
<pre>WeaponStation 0 "M61A1" 500 0 0</pre>
</td>
<td>
<b>M-61A1</b> "Vulcan" cannon. 500 is the number of rounds initially supplied to
the cannon. The other two numbers are ignored.
</td>
</tr>
<tr>
<td valign=top align=left>
<pre>WeaponStation 1 "AIM9M" 0 0 0</pre>
</td>
<td>
<b>AIM9M</b> "Sidewinder", infra-red guided, short range, air-to-air missile.
The other 3 numbers are ignored.
</td>
</tr>
<tr>
<td valign=top align=left>
<pre>WeaponStation 2 "AIM120" 0 0 0</pre>
</td>
<td>
<b>AIM-120</b> "AMRAAM", radar guided, medium-range, air-to-air missile.
The other 3 numbers are ignored.
</td>
</tr>
<tr>
<td valign=top align=left>
<pre>WeaponStation 6 "MK82" 0 0 0</pre>
</td>
<td>
<b>Mark 82</b> 250 lb standard drop bomb.
The other 3 numbers are ignored.
</td>
</tr>
</table>
<p>
Remote aircraft may submit detonation PDUs claiming you have been hit by something. The <tt>objects/munition-map.txt</tt> lists all the recognized munitions by their DIS entity type, along with their effectiveness.
</p>
<a name=definingnewobjects></a>
<h2>Defining new objects</h2>
<p>
An "object" can be the shape of an aircraft (field "Object" of the
aircraft inventory file) or a feature of the scene file (record
"FEATURE"). The format of the object file, described here, is copied
from the <i>ACM Flight Simulator Frequently-Asked Questions List</i>
maintained by Brad Bass (bass@convex.com).
</p>
<p>
The first line is the object name.
</p>
<p>
The 2nd line has two integers which represent the number of points
and the number of polygons in the object.
</p>
<p>
Then, the points are listed, one per line, with the point id number
followed by the X, Y and Z coordinate (North, East, Down). The
units are feet.
</p>
<p>
Finally, the polygons are listed as color, number of corners,
and the list of point id numbers. The color may be expressed
as a single color or as a front and back color. Also, the polygon
can be flagged as "one-sided" -- that is visible only
from the front.
</p>
<p>
For example:
</p>
<blockquote>
<pre>
pyramid # the object name
5 4 # the point count and polygon count
1 28000 0 0 # the
2 0 16000 0 # points that
3 -35000 0 0 # describe
4 0 -14000 0 # the
5 0 0 -8200 # corners
#788b63 3 1 2 5 # side 1 - color (X-style), cornercount,
# corner list
violet 3 2 3 5 # side 2
(blue red) 3 3 4 5 # side 3 - front is blue, back is red
(green clip) 3 4 1 5 # side 4 - one-sided polygon
</pre>
</blockquote>
<a name=ieee12781discompliance></a>
<h2>IEEE 1278.1 (DIS) compliance</h2>
<p>ACM emits and recognizes the following DIS PDU Types:</p>
<p><br><br>
</p>
<p>Entity
State</p>
<p>Fire</p>
<p>Detonation</p>
<p>Electromagnetic
Emission</p>
<p><br><br>
</p>
<p>
The ACM program may simulate several entities, including the aircraft
the
user is flying on, each missile it fires and possibly other
entities. For
each entity it simulates, the program sends a state
packet to all the
other participants every 4.8 seconds along with the
<i>dead reckoning</i>
algorithm each remote program must apply in the
meanwhile. The program
also sends an updated state packet when it detects the actual
state of
the entity deviates from the stated dead reckoning method for
more than
a given amount.
</p>
<p>The following dead reckoning methods are recognized:</p>
<table border=1 cellspacing="0" cellpadding="5" align="center">
<tr>
<th>Name</th>
<th>DIS enumeration</th>
<th>Description</th>
</tr>
<tr>
<td>Other</td>
<td>0</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>Static</td>
<td>1</td>
<td>Entity does not move.</td>
</tr>
<tr>
<td>FPW</td>
<td>2</td>
<td>Entity moves at constant speed.</td>
</tr>
<tr>
<td>RPW</td>
<td>3</td>
<td>Entity moves at constant speed and rotates at constant speed.</td>
</tr>
<tr>
<td>RVW</td>
<td>4</td>
<td>Entity moves at constant acceleration and rotates at constant speed.</td>
</tr>
<tr>
<td>FVW</td>
<td>5</td>
<td>Entity moves at constant acceleration and does not rotate.</td>
</tr>
<tr>
<td>FPB</td>
<td>6</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>RPB</td>
<td>7</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>RVB</td>
<td>8</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>FVB</td>
<td>9</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>RPW</td>
<td>10</td>
<td>NOT SUPPORTED.</td>
</tr>
<tr>
<td>RVW</td>
<td>11</td>
<td>NOT SUPPORTED.</td>
</tr>
</table>
<p>
For methods not supported, the state is not updated and the remote entity looks moving by steps.
</p>
<p>Entities
managed by ACM emit entity state PDU’s specifying the RPW(3)
dead reckoning method. No articulation parameters are currently sent
by entities managed by ACM. As with most current DIS-based
simulations, altitudes are expressed as heights above the WGS-84
ellipsoid, not as heights above the geoid. Dead reckoning threshold
values are hard-coded in the <tt>src/acm/manifest.h</tt> file:</p>
<table border="1" cellspacing=0 cellpadding="5" align="center">
<tr><th>Description</th><th>Value</th></tr>
<tr><td>Maximum time between Entity State PDUs</td><td>4.8 seconds</td></tr>
<tr><td>Dead Reckoning Cartesian distance error</td><td>3 meters</td></tr>
<tr><td>Dead Reckoning angular orientation error</td><td>2 degrees</td></tr>
</table>
<p>When
operating in IEEE 1278 mode, the default UDP port number for PDU
transmission is 3000.</p>
<h3>The Transfer Control Protocol</h3>
<p>ACM
supports an experimental DIS 2.1.4++ control request protocol. It
permits it to “take over” aircraft of similarly enabled
applications. For ACM to take over an aircraft, it must have model
information in the “inventory” file describing the
characteristics of that aircraft type. Those aircraft entities will
be marked with an asterisk (*) in the left margin of the stealth
browser display. Double click an aircraft entity id to take control.
</p>
<h3>Transfer Control Request PDU</h3>
<center>
<table border="1" cellpadding="4" cellspacing="0">
<thead>
<tr>
<th>
Field<br>
Size<br>
(bits)
</th>
<th colspan=2>
Transfer Control Request PDU fields
</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan=4>
96
</td>
<td rowspan=4>
PDU Header
</td>
<td>
<p>Protocol Version—8-bit enumeration</p>
</td>
</tr>
<tr>
<td>
<p>Exercise ID—8-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>PDU Type —8-bit unsigned integer</p>
<p>Protocol Family—8-bit enumeration</p>
<p>Timestamp—32-bit unsigned integer</p>
<p>Length—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>Padding—16-bits unused</p>
</td>
</tr>
<tr>
<td rowspan=3>
48
</td>
<td rowspan=3>
Originating Entity ID
</td>
<td>
<p>Site—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>Application—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>Entity—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td rowspan=2>
48
</td>
<td rowspan=2>
Receiving Entity ID
</td>
<td>
Site—16-bit unsigned integer
</td>
</tr>
<tr>
<td>
<p>Application—16-bit unsigned integer</p>
<p>Entity—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
32
</td>
<td>
Request ID
</td>
<td>
32-bit unsigned integer
</td>
</tr>
<tr>
<td>
8
</td>
<td>
Required Reliability Service
</td>
<td>
8-bit enumeration
</td>
</tr>
<tr>
<td>
8
</td>
<td>
Transfer Type
</td>
<td>
8-bit enumeration
</td>
</tr>
<tr>
<td rowspan=2>
48
</td>
<td rowspan=2>
Entity ID to be Transferred
</td>
<td>
Site—16-bit unsigned integer
</td>
</tr>
<tr>
<td>
<p>Application—16-bit unsigned integer</p>
<p>Entity—16-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
32
</td>
<td>
Number of Record Sets (R)
</td>
<td>
32-bit unsigned integer
</td>
</tr>
<tr>
<td rowspan=6>
96 +<br>
(L<sub>1</sub> x Q<sub>1</sub>)<br>
+ P<sub>1</sub>
</td>
<td rowspan=6>
Record Set #1
</td>
<td>
Datum ID—32-bit enumeration
</td>
</tr>
<tr>
<td>
<p>Record
Set Serial Number—32-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>Record
Length—16-bit unsigned integer (L<sub>1</sub>)</p>
</td>
</tr>
<tr>
<td>
<p>Record
Count—16-bit unsigned integer (Q<sub>1</sub>)</p>
</td>
</tr>
<tr>
<td>
<p>Record
Values—(L<sub>1</sub> x Q<sub>1</sub>) bits</p>
</td>
</tr>
<tr>
<td>
<p>Padding—P<sub>1</sub>
bits</p>
</td>
</tr>
<tr>
<td colspan=3 align=center>
<p>·</p>
<p>·</p>
<p>·</p>
</td>
</tr>
<tr>
<td rowspan=6>
96 +<br>
(L<sub>R</sub> x Q<sub>R</sub>)<br>
+ P<sub>R</sub>
</td>
<td rowspan=6>
Record Set #R
</td>
<td>
<p>Datum
ID—32-bit enumeration</p>
</td>
</tr>
<tr>
<td>
<p>Record
Set Serial Number—32-bit unsigned integer</p>
</td>
</tr>
<tr>
<td>
<p>Record
Length—16-bit unsigned integer (L<sub>R</sub>)</p>
</td>
</tr>
<tr>
<td>
<p>Record
Count—16-bit unsigned integer (Q<sub>R</sub>)</p>
</td>
</tr>
<tr>
<td>
<p>Record
Values—(L<sub>R</sub> x Q<sub>R</sub>) bits</p>
</td>
</tr>
<tr>
<td>
<p>Padding—P<sub>R</sub>
bits</p>
</td>
</tr>
</tbody>
</table>
</center>
<p>
Total Transfer Control Request PDU size:
</p>
<blockquote>
320 + Sum<sub>i=1..R</sub>(96
+ (L<sub>i </sub>x Q<sub>i</sub>) + P<sub>i</sub>) bits
</blockquote>
<p>
where R is the number of Record Sets.
</p>
<p>ACM currently ignores record sets in a transfer control PDU.</p>
<h3>Transfer Type</h3>
<p>This
section specifies the 8-bit enumeration for the Transfer Type field
of the Transfer Control PDU.</p>
<p><br><br>
</p>
<table align=center border="1" cellpadding="4" cellspacing="0">
<thead>
<tr>
<th>Field Value</th>
<th>Nature</th>
</tr>
</thead>
<tbody>
<tr>
<td>0</td>
<td>Other</td>
</tr>
<tr>
<td>
1
</td>
<td>
Controlling
application requests transfer of an entity
</td>
</tr>
<tr>
<td>
2
</td>
<td>
Application
desiring control requests transfer of an entity
</td>
</tr>
<tr>
<td>
3
</td>
<td>
Mutual
exchange / swap of an entity
</td>
</tr>
<tr>
<td>
4
</td>
<td>
Controlling
application requests transfer of an environmental process
</td>
</tr>
<tr>
<td>
5
</td>
<td>
Application
desiring controls request transfer of an environmental process
</td>
</tr>
<tr>
<td>
6
</td>
<td>
Mutual exchange of an environmental
</td>
</tr>
</tbody>
</table>
<p><br><br>
</p>
<p>
<i><b>Assuming Ownership</b></i></p>
<p><img src="acmdoc_html_assuming-ownership.gif"></p>
<p><br><br>
</p>
<p>
<i><b>Accepting Ownership
by Request</b></i></p>
<p>Another
feature of ACM’s transfer control protocol support is that
other applications may request that ACM take control of an aircraft
that the other application controls.</p>
<a name=suggestedfurtherreading></a>
<h2>Suggested further reading</h2>
<ul>
<li>3D Graphics Programming for Windows 95<br>
by Nigel Thompson ISBN 1-57231-345-5</li>
<li>Fighter Combat<br>
by Robert L. Shaw ISBN 0-87021-059-9</li>
<li>Aircraft Control and Simulation<br>
by Brian L. Stevens and Frank L. Lewis ISBN 0-471-61397-5</li>
<li>1278.1-1995 IEEE Standard for Distributed Interactive
Simulation--Application Protocols ISBN 1-55937-572-8</li>
<li>The Surveying Handbook, second edition<br>
edited by Russel Brinker and Roy Minnick ISBN 0-412-98511-X</li>
</ul>
<p>
...and the source code of ACM which contains lots of interesting comments!
</p>
</body>
</html>
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