1 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
7 ------------- Copyright (C) 1999 Jon S. Berndt (jsb@hal-pc.org) -------------
9 This program is free software; you can redistribute it and/or modify it under
10 the terms of the GNU General Public License as published by the Free Software
11 Foundation; either version 2 of the License, or (at your option) any later
14 This program is distributed in the hope that it will be useful, but WITHOUT
15 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
16 FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
19 You should have received a copy of the GNU General Public License along with
20 this program; if not, write to the Free Software Foundation, Inc., 59 Temple
21 Place - Suite 330, Boston, MA 02111-1307, USA.
23 Further information about the GNU General Public License can also be found on
24 the world wide web at http://www.gnu.org.
27 --------------------------------------------------------------------------------
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42 # include <simgear/compiler.h>
46 #include "FGConfigFile.h"
48 #include "FGFDMExec.h"
51 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
53 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
55 #define ID_LGEAR "$Header"
57 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
59 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
65 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
66 COMMENTS, REFERENCES, and NOTES [use "class documentation" below for API docs]
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69 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
71 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
73 /** Landing gear model.
74 Calculates forces and moments due to landing gear reactions. This is done in
75 several steps, and is dependent on what kind of gear is being modeled. Here
76 are the parameters that can be specified in the config file for modeling
79 <b><u>Physical Characteristics</u></b><br>
81 <li>X, Y, Z location, in inches in structural coordinate frame</li>
82 <li>Spring constant, in lbs/ft</li>
83 <li>Damping coefficient, in lbs/ft/sec</li>
84 <li>Dynamic Friction Coefficient</li>
85 <li>Static Friction Coefficient</li>
87 <b><u>Operational Properties</b></u><br>
90 <li>Steerability attribute {one of STEERABLE | FIXED | CASTERED}</li>
91 <li>Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE}</li>
92 <li>Max Steer Angle, in degrees</li>
95 <b><u>Algorithm and Approach to Modeling</u></b><br>
97 <li>Find the location of the uncompressed landing gear relative to the CG of
98 the aircraft. Remember, the structural coordinate frame that the aircraft is
99 defined in is: X positive towards the tail, Y positive out the right side, Z
100 positive upwards. The locations of the various parts are given in inches in
101 the config file.</li>
102 <li>The vector giving the location of the gear (relative to the cg) is
103 rotated 180 degrees about the Y axis to put the coordinates in body frame (X
104 positive forwards, Y positive out the right side, Z positive downwards, with
105 the origin at the cg). The lengths are also now given in feet.</li>
106 <li>The new gear location is now transformed to the local coordinate frame
107 using the body-to-local matrix. (Mb2l).</li>
108 <li>Knowing the location of the center of gravity relative to the ground
109 (height above ground level or AGL) now enables gear deflection to be
110 calculated. The gear compression value is the local frame gear Z location
111 value minus the height AGL. [Currently, we make the assumption that the gear
112 is oriented - and the deflection occurs in - the Z axis only. Additionally,
113 the vector to the landing gear is currently not modified - which would
114 (correctly) move the point of contact to the actual compressed-gear point of
115 contact. Eventually, articulated gear may be modeled, but initially an
116 effort must be made to model a generic system.] As an example, say the
117 aircraft left main gear location (in local coordinates) is Z = 3 feet
118 (positive) and the height AGL is 2 feet. This tells us that the gear is
119 compressed 1 foot.</li>
120 <li>If the gear is compressed, a Weight-On-Wheels (WOW) flag is set.</li>
121 <li>With the compression length calculated, the compression velocity may now
122 be calculated. This will be used to determine the damping force in the
123 strut. The aircraft rotational rate is multiplied by the vector to the wheel
124 to get a wheel velocity in body frame. That velocity vector is then
125 transformed into the local coordinate frame.</li>
126 <li>The aircraft cg velocity in the local frame is added to the
127 just-calculated wheel velocity (due to rotation) to get a total wheel
128 velocity in the local frame.</li>
129 <li>The compression speed is the Z-component of the vector.</li>
130 <li>With the wheel velocity vector no longer needed, it is normalized and
131 multiplied by a -1 to reverse it. This will be used in the friction force
133 <li>Since the friction force takes place solely in the runway plane, the Z
134 coordinate of the normalized wheel velocity vector is set to zero.</li>
135 <li>The gear deflection force (the force on the aircraft acting along the
136 local frame Z axis) is now calculated given the spring and damper
137 coefficients, and the gear deflection speed and stroke length. Keep in mind
138 that gear forces always act in the negative direction (in both local and
139 body frames), and are not capable of generating a force in the positive
140 sense (one that would attract the aircraft to the ground). So, the gear
141 forces are always negative - they are limited to values of zero or less. The
142 gear force is simply the negative of the sum of the spring compression
143 length times the spring coefficient and the gear velocity times the damping
145 <li>The lateral/directional force acting on the aircraft through the landing
146 gear (along the local frame X and Y axes) is calculated next. First, the
147 friction coefficient is multiplied by the recently calculated Z-force. This
148 is the friction force. It must be given direction in addition to magnitude.
149 We want the components in the local frame X and Y axes. From step 9, above,
150 the conditioned wheel velocity vector is taken and the X and Y parts are
151 multiplied by the friction force to get the X and Y components of friction.
153 <li>The wheel force in local frame is next converted to body frame.</li>
154 <li>The moment due to the gear force is calculated by multiplying r x F
155 (radius to wheel crossed into the wheel force). Both of these operands are
158 @author Jon S. Berndt
160 @see Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at
161 NASA-Ames", NASA CR-2497, January 1975
162 @see Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics",
163 Wiley & Sons, 1979 ISBN 0-471-03032-5
166 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
168 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
173 /// Brake grouping enumerators
174 enum eBrakeGroup {bgNone=0, bgLeft, bgRight, bgCenter, bgNose, bgTail };
176 @param Executive a pointer to the parent executive object
177 @param File a pointer to the config file instance */
178 FGLGear(FGConfigFile* File, FGFDMExec* Executive);
180 @param lgear a reference to an existing FGLGear object */
181 FGLGear(const FGLGear& lgear);
186 /// The Force vector for this gear
187 FGColumnVector Force(void);
188 /// The Moment vector for this gear
189 FGColumnVector Moment(void) {return vMoment;}
190 /// Gets the location of the gear in Body axes
191 FGColumnVector GetBodyLocation(void) { return vWhlBodyVec; }
193 FGColumnVector GetLocalGear(void) { return vLocalGear; }
195 /// Gets the name of the gear
196 inline string GetName(void) {return name; }
197 /// Gets the Weight On Wheels flag value
198 inline bool GetWOW(void) {return WOW; }
199 /// Gets the current compressed length of the gear in feet
200 inline float GetCompLen(void) {return compressLength;}
201 /// Gets the current gear compression velocity in ft/sec
202 inline float GetCompVel(void) {return compressSpeed; }
203 /// Gets the gear compression force in pounds
204 inline float GetCompForce(void) {return Force()(3); }
206 /// Sets the brake value in percent (0 - 100)
207 inline void SetBrake(double bp) {brakePct = bp;}
209 /** Set the console touchdown reporting feature
210 @param flag true turns on touchdown reporting, false turns it off */
211 inline void SetReport(bool flag) { ReportEnable = flag; }
212 /** Get the console touchdown reporting feature
213 @return true if reporting is turned on */
214 inline bool GetReport(void) { return ReportEnable; }
219 FGColumnVector vMoment;
220 FGColumnVector vWhlBodyVec;
221 FGColumnVector vLocalGear;
224 float compressLength;
226 float staticFCoeff, dynamicFCoeff;
231 double DistanceTraveled;
232 double MaximumStrutForce;
233 double MaximumStrutTravel;
241 eBrakeGroup eBrakeGrp;
246 FGAircraft* Aircraft;
247 FGPosition* Position;
248 FGRotation* Rotation;
253 #include "FGAircraft.h"
254 #include "FGPosition.h"
255 #include "FGRotation.h"
257 //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%