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"
47 #include "FGMatrix33.h"
48 #include "FGColumnVector3.h"
49 #include "FGColumnVector4.h"
50 #include "FGFDMExec.h"
51 #include "FGJSBBase.h"
53 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
55 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
57 #define ID_LGEAR "$Id$"
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70 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
71 COMMENTS, REFERENCES, and NOTES [use "class documentation" below for API docs]
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78 /** Landing gear model.
79 Calculates forces and moments due to landing gear reactions. This is done in
80 several steps, and is dependent on what kind of gear is being modeled. Here
81 are the parameters that can be specified in the config file for modeling
84 <b><u>Physical Characteristics</u></b><br>
86 <li>X, Y, Z location, in inches in structural coordinate frame</li>
87 <li>Spring constant, in lbs/ft</li>
88 <li>Damping coefficient, in lbs/ft/sec</li>
89 <li>Dynamic Friction Coefficient</li>
90 <li>Static Friction Coefficient</li>
92 <b><u>Operational Properties</b></u><br>
95 <li>Steerability attribute {one of STEERABLE | FIXED | CASTERED}</li>
96 <li>Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE}</li>
97 <li>Max Steer Angle, in degrees</li>
100 <b><u>Algorithm and Approach to Modeling</u></b><br>
102 <li>Find the location of the uncompressed landing gear relative to the CG of
103 the aircraft. Remember, the structural coordinate frame that the aircraft is
104 defined in is: X positive towards the tail, Y positive out the right side, Z
105 positive upwards. The locations of the various parts are given in inches in
106 the config file.</li>
107 <li>The vector giving the location of the gear (relative to the cg) is
108 rotated 180 degrees about the Y axis to put the coordinates in body frame (X
109 positive forwards, Y positive out the right side, Z positive downwards, with
110 the origin at the cg). The lengths are also now given in feet.</li>
111 <li>The new gear location is now transformed to the local coordinate frame
112 using the body-to-local matrix. (Mb2l).</li>
113 <li>Knowing the location of the center of gravity relative to the ground
114 (height above ground level or AGL) now enables gear deflection to be
115 calculated. The gear compression value is the local frame gear Z location
116 value minus the height AGL. [Currently, we make the assumption that the gear
117 is oriented - and the deflection occurs in - the Z axis only. Additionally,
118 the vector to the landing gear is currently not modified - which would
119 (correctly) move the point of contact to the actual compressed-gear point of
120 contact. Eventually, articulated gear may be modeled, but initially an
121 effort must be made to model a generic system.] As an example, say the
122 aircraft left main gear location (in local coordinates) is Z = 3 feet
123 (positive) and the height AGL is 2 feet. This tells us that the gear is
124 compressed 1 foot.</li>
125 <li>If the gear is compressed, a Weight-On-Wheels (WOW) flag is set.</li>
126 <li>With the compression length calculated, the compression velocity may now
127 be calculated. This will be used to determine the damping force in the
128 strut. The aircraft rotational rate is multiplied by the vector to the wheel
129 to get a wheel velocity in body frame. That velocity vector is then
130 transformed into the local coordinate frame.</li>
131 <li>The aircraft cg velocity in the local frame is added to the
132 just-calculated wheel velocity (due to rotation) to get a total wheel
133 velocity in the local frame.</li>
134 <li>The compression speed is the Z-component of the vector.</li>
135 <li>With the wheel velocity vector no longer needed, it is normalized and
136 multiplied by a -1 to reverse it. This will be used in the friction force
138 <li>Since the friction force takes place solely in the runway plane, the Z
139 coordinate of the normalized wheel velocity vector is set to zero.</li>
140 <li>The gear deflection force (the force on the aircraft acting along the
141 local frame Z axis) is now calculated given the spring and damper
142 coefficients, and the gear deflection speed and stroke length. Keep in mind
143 that gear forces always act in the negative direction (in both local and
144 body frames), and are not capable of generating a force in the positive
145 sense (one that would attract the aircraft to the ground). So, the gear
146 forces are always negative - they are limited to values of zero or less. The
147 gear force is simply the negative of the sum of the spring compression
148 length times the spring coefficient and the gear velocity times the damping
150 <li>The lateral/directional force acting on the aircraft through the landing
152 gear (along the local frame X and Y axes) is calculated next. First, the
153 friction coefficient is multiplied by the recently calculated Z-force. This
154 is the friction force. It must be given direction in addition to magnitude.
155 We want the components in the local frame X and Y axes. From step 9, above,
156 the conditioned wheel velocity vector is taken and the X and Y parts are
157 multiplied by the friction force to get the X and Y components of friction.
159 <li>The wheel force in local frame is next converted to body frame.</li>
160 <li>The moment due to the gear force is calculated by multiplying r x F
161 (radius to wheel crossed into the wheel force). Both of these operands are
164 @author Jon S. Berndt
166 @see Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at
167 NASA-Ames", NASA CR-2497, January 1975
168 @see Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics",
169 Wiley & Sons, 1979 ISBN 0-471-03032-5
170 @see W. A. Ragsdale, "A Generic Landing Gear Dynamics Model for LASRS++",
174 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
176 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
178 class FGLGear : public FGJSBBase
181 /// Brake grouping enumerators
182 enum BrakeGroup {bgNone=0, bgLeft, bgRight, bgCenter, bgNose, bgTail };
183 /// Steering group membership enumerators
184 enum SteerType {stSteer, stFixed, stCaster};
186 @param Executive a pointer to the parent executive object
187 @param File a pointer to the config file instance */
188 FGLGear(FGConfigFile* File, FGFDMExec* Executive);
190 @param lgear a reference to an existing FGLGear object */
191 FGLGear(const FGLGear& lgear);
196 /// The Force vector for this gear
197 FGColumnVector3& Force(void);
198 /// The Moment vector for this gear
199 FGColumnVector3& Moment(void) {return vMoment;}
201 /// Gets the location of the gear in Body axes
202 FGColumnVector3& GetBodyLocation(void) { return vWhlBodyVec; }
203 double GetBodyLocation(int idx) { return vWhlBodyVec(idx); }
205 FGColumnVector3& GetLocalGear(void) { return vLocalGear; }
206 double GetLocalGear(int idx) { return vLocalGear(idx); }
208 /// Gets the name of the gear
209 inline string GetName(void) {return name; }
210 /// Gets the Weight On Wheels flag value
211 inline bool GetWOW(void) {return WOW; }
212 /// Gets the current compressed length of the gear in feet
213 inline double GetCompLen(void) {return compressLength;}
214 /// Gets the current gear compression velocity in ft/sec
215 inline double GetCompVel(void) {return compressSpeed; }
216 /// Gets the gear compression force in pounds
217 inline double GetCompForce(void) {return Force()(3); }
218 inline double GetBrakeFCoeff(void) {return BrakeFCoeff;}
220 /// Sets the brake value in percent (0 - 100)
221 inline void SetBrake(double bp) {brakePct = bp;}
223 /** Set the console touchdown reporting feature
224 @param flag true turns on touchdown reporting, false turns it off */
225 inline void SetReport(bool flag) { ReportEnable = flag; }
226 /** Get the console touchdown reporting feature
227 @return true if reporting is turned on */
228 inline bool GetReport(void) { return ReportEnable; }
229 inline double GetSteerAngle(void) { return SteerAngle;}
230 inline double GetstaticFCoeff(void) { return staticFCoeff;}
232 inline int GetBrakeGroup(void) { return (int)eBrakeGrp; }
233 inline int GetSteerType(void) { return (int)eSteerType; }
235 inline bool GetRetractable(void) { return isRetractable; }
236 inline bool GetGearUnitUp(void) { return GearUp; }
237 inline bool GetGearUnitDown(void) { return GearDown; }
238 inline double GetWheelSideForce(void) { return SideForce; }
239 inline double GetWheelRollForce(void) { return RollingForce; }
240 inline double GetBodyXForce(void) { return vLocalForce(eX); }
241 inline double GetBodyYForce(void) { return vLocalForce(eY); }
242 inline double GetWheelSlipAngle(void) { return WheelSlip; }
245 FGColumnVector3 vXYZ;
246 FGColumnVector3 vMoment;
247 FGColumnVector3 vWhlBodyVec;
248 FGColumnVector3 vLocalGear;
249 FGColumnVector3 vForce;
250 FGColumnVector3 vLocalForce;
251 FGColumnVector3 vWhlVelVec; // Velocity of this wheel (Local)
255 double compressLength;
256 double compressSpeed;
257 double staticFCoeff, dynamicFCoeff, rollingFCoeff;
263 double DistanceTraveled;
264 double MaximumStrutForce;
265 double MaximumStrutTravel;
266 double SideWhlVel, RollingWhlVel;
267 double RollingForce, SideForce, FCoeff;
275 bool GearUp, GearDown;
281 BrakeGroup eBrakeGrp;
282 SteerType eSteerType;
283 double maxSteerAngle;
287 FGAircraft* Aircraft;
288 FGPosition* Position;
289 FGRotation* Rotation;
291 FGMassBalance* MassBalance;
294 void Debug(int from);
297 #include "FGAircraft.h"
298 #include "FGPosition.h"
299 #include "FGRotation.h"
301 #include "FGMassBalance.h"
303 //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%