1 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
7 ------------- Copyright (C) 1999 Jon S. Berndt (jon@jsbsim.org) -------------
9 This program is free software; you can redistribute it and/or modify it under
10 the terms of the GNU Lesser 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 Lesser General Public License for more
19 You should have received a copy of the GNU Lesser 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 Lesser General Public License can also be found on
24 the world wide web at http://www.gnu.org.
27 --------------------------------------------------------------------------------
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32 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
37 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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41 #include "FGFDMExec.h"
42 #include "models/propulsion/FGForce.h"
43 #include "input_output/FGXMLElement.h"
44 #include "math/FGColumnVector3.h"
45 #include "math/FGTable.h"
48 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
50 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
52 #define ID_LGEAR "$Id$"
54 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
56 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
67 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
69 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
71 /** Landing gear model.
72 Calculates forces and moments due to landing gear reactions. This is done in
73 several steps, and is dependent on what kind of gear is being modeled. Here
74 are the parameters that can be specified in the config file for modeling
77 <h3>Physical Characteristics</h3>
79 <li>X, Y, Z location, in inches in structural coordinate frame</li>
80 <li>Spring constant, in lbs/ft</li>
81 <li>Damping coefficient, in lbs/ft/sec</li>
82 <li>Dynamic Friction Coefficient</li>
83 <li>Static Friction Coefficient</li>
85 <h3>Operational Properties</h3>
88 <li>Steerability attribute {one of STEERABLE | FIXED | CASTERED}</li>
89 <li>Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE}</li>
90 <li>Max Steer Angle, in degrees</li>
93 <h3>Algorithm and Approach to Modeling</h3>
95 <li>Find the location of the uncompressed landing gear relative to the CG of
96 the aircraft. Remember, the structural coordinate frame that the aircraft is
97 defined in is: X positive towards the tail, Y positive out the right side, Z
98 positive upwards. The locations of the various parts are given in inches in
100 <li>The vector giving the location of the gear (relative to the cg) is
101 rotated 180 degrees about the Y axis to put the coordinates in body frame (X
102 positive forwards, Y positive out the right side, Z positive downwards, with
103 the origin at the cg). The lengths are also now given in feet.</li>
104 <li>The new gear location is now transformed to the local coordinate frame
105 using the body-to-local matrix. (Mb2l).</li>
106 <li>Knowing the location of the center of gravity relative to the ground
107 (height above ground level or AGL) now enables gear deflection to be
108 calculated. The gear compression value is the local frame gear Z location
109 value minus the height AGL. [Currently, we make the assumption that the gear
110 is oriented - and the deflection occurs in - the Z axis only. Additionally,
111 the vector to the landing gear is currently not modified - which would
112 (correctly) move the point of contact to the actual compressed-gear point of
113 contact. Eventually, articulated gear may be modeled, but initially an
114 effort must be made to model a generic system.] As an example, say the
115 aircraft left main gear location (in local coordinates) is Z = 3 feet
116 (positive) and the height AGL is 2 feet. This tells us that the gear is
117 compressed 1 foot.</li>
118 <li>If the gear is compressed, a Weight-On-Wheels (WOW) flag is set.</li>
119 <li>With the compression length calculated, the compression velocity may now
120 be calculated. This will be used to determine the damping force in the
121 strut. The aircraft rotational rate is multiplied by the vector to the wheel
122 to get a wheel velocity in body frame. That velocity vector is then
123 transformed into the local coordinate frame.</li>
124 <li>The aircraft cg velocity in the local frame is added to the
125 just-calculated wheel velocity (due to rotation) to get a total wheel
126 velocity in the local frame.</li>
127 <li>The compression speed is the Z-component of the vector.</li>
128 <li>With the wheel velocity vector no longer needed, it is normalized and
129 multiplied by a -1 to reverse it. This will be used in the friction force
131 <li>Since the friction force takes place solely in the runway plane, the Z
132 coordinate of the normalized wheel velocity vector is set to zero.</li>
133 <li>The gear deflection force (the force on the aircraft acting along the
134 local frame Z axis) is now calculated given the spring and damper
135 coefficients, and the gear deflection speed and stroke length. Keep in mind
136 that gear forces always act in the negative direction (in both local and
137 body frames), and are not capable of generating a force in the positive
138 sense (one that would attract the aircraft to the ground). So, the gear
139 forces are always negative - they are limited to values of zero or less. The
140 gear force is simply the negative of the sum of the spring compression
141 length times the spring coefficient and the gear velocity times the damping
143 <li>The lateral/directional force acting on the aircraft through the landing
145 gear (along the local frame X and Y axes) is calculated next. First, the
146 friction coefficient is multiplied by the recently calculated Z-force. This
147 is the friction force. It must be given direction in addition to magnitude.
148 We want the components in the local frame X and Y axes. From step 9, above,
149 the conditioned wheel velocity vector is taken and the X and Y parts are
150 multiplied by the friction force to get the X and Y components of friction.
152 <li>The wheel force in local frame is next converted to body frame.</li>
153 <li>The moment due to the gear force is calculated by multiplying r x F
154 (radius to wheel crossed into the wheel force). Both of these operands are
158 <h3>Configuration File Format:</h3>
160 <contact type="{BOGEY | STRUCTURE}" name="{string}">
161 <location unit="{IN | M}">
166 <static_friction> {number} </static_friction>
167 <dynamic_friction> {number} </dynamic_friction>
168 <rolling_friction> {number} </rolling_friction>
169 <spring_coeff unit="{LBS/FT | N/M}"> {number} </spring_coeff>
170 <damping_coeff unit="{LBS/FT/SEC | N/M/SEC}"> {number} </damping_coeff>
171 <damping_coeff_rebound unit="{LBS/FT/SEC | N/M/SEC}"> {number} </damping_coeff_rebound>
172 <max_steer unit="DEG"> {number | 0 | 360} </max_steer>
173 <brake_group> {NONE | LEFT | RIGHT | CENTER | NOSE | TAIL} </brake_group>
174 <retractable>{0 | 1}</retractable>
175 <table type="{CORNERING_COEFF}">
177 <relaxation_velocity>
178 <rolling unit="{FT/SEC | KTS | M/S}"> {number} </rolling>
179 <side unit="{FT/SEC | KTS | M/S}"> {number} </side>
180 </relaxation_velocity>
182 <rolling> {number} </rolling>
183 <side> {number} </side>
185 <wheel_slip_filter> {number} </wheel_slip_filter>
188 @author Jon S. Berndt
190 @see Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at
191 NASA-Ames", NASA CR-2497, January 1975
192 @see Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics",
193 Wiley & Sons, 1979 ISBN 0-471-03032-5
194 @see W. A. Ragsdale, "A Generic Landing Gear Dynamics Model for LASRS++",
198 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
200 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
202 class FGLGear : public FGForce
205 /// Brake grouping enumerators
206 enum BrakeGroup {bgNone=0, bgLeft, bgRight, bgCenter, bgNose, bgTail };
207 /// Steering group membership enumerators
208 enum SteerType {stSteer, stFixed, stCaster};
209 /// Contact point type
210 enum ContactType {ctBOGEY, ctSTRUCTURE};
211 /// Report type enumerators
212 enum ReportType {erNone=0, erTakeoff, erLand};
214 enum DampType {dtLinear=0, dtSquare};
216 @param el a pointer to the XML element that contains the CONTACT info.
217 @param Executive a pointer to the parent executive object
218 @param number integer identifier for this instance of FGLGear
220 FGLGear(Element* el, FGFDMExec* Executive, int number);
224 /// The Force vector for this gear
225 FGColumnVector3& GetBodyForces(void);
227 /// Gets the location of the gear in Body axes
228 FGColumnVector3& GetBodyLocation(void) { return vWhlBodyVec; }
229 double GetBodyLocation(int idx) const { return vWhlBodyVec(idx); }
231 FGColumnVector3& GetLocalGear(void) { return vLocalGear; }
232 double GetLocalGear(int idx) const { return vLocalGear(idx); }
234 /// Gets the name of the gear
235 string GetName(void) const {return name; }
236 /// Gets the Weight On Wheels flag value
237 bool GetWOW(void) const {return WOW; }
238 /// Gets the current compressed length of the gear in feet
239 double GetCompLen(void) const {return compressLength;}
240 /// Gets the current gear compression velocity in ft/sec
241 double GetCompVel(void) const {return compressSpeed; }
242 /// Gets the gear compression force in pounds
243 double GetCompForce(void) const {return StrutForce; }
244 double GetBrakeFCoeff(void) const {return BrakeFCoeff;}
246 /// Gets the current normalized tire pressure
247 double GetTirePressure(void) const { return TirePressureNorm; }
248 /// Sets the new normalized tire pressure
249 void SetTirePressure(double p) { TirePressureNorm = p; }
251 /// Sets the brake value in percent (0 - 100)
252 void SetBrake(double bp) {brakePct = bp;}
254 /// Sets the weight-on-wheels flag.
255 void SetWOW(bool wow) {WOW = wow;}
257 /** Set the console touchdown reporting feature
258 @param flag true turns on touchdown reporting, false turns it off */
259 void SetReport(bool flag) { ReportEnable = flag; }
260 /** Get the console touchdown reporting feature
261 @return true if reporting is turned on */
262 bool GetReport(void) const { return ReportEnable; }
263 double GetSteerNorm(void) const { return radtodeg/maxSteerAngle*SteerAngle; }
264 double GetDefaultSteerAngle(double cmd) const { return cmd*maxSteerAngle; }
265 double GetstaticFCoeff(void) const { return staticFCoeff; }
267 int GetBrakeGroup(void) const { return (int)eBrakeGrp; }
268 int GetSteerType(void) const { return (int)eSteerType; }
270 bool GetSteerable(void) const { return eSteerType != stFixed; }
271 bool GetRetractable(void) const { return isRetractable; }
272 bool GetGearUnitUp(void) const { return GearUp; }
273 bool GetGearUnitDown(void) const { return GearDown; }
274 double GetWheelRollForce(void) {
275 FGColumnVector3 vForce = mTGear.Transposed() * FGForce::GetBodyForces();
276 return vForce(eX)*cos(SteerAngle) + vForce(eY)*sin(SteerAngle); }
277 double GetWheelSideForce(void) {
278 FGColumnVector3 vForce = mTGear.Transposed() * FGForce::GetBodyForces();
279 return vForce(eY)*cos(SteerAngle) - vForce(eX)*sin(SteerAngle); }
280 double GetWheelRollVel(void) const { return vWhlVelVec(eX)*cos(SteerAngle)
281 + vWhlVelVec(eY)*sin(SteerAngle); }
282 double GetWheelSideVel(void) const { return vWhlVelVec(eY)*cos(SteerAngle)
283 - vWhlVelVec(eX)*sin(SteerAngle); }
284 double GetWheelSlipAngle(void) const { return WheelSlip; }
285 double GetWheelVel(int axis) const { return vWhlVelVec(axis);}
286 bool IsBogey(void) const { return (eContactType == ctBOGEY);}
287 double GetGearUnitPos(void);
293 static const FGMatrix33 Tb2s;
295 FGColumnVector3 vGearOrient;
296 FGColumnVector3 vWhlBodyVec;
297 FGColumnVector3 vLocalGear;
298 FGColumnVector3 vWhlVelVec, vLocalWhlVel; // Velocity of this wheel
299 FGColumnVector3 normal, cvel, vGroundNormal;
300 FGLocation contact, gearLoc;
301 FGTable *ForceY_Table;
307 double compressLength;
308 double compressSpeed;
309 double staticFCoeff, dynamicFCoeff, rollingFCoeff;
310 double Stiffness, Shape, Peak, Curvature; // Pacejka factors
316 double TakeoffDistanceTraveled;
317 double TakeoffDistanceTraveled50ft;
318 double LandingDistanceTraveled;
319 double MaximumStrutForce, StrutForce;
320 double MaximumStrutTravel;
323 double TirePressureNorm;
329 bool StartedGroundRun;
330 bool LandingReported;
331 bool TakeoffReported;
334 bool GearUp, GearDown;
342 BrakeGroup eBrakeGrp;
343 ContactType eContactType;
344 SteerType eSteerType;
346 DampType eDampTypeRebound;
347 double maxSteerAngle;
348 double RFRV; // Rolling force relaxation velocity
349 double SFRV; // Side force relaxation velocity
350 double LongForceLagFilterCoeff; // Longitudinal Force Lag Filter Coefficient
351 double LatForceLagFilterCoeff; // Lateral Force Lag Filter Coefficient
352 double WheelSlipLagFilterCoeff; // Wheel slip angle lag filter coefficient
354 Filter LongForceFilter;
355 Filter LatForceFilter;
356 Filter WheelSlipFilter;
359 FGAircraft* Aircraft;
360 FGPropagate* Propagate;
361 FGAuxiliary* Auxiliary;
363 FGMassBalance* MassBalance;
365 void ComputeRetractionState(void);
366 void ComputeBrakeForceCoefficient(void);
367 void ComputeSteeringAngle(void);
368 void ComputeSlipAngle(void);
369 void ComputeSideForceCoefficient(void);
370 void ComputeVerticalStrutForce(void);
371 void ComputeGroundCoordSys(void);
372 void CrashDetect(void);
373 void InitializeReporting(void);
374 void ResetReporting(void);
375 void ReportTakeoffOrLanding(void);
376 void Report(ReportType rt);
377 void Debug(int from);
380 #include "FGAircraft.h"
381 #include "FGPropagate.h"
382 #include "FGAuxiliary.h"
384 #include "FGMassBalance.h"
387 //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%