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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32 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
37 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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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 "$Id$"
59 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
61 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
72 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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76 /** Landing gear model.
77 Calculates forces and moments due to landing gear reactions. This is done in
78 several steps, and is dependent on what kind of gear is being modeled. Here
79 are the parameters that can be specified in the config file for modeling
82 <b><u>Physical Characteristics</u></b><br>
84 <li>X, Y, Z location, in inches in structural coordinate frame</li>
85 <li>Spring constant, in lbs/ft</li>
86 <li>Damping coefficient, in lbs/ft/sec</li>
87 <li>Dynamic Friction Coefficient</li>
88 <li>Static Friction Coefficient</li>
90 <b><u>Operational Properties</b></u><br>
93 <li>Steerability attribute {one of STEERABLE | FIXED | CASTERED}</li>
94 <li>Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE}</li>
95 <li>Max Steer Angle, in degrees</li>
98 <b><u>Algorithm and Approach to Modeling</u></b><br>
100 <li>Find the location of the uncompressed landing gear relative to the CG of
101 the aircraft. Remember, the structural coordinate frame that the aircraft is
102 defined in is: X positive towards the tail, Y positive out the right side, Z
103 positive upwards. The locations of the various parts are given in inches in
104 the config file.</li>
105 <li>The vector giving the location of the gear (relative to the cg) is
106 rotated 180 degrees about the Y axis to put the coordinates in body frame (X
107 positive forwards, Y positive out the right side, Z positive downwards, with
108 the origin at the cg). The lengths are also now given in feet.</li>
109 <li>The new gear location is now transformed to the local coordinate frame
110 using the body-to-local matrix. (Mb2l).</li>
111 <li>Knowing the location of the center of gravity relative to the ground
112 (height above ground level or AGL) now enables gear deflection to be
113 calculated. The gear compression value is the local frame gear Z location
114 value minus the height AGL. [Currently, we make the assumption that the gear
115 is oriented - and the deflection occurs in - the Z axis only. Additionally,
116 the vector to the landing gear is currently not modified - which would
117 (correctly) move the point of contact to the actual compressed-gear point of
118 contact. Eventually, articulated gear may be modeled, but initially an
119 effort must be made to model a generic system.] As an example, say the
120 aircraft left main gear location (in local coordinates) is Z = 3 feet
121 (positive) and the height AGL is 2 feet. This tells us that the gear is
122 compressed 1 foot.</li>
123 <li>If the gear is compressed, a Weight-On-Wheels (WOW) flag is set.</li>
124 <li>With the compression length calculated, the compression velocity may now
125 be calculated. This will be used to determine the damping force in the
126 strut. The aircraft rotational rate is multiplied by the vector to the wheel
127 to get a wheel velocity in body frame. That velocity vector is then
128 transformed into the local coordinate frame.</li>
129 <li>The aircraft cg velocity in the local frame is added to the
130 just-calculated wheel velocity (due to rotation) to get a total wheel
131 velocity in the local frame.</li>
132 <li>The compression speed is the Z-component of the vector.</li>
133 <li>With the wheel velocity vector no longer needed, it is normalized and
134 multiplied by a -1 to reverse it. This will be used in the friction force
136 <li>Since the friction force takes place solely in the runway plane, the Z
137 coordinate of the normalized wheel velocity vector is set to zero.</li>
138 <li>The gear deflection force (the force on the aircraft acting along the
139 local frame Z axis) is now calculated given the spring and damper
140 coefficients, and the gear deflection speed and stroke length. Keep in mind
141 that gear forces always act in the negative direction (in both local and
142 body frames), and are not capable of generating a force in the positive
143 sense (one that would attract the aircraft to the ground). So, the gear
144 forces are always negative - they are limited to values of zero or less. The
145 gear force is simply the negative of the sum of the spring compression
146 length times the spring coefficient and the gear velocity times the damping
148 <li>The lateral/directional force acting on the aircraft through the landing
150 gear (along the local frame X and Y axes) is calculated next. First, the
151 friction coefficient is multiplied by the recently calculated Z-force. This
152 is the friction force. It must be given direction in addition to magnitude.
153 We want the components in the local frame X and Y axes. From step 9, above,
154 the conditioned wheel velocity vector is taken and the X and Y parts are
155 multiplied by the friction force to get the X and Y components of friction.
157 <li>The wheel force in local frame is next converted to body frame.</li>
158 <li>The moment due to the gear force is calculated by multiplying r x F
159 (radius to wheel crossed into the wheel force). Both of these operands are
162 @author Jon S. Berndt
164 @see Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at
165 NASA-Ames", NASA CR-2497, January 1975
166 @see Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics",
167 Wiley & Sons, 1979 ISBN 0-471-03032-5
168 @see W. A. Ragsdale, "A Generic Landing Gear Dynamics Model for LASRS++",
172 /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
174 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
176 class FGLGear : public FGJSBBase
179 /// Brake grouping enumerators
180 enum BrakeGroup {bgNone=0, bgLeft, bgRight, bgCenter, bgNose, bgTail };
181 /// Steering group membership enumerators
182 enum SteerType {stSteer, stFixed, stCaster};
183 /// Report type enumerators
184 enum ReportType {erNone=0, erTakeoff, erLand};
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 /// Gets the current normalized tire pressure
221 inline double GetTirePressure(void) { return TirePressureNorm; }
222 /// Sets the new normalized tire pressure
223 inline void SetTirePressure(double p) { TirePressureNorm = p; }
225 /// Sets the brake value in percent (0 - 100)
226 inline void SetBrake(double bp) {brakePct = bp;}
228 /** Set the console touchdown reporting feature
229 @param flag true turns on touchdown reporting, false turns it off */
230 inline void SetReport(bool flag) { ReportEnable = flag; }
231 /** Get the console touchdown reporting feature
232 @return true if reporting is turned on */
233 inline bool GetReport(void) { return ReportEnable; }
234 inline double GetSteerAngle(void) { return SteerAngle;}
235 inline double GetstaticFCoeff(void) { return staticFCoeff;}
237 inline int GetBrakeGroup(void) { return (int)eBrakeGrp; }
238 inline int GetSteerType(void) { return (int)eSteerType; }
240 inline bool GetRetractable(void) { return isRetractable; }
241 inline bool GetGearUnitUp(void) { return GearUp; }
242 inline bool GetGearUnitDown(void) { return GearDown; }
243 inline double GetWheelSideForce(void) { return SideForce; }
244 inline double GetWheelRollForce(void) { return RollingForce; }
245 inline double GetBodyXForce(void) { return vLocalForce(eX); }
246 inline double GetBodyYForce(void) { return vLocalForce(eY); }
247 inline double GetWheelSlipAngle(void) { return WheelSlip; }
248 double GetWheelVel(int axis) { return vWhlVelVec(axis);}
251 FGColumnVector3 vXYZ;
252 FGColumnVector3 vMoment;
253 FGColumnVector3 vWhlBodyVec;
254 FGColumnVector3 vLocalGear;
255 FGColumnVector3 vForce;
256 FGColumnVector3 vLocalForce;
257 FGColumnVector3 vWhlVelVec; // Velocity of this wheel (Local)
261 double compressLength;
262 double compressSpeed;
263 double staticFCoeff, dynamicFCoeff, rollingFCoeff;
269 double TakeoffDistanceTraveled;
270 double TakeoffDistanceTraveled50ft;
271 double LandingDistanceTraveled;
272 double MaximumStrutForce;
273 double MaximumStrutTravel;
274 double SideWhlVel, RollingWhlVel;
275 double RollingForce, SideForce, FCoeff;
277 double lastWheelSlip;
278 double TirePressureNorm;
282 bool StartedGroundRun;
283 bool LandingReported;
284 bool TakeoffReported;
287 bool GearUp, GearDown;
294 BrakeGroup eBrakeGrp;
295 SteerType eSteerType;
296 double maxSteerAngle;
300 FGAircraft* Aircraft;
301 FGPosition* Position;
302 FGRotation* Rotation;
304 FGMassBalance* MassBalance;
306 void Report(ReportType rt);
307 void Debug(int from);
310 #include "FGAircraft.h"
311 #include "FGPosition.h"
312 #include "FGRotation.h"
314 #include "FGMassBalance.h"
316 //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%