/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Header: FGLGear.h Author: Jon S. Berndt Date started: 11/18/99 ------------- Copyright (C) 1999 Jon S. Berndt (jsb@hal-pc.org) ------------- This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. Further information about the GNU General Public License can also be found on the world wide web at http://www.gnu.org. HISTORY -------------------------------------------------------------------------------- 11/18/99 JSB Created %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% SENTRY %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ #ifndef FGLGEAR_H #define FGLGEAR_H /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% INCLUDES %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ #ifdef FGFS # include #endif #include #include "FGConfigFile.h" #include "FGMatrix33.h" #include "FGColumnVector3.h" #include "FGColumnVector4.h" #include "FGFDMExec.h" #include "FGJSBBase.h" /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% DEFINITIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ #define ID_LGEAR "$Id$" /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% FORWARD DECLARATIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ class FGAircraft; class FGPosition; class FGRotation; class FGFCS; class FGState; class FGMassBalance; /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% COMMENTS, REFERENCES, and NOTES [use "class documentation" below for API docs] %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CLASS DOCUMENTATION %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ /** Landing gear model. Calculates forces and moments due to landing gear reactions. This is done in several steps, and is dependent on what kind of gear is being modeled. Here are the parameters that can be specified in the config file for modeling landing gear:

Physical Characteristics

  1. X, Y, Z location, in inches in structural coordinate frame
  2. Spring constant, in lbs/ft
  3. Damping coefficient, in lbs/ft/sec
  4. Dynamic Friction Coefficient
  5. Static Friction Coefficient

Operational Properties

  1. Name
  2. Steerability attribute {one of STEERABLE | FIXED | CASTERED}
  3. Brake Group Membership {one of LEFT | CENTER | RIGHT | NOSE | TAIL | NONE}
  4. Max Steer Angle, in degrees

Algorithm and Approach to Modeling

  1. Find the location of the uncompressed landing gear relative to the CG of the aircraft. Remember, the structural coordinate frame that the aircraft is defined in is: X positive towards the tail, Y positive out the right side, Z positive upwards. The locations of the various parts are given in inches in the config file.
  2. The vector giving the location of the gear (relative to the cg) is rotated 180 degrees about the Y axis to put the coordinates in body frame (X positive forwards, Y positive out the right side, Z positive downwards, with the origin at the cg). The lengths are also now given in feet.
  3. The new gear location is now transformed to the local coordinate frame using the body-to-local matrix. (Mb2l).
  4. Knowing the location of the center of gravity relative to the ground (height above ground level or AGL) now enables gear deflection to be calculated. The gear compression value is the local frame gear Z location value minus the height AGL. [Currently, we make the assumption that the gear is oriented - and the deflection occurs in - the Z axis only. Additionally, the vector to the landing gear is currently not modified - which would (correctly) move the point of contact to the actual compressed-gear point of contact. Eventually, articulated gear may be modeled, but initially an effort must be made to model a generic system.] As an example, say the aircraft left main gear location (in local coordinates) is Z = 3 feet (positive) and the height AGL is 2 feet. This tells us that the gear is compressed 1 foot.
  5. If the gear is compressed, a Weight-On-Wheels (WOW) flag is set.
  6. With the compression length calculated, the compression velocity may now be calculated. This will be used to determine the damping force in the strut. The aircraft rotational rate is multiplied by the vector to the wheel to get a wheel velocity in body frame. That velocity vector is then transformed into the local coordinate frame.
  7. The aircraft cg velocity in the local frame is added to the just-calculated wheel velocity (due to rotation) to get a total wheel velocity in the local frame.
  8. The compression speed is the Z-component of the vector.
  9. With the wheel velocity vector no longer needed, it is normalized and multiplied by a -1 to reverse it. This will be used in the friction force calculation.
  10. Since the friction force takes place solely in the runway plane, the Z coordinate of the normalized wheel velocity vector is set to zero.
  11. The gear deflection force (the force on the aircraft acting along the local frame Z axis) is now calculated given the spring and damper coefficients, and the gear deflection speed and stroke length. Keep in mind that gear forces always act in the negative direction (in both local and body frames), and are not capable of generating a force in the positive sense (one that would attract the aircraft to the ground). So, the gear forces are always negative - they are limited to values of zero or less. The gear force is simply the negative of the sum of the spring compression length times the spring coefficient and the gear velocity times the damping coefficient.
  12. The lateral/directional force acting on the aircraft through the landing gear (along the local frame X and Y axes) is calculated next. First, the friction coefficient is multiplied by the recently calculated Z-force. This is the friction force. It must be given direction in addition to magnitude. We want the components in the local frame X and Y axes. From step 9, above, the conditioned wheel velocity vector is taken and the X and Y parts are multiplied by the friction force to get the X and Y components of friction.
  13. The wheel force in local frame is next converted to body frame.
  14. The moment due to the gear force is calculated by multiplying r x F (radius to wheel crossed into the wheel force). Both of these operands are in body frame.
@author Jon S. Berndt @version $Id$ @see Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at NASA-Ames", NASA CR-2497, January 1975 @see Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics", Wiley & Sons, 1979 ISBN 0-471-03032-5 @see W. A. Ragsdale, "A Generic Landing Gear Dynamics Model for LASRS++", AIAA-2000-4303 */ /*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% CLASS DECLARATION %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/ class FGLGear : public FGJSBBase { public: /// Brake grouping enumerators enum BrakeGroup {bgNone=0, bgLeft, bgRight, bgCenter, bgNose, bgTail }; /// Steering group membership enumerators enum SteerType {stSteer, stFixed, stCaster}; /** Constructor @param Executive a pointer to the parent executive object @param File a pointer to the config file instance */ FGLGear(FGConfigFile* File, FGFDMExec* Executive); /** Constructor @param lgear a reference to an existing FGLGear object */ FGLGear(const FGLGear& lgear); /// Destructor ~FGLGear(); /// The Force vector for this gear FGColumnVector3& Force(void); /// The Moment vector for this gear FGColumnVector3& Moment(void) {return vMoment;} /// Gets the location of the gear in Body axes FGColumnVector3& GetBodyLocation(void) { return vWhlBodyVec; } float GetBodyLocation(int idx) { return vWhlBodyVec(idx); } FGColumnVector3& GetLocalGear(void) { return vLocalGear; } float GetLocalGear(int idx) { return vLocalGear(idx); } /// Gets the name of the gear inline string GetName(void) {return name; } /// Gets the Weight On Wheels flag value inline bool GetWOW(void) {return WOW; } /// Gets the current compressed length of the gear in feet inline float GetCompLen(void) {return compressLength;} /// Gets the current gear compression velocity in ft/sec inline float GetCompVel(void) {return compressSpeed; } /// Gets the gear compression force in pounds inline float GetCompForce(void) {return Force()(3); } inline float GetBrakeFCoeff(void) {return BrakeFCoeff;} /// Sets the brake value in percent (0 - 100) inline void SetBrake(double bp) {brakePct = bp;} /** Set the console touchdown reporting feature @param flag true turns on touchdown reporting, false turns it off */ inline void SetReport(bool flag) { ReportEnable = flag; } /** Get the console touchdown reporting feature @return true if reporting is turned on */ inline bool GetReport(void) { return ReportEnable; } inline float GetSteerAngle(void) { return SteerAngle;} inline float GetstaticFCoeff(void) { return staticFCoeff;} inline int GetBrakeGroup(void) { return (int)eBrakeGrp; } inline int GetSteerType(void) { return (int)eSteerType; } private: FGColumnVector3 vXYZ; FGColumnVector3 vMoment; FGColumnVector3 vWhlBodyVec; FGColumnVector3 vLocalGear; FGColumnVector3 vForce; FGColumnVector3 vLocalForce; FGColumnVector3 vWhlVelVec; // Velocity of this wheel (Local) float SteerAngle; float kSpring; float bDamp; float compressLength; float compressSpeed; float staticFCoeff, dynamicFCoeff, rollingFCoeff; float brakePct; float BrakeFCoeff; float maxCompLen; double SinkRate; double GroundSpeed; double DistanceTraveled; double MaximumStrutForce; double MaximumStrutTravel; bool WOW; bool FirstContact; bool Reported; bool ReportEnable; string name; string sSteerType; string sBrakeGroup; BrakeGroup eBrakeGrp; SteerType eSteerType; float maxSteerAngle; FGFDMExec* Exec; FGState* State; FGAircraft* Aircraft; FGPosition* Position; FGRotation* Rotation; FGFCS* FCS; FGMassBalance* MassBalance; void Report(void); void Debug(void); }; #include "FGAircraft.h" #include "FGPosition.h" #include "FGRotation.h" #include "FGFCS.h" #include "FGMassBalance.h" //%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% #include "FGState.h" #endif