JSBSim::FGLGear

FGLGear Class Reference

#include <FGLGear.h>

Inherits FGJSBBase.

List of all members.

Detailed Description

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.

Configuration File Format:

        <contact type="{BOGEY | STRUCTURE}" name="{string}">
            <location unit="{IN | M}">
                <x> {number} </x>
                <y> {number} </y>
                <z> {number} </z>
            </location>
            <static_friction> {number} </static_friction>
            <dynamic_friction> {number} </dynamic_friction>
            <rolling_friction> {number} </rolling_friction>
            <spring_coeff unit="{LBS/FT | N/M}"> {number} </spring_coeff>
            <damping_coeff unit="{LBS/FT/SEC | N/M/SEC}"> {number} </damping_coeff>
            <damping_coeff_rebound unit="{LBS/FT/SEC | N/M/SEC}"> {number} </damping_coeff_rebound>
            <max_steer unit="DEG"> {number | 0 | 360} </max_steer>
            <brake_group> {NONE | LEFT | RIGHT | CENTER | NOSE | TAIL} </brake_group>
            <retractable>{0 | 1}</retractable>
            <table type="{CORNERING_COEFF}">
            </table>
            <relaxation_velocity>
               <rolling unit="{FT/SEC | KTS | M/S}"> {number} </rolling>
               <side unit="{FT/SEC | KTS | M/S}"> {number} </side>
            </relaxation_velocity>
            <force_lag_filter>
               <rolling> {number} </rolling>
               <side> {number} </side>
            </force_lag_filter>
            <wheel_slip_filter> {number} </wheel_slip_filter>  
        </contact>
Author:
Jon S. Berndt
Version:
Id
FGLGear.h,v 1.21 2008/05/04 18:22:55 dpculp Exp
See also:
Richard E. McFarland, "A Standard Kinematic Model for Flight Simulation at NASA-Ames", NASA CR-2497, January 1975

Barnes W. McCormick, "Aerodynamics, Aeronautics, and Flight Mechanics", Wiley & Sons, 1979 ISBN 0-471-03032-5

W. A. Ragsdale, "A Generic Landing Gear Dynamics Model for LASRS++", AIAA-2000-4303

Definition at line 206 of file FGLGear.h.

Public Types

enum  BrakeGroup {
  bgNone = 0, bgLeft, bgRight, bgCenter,
  bgNose, bgTail
}
 Brake grouping enumerators.
enum  ContactType { ctBOGEY, ctSTRUCTURE, ctUNKNOWN }
 Contact point type.
enum  ReportType { erNone = 0, erTakeoff, erLand }
 Report type enumerators.
enum  SteerType { stSteer, stFixed, stCaster }
 Steering group membership enumerators.

Public Member Functions

void bind (void)
 FGLGear (const FGLGear &lgear)
 Constructor.
 FGLGear (Element *el, FGFDMExec *Executive, int number)
 Constructor.
FGColumnVector3 & Force (void)
 The Force vector for this gear.
double GetBodyLocation (int idx) const
FGColumnVector3 & GetBodyLocation (void)
 Gets the location of the gear in Body axes.
double GetBodyXForce (void) const
double GetBodyYForce (void) const
double GetBrakeFCoeff (void) const
int GetBrakeGroup (void) const
double GetCompForce (void) const
 Gets the gear compression force in pounds.
double GetCompLen (void) const
 Gets the current compressed length of the gear in feet.
double GetCompVel (void) const
 Gets the current gear compression velocity in ft/sec.
double GetDefaultSteerAngle (double cmd) const
bool GetGearUnitDown (void) const
double GetGearUnitPos (void)
bool GetGearUnitUp (void) const
double GetLocalGear (int idx) const
FGColumnVector3 & GetLocalGear (void)
string GetName (void) const
 Gets the name of the gear.
bool GetReport (void) const
 Get the console touchdown reporting feature.
bool GetRetractable (void) const
double GetstaticFCoeff (void) const
bool GetSteerable (void) const
double GetSteerNorm (void) const
int GetSteerType (void) const
double GetTirePressure (void) const
 Gets the current normalized tire pressure.
double GetWheelRollForce (void) const
double GetWheelRollVel (void) const
double GetWheelSideForce (void) const
double GetWheelSideVel (void) const
double GetWheelSlipAngle (void) const
double GetWheelVel (int axis) const
bool GetWOW (void) const
 Gets the Weight On Wheels flag value.
double GetZPosition (void) const
bool IsBogey (void) const
FGColumnVector3 & Moment (void)
 The Moment vector for this gear.
void SetBrake (double bp)
 Sets the brake value in percent (0 - 100).
void SetReport (bool flag)
 Set the console touchdown reporting feature.
void SetTirePressure (double p)
 Sets the new normalized tire pressure.
void SetZPosition (double z)
void unbind (void)
 ~FGLGear ()
 Destructor.

Constructor & Destructor Documentation

FGLGear ( Element *  el,
FGFDMExec *  Executive,
int  number 
)

Parameters:
el a pointer to the XML element that contains the CONTACT info.
Executive a pointer to the parent executive object
number integer identifier for this instance of FGLGear

Definition at line 60 of file FGLGear.cpp.

References Element::FindElement(), Element::FindElementTripletConvertTo(), Element::FindElementValue(), Element::FindElementValueAsNumber(), Element::FindElementValueAsNumberConvertTo(), Element::FindNextElement(), FGFDMExec::GetAircraft(), Element::GetAttributeValue(), FGFDMExec::GetAuxiliary(), Element::GetDataAsNumber(), FGFDMExec::GetFCS(), FGFDMExec::GetMassBalance(), FGFDMExec::GetPropagate(), FGFDMExec::GetPropertyManager(), FGFDMExec::GetState(), FGPropagate::GetTb2l(), and FGMassBalance::StructuralToBody().

FGLGear ( const FGLGear &  lgear  ) 

Parameters:
lgear a reference to an existing FGLGear object

Definition at line 232 of file FGLGear.cpp.

References FGLGear::Aircraft, FGLGear::Auxiliary, FGLGear::bDamp, FGLGear::bDampRebound, FGLGear::brakePct, FGLGear::compressLength, FGLGear::compressSpeed, FGLGear::CosWheel, FGLGear::dynamicFCoeff, FGLGear::eBrakeGrp, FGLGear::eContactType, FGLGear::eSteerType, FGLGear::Exec, FGLGear::FCS, FGLGear::FirstContact, FGLGear::ForceY_Table, FGLGear::GearDown, FGLGear::GearNumber, FGLGear::GearPos, FGLGear::GearUp, FGLGear::GroundSpeed, FGLGear::isRetractable, FGLGear::kSpring, FGLGear::LandingDistanceTraveled, FGLGear::LandingReported, FGLGear::lastWOW, FGLGear::LatForceLagFilterCoeff, FGLGear::LongForceLagFilterCoeff, FGLGear::MassBalance, FGLGear::maxCompLen, FGLGear::MaximumStrutForce, FGLGear::MaximumStrutTravel, FGLGear::maxSteerAngle, FGLGear::name, FGLGear::prevIn, FGLGear::prevOut, FGLGear::prevSlipIn, FGLGear::prevSlipOut, FGLGear::Propagate, FGLGear::ReportEnable, FGLGear::RFRV, FGLGear::rollingFCoeff, FGLGear::RollingForce, FGLGear::sBrakeGroup, FGLGear::sContactType, FGLGear::Servicable, FGLGear::SFRV, FGLGear::SideForce, FGLGear::SinkRate, FGLGear::SinWheel, FGLGear::sRetractable, FGLGear::sSteerType, FGLGear::StartedGroundRun, FGLGear::State, FGLGear::staticFCoeff, FGLGear::TakeoffDistanceTraveled, FGLGear::TakeoffDistanceTraveled50ft, FGLGear::TakeoffReported, FGLGear::TirePressureNorm, FGLGear::useFCSGearPos, FGLGear::vLocalGear, FGLGear::vMoment, FGLGear::vWhlBodyVec, FGLGear::vXYZ, FGLGear::WheelSlip, FGLGear::WheelSlipLagFilterCoeff, and FGLGear::WOW.

Member Function Documentation

bool GetReport ( void   )  const [inline]

Returns:
true if reporting is turned on

Definition at line 267 of file FGLGear.h.

void SetReport ( bool  flag  )  [inline]

Parameters:
flag true turns on touchdown reporting, false turns it off

Definition at line 264 of file FGLGear.h.

Referenced by FGTrimAnalysis::DoTrim(), and FGTrim::DoTrim().

The documentation for this class was generated from the following files:
Generated on Sun May 4 15:10:30 2008 for JSBSim Flight Dynamics Model by  doxygen 1.5.1