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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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------------- Copyright (C) 1999 Anthony K. Peden (apeden@earthlink.net) -------------
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This program is free software; you can redistribute it and/or modify it under
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the terms of the GNU General Public License as published by the Free Software
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Foundation; either version 2 of the License, or (at your option) any later
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This program is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
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You should have received a copy of the GNU General Public License along with
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this program; if not, write to the Free Software Foundation, Inc., 59 Temple
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Place - Suite 330, Boston, MA 02111-1307, USA.
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Further information about the GNU General Public License can also be found on
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the world wide web at http://www.gnu.org.
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--------------------------------------------------------------------------------
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FUNCTIONAL DESCRIPTION
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--------------------------------------------------------------------------------
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The purpose of this class is to provide storage for computed forces and
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encapsulate all the functionality associated with transforming those
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forces from their native coord system to the body system. This includes
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computing the moments due to the difference between the point of application
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CAVEAT: if the custom transform is used for wind-to-body transforms then the
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user *must* always pass this class the negative of beta. This is true
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because sideslip angle does not follow the right hand rule i.e. it is
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positive for aircraft nose left sideslip. Note that use of the custom
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transform for this purpose shouldn't be necessary as it is already
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provided by SetTransform(tWindBody) and is not subject to the same
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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#include "FGFDMExec.h"
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#include "FGJSBBase.h"
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#include "FGMatrix33.h"
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#include "FGColumnVector3.h"
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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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#define ID_FORCE "$Id: FGForce.h,v 1.6 2004/06/14 11:40:45 ehofman Exp $"
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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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/** Utility class that aids in the conversion of forces between coordinate systems
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and calculation of moments.
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<br><h3>Resolution of Applied Forces into Moments and Body Axes Components</h3>
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All forces acting on the aircraft that cannot be considered a change in weight
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need to be resolved into body axis components so that the aircraft acceleration
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vectors, both translational and rotational, can be computed. Furthermore, the
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moments produced by each force that does not act at a location corresponding to
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the center of gravity also need to be computed. Unfortunately, the math required
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to do this can be a bit messy and errors are easily introduced so the class
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FGForce was created to provide these services in a consistent and reusable
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<p>FGForce requires that its users supply it with the location of the applied
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force vector in JSBSim structural coordinates, the sense of each axis in that
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coordinate system relative to the body system, the orientation of the vector
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also relative to body coordinates and, of course, the force vector itself. With
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this information it will compute both the body axis force components and the
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resulting moments. Any moments inherently produced by the native system can be
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supplied as well and they will be summed with those computed.</p>
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<p>A good example for demonstrating the use of this class are the aerodynamic
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forces: lift, drag, and side force and the aerodynamic moments about the pitch,
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roll and yaw axes. These "native" forces and moments are computed and stored
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in the FGColumnVector objects vFs and vMoments. Their native coordinate system
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is often referred to as the wind system and is defined as a right-handed system
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having its x-axis aligned with the relative velocity vector and pointing towards
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the rear of the aircraft , the y-axis extending out the right wing, and the
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z-axis directed upwards. This is different than body axes; they are defined such
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that the x-axis is lies on the aircraft's roll axis and positive forward, the
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y-axis is positive out the right wing, and the z-axis is positive downwards. In
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this instance, JSBSim already provides the needed transform and FGForce can make
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use of it by calling SetTransformType() once an object is created:</p>
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<p><tt>FGForce fgf(FDMExec);</tt><br>
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<tt>fgf.SetTransformType(tWindBody);</tt><br><br>
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This call need only be made once for each object. The available transforms are
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defined in the enumerated type TransformType and are tWindBody, tLocalBody,
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tCustom, and tNone. The local-to-body transform, like the wind-to-body, also
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makes use of that already available in JSBSim. tNone sets FGForce to do no
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angular transform at all, and tCustom allows for modeling force vectors at
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arbitrary angles relative to the body system such as that produced by propulsion
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systems. Setting up and using a custom transform is covered in more detail below.
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Continuing with the example, the point of application of the aerodynamic forces,
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the aerodynamic reference point in JSBSim, also needs to be set:</p>
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fgf.SetLocation(x, y, z)</tt></p>
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<p>where x, y, and z are in JSBSim structural coordinates.</p>
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<p>Initialization is complete and the FGForce object is ready to do its job. As
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stated above, the lift, drag, and side force are computed and stored in the
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vector vFs and need to be passed to FGForce:</p>
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<p><tt>fgf.SetNativeForces(vFs);</tt> </p>
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<p>The same applies to the aerodynamic pitching, rolling and yawing moments:</p>
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<p><tt>fgf.SetNativeMoments(vMoments);</tt></p>
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<p>Note that storing the native forces and moments outside of this class is not
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strictly necessary, overloaded SetNativeForces() and SetNativeMoments() methods
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which each accept three doubles (rather than a vector) are provided and can be
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repeatedly called without incurring undue overhead. The body axes force vector
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can now be retrieved by calling:</p>
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<p><tt>vFb=fgf.GetBodyForces();</tt></p>
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<p>This method is where the bulk of the work gets done so calling it more than
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once for the same set of native forces and moments should probably be avoided.
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Note that the moment calculations are done here as well so they should not be
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retrieved after calling the GetBodyForces() method:</p>
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<p><tt>vM=fgf.GetMoments();</tt> </p>
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<p>As an aside, the native moments are not needed to perform the computations
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correctly so, if the FGForce object is not being used to store them then an
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alternate approach is to avoid the SetNativeMoments call and perform the sum</p>
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<p><tt>vMoments+=fgf.GetMoments();</tt> <br><br>
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after the forces have been retrieved. </p>
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<h4>Use of the Custom Transform Type</h4>
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<p>In cases where the native force vector is not aligned with the body, wind, or
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local coordinate systems a custom transform type is provided. A vectorable engine
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nozzle will be used to demonstrate its usage. Initialization is much the same:</p>
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<p><tt>FGForce fgf(FDMExec);</tt> <br>
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<tt>fgf.SetTransformType(tCustom);</tt> <br>
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<tt>fgf.SetLocation(x,y,z);</tt> </p>
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<p>Except that here the tCustom transform type is specified and the location of
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the thrust vector is used rather than the aerodynamic reference point. Thrust is
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typically considered to be positive when directed aft while the body x-axis is
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positive forward and, if the native system is right handed, the z-axis will be
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reversed as well. These differences in sense need to be specified using by the
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<p><tt>fgf.SetSense(-1,1,-1);</tt></p>
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<p>The angles are specified by calling the method: </p>
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<p><tt>fgf.SetAnglesToBody(pitch, roll, yaw);</tt> </p>
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<p>in which the transform matrix is computed. Note that these angles should be
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taken relative to the body system and not the local as the names might suggest.
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For an aircraft with vectorable thrust, this method will need to be called
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every time the nozzle angle changes, a fixed engine/nozzle installation, on the
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other hand, will require it to be be called only once.</p>
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<p>Retrieval of the computed forces and moments is done as detailed above.</p>
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<p><i>CAVEAT: If the custom system is used to compute
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the wind-to-body transform, then the sign of the sideslip
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angle must be reversed when calling SetAnglesToBody().
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This is true because sideslip angle does not follow the right
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hand rule. Using the custom transform type this way
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should not be necessary, as it is already provided as a built
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in type (and the sign differences are correctly accounted for).</i>
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<h4>Use as a Base Type</h4>
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<p>For use as a base type, the native force and moment vector data members are
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defined as protected. In this case the SetNativeForces() and SetNativeMoments()
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methods need not be used and, instead, the assignments to vFn, the force vector,
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and vMn, the moments, can be made directly. Otherwise, the usage is similar.<br>
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@version $Id: FGForce.h,v 1.6 2004/06/14 11:40:45 ehofman Exp $
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/*%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%*/
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class FGForce : public FGJSBBase
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FGForce(FGFDMExec *FDMExec);
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enum TransformType { tNone, tWindBody, tLocalBody, tCustom } ttype;
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inline void SetNativeForces(double Fnx, double Fny, double Fnz) {
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inline void SetNativeForces(FGColumnVector3 vv) { vFn = vv; };
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inline void SetNativeMoments(double Ln,double Mn, double Nn) {
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inline void SetNativeMoments(FGColumnVector3 vv) { vMn = vv; }
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inline FGColumnVector3& GetNativeForces(void) { return vFn; }
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inline FGColumnVector3& GetNativeMoments(void) { return vMn; }
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FGColumnVector3& GetBodyForces(void);
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inline FGColumnVector3& GetMoments(void) { return vM; }
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// Normal point of application, JSBsim structural coords
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// (inches, x +back, y +right, z +up)
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inline void SetLocation(double x, double y, double z) {
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SetActingLocation(x, y, z);
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/** Acting point of application.
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JSBsim structural coords used (inches, x +back, y +right, z +up).
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This function sets the point at which the force acts - this may
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not be the same as where the object resides. One area where this
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is true is P-Factor modeling.
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@param x acting location of force
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@param y acting location of force
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@param z acting location of force */
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inline void SetActingLocation(double x, double y, double z) {
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inline void SetLocationX(double x) {vXYZn(eX) = x; vActingXYZn(eX) = x;}
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inline void SetLocationY(double y) {vXYZn(eY) = y; vActingXYZn(eY) = y;}
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inline void SetLocationZ(double z) {vXYZn(eZ) = z; vActingXYZn(eZ) = z;}
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inline double SetActingLocationX(double x) {vActingXYZn(eX) = x; return x;}
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inline double SetActingLocationY(double y) {vActingXYZn(eY) = y; return y;}
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inline double SetActingLocationZ(double z) {vActingXYZn(eZ) = z; return z;}
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inline void SetLocation(FGColumnVector3 vv) { vXYZn = vv; SetActingLocation(vv);}
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inline void SetActingLocation(FGColumnVector3 vv) { vActingXYZn = vv; }
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inline double GetLocationX( void ) { return vXYZn(eX);}
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inline double GetLocationY( void ) { return vXYZn(eY);}
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inline double GetLocationZ( void ) { return vXYZn(eZ);}
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inline double GetActingLocationX( void ) { return vActingXYZn(eX);}
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inline double GetActingLocationY( void ) { return vActingXYZn(eY);}
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inline double GetActingLocationZ( void ) { return vActingXYZn(eZ);}
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FGColumnVector3& GetLocation(void) { return vXYZn; }
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FGColumnVector3& GetActingLocation(void) { return vActingXYZn; }
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//these angles are relative to body axes, not earth!!!!!
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//I'm using these because pitch, roll, and yaw are easy to visualize,
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//there's no equivalent to roll in wind axes i.e. alpha, ? , beta
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//making up new names or using these is a toss-up: either way people
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//are going to get confused.
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//They are in radians.
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void SetAnglesToBody(double broll, double bpitch, double byaw);
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inline void SetAnglesToBody(FGColumnVector3 vv) {
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SetAnglesToBody(vv(eRoll), vv(ePitch), vv(eYaw));
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inline void SetSense(double x, double y, double z) { vSense(eX)=x, vSense(eY)=y, vSense(eZ)=z; }
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inline void SetSense(FGColumnVector3 vv) { vSense=vv; }
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inline FGColumnVector3& GetSense(void) { return vSense; }
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inline void SetTransformType(TransformType ii) { ttype=ii; }
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inline TransformType GetTransformType(void) { return ttype; }
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FGMatrix33 Transform(void);
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FGColumnVector3 vXYZn;
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FGColumnVector3 vActingXYZn;
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FGColumnVector3 vDXYZ;
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FGColumnVector3 vSense;
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void Debug(int from);
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src/FDM/JSBSim/FGForce.h
