Aero input and outputs
Properties aerodynamic functions in JSBSim can read and output.
Function inputs
Location of the aerodynamic reference point in the aircraft structural frame, measured in inches:
/fdm/jsbsim/metrics/aero-rp-x-in /fdm/jsbsim/metrics/aero-rp-y-in /fdm/jsbsim/metrics/aero-rp-z-in
Maximum and minimum alpha (angle of attack) in degrees:
/fdm/jsbsim/aero/alpha-max-deg /fdm/jsbsim/aero/alpha-min-deg
Wing span/(2*Velocity)
/fdm/jsbsim/aero/bi2vel
Wing chord/(2*Velocity)
/fdm/jsbsim/aero/ci2vel
Alpha of the wing in radians (includes the angle of incidence of the wing, if input):
/fdm/jsbsim/aero/alpha-wing-rad
Altitude of CG divided by wing span:
/fdm/jsbsim/aero/h_b-cg-ft
Altitude of MAC divided by wing span:
/fdm/jsbsim/aero/h_b-mac-ft
Aerodynamic rotational rates in rad/sec (includes wind effects, if any):
/fdm/jsbsim/velocities/p-aero-rad_sec /fdm/jsbsim/velocities/q-aero-rad_sec /fdm/jsbsim/velocities/r-aero-rad_sec
Aerodynamic velocities in ft/sec (includes wind effects, if any):
/fdm/jsbsim/velocities/u-aero-fps /fdm/jsbsim/velocities/v-aero-fps /fdm/jsbsim/velocities/w-aero-fps
The body axis aerodynamic force vector of the aircraft in lbs:
/fdm/jsbsim/forces/fbx-aero-lbs /fdm/jsbsim/forces/fby-aero-lbs /fdm/jsbsim/forces/fbz-aero-lbs
The stability axis aerodynamic force vector of the aircraft in lbs:
/fdm/jsbsim/forces/fwx-aero-lbs /fdm/jsbsim/forces/fwy-aero-lbs /fdm/jsbsim/forces/fwz-aero-lbs
The body axis aerodynamic moment vector of the aircraft in ft-lbs:
/fdm/jsbsim/moments/l-aero-lbsft /fdm/jsbsim/moments/m-aero-lbsft /fdm/jsbsim/moments/n-aero-lbsft
The lift coefficient squared:
/fdm/jsbsim/aero/cl-squared
Dynamic pressure times reference area:
/fdm/jsbsim/aero/qbar-area
Angles of attack and sideslip in radians and degrees (and the magnitudes):
/fdm/jsbsim/aero/alpha-rad /fdm/jsbsim/aero/beta-rad /fdm/jsbsim/aero/mag-beta-rad /fdm/jsbsim/aero/alpha-deg /fdm/jsbsim/aero/beta-deg /fdm/jsbsim/aero/mag-beta-deg
Dynamic pressure in psf:
/fdm/jsbsim/aero/qbar-psf
Dynamic pressure in the body axis XZ plane:
/fdm/jsbsim/aero/qbarUW-psf
Dynamic pressure in the body axis XY plane:
/fdm/jsbsim/aero/qbarUV-psf
Others:
/fdm/jsbsim/metrics/cbarw-ft /fdm/jsbsim/metrics/bw-ft
Function outputs
Ground effect factors for lift and drag:
/fdm/jsbsim/aero/function/ground-effect-factor-lift /fdm/jsbsim/aero/function/ground-effect-factor-drag
Runtime-created aerodynamic force and moment components from definitions in the <aerodynamics> section of the configuration file (varies per-aircraft):
Lift coefficients:
/fdm/jsbsim/aero/coefficient/CL - Lift coefficient /fdm/jsbsim/aero/coefficient/CLwbh /fdm/jsbsim/aero/coefficient/CLDf /fdm/jsbsim/aero/coefficient/CLDe /fdm/jsbsim/aero/coefficient/CLadot /fdm/jsbsim/aero/coefficient/CLq /fdm/jsbsim/aero/coefficient/CLDsb - Lift coefficient of airbrake /fdm/jsbsim/aero/coefficient/CLDflaps - Lift coefficient of flaps
Alan wrote about lift:
Lift = qbar-psf x Sw-sqft x CL
Lift is a force, CL is the non-dimensional coefficient of lift, qbar-psf x Sw-sqft is the dimensionalisation factor
It is common practice to build up the lift force from several sources.
e.g., for the Lift Axis
Lift = qbar-psf x Sw-sqft x (CLwing +CLtail +CLelevator +CLfuselage +... +...)
here the various lift coefficients are summed before being dimensionalised.
or you can also write
Lift(wing) = qbar-psf x Sw-sqft x CLwing Lift(tail) = qbar-psf x Sw-sqft x CLtail Lift(fuselage) = qbar-psf x Sw-sqft x CLfuselage .... Lift(total) = Lift(wing)+Lift(tail)+Lift(fuselage) +.... +...
here the various lift coefficients are separately dimensionalised to produce a set of forces. These forces are then added to produce the total lift force.
You can write CLwing = CLo + Alpha* dCL/Alpha, or alternatively use a table to generate CL as a function of alpha, Mach no etc.
It is not necessary to slavishly follow the examples in the JSBSim reference manual, or copy other Flightgear examples. It is best to follow the conventions that apply to the data you have available.
JSBsim allows you to specify these equations in any way that suits you, or suits the data that is available.
One thing to be careful about is not including a component twice. e.g. the CL that you are given may already include the tail and fuselage effects, and this case they should not be added in again.
Drag coefficients:
/fdm/jsbsim/aero/coefficient/CD - Drag coefficient /fdm/jsbsim/aero/coefficient/CDo /fdm/jsbsim/aero/coefficient/CDDf /fdm/jsbsim/aero/coefficient/CDwbh /fdm/jsbsim/aero/coefficient/CDDe /fdm/jsbsim/aero/coefficient/CDbeta /fdm/jsbsim/aero/coefficient/CDDsb - Drag coefficient of airbrake /fdm/jsbsim/aero/coefficient/CDgear - Drag coefficient of gear /fdm/jsbsim/aero/coefficient/CDDflaps - Drag coefficient of flaps
Side force coefficients:
/fdm/jsbsim/aero/coefficient/CYb - Side Force coefficient due to Sideslip /fdm/jsbsim/aero/coefficient/CYda /fdm/jsbsim/aero/coefficient/CYdr /fdm/jsbsim/aero/coefficient/CYp /fdm/jsbsim/aero/coefficient/CYr
Roll moment coefficients:
/fdm/jsbsim/aero/coefficient/Clb /fdm/jsbsim/aero/coefficient/Clp /fdm/jsbsim/aero/coefficient/Clr /fdm/jsbsim/aero/coefficient/Clda /fdm/jsbsim/aero/coefficient/Cldr
Pitch moment coefficients:
/fdm/jsbsim/aero/coefficient/Cmalpha /fdm/jsbsim/aero/coefficient/CmDf /fdm/jsbsim/aero/coefficient/Cmq /fdm/jsbsim/aero/coefficient/Cmadot /fdm/jsbsim/aero/coefficient/Cmo /fdm/jsbsim/aero/coefficient/Cmde /fdm/jsbsim/aero/coefficient/CmDsb -
Yaw moment coefficients:
/fdm/jsbsim/aero/coefficient/Cnb /fdm/jsbsim/aero/coefficient/Cnp /fdm/jsbsim/aero/coefficient/Cnr /fdm/jsbsim/aero/coefficient/Cnda /fdm/jsbsim/aero/coefficient/Cndr
Stall hysteresis, normalized:
/fdm/jsbsim/aero/stall-hyst-norm
Time rate of change in alpha and sideslip in deg/sec and rad/sec:
/fdm/jsbsim/aero/alphadot-rad_sec /fdm/jsbsim/aero/betadot-rad_sec /fdm/jsbsim/aero/alphadot-deg_sec /fdm/jsbsim/aero/betadot-deg_sec