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

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

Function outputs

Runtime-created aerodynamic force and moment components from definitions in the <aerodynamics> section of the configuration file (varies per-aircraft):

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.