On Saturday 21 August 2010 15:57:30 Jon S. Berndt wrote: > > Now I am getting confused. I need an example. > > > > Say if I have the following entry: > > <function name="aero/coefficient/CDo"> > > <description>Drag_at_zero_lift</description> > > <product> > > <property>aero/qbar-psf</property> > > <property>metrics/Sw-sqft</property> > > <value>0.016</value> > > </product> > > </function> > > > > What does this do? What does 0.016 signify? > > > > Ampere > > Ampere, > > In your example, the following modified version would be a *coefficient* > definition, > > <function name="aero/coefficient/CDo"> > <description>Drag_at_zero_lift</description> > <product> > <value> 0.016 </value> > </product> > </function> > > But, that has to be turned into a force through multiplication by wing area > and dynamic pressure: > > <property>aero/qbar-psf</property> > <property>metrics/Sw-sqft</property> > > We are assuming that the CDo coefficient value is a constant 0.016. So, the > definition of the example you gave, above, is actually a force. So, more > properly, this would be defined as a "force", e.g.: > > <function name="aero/force/CDo"> > <description>Drag_at_zero_lift</description> > <product> > <property>aero/qbar-psf</property> > <property>metrics/Sw-sqft</property> > <value>0.016</value> > </product> > </function> > > Is that more clear? > > Jon
Or in computer-ish math: CDo = Qbar * WingArea * 0.016; // QBar = Velocity^2 * air_density / 2; Drag = CDo + All_Other_Drag_Function_Coefficients; Looking at our units (forgive the American engineering units, I spend much of my engineering study efforts reading NACA documents and I find units like the Newton and Pascal to be obtuse and inane) Velocity squared = (ft/s)^2 Density = slug/ft^3 Area = ft^2 (ft/s)^2 * (slug/ft^3) * ft^2 = slug * (ft/s^2) = mass * acceleration = force The 0.016 would be a non-dimensional coefficient of drag. http://en.wikipedia.org/wiki/Drag_coefficient gives us the equation Cd = 2 * Force / ( density * velocity^2 * Area) Solving for force, which is what we need to "fly": Force = Cd * Area * ( density * velocity^2 ) / 2 But QBar = ( density * velocity^2 ) / 2 So Force = Cd * Area * QBar Which brings us back to the original xml function fragment. Thanks, Ron ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
