On August 21, 2010 11:39:52 pm Ron Jensen wrote: > 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
Thank you very much Jon and Ron. That certainly cleared things up for me. I only have one question left, which is my original question regarding turbofan in JSBsim. How does JSBsim simulates a turbofan engine? Ampere ------------------------------------------------------------------------------ Sell apps to millions through the Intel(R) Atom(Tm) Developer Program Be part of this innovative community and reach millions of netbook users worldwide. Take advantage of special opportunities to increase revenue and speed time-to-market. Join now, and jumpstart your future. http://p.sf.net/sfu/intel-atom-d2d _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
