[Flightgear-devel] 747-100 config problem

[Dave Culp]

It looks like the main problem with the 747-100 configuration file is the 
ground effect factor in the <drag> section.  Here it is:

         <FACTOR NAME="kCDge" TYPE="VECTOR">
            Change_in_drag_due_to_ground_effect
            11
            aero/h_b-mac-ft
            none
            0   0.048
            8   0.515
            16  0.709
            24  0.815
            32  0.882
            40  0.928
            48  0.962
            56  0.988
            64  1.0
            72  1.0
            80  1.0
         </FACTOR>

The property used is called h_b-mac-ft.  This is what is actually calculated 
in FGAuxiliary:


    FGColumnVector3 vMac = 
Propagate->GetTb2l()*MassBalance->StructuralToBody(Aircraft->GetXYZrp());
    hoverbmac = (Propagate->GetDistanceAGL() + vMac(3)) / 
Aircraft->GetWingSpan();


Which is basically the altitude AGL divided by the wing span.  Using a chart 
like this one:

      http://home.comcast.net/~davidculp2/groundeffect.png

We see that ground effect starts at a height of about one wingspan.  In the 
747-100 config file the ground effect is occurring  at 64 wingspans, which 
for a 747 means it occurs everywhere under 12000 feet.  So, the author is off 
by a factor of 64.  A better factor would be:

         <FACTOR NAME="kCDge" TYPE="VECTOR">
            Change_in_drag_due_to_ground_effect
            9
            aero/h_b-mac-ft
            none
            0          0.048
            0.125  0.515
            0.250  0.709
            0.375  0.815
            0.500  0.882
            0.625  0.928
            0.750  0.962
            0.875  0.988
            1.000  1.0
         </FACTOR>

I've flight-tested it and the drag looks much better.  What's the protocol for 
fixing configs?


Dave

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