Re: [Flightgear-flightmodel] New properties in c172.xml

[Tony P. <ap...@ea...>]

On Friday 07 September 2001 05:01 am, you wrote:
> > Thanx Tony for your reply
> > but could you please explain what does this values mean?
> >
> > > They are parameters needed for tail-off aero models.  If you don't use:
> > > FG_LBARH
> > > FG_LBARV
> > > FG_HTAILAREA
> > > FG_VTAILAREA
> > > FG_VBARH
> > > FG_VBARV
> > >
> > > in any of your coefficients then you need not supply values for these.
>
> Could you restate what tail-off aero is, too, while you answer Roman's
> question?

For an aircraft like the c172, the pitching moment sum is primarily composed 
of contributions from the wing, fuselage, thrust and horizontal tail.   So I 
can write:
Cm = Cmwing + Cmbody + Cmthrust + Cmhtail
The primary sources of lift are:
CL = CLwing + CLbody + CLhtail
That lift from the tail is generally the largest contributor to tail pitching 
moment (there will be a relatively small Cmo from the tail):
Cmhtail = -lh / cbar * CLhtail
where lh is the distance from the aero reference point to the MAC of the 
horizontal tail.  Note the minus sign, positve tail lift produces negative or 
nose down pitching moment.  

OK, so now if I derive Cmhtail offline and separate out the elevator effects 
so that I've got:
Cmhtail = Cmstabilizer + Cmde
Then realize that the  stabilizer is fixed relative to the body in the case 
of the c172, I can combine some terms and reduce the amount of data I need in 
the config file:
Cm = Cmwbh + Cmthrust + Cmde
where
Cmwbh = Cmwing + Cmbody + Cmstabilizer
A similar sort of reasoning can be followed for the lift equation:
CL = CLwbh + CLde

That's a tail-on model.  

On the other hand, if I wanted to take a little more fundamental approach I 
can build an aero model that always calculates the tail pitching moment from 
the tail lift.  Many people like this approach because it preserves the 
physical relationship that we know must exist between the tail lift and its 
contribution to the total aircraft pitching moment.  Also, if you design an 
airplane, you pretty much have to look at it this way so that you can make 
good choices regarding the tail airfoil section and planform.

The downside to a tail-off model is that it can be harder to make changes to 
the model data to get the simulation to match flight.  How much harder 
depends on both what you can measure on board and how you go about
 reducing the flight data down into something that can be applied to the 
simulation.

A similar sort of approach can be applied to the vertical tail (though my 
understanding is that tail-off vertical models are even harder to get right)

Does that help?




 
>
> Jon
>
>
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-- 
Tony Peden
ap...@ea...
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