Re: [Jsbsim-devel] Moment axis versus force calculation of moments

[Tony P. <ton...@co...>]

That's a fine way to build a sim.

The difficulty in both obtaining the data and the complexity it introduces are the reasons we don't.  

It's also true that either way can get you acceptable fidelity.



--
Tony
Follow your heart, don't give up, and try like hell to not be stupid.

On Aug 19, 2010, at 1:27 PM, Sean McLeod <se...@se...> wrote:

> Hi
> 
>  
> 
> I have a general question regarding the use of the moment axes like the pitch axis to calculate a moment as opposed to modeling the relevant aerodynamic forces and using those to derive the relevant moment during simulation.
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> For now just talking about the pitch axis, which is probably the easiest case.
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> So instead of having a pitch axis with entries for Cm-alpha, Cm-q, Cm-de etc. couldn’t JSBSim model the lift of the main wing and tail and then given their relative arms relative to the current cg calculate the pitching moment? Ignoring contributions from the fuselage etc. at the moment.
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> Now JSBSim is already modeling the lift of the wing given the AoA, wing incidence angle, Cl lift curve for the wing etc.
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> So assuming the incidence angle of the tail and it’s Cl lift curve are known and it’s feasible to calculate the downwash angle then the lift on the tail could be calculated as well and given the 2 lift forces and their arms the pitching moment would then be calculated.
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> Given the pitch rate, velocity and tail arm the AoA-delta at the tail due to pitch rate could be calculated and be used to adjust the lift generated by the tail to replace Cm-q.
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> Elevator deflection would have an incremental effect on the tail lift replacing Cm-de.
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> Is the reason this approach isn’t taken because it’s difficult to source the relevant tail data, e.g. tail incidence, tail Cl curve and therefore it’s easier to source and use aero data like Cm-alpha, Cm-q to calculate pitching moments?
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> Or is the tail data generally  available but it’s difficult to model effects like downwash angle, or all the other contributions like the fuselage etc.?
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> I haven’t used Datcom+ yet but my rough understanding is that you feed in aircraft geometry into Datcom+ and it will then generate the stability coefficients like Cm-alpha, Cm-q etc. So presumably it’s taking into account effects like downwash angle, the tail’s lift etc. into account in order to generate Cm-alpha etc.?
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> Or have I missed some other underlying techniques or issues in terms of aero modeling?
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> Cheers
> 
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