Re: stability vs damping; pitch vs AoA

[brian whatcott <inet@INTELLISYS.NET>]

At 12:59 10/28/00 -0400, John Denker wrote:
> ... rather than calling it
> "pitch-axis damping", an even better term would be "angle-of-attack
> damping".  An ordinary airplane in flight has
>   gobs of AoA stability,
>   marginal AoA damping, and
>   very little (ideally zero) sensitivity to pitch except via AoA;
>     i.e. AoA is a "sufficient statistic":
>       if you know the AoA you don't need to know the pitch.

Aerodynamics has traps for the unwary:
it is not difficult to produce zero pitch rate with changing AoA,
but it is easy to produce varying pitch rate with constant AoA - and
 this case needs consideration for stability analysis.
 Specifically, change in pitching moment coefficient due to change
in pitching rate is the stab derivative Cm subq
 This is sometimes called the 'pitch damping' derivative
 - usually negative.
 Worth considering for its effect on damping the short period mode.

Ref: Smetana, Computer Asssisted Analysis of Aircraft Performance,
 Stability & Control. McGraw-Hill
brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!