Re: [Jsbsim-devel] Aitfoils at high alpha

[Ron F. <ron...@at...>]

----- Original Message ----- 
From: "Jon S Berndt" <js...@ha...>
To: <jsb...@li...>
Sent: Wednesday, July 21, 2004 6:17 PM
Subject: Re: [Jsbsim-devel] Aitfoils at high alpha


> On Wed, 21 Jul 2004 18:03:02 -0500
>   "Ron Freimuth" <ron...@at...> wrote:
> >
> >     I think the shape of the CL curve is important in modeling spins
(also
> >Cm vs alpha).  I suspect CL  gets stuck a bit past the bottom of the dip
at
> >20 degs in a stall/spin.  Something has to keep Alpha relatively constant
> >since a spin tends to stay in a constant state.
>
> Ron:
>
> Given what you know about simulation, let me state these questions to
> you (and anyone else that wants to answer):
>
> 1) What do you think is the value of providing some modeling of high
> alpha lift/drag characteristics - even if the data is sort of generic,
> providing perhaps (at best) only "plausible" dynamics?

    I've been sticking the humped CL vs AoA curve in my FS AC for some time
now.  Even in jet transports, which are never purposely stalled.

    MSFS does not set Cdi to IDK * CL^2.  Rather, it bases it on
{m*Alpha)^2.   Which gives virtually the same Cdi in the Linear region of
the lift polar, but continues to increase Cdi past stall.

    I saw one curve of Cdi in some image that suggested Cdi continued to
increase even faster than alpha^2 past stall.    JSBSim's CL, CD table
should probably be set for an increasing CD past stall, rather than dropping
as CL drops.

    Cm(alpha) is clearly important.

            ---------------
    Side note: I've finally figured that Cdo for Flaps always varies as
sin^2 (deflection).  Essentially deflection (rad) up to 30 deg.

    That works for split, ordinary, and fowler flaps.  Only the magnitude of
the sin^2 (def) need be set.  That's given in a 1930's formula and depends
on flapped width, 'depth', and type.


> 2) Do you have any insight or suggestions about modeling high alpha
> lift/drag characteristics, given the use of the JSBSim coefficient
> buildup method?
 > Jon

    JSBSim and MSFS have similar flight models.  FS has many tables that
multiply the basic SD's by a factor which varies with alpha.  It's 1.00 or
near at alpha =0, but typically drops as alpha increases.  JSBSim uses an
array for a SD that has rows for each alpha.

    Default tables in the FS AIR file are triangular shaped, and drop Cl_p,
Cl_beta, Cn_r, and Cn_beta on each side of AoA = 0.   I've modified them as
I found information on realistic variations.  Cl_p(alpha) typically
increases between alpha = -12 deg to alpha =  +12 deg.  But, drops rapidly
in the stall regions.

    Cl_beta increases over that range, at least if the wing has quite a bit
of sweep.  I've typically set the factor to 0.7 at alpha = 0 and 1.4 at
alpha = 12 deg. The increase in Dihedral Effect is quite apparent at lower
speeds, where AoA is high.

    Similar tables modify Cn_r and Cn_beta vs alpha.

    All four of the above affect the tendency to spin.  However, they are
not enough.

    While some CFS2 AC, and FS AC may develop spins of a sort, I don't think
the flight model has what it takes to model them correctly.

    I need to get my email out.  I'll go more into what I think is required
to model at least some spin characteristics later.

    NACA Report 517 goes into some aspects of spins.

    Ron