Re: [Flightgear-devel] J7W Shinden

[Dave P. <ski...@mi...>]

On Thu, 2007-02-01 at 13:43 -0800, Tatsuhiro Nishioka wrote:

> I hope many people fly it and tell me how to improve the stability  
> without doing this. I know it's supposed to be a bit unstable but  
> it's too unstable without
> this workaround.
Hi Tat,

Thanks for this great addition!

I took a look at the j7w Shindon including the j7w.xml yasim
configuration.  Did you know there is a command line yasim in
$FG_ROOT/bin that allows you to get a lot of feedback from a trial
config xml file like j7w.xml w/o running fgfs?  After a flight in the
Shindon, I was fairly certain that the CG (center of gravity) was too
far forward.  This required you to use an effectiveness of 3.5 on the
cannard.  And you did get the desired increase in stability.  So I made
an estimate from the 3D model as to approximately where the CG should
be.  The main wing should cary most of the weight, so since the wing is
swept, I tried to change the ballast to move the CG to about x = -1.7 m
(yours is at x = -0.666), or approximately at the leading edge root.
This allowed me to reduce the cannard effectiveness to 1.5. Also, from
your data, I see that the power off dirty stall is 67 knots, so I used
that in the approach configuration (yasim assumes that the approach
speed is the dirty stall speed).  This allows a slower stall and better
short field landings with approach at 85 knots.  I flew from KSFO to
KHLF and landed and turned off before the tower with little braking.

Bert Rutan's Long-EZ, etc. were designed so the cannard stalls first.
This is true for your model of the j7w also.

I moved the x coordinate of the main gear aft for the duration of this
experiment to assure the AC sat with the nose gear on the ground as I
moved the CG back and forth.  Give this config a try.
-- 
Dave Perry