There is an excellent article on flat spins by Peter Garrison in his
"Aftermath" column in this month's Flying. Garrison does a really fine
job explaining the aerodynamics involved, and also the types of mental
mistakes a pilot can make that can lead to an inadvertent flat spin -
especially when an aft CG is involved.
This is critical stuff, especially for homebuilt airplanes, and even
more so when a non-standard engine is involved, because the airplane has
usually been designed around the weight and CG of a Lycoming or other
"standard" engine. Looking back on the discussion we had here recently
with Baz and his white-knuckle test flights, it seems that some of us
will take off without having first done an accurate CG - as incredible
as that sounds.
Garrison presents no less than three flat spin accidents in his report,
all of them in fully aerobatic airplanes, a Pitts, a Christen Eagle and
a Sukhoi. With his usual precision and thoughtfulness, he analyzes each
of these accidents and homes in on the critical issue: the aft CG, in
combination with excess weight.
The article does perhaps leave the impression that flat spins are always
to be avoided, although, in truth, they are an aerobatic manoeuvre that
can be entered and recovered from quite predictably. Once you know the
control inputs required for the particular airplane being flown, they
are no more difficult than the regular, nose-down spin -- provided you
don't have an aft CG or overloaded plane.
Although the flat spin is not a competition manoeuvre, some pilots do
fly them in air shows and they are a real crowd pleaser. A local pilot,
Rick Volker, performs them regularly in his Sukhoi. (This airplane will
also do a vertical hover, which is something to see - it has a thrust to
weight ratio of better than 1.)
http://www.rvairshows.com/index.html
Regards,
Gordon.
Just an update from Baz.
I have been home a couple of weeks now and after checking that the
information I had was correct as far as the C of G envelope was concerned
I
physically balanced the Cobra. Although it came out inside the limits, the
balance point wasn't where I thought it would be according to
calculations.
I made some adjustments to ballast in the tail and have balanced with 1
pilot, full fuel, pilot, passenger, full fuel, pilot passenger no
fuel ( most aft position ). I am now waiting for details of anti servo
tabs
and will not fly again until they are fitted.
Baz
Baz,
Was your CG aft of where you calculated? How close to aft limit?
With a relatively new design as this appears to be, I would be hesitant
about going close to the limit. Designers have been known to change CG
envelope after flight testing. They should be paying for your test pilot
services.
And just a thought about the anti-servo -- does the factory prototype have
one fitted and flight tested? It seems curious that they let kit buyers
decide whether or not the plane needs one.
Regards,
Gordon.
G'day Gordon,
Yes the C of g was more aft than I had calculated but still inside the
envelope but getting close to the rear limit.
The prototype airframe has been flying for a number of years and according
to previous pilots it flies very well. Of course that was with a Subaru
engine. The factory did have a model with the rotary which flew well but was
destroyed after a bad landing and caught fire. All previous planes have had
anti servo tabs and so will mine soon. I don't know why I missed out on the
tabs, I can only guess as mine is the first truely homebuilt one so far it
was missed somewhere along the way. Factory support has been excellent right
from the beginning.
All part of the fun I reckon.
Baz
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