Negative G
Aerobatics
When you
start doing aerobatics, the first thing you learn is to tense up before you do
a maneuver, to keep the blood up to your brain during the positive G pull.
That's good
for pulling on the stick, but really bad for pushing on the stick. If you grunt
before a negative G push, you're really going to pressurize your poor skull
with blood.
You want to
set up the cockpit so that you can totally relax before a push, to keep your
blood pressure as low as possible. The jets that I flew, for example, have very
nice stirrups to keep your feet on the rudder pedals under negative G. And you
want your lap belt insanely tight, and your shoulder harnesses reasonably
tight. Tiny details, like your sunglasses and mike boom moving
around under negative G, need to be addressed.
Your headset must be tethered in some manner. I like the Perrone mesh
"snoopy" helmets, but some people use chin straps. Heck, a strip of
duct tape will even do it!
Positive G is
great fun. It's sort of like lifting weights, except with your whole body. Once you get onto the "grunt", you
can rapidly increase the positive G that you pull: 2, then 4, then 6, then 8,
then 10.
A guy I know
pulls +11 and pushes -7 on every flight.
He's broken every part of his Pitts, and reinforced it.
Negative G is
really strange. You must build the negative G very slowly over time, to avoid
really screwing yourself up. Your blood pressure soars to insane levels, so if
you have any kind of flaw in your brain - aneurysm, etc - you will find it
right away. If I overdo it, I blow blood vessels in the skin on my face, especially
underneath my eyes. Looks really weird.
But the real
danger of ramping up the negative G too fast is tearing up your inner ear - the
wobblies. No one knows much about them, because the western military doesn't do
negative G, and the civilians that get the wobblies don't want to talk about
it, and have their medicals pulled.
Start with
-1G. Roll upside down, a 2 pt roll, and stay there for a bit. Try to figure out
– remember attitudes and movements? - what pitch attitude you need to maintain
altitude. Common error is to not push -1G and to lose altitude. A little bit of
nose down trim can help reduce the stick push force. Obviously this is speed and aircraft type
(flat bottom wing?) dependent.
Ok, so try to
fly straight and level inverted for 10 seconds, then 20 seconds, then 30
seconds, hesitating during that 2 pt roll. Obviously you need to worry about
aircraft systems limitations. Not all aircraft can tolerate indefinite negative
G.
You're going
to get disoriented upside down. You are going to get lost. Seriously. Best
thing you can do, is after taking off, is roll inverted and navigate to the practice
area and back again. It's like learning to fly all over again.
Some level
inverted turns are a good idea. Start with 30, then 45, then 60 degree inverted
turns. Remember to co-ordinate with OPPOSITE rudder. It is really easy to
slip/skip inverted turns. Some people install inverted balls (inclinometers).
Watch the altitude! It is really easy to spiral dive out of inverted steep
turns.
60 degree
inverted turns is -2G, which is getting better.
You want to start with -1G, then -1.5G, then -2G, then -2.5G, then -3G,
etc. If you go right to -6G you are NOT going to be a happy puppy.
Another
exercise, once you can fly inverted, is to push to the inverted 45 upline, hold
it for a moment, then level off inverted again at a higher altitude and slower
airspeed. Pause, then let the nose drop to the inverted 45 downline and push level
again.
This is
really excellent practice. One very simple maneuver I like is max airspeed at
the surface, and pull for an inside 1/2 cuban-8. But instead of 1/2 rolling
upright from the inverted 45 downline at the end, push to level inverted. Or, keep
pushing to a 45 inverted upline, and do a nice 1 ½ roll to the upright 45.
Once you have
mastered the inverted pushes to 45, time to take the next step to the outside
1/2 cuban-8. Dive for some airspeed. 1/2 roll inverted, and push through the vertical
- lots of right rudder required, especially with a metal prop - to the 45
upright downline. Call it quits there. Practice until you've got it nailed, no
spiraling. If anything goes wrong, you are going up with decreasing airspeed
which is good for the Vg diagram; you will "fall upwards", which is
unlikely to bend your airplane.
Once you
master the outside 1/2 cu-8, time to continue the push for the complete outside
loop, which is interesting because your altitude is going away fast, the
airspeed is increasing, and the negative G is building up fast. This is a much
more dangerous maneuver than the outside 1/2 cuban-8, and care must be taken
with the top gate - is the altitude at least the minimum, and is the airspeed
down below the maximum?
Not many
people master the outside loop. If you do, you are in a pretty elite group of
masochistic pilots!
--
Dec 2012