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 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!

 

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acboyd@gmail.com

Dec 2012