Introduction to Aerobatics
Aerobatics is often viewed as some sort of exotic, mysterious and dangerous thing to do, like supplying arms to one side of a Burmese civil war. This is unfortunate, because aerobatics, like tailwheel flying, formation, instrument flying, radial engines, float flying, glider flying, etc all are great fun, and will really improve your pilot skill and knowledge.
Anyways, before you go fly aerobatics, there’s some knowledge that you need to learn, usually via some ground instruction. If your aerobatic instructor doesn’t know this stuff – and explain it to you - consider moving on to one that does.
Angle Of Attack vs Pitch Attitude
First thing you need to do, to break away from the pre-conceived assumptions of straight and level pilots, is that pitch attitude is a proxy for angle of attack. For example, in a light trainer, if you set the nose 30 degrees nose up, you know you’re going to stall. But that’s not always true, especially during aerobatics.
When I fly straight up (vertical upline) or straight down (vertical downline) I actually have a zero degree angle of attack! Remember that a wing has no eyeballs. It cannot see anything and does not get scared in unusual attitudes. It always stalls at the same angle of attack.
Also, over the top of a big loop (either inside or outside, doesn’t really matter) I might have an airspeed indication of nearly zero, but despite that, the wing is nowhere near the stalling angle of attack, because I am not attempting to produce any lift with the wing. The aircraft is actually ballistic, like a baseball at the top of it’s arc.
Sitting in the hangar, the airspeed is zero, but if you turn the master switch on, the stall warning will not actuate, because the wing isn’t doing any work – the weight of the aircraft is being supported by the landing gear.
So the “One Gee” stall speed, which in your POH/AFM and marked at the bottom of the green arc on your airspeed indicator, is really only of passing interest when you are flying aerobatics, because you are rarely pulling one G.
Stall Speed and G
One concept that every aerobatic pilot needs to understand is the implications of this formula:
Vs (G) = Vs(1G) x sqrt(G)
It’s derived from the lift equation, and it governs how we fly aerobatics. In fact, a civilian airshow pilot just died recently because he didn’t correctly apply the above formula at low altitude. You might find that somewhat motivating.
The above formula tells us that our stall speed and G are intimately and simply related.
Note that Va or maneuvering speed is calculated exactly as above, and should already be a concept familiar to you from your Private Pilot training. I hope.
Let’s look at the Pitts S-2B, which stalls around 70 mph straight and level (1 G). What is it’s stall speed at 4G?
Vs(4G) = 70 mph x sqrt(4) = 70 x 2 = 140 mph
Very simply, if we try to pull more than 4G slower than 140 mph, we will stall. If we fly faster than 140 mph, we can pull 4G and not stall. If you pull 4G at precisely 140 mph, we will stall, because the wing has reached it’s stalling angle of attack, just as if it was doing 70 mph straight and level.
The above is really, really important for any aerobatic pilot to thoroughly understand and continually apply during aerobatic flight. If you don’t, you will stall/spin at slower speeds when you try to pull too much G, or even damage the aircraft when you try to pull too much G at high speeds. This leads nicely to a discussion of the Vg or Vn diagram, known as the “flight envelope”.
Get out the POH/AFM for any aerobatic aircraft, and find the Vg diagram. If you fly to the left of the envelope, you stall. If you fly above, below or to the right of the envelope, you will overstress the aircraft and when you land, possibly bits will be missing. The Vg diagram comes from the above equation and will tell you how to fly your aircraft without stalling it or breaking it.
Probably the most important component of an aerobatic aircraft is the pilot. Even if the engine fails, a good pilot can still do some pretty nice aerobatics. See Bob Hoover.
Aerobatics is different from normal straight and level flying in that it can be pretty hard on the pilot. From the ground, looking up at an aircraft doing aerobatics you might think it’s pretty, but inside the cockpit it be pretty hard work. Let’s look at some considerations for a new aerobatic pilot
Nervous / Scared
When a pilot starts aerobatic training, it is natural to be nervous or outright scared. If you aren’t, I’m a bit worried about you, because you have no normal morbid fear of death.
But don’t let your fear stop you from doing something worthwhile. If you have a good, experienced instructor pilot, a good aircraft and good weather, you shouldn’t be afraid of doing some gentle introductory aerobatics. The problem with fear is that people naturally curl up into the fetal position when they are scared, and that’s bad – see below.
Motion sickness or nausea can be a real problem for many beginning aerobatic pilots. The good news is that your tolerance is easily increased. Every flight, fly just to your limit of nausea, then stop aerobatics, and fly gently straight and level home again. You should not barf. In case you do, velcro a Ziploc back to the side of the cockpit for quick access.
Nausea is actually caused by a disagreement between your eyeballs and your inner ear. If you curl up (fear – above) you look inside the aircraft, and your eyeballs tell your brain that you aren’t moving, but your inner ear disagrees – it can sense the aircraft movement – and you barf.
It is this disagreement between your eyeballs and your inner ear that decreases your nausea tolerance. What you can do to help is always LOOK OUTSIDE at the horizon, so that your eyeballs and inner ear agree on what’s going on.
Some people like ginger, and claim that helps. Try it if you want. Whatever works for you. All I know is that you need to look outside when doing aerobatics, and your nausea tolerance will increase with every flight.
You’re a lot more likely to barf if you’re a passenger doing aerobatics. It helps enormously when you are flying.
Positive G Tolerance
As a new aerobatic pilot, you’re almost certainly going to want to work on your positive G tolerance. Positive G is the “nice” G, which you get when you do a steep turn, or you pull the stick back to enter an inside loop.
What happens when you pull positive G is that your blood goes to your feet. Actually, your blood stays in the same place, and the aircraft pulls your body up, but that’s just frame of reference trickery.
The problem is that your feet don’t need all that blood. Your eyes are actually very sensitive to oxygen deficiency, which you might be aware of if you fly at night at high altitudes.
If you gently pull some G and the blood starts to gradually drain out of your skull, the first thing you will notice is your vision. You will start to lose peripheral vision, and you will lose your colour vision – everything will go black & white (really) – and then everything will go black – total loss of vision. If you keep pulling G, you will go unconscious which is bad if you’re the one that supposed to be flying the airplane. A couple years back a Blue Angel pilot pulled too many G’s down low, passed out and crashed. Don’t do that.
If someone climbs into an aerobatic aircraft and doesn’t
know anything about G, usually somewhere around +4G they will go to sleep. Most people think you need a G suit, but
actually they’re almost certainly not necessary for the kind of aerobatic
flying you’re going to do. When my
father was in
With a little practice, you can learn to pull quite a lot of positive G and keep your vision and stay conscious.
Imagine squeezing your body like a vertical toothpaste tube with the opening at the top.
That’s what you’re going to do. Before the positive G comes on, you’re going to scrunch your toes, then flex your leg and bum muscles, then your stomach muscles to keep the blood up in your brain.
The bad news is that after you’ve put the positive G on, if you’ve have forgotten to scrunch, you can’t force the blood up again. You’re going to have to unload, scrunch and then put the positive G back on again.
This sounds complicated, but so is shifting gears with a manual transmission in a car, and you can probably do that ok. After a while, it will become automatic to you to scrunch before you pull back on the stick.
All things being equal – which they rarely are – your positive G tolerance is a function of the vertical height of the column of blood from your heart to your brain. This is why the F-16 seat is inclined, to reduce that height. It also screwed up all the pilot’s necks. Oops.
For positive G tolerance, you don’t want low blood pressure! Funny as it sounds, you want high blood pressure for positive G tolerance. Working out in the gym helps – it develops specific muscle sets – but pulling positive G works your whole body – your internal organs are distended – and there is no substitute for getting into an airplane.
One more trick to increase your positive G tolerance … you can enrichen the oxygen in your blood, so that with the same blood pressure, your eyes and brain are fed oxygen better. How do you do that? Grunt breathing.
If you fly aircraft up high, you might be aware that above 30,000 feet, the partial pressure of the atmosphere isn’t high enough to force oxygen into your lungs. So if you lose pressurization, you will put on a mask which will uncomfortably blow oxygen into your lungs, at high altitude.
You can use the same trick down low, to enrich the oxygen in your blood. While you are pulling G, in addition to scrunching your body, take a breath and force to slowly out under pressure by almost trying to stop it leaving your lungs. This will result in a higher than ambient pressure in your lungs, and will pump more oxygen into your blood, and get you another 1 G or so of tolerance. Neat, eh?
One last thing I should mention about positive G … your tolerance is not only a function of how much positive G you pull, but importantly also how long you pull it for. Civilian prop aircraft simply don’t have the massive thrust required overcome the drag produced by sustained very high G. With prop aircraft, the G goes on, then off. If you don’t, you stop in the sky as you run out of airspeed. You can rapidly give away altitude to try to maintain a very high positive G, but obviously that’s not going to work for very long.
In contrast, military fighter jets – especially with afterburners – can produce massive thrust and sustain high positive G for a very long time. +6G isn’t much in a prop aircraft, but it’s a lot harder to sustain +6G for a minute in a jet!
Negative G Tolerance
Negative G is what happens when you are flying straight and level, and you push the stick forward – the English call it a “bunt”. You feel as if you are being centrifuged out of the cockpit, through the canopy. Most people think this is pretty unpleasant. Like beer and coffee, negative G is an acquired taste.
During negative G, the blood is being forced into your skull under extremely high pressure. God knows what your blood pressure is at the time. The important thing to do during negative G is to RELAX. When people start pushing negative G, they always grunt before a push (just as they would for a pull) but that’s exactly the wrong thing to do! You must totally relax during a push, which is easy to say but hard to do.
I don’t know of any exercises to increase negative G tolerance. Maybe you could get heel clamps and hang upside down like a bat from a horizontal pole. All I can suggest is that you increase your negative G very, very slowly. Start with -1G, straight and level, and do some gentle pushes to a 45 degree inverted upline, or perhaps a 60 degree banked inverted level turn (remember to use opposite rudder to co-ordinate).
If you hammer the negative G, you can blow blood vessels and tear up your inner ear, resulting in the “wobblies”, which you don’t want to get. Be patient and slowly work up the level of negative G over time, otherwise you’re going to hurt yourself.
There are a few more things I should mention about negative G.
One thing is that western military pilots have a multi-generational cultural bias against negative G. Their airplanes wouldn’t do it, so the planes wouldn’t do it, so the next generation of military pilots didn’t do it, so the next generation of airplanes specified by those pilots didn’t do it. “No tactical advantage”, they will mumble. Male bovine excrement. East bloc pilots have no such cultural bias against negative G, and it shows in their aircraft design. It takes time and airspeed to half-roll to pull instead of push, and the half-roll can disorient you. A good example is a hand bomb toss with a low-level ingress. Approaching the target, you pull up to gain altitude. A western military pilot would now half-roll inverted and pull to the inverted downline to line up with the target, then half-roll upright again for bomb release. Or you could pass on all those disorienting half-rolls and simply push over the top, but that’s not what western military pilots do, because negative G is of “no tactical advantage”.
Another important detail about negative G is that it very temporarily decreases your positive G tolerance afterwards because your capillaries have been dilated. What this means is that you CANNOT do a big pull after a push – you must give your body a few seconds to recover. If you must do a push after a pull, try to make it a gentle one – say +4G – and be sure to grunt as if you were going to pull +8G.
Because of all of the above, not very many pilots in
One thing I should warn you about is that before you push negative G in a strange airplane, be sure to pull the access panels in the tail and fish out all the junk that seems to migrate there. Whenever you fly aerobatics, your pockets should be empty, but you would be amazed at the collection of keys, barf bags, pocket knives, maps, sunglasses etc that I have fished out of the tails of aerobatic aircraft.
The problem with all the junk is that it can jam up your elevator. It will stay put during positive G, but if you take an aerobatic airplane that has only flown gentleman positive G aerobatics and do some pushes in it, the junk in the tail can float up and jam your elevator.
It’s happened to me. A few years back, I was flying a strange-to-me Pitts S-2B and the elevator jammed in a vertical downline, which was annoying, as was the following landing. Turned out it was a BNC T-connector wedged in the elevator crank.
A friend of mine has had his stick jam twice now due to junk in the tail. The first time was a set of keys that slipped out of passenger’s pocket, and the second time was a half-inch 10-32 machine screw that popped up into the linkage during a double outside snap after takeoff.
I will leave it as an exercise for the reader how to recover from a vertical downline (and land) with no back elevator.
email@example.com Sept 2011