Jun 2, 2011

How to do Aerobatics from A to Z

How to do Aerobatics from A to Z

Any aerial maneuver involving abrupt or extreme bank or pitch angles, unnecessary for normal flight
Significance: Aerobatics are an integral part of military flight tactics, air show demonstrations, and sport flying. An aerobatic pilot’s ability to retain spatial orientation and control an airplane in three dimensions provides an extra measure of safety in the event of an accidental upset.
Most aerobatic flying is for pleasure, but regional and national contests are held every year, and a world championship contest is held every other year. Although there is no separate aerobatic rating, aerobatics can be safely learned only in an aircraft that is certified for the maneuvers and only under the tutelage of an experienced instructor.
Specifically, the U.S. Federal Aviation Regulations require approved parachutes when two or more occupants in an airplane intentionally exceed a bank of 60 degrees or a pitch angle of 30 degrees relative to the horizon.

The basic aerobatic maneuvers are the slow roll, loop, spin, snap roll, aileron or barrel roll, and the wingover/hammerhead stall. Competition and air show figures combine these basic maneuvers into complex upright and inverted versions.
In the absence of a specialwaiver and to protect passengers and the general populace, intentional aerobatic maneuvers must be performed away from crowded air space, above only sparsely populated areas, and at altitudes greater than 1,500 feet above the surface.

Aerobatic aircraft include some gliders and helicopters.
Because aerobatics places extra structural and stability demands on an aircraft, only approved maneuvers may be performed in a particular aircraft. For aerobatic certification in the United States, an airplane must be capable of withstanding g-load factors from minus 3 to 6 without permanent deformation and loads of up to 50 percent greater (minus 4.5 to 9) without structural failure.

The g-load factor, popularly known as the number of “g’s,” refers to the acceleration of gravity. Sitting still on Earth, one experiences an acceleration of 1 g, or a gravitational force of 1, the normal sensation of gravity. During periods of changing acceleration, such as a banking turn in an airplane, the so-called g-loading will change. Although the g-load factor in upright level flight is 1, it becomes minus 1 in inverted level flight. The best aerobatic aircraft, including those suitable for competition at the highest level, are stressed for load factors of 12 or more g’s.
Aerobatics places extra physical demands on the pilot as well: loss of consciousness (positive g-load factors) or burst blood vessels (negative g-load factors) result from sustained high load factors. 

Military pilots have g-suits that help keep blood in their heads during positive load factors, whereas competition pilots use reclined seats and muscle tensing. A pilot’s tolerance to g-loads increases with practice.

Slow Roll:
The slow roll is the most basic roll maneuver and the hardest to learn. It must be mastered before solo aerobatic flight should be considered. In this maneuver, the aircraft is rolled about its longitudinal, or nose-to-tail, axis without altering the direction of flight.
Differential aileron deflection provides the torque that produces the roll. The other two controls, the elevator and the rudder, are used to keep the airplane from turning.
When the roll is initiated, the opposing rudder must be used, and this reaches a maximum at about one-quarter, or about 90 degrees, through the roll. As the wings lose lift, the elevator must simultaneously be moved toward neutral.
For the next quarter-roll, the rudder pressure is reduced, and forward elevator is added, as the wings are asked to generate negative lift. For the next 90 degrees of roll, rudder pressure in the direction of the roll is added and the elevator is gradually neutralized. In the last 90 degrees, elevator pressure is increased to the value before the roll was initiated, in level flight. The roll can be stopped at any point by neutralizing the ailerons; a momentary stop every 90 degrees, for example, yields a four-point roll.

The slow roll is difficult to learn because elevator and rudder inputs are constantly changing in a manner completely different from those of other maneuvers, because the forces on the pilot are so different and constantly changing, and because even a small error can place the aircraft in an inverted dive from which a safe recovery can be difficult. If the roll is initiated from level flight, the pilot senses an apparent weight that varies from normal to zero to upside down to zero to normal, corresponding to g-load factors varying from (at least) 1 to 0 to -1 to 0 to 1.
Attempts to teach oneself this maneuver will almost certainly cost a great deal of altitude and exceed design speeds and loads. Beginning pilots often fail to add enough opposite elevator as they near inverted flight, causing the nose to drop and allowing the speed to drop and then build very rapidly. At this point, pilots are disoriented and distracted by hanging from their shoulder harness and will relax the aileron pressure, causing the roll to stop.

The natural and almost guaranteed reaction is then to pull back on the stick or wheel, attempting a recovery with a dangerous half-loop. A similar disastrous reaction can be expected from a nonaerobatic pilot when wingtip vortices or atmospheric turbulence flips the plane well past a 90-degree bank.
The slow roll has been mastered when the control inputs are instinctive, based on what the pilot wants the nose to do. Rudder pressure on one side always moves the nose in that direction, and forward movement of the stick or wheel always moves the nose away from the pilot.
Once this concept is learned, slow rolls in any direction— straight up, straight down, or at an angle to the horizon— can be safely executed. However, the vertical, climbing roll is always a challenge, because it lacks a forward reference point and poses the danger of an inadvertent tail slide.

A slow roll is anything but slow in a modern, competition aerobatic airplane, in which roll rates of 720 degrees per second are not uncommon. The roll can be completed so rapidly that there is little time in which to encounter difficulties.
Jet fighters can roll very rapidly without requiring rudder input.

A loop is one of the prettiest and most enjoyable aerobatic maneuvers, but skill is required to perform it safely and well. If the pull-up is made too abruptly, the aircraft can suffer either structural damage or a high-speed stall and will not complete the top of the loop. If the pull-up is too gradual, or if there is inadequate speed, the aircraft will run out of speed and fall inverted out of the maneuver.
A smooth but noncircular loop requires a g-load factor of 3 to 3.5, whereas a competition-quality circular loop may require a g-load factor of 6. Good aerobatic aircraft are fully symmetrical and can loop from level flight from either erect or inverted flight.

A wingtip can be used for spatial reference during the second quarter of the loop, when the horizon will be hidden but, once over the top, the pilot will look overhead for the beautiful sight of the reappearing horizon. Competition-quality “square” loops can generate momentary g-load factors of 10 or more.
The first pilots to perform the loop, in 1913, were Petr Nesterov of Russia and Adolphe Pégoud of France.
In 1928, Speed Holman of Minnesota broke the world’s upright looping record by performing 1,433 consecutive loops in a five-hour period.

A spin’s downward spiral makes it a crowd pleaser, although it is not a particularly pleasant maneuver for the occupants of the plane. A spin is normally initiated at a speed close to the stall speed with power off, neutral aileron, and full rudder and elevator deflection.
After about one turn, the spin should stabilize in a nose-low position, and the airspeed should stabilize at a relatively slowspeed, because both wings should be stalled, one more than the other, creating considerable drag. Recovery is usually effected with full opposite rudder to stop the rotation and then at least a relaxation, if not a reverse deflection, of the elevator control. Pulling out of the resulting dive generates a g-load factor of 2 or more.
All aerobatic pilots must be very well versed in the spin characteristics of their aircraft, because any failed maneuver often degenerates into a spin.
In a true, stable spin, the spin can be continued as long as altitude remains and the airspeed does not increase.
Utility aircraft certificated for spinning may appear to give a good spin entry, but the spin may become a diving spiral, increasing the speed. The same will happen in a good aerobatic airplane if the pilot does not hold full elevator and rudder deflection.
Heavy aircraft such as fighter aircraft may show wild gyrations upon spin entry and an oscillating pitch attitude once the spin is established. TheWorldWar II P-51D Mustang, for example, would oscillate from near-vertical to above the horizon and would lose about 1,000 feet per turn; spins were not to be performed below 12,000 feet.

Fully aerobatic aircraft can perform inverted spins as well as upright spins, but the aircraft recovers to inverted, stalled flight when the rotation is stopped, from which recovery to level flight should be made with a slow roll to minimize altitude loss. The rudder may suffer less blanking in inverted spins, allowing recovery to be faster. The inverted spin is much more disorienting than an upright spin and the pilot must concentrate on maintaining full elevator deflection, or the spin will transition to a diving spiral with rapidly increasing speed. If recovery from an upright spin is forced with down elevator and power, some aircraft will flick into an inverted spin.
If the aircraft is not certified for spins, or if the center of gravity is too far aft, the spin may be an unrecoverable flat spin with the nose on the horizon, yawing almost entirely rather than exhibiting nearly equal yaw and roll.
Modern aerobatic aircraft with fully inverted fuel and oil systems, however, can force an upright or inverted spin to go flat with power and aileron deflection against the spin.
These flat spins not only are recoverable but also form an important part of many air show routines.
Because it is such an important maneuver, the spin is the only aerobatic maneuver required of pilots seeking to become flight instructors. The requirement for parachutes is waived if an instructor is teaching an instructor-student.
Considering that a low-altitude stall that degenerates into even an incipient spin remains a leading cause of fatal accidents, it would seem reasonable for more pilots to become familiar and comfortable with efficient recoveries from incipient spins, entered in the same fashion as accidental spins. Lieutenant Wilfred Parke of England is generally credited with first using what became the classic spin recovery method, in 1912.

Snap Roll:
A snap roll, also known in England as a flick roll, uses the same control inputs as the spin, but in a snap roll, the controls are applied with power on and at speeds well above the unaccelerated stall speed. The resulting differential lift of the wings produces a rapid roll that can be very difficult to stop at a precise point. Good aerobatic aircraft can execute three or more consecutive snap rolls, both upright and inverted, before the axis of the roll changes excessively and the roll degenerates into a power spin.

The load factor varies, as the square of the entry speed is divided by the unaccelerated stall speed, but a considerable twisting moment is also applied to the fuselage. This maneuver, among others, teaches the aerobatic pilot that an aircraft can exceed the critical angle of attack at any airspeed and at any angle relative to the horizon.

Aileron Roll:
The most comfortable rolling maneuver is the aileron roll, also known as the barrel roll. It is performed through coordinated use of the ailerons and rudder, basically continuing a climbing steep turn to a 90-degree bank, letting the nose fall through the horizon with reducing elevator pressure as the roll continues to inverted flight and then recovering with increasing elevator pressure back to upright flight.

The nose will trace out a sort of circle around a point on the horizon. The radius of the circle depends on the roll rate; if the roll is slow, the circle must be large and the top of the circle must be far above the horizon to keep the nose from dropping too low and building up a great deal of speed in the lower half of the maneuver. G-load factors of close to 1 throughout the maneuver are achievable.
An expert pilot can perform this kind of roll in almost any airplane; in 1955, test pilot Tex Johnston barrel-rolled the prototype Boeing 707 airliner at a flight demonstration for potential customers.

Wingover and Hammerhead Stall:
The hammerhead stall and the wingover are the most common turnaround maneuvers used by air show performers to maintain their presence in front of the audience.
A wingover is a maneuver that changes the flight direction through 180 degrees with negligible net change in altitude. It is performed by simultaneously raising the nose and smoothly banking to a 90-degree bank angle as the flight direction changes by 90 degrees and then smoothly reducing the bank angle to 0 degrees in a descending turn to level flight in the opposite direction. Load factors should be in the range of 0 to
2 for a smoothly executed wingover, because there is no attempt to maintain level flight in the steeply banked turn.
In the hammerhead stall, known in England as the stall turn, the aircraft is pitched straight up with power on until it is pointing straight up. Shortly before the craft runs out of airspeed, full rudder is used to rotate the nose to the right or the left, and the rotation is stopped when the aircraft is heading straight down. Recovery may be to either upright or inverted level flight. Load factors need not exceed 2 or 3 if the initial entry and the pullout in recovering are smooth and to upright flight.

The “stall” part of the maneuver’s name is a misnomer, because the angle of attack is close to zero during the maneuver, and no stall buffet should be felt. An aircraft with a clockwise propeller rotation from the pilot’s viewwill rotate best to the left. The greatest danger iswaiting too long to use full rudder, allowing the aircraft to slide back on its tail, known as a tailslide, which could damage some of the control surfaces on otherwise aerobatic aircraft.

Advanced Aerobatic Maneuvers:
The Immelmann turn, named after German World War I fighter pilot Max Immelmann, is a half-loop followed by a half-roll to upright flight. If the speed is low or the loop is stopped too abruptly, a sudden flick into an inverted spin is possible.
The Cuban Eight combines three-quarters of a loop, a roll to upright, another three quarters of a loop, and a roll to upright again. From the ground it appears in the form of a horizontal eight.
The rolling turn, a very demanding maneuver to do well, combines a 360-degree turn with a roll, either to the inside or the outside of the turn. The square loop attempts to minimize the radius of the turns at the top and bottom of the loop and generates some of the highest momentary load factors.
The lomcovàk is a spectacular, twisting, tumbling maneuver invented by the Czech Ladislav Bezák in 1957. It is usually entered from a climbing, inverted snap roll and is commonly demonstrated at air shows.

Another spectacular maneuver is the torque roll, in which the airplane is rolled pointing straight up, and the roll is continued, with the help of engine torque, for a number of fuselage lengths in the ensuing tailslide.
Powerful aerobatic airplanes can generate enough fuselage lift and horizontal thrust component to maintain level flight in a 90-degree bank, known as knife-edge flight.
Russian pilots have demonstrated the cobra maneuver, in which a jet fighter, flying in level flight, is abruptly pitched up through 90 degrees of rotation or more, recovering to level flight with relaxation of the stick.

Aresti Symbols:
The distinguished Spanish aerobatic pilot Colonel José Luis de Aresti Aguirre developed a shorthand notation for aerobatic maneuvers. First published in 1961, Aresti symbols have become universally used to outline aerobatic routines for both contests and air shows. Each figure in Aresti’s dictionary includes a difficulty, or “K,” factor, by which, in contests, judges’ scores—from 0 to 10—are multiplied.

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