Spin (flight)

In flying, a spin is a special case of a stall, with the aircraft descending rapidly and rotating about its vertical and longitudinal axes. It is characterized by low airspeed, a large rate of descent, and high yaw and roll rates. In most aircraft, a spin is a stable condition that will continue until the aircraft descends into the ground unless the pilot takes action to recover from it.

Types of spin

A spinning aircraft usually has a large positive angle of attack (resulting in the stall) and usually a large nose-down pitch angle. A spin can be entered in any attitude, however; a negative angle of attack in a spin is called an "inverted spin". A spin in which the nose is more-or-less level with the horizon is a "flat spin." Flat spins are very difficult to recover from because there is little or no smooth airflow over the control surfaces. Flat spins are mainly caused by the center of gravity of the aircraft being too far back, which is why when carrying out spin exercises in flight the center of gravity has to be carefully calculated. Most aircraft cannot recover from a flat spin once entered.

One needs to be very careful to distinguish spins from spiral dives. In both cases the aircraft is pointing down, rotating, and descending rapidly, but in a spiral dive the wings are not stalled, and the speed builds up rapidly. Also, the rate of rotation is usually slower. A spin is usually a stable state, and can be maintained indefinitely provided there is sufficient height. In a spiral dive the aircraft can quickly exceed the aircraft's maximum speed, resulting in flutter, and major structural failure.

A snap roll (or flick roll) is a maneuver related to the spin, and is characterized by a marked pitch change followed by rapid rotation about the roll axis. A snap roll may be flown on a straight line or arc, horizontal or otherwise. The pilot initiates a snap roll (usually at about twice the normal stall speed) with pitch and rudder inputs. As with any spin, both wings must be stalled to enter autorotation.

How a spin occurs

A spin usually occurs if an aircraft stalls while yawing. The yawing can be aerodynamic (slip angle, rudder, adverse yaw from the ailerons), the slip stream off the propeller or thrust related (one engine inoperative on a multi-engine aircraft where the engines are mounted away from the centreline).

This may happen during an uncoordinated turn or other maneuver, normally a skidding turn. The most common (and most deadly) time for a spin to occur is while the pilot is making his final turn before landing - he may use too much rudder while flying too slowly and have his nose in high angle of attack, thus stalling while yawing.

When the yawing effect occurs, a rolling moment is induced. This drives the inside wing down and backward. Likewise, the outer wing moves up and forward. This causes the outboard (high) wing to develop more lift and less drag, becoming less stalled, and the inboard (low) wing to develop more drag and still less lift, becoming even more stalled. It is a common misconception that the outboard wing is still flying while the inner wing is stalled; in reality both wings are stalled, one is simply deeper in the stall than the other. The lift differential between the wings induces a rolling moment, and the drag differential induces a yawing moment, creating the spinning tendency. Once these aerodynamic forces have developed, the aircraft has entered a spin and will continue to spin with no control input from the pilot.

An aircraft may be deliberately spun for purposes of training, test flying, or aerobatics. A spin is usually entered by flying the aircraft into a stall condition. As the stall point is reached, the rudder (and sometimes opposite aileron) is used to yaw the aircraft. Spins can also be entered by going into slow flight and having too large an aileron input, thus stalling one of the wings.

How to perform a spin

The most common way to enter a spin for training and aerobatics is following:

Idle the engine and begin to bring the nose up to maintain altitude.
When the stall warning light or alarm triggers, immediately apply full rudder in the direction of the desired spin and pull back on the control column.
Hold these control inputs until the spin is established.

Recovery

To recover from spins in most aircraft, most pilots are trained to:

Reduce throttle to idle
Apply full opposite rudder to stop the rotation, centre the ailerons, pause for the rudder to take effect.
Then Push the control column forward until the stall breaks.
When the rotation stops, neutralize the rudder and gently pull out of the resulting dive. (g-forces will be felt during the pull-out)
Apply full power once the nose is above the horizon and climb to altitude

Pulling out too quickly can cause a secondary stall. Also if the spin is carried out in the same direction of the rotation of the aircraft's attitude indicator, this instrument will be rendered useless until it has time to level out again. Finally, this procedure is not universal; some aircraft may require a different method to recover from a spin, while others simply cannot recover.

Warnings when attempting spins

Never attempt a spin over a populated area, such as a city or town.
Do not attempt a spin below a safe altitude of 3,000 feet, or higher as stated in the aircraft manual or pilot's operating handbook.
The aircraft will lose a lot of altitude as it will be descending at 2,000 feet per minute or more, so make sure there are no aircraft in the immediate vicinity, especially below.
Always conduct a HASELL check (Height, Airframe, cockpit Security, Engine, Lookout & Location) or any other check of the area around your aircraft and the aircraft itself.
Never conduct a spin with passengers, excess weight, a rearward center of gravity, or any loose items in the aircraft.
Carefully read the pilots operating handbook of that aircraft as some aircraft are not designed to intentionally spin.

Aircraft design

A spin should only be performed deliberately on an aircraft that is certified for it, otherwise the stress could damage the airframe. It is also possible that the aircraft may not recover from the spin, especially if the centre of gravity is too rearward. In some countries it is only required to be taught and carried out for a commercial license, however always check the regulations of that country before carrying one out.

Some powered aircraft and gliders are designed to be difficult or impossible to spin, even in an uncoordinated stall. These designs have a greater angle of attack towards the center of the wing than the tips, so that the centre stalls first and the ailerons remain effective. This wing design is known as 'washout', a good example of which can be found on the Grob 103 glider.

Many training aircraft are highly resistant to entering a spin. However, some other aircraft spin very easily and require correct recovery procedure to exit the spin. Of those aircraft that do spin, many are not rated for this maneuver and may be very difficult to recover or may sustain structural damage if it is attempted. A well known example of this is the Piper Tomahawk, which was initially certified for spins, but this certification was removed due to a number of fatal accidents which occurred.

A spin may sound scary at first, however most flight instructors and some other pilots find that safely spinning is an enjoyable experience. Although it has been removed from most flight test syllabuses, there are some countries that still require you to have been taught how to recover from a spin. Of those that do not, the rationale is usually that since spin recovery training requires an actual spin, an improper recovery could result in a crash. The counter-argument is that it is far better to have tried it in practice than just learnt about it in theory, as you will be able to tell when you are about to spin in real-world flying and possess the skills to stop the spin from occurring.

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