Spin

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Aresti symbol for spin

Trudeln (English spin , French vrille , also in German-speaking Switzerland Vrille ) is a flight condition in which the flight object moves in a steep spiral or helical line around the vertical axis towards the ground after a one-sided stall on a wing .

There are many indications that the crash of gliding pioneer Otto Lilienthal in 1896 was the first spin accident in the history of modern aviation.

Initiate the spin

Spin as part of an aerobatic demonstration

An aircraft goes into a tailspin if the flow breaks off on one side of a wing at a large angle of attack . As a result, the lift on this wing collapses, while at the same time the resistance increases sharply, whereby this surface tilts downwards and the aircraft begins to turn downwards like a corkscrew. The flow on the inner wing remains interrupted, while depending on the aircraft type, it can at least partially continue to rest on the outer wing or even tear off completely. In any case, the angle of attack of the two wings and thus also their air resistance is different, so that the outer wing flies around the inner one in a circular path. Even if the sink rate is high during a spin, the actual airspeed and the g load remain low due to the high flow resistance.

The stall does not depend on the speed, but on the angle of attack . Only the condition of constant lift provides a relationship between speed and angle of attack, which is why we normally speak of the (1 g ) stall speed in relation to normal flight .

In aerobatics , the spin is deliberately initiated by increasing the angle of attack by pulling the elevator further at a low airspeed and initiating a rotation around the vertical axis by deflecting the rudder, causing the flow to stall on one side.

The standard method of initiating the spin:

  1. Straight flight - all rudders neutral
  2. Bring the engine to idle
  3. Nose slightly above the horizon so that the airspeed slowly decreases. Wait until spin initiation speed is reached
  4. Fully deflect the rudder in the desired direction of rotation and pull the elevator through
  5. Ideally, the plane should go straight into a spin from a straight flight without rearing up first. Rising gives deductions in the competition.

The spin can also be initiated at high speed by increasing the wing loading by pulling, whereby the critical angle of attack is already reached at higher speeds. This situation can (mostly unintentionally) occur with banked turns. In aerobatics, it is deliberately brought about by a sudden pull ( torn roll ), but this places considerable demands on the torsional strength of the fuselage due to the high rotational acceleration around the longitudinal axis and is therefore not permissible with every aircraft.

Eliminating the spin

The licensing regulations stipulate that the spin of every aircraft - provided the load and center of gravity are within the permitted limits - must be able to be ended at any time using the standard method.

The standard method for eliminating the spin is defined according to JAR as follows:

  1. Engine idling (if applicable)
  2. Aileron neutral
  3. Full deflection of the rudder against the direction of rotation
  4. short break
  5. Push the elevator forward
  6. As soon as the current is present again: all rudders neutral and the dive softly intercept

The order (first rudder then elevator) is important, otherwise the rudder will be shaded by the elevator, which can delay or even completely prevent the exit. The pause between rudder and elevator listed under point 4 is not included in the JAR (as of 2006). It is important, however, because pressing too early and / or too hard because of the shading mentioned above can trigger an "accelerated spin" instead of releasing it, which is no longer released directly, but only through the previous restoration of the "normal" spin by pulling can. Many spin accidents, especially those in which the aircraft started to spin at low altitude, are probably due to the fact that the pilot pressed too early in the rush.

In addition, other rejection procedures have been described (e.g. "Müller-Beggs" is known). However, none of these work with all aircraft types or in all spin states. (Which, incidentally, also Eric Müller himself, the inventor of the above-mentioned method, emphasizes in his textbook "Flight Unlimited".)

The standard method works for all aircraft that have been approved under JAR. However, it is only formulated clearly at first glance; on closer inspection, many questions remain unanswered. How long does the break have to be? How far should the altitude tax be reduced? There are JAR-approved aircraft that can only be brought out of a spin with the elevator control fully depressed. Others, however, turn seamlessly into a backspin when the elevator control is fully depressed, instead of restoring, so that with them the stick should only be released to the neutral position. Still others restore with the elevator control depressed, but only after a delay of several revolutions, while with the elevator control pulled they immediately go out of a spin. Therefore, despite the defined standard method, it is essential to read the flight manual and to know the rejection method described therein. This applies even more to aircraft types that have been approved according to older standards. These may not even be restored using the standard method. Some aircraft types can be relieved again or better by accelerating, better flow rudders. The acceleration counteracts this in such a way that the fuel supply to the engine is often interrupted.

Flat spin

Flat spin

Depending on the inclination of the longitudinal axis of the aircraft, a distinction is made between a steep (or normal) spin and a flat spin. In some sources, a certain longitudinal inclination is mentioned as the boundary between the two states (usually 45 ° or 60 °). However, this is wrong for the following reasons: Firstly, many aircraft swing more or less strongly around the transverse axis during the spin, so that specifying a certain angle of inclination is meaningless from the outset. In addition, the main difference is not in a certain angle of inclination, but in the flow conditions at the rudder.

During normal (steep) spin, there is always a current on the rudder, even during the spin. This means that the spin can be avoided at any time by a courageous use of the rudder (see "Standard method" above). This is not the case with a flat spin. If an aircraft goes into a flat spin due to incorrect loading (center of gravity too far back), there is often no longer any possibility of removing it. First you should try to increase the pitch by pushing abruptly as far as it will go in order to return to normal spin. There are also said to have been cases in which the pilot is said to have succeeded in ending the flat spin by unbuckling the seat belt and shifting the center of gravity forward, but the success of this act of desperation is by no means guaranteed.

In aerobatics in propeller-driven aircraft, the power setting and the resulting different gyroscopic effects of the engine can be used to influence the spin position and thus produce a flat spin even with the correct center of gravity. This then reverts to a normal spin as soon as the power is reduced. With some glider types, a flat spin can be brought about with the help of suitable aileron deflections, which can also be ended again by suitable aileron deflections. However, a pilot must know exactly how his aircraft behaves with the current load.

The licensing regulations with regard to spin behavior imply that an aircraft must not go into a flat spin on its own if it is correctly loaded.

Spiral fall

The spiral fall is not to be confused with the spin. Here, too, the aircraft turns down a narrow vertical spiral, but in contrast to a spin, the flow is applied to both wings in a spiral dive, which means that the large flow resistance of the "designed" inner wing is eliminated and the speed and the g- load very quickly increase. The spiral dive must be stopped immediately, otherwise the load limits of the aircraft (high g load, exceeding the maximum speed) and those of the pilot ( loss of consciousness due to high g load) will be exceeded very quickly.

Some aircraft, especially if they are loaded in the front center of gravity, go from spin to spiral dive after a short time. An attentive pilot will notice this immediately, as the two flight modes feel completely different. When spinning, you can feel the acceleration to the side against the turning movement, and when you fall in a spiral, it is almost perpendicular to the seat. Because of the danger of transition into a spiral fall, it is now expressly advised against using the spin as a rescue measure in the event of an unintentional flight into a cloud, as was sometimes still recommended in the older literature.

etymology

The word spin has been used since the 18th century. Its origin is unclear. At that time it had nothing to do with flying, of course, but generally meant “rolling slowly and unevenly” or “falling slowly turning around itself”. For example, like autumn leaves falling from trees without a safe destination and swirling. This general meaning still lives on in the word "roll in". Spinning itself no longer exists in this general sense today; in landscape terms it is still used for rolling dice, based on the "spinning" movement of the dice.

Apart from this special landscape case, the word “spin” is only used today as a precisely defined technical term in aviation with the meaning described above. In particular, an aircraft does not "spin down" with any loss of control, as one often reads. In most cases, excessive flight conditions (aircraft too slow, pilot does not want to go down and involuntarily pulls on the elevator) lead to a stall and the aircraft falls to the ground like a stone.

theory

A physical theory of spin was first formulated by Frederick Lindemann during the First World War. Lindemann was a member of the Royal Flying Corps and verified his theory in a successful self-experiment .

In the 1930s, the Trudelturm and the 130 m long large wind tunnel were built in Berlin .

There are numerous books and writings on the subject of a spin. However, the spin depends on so many parameters and the possible processes are so diverse that there is still no final theory that encompasses all possible modes.

literature

  • Eric Müller, Arnette Carson: Flight unlimited '95 . Penrose Press, 1994, ISBN 0-620-18774-3 , Chapter 15: The Spin.

Web links

Commons : Spin (flight)  - Collection of images, videos and audio files
Wiktionary: Trudeln  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Duden, Vol. 7