Stability (aeronautical)

The stability of a Avion is its aptitude to return in its state of initial balance when it was modified by the pilot or an external agent (ascent, turbulence).
The return to initial balance requires to provide a moment , i.e. a force acting with an arm of lever:

the produced forces are of aerodynamic origin,
the distances depend on the configuration (of the geometry) of the plane.

Axes of stability

One studies the stability of a Aéronef on three axes:

stability in lace, capacity of the aircraft to preserve its direction
stability in pitching, capacity of the aircraft to preserve its plate
stability in rolling, capacity of the aircraft to preserve its slope in straight line and turn

When the aircraft is stable on an axis, it preserves its balance on this axis without action on the orders of vol.
On the contrary, an unstable aircraft deviates from this axis, which requires to operate the orders to correct.

Active and passive stability

Stability of surface fixed, compromised stability/performances, controlled instability = artificial stability. To develop. The stability of origin aerodynamic is of passive type: the plane answers the modifications of the mass of air.

Stability and performances

Passive stability in pitching involves a trail suppémentaire known as Traînée balancing. The CG being in front of the center of bearing pressure, the empennage is spoiler (it pushes downwards). That increases the induced Traînée (by the bearing pressure) of the wing which must carry this déportance in addition to the weight of the plane. The wing must be a little larger, heavier, etc… stabilization activates in pitching makes it possible to move back the center of gravity; the stabilizer can be slightly carrying in vol. On the plane has 310-300 equipped with a back tank (placed in the horizontal Empennage), the retreat of the CG in flight and the additional fuel makes it possible to increase the passable distance from 16% compared to the model has 310-200 not equipped with back tanks.

Stability and comfort

The more the air is agitated, the more the plane has passive stability, the more it is uncomfortable. On the commercial transport aircraft, like the Airbus, active stability is an essential component of safety and comfort in vol.

Stability and handiness

On a Fighter or Aerobatics, passive stability decreases the capacity of operation in evolution. to develop

Stability of the planes

Concerning stability in pitching, there is a factor common to the planes below: the positioning of the center of gravity compared to the bearing pressure of the wings. To be more precise, it would be necessary to speak about the aerodynamic Foyer. This factor will not be repeated below.

Empennage planes (traditional)

The empennage being placed with the back by definition:

stability in lace is ensured by the Dérive (vertical stabilizer),
stability in pitching is ensured by the stabilizer (horizontal stabilizer),
stability in rolling in straight line is slightly positive, null or slightly negative.
stability in rolling in turn depends on the coupling on the dihedral effect with the stability of lace.

Plane with ground effect

See Ekranoplane.

stability in pitching and the maintenance of altitude are ensured by the differential of the slopes of bearing pressure of two surfaces.

Plane duck

stability in pitching is ensured by the difference of Wing load and by the differential of the slopes of bearing pressure of two surfaces,
stability in lace is ensured by Dérive S in end of wing (Winglet S).

Delta wing

stability in pitching is ensured by the choice of a carrying profile not destabilizing (with coefficient of positive moment).

the flight at supersonic speed modifies the Moment of pitching of the wing: the Concorde varied its Centrage in vol.

stability in lace is classically assured by the Dérive.

Hang-glider

passive stability in lace is generally ensured by drifts in end of wing. When these drifts are removed (to reduce the signature radar), stability in lace is obtained by an active stability (artificial): a differential trail of air-brakes (AF) or a differential push if the speed is too low so that the AF are effective.
stability in rolling in turn depends on the coupling with the stability of lace. The differential trail of the ailerons to the setting in turn is accompanied one moment of Lacet reverses not easily compensated by low stability in lace. It is desirable to obtain rolling with systems not generating an opposite lace (Spoiler).
stability in pitching is reduced by the absence of empennage. The usual carrying profiles present a coefficient of Moment destabilizing stitcher, which must then be balanced by:

- the use on all the scale of a profile of wing in " S" with coefficient of Moment of pitching (Cm) positive. These profiles are less carrying.

- the shape in plan of the wing in back arrow, ends of wing presenting a negative twist (reduction in the chock of the profiles in end of wing). One then transformed the end of wing into traditional empennage, noncarrying. The back arrow gives a dihedral effective, which reinforces the induced Roulis and the coupling rolling - lace.

Stabilities on all the axes being weak, it is particularly necessary to limit the destabilizing effects of the propulsion in pitching and lace (effects of blasts, vertical shift between the axis of traction and the center of trail, dissymmetry in the event of breakdown of a side engine).

Helicopter

The Hélicoptère is unstable in pitching and rolling, but the setting in balance of the various forces transmitted by the Rotor S and fixed aerodynamic elements, drift S of Depth and Direction returns this instability very limited: one can measure it by time that puts the apparatus to double the angle of the rotor with the horizontal plane.

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