этапы отбора и ответы. Этапы отбора и ответы. REV2. Этапы отбора в Аэрофлот
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УправляемостьThe term controllability refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. A contradiction exists between stability and controllability. A high degree of stability gives reduced controllability. Controllability — the capability of an aircraft to respond to the pilot’s control, especially with regard to flightpath and attitude. It is brought about by the use of devices that alter the lift force on the surface to which they are attached. The familiar controls include the elevator to provide longitudinal control (in pitch), the ailerons to provide lateral control (in roll), and the rudder to provide directional control (in yaw). From the pilot's point of view, if he pulls the control stick back, the elevator turns upward, this movement gives a negative camber to the entire horizontal-tail surface and a downward lift is produced. This, in turn, produces a nose-up moment about the airplane center of gravity and the airplane pitches upwards. A side motion of the control stick results in the movement of one aileron up and the other down. This reduces the camber of one wing while it increases the camber of the other wing. One wing then produces more lift than the other and a rolling moment results. This condition causes the airplane to roll about its longitudinal axis in the direction toward which the control stick was pushed. Applying pressure to the rudder pedals will deflect the rudder. If the pilot pushes the right pedal forward (the left pedal comes back), the rudder deflects to the right. This movement increases the vertical tail camber and a tail force to the left results. A moment arises that yaws the nose to the right and hence, the airplane turns right. Centre of gravity limitThe center of gravity range relates to the furthest forward and aft center of gravity positions along the aircraft’s longitudinal axis, inside which the aircraft is permitted to fly. This is so because the horizontal tailplane can generate a sufficient lift force to balance the aircraft’s lift-weight moment couple so that it remains longitudinally stable and retains a manageable pitch control. The CG limits are indicated in the airplane flight manual. The forward position of the center of gravity is limited to: - Ensure that the aircraft is not too nose heavy so that the horizontal tailplane has a sufficient turning moment available to overcome its natural longitudinal stability. The aft position of the center of gravity is limited to: - Ensure that the aircraft is not too tail heavy so that the horizontal tailplane has a sufficient turning moment available to make the aircraft longitudinally stable. Как изменяется управляемость и устойчивость ВС при изменении положения центра тяжести.The CG is ahead of the center of pressure. And therefore an airplane would nose down except the aerodynamic downforce on the tail. If CoG moves forward stability increases and controllability decreases. If CoG moves rearward stability decreases (in both pitch and yaw) and controllability increases. AFT CG: ↑ Controllable ↓ Stability ↑ Cruise speed ↑ Efficient ↓ Lowest stall speed (the less downforce on the tail - the wing has to support less weight), but recovery from stall becomes progressively more difficult FWD CG: ↑ Stability ↓Controllable ↑ Stall speed ↓ Efficient (most critical on landing, the wing has to support more weight) Dutch roll, spiral stability, sweep back, keel effect, shock waveDutch roll is a coupled lateral/directional oscillation that is usually dynamically stable but is unsafe in an aircraft because of the oscillatory nature. Dutch roll is an oscillatory instability associated with swept-wing jet aircraft. It is the combination of yawing and rolling motions. When the aircraft yaws, it will develop into a roll. The yaw itself is not too significant, but the roll is much more noticeable and unstable. This is so because the aircraft suffers from a continuous reversing rolling action. An aircraft with strong “dihedral effect” and weak directional stability will have a tendency towards dutch roll instability. Too much static lateral stability could result in dynamic instability - Dutch Roll. Dutch roll will occur when the “dihedral effect” is large when compared to static directional stability. Spiral stability (or a spirally stable aircraft) is defined as the tendency of an aircraft in a properly coordinated banked turn to return to a laterally level flight attitude on release of the ailerons. Spirally stable aircraft have dominant lateral surfaces (e.g., wings). If an airplane has strong directional stability and weak lateral stability (large fin and no dihedral), then the aircraft will tend to bank further into the sideslip, towards the lower wing, with the nose continuing to droop, until the airplane is in a spiral dive. It is call a spiral instability. This is preferable to the reverse situation - Dutch roll. This is so because the fin/tailplane area (outside) becomes exposed to the relative airflow, which exerts two forces on the aircraft: 1. Around the vertical axis, which straightens the aircraft directionally 2. Around the longitudinal axis, which increases the bank This accelerates the outer (upper) wing and causes the bank to be increased further. The increased bank causes another slip, which the fin again straightens. This sequence repeats, and the turn is thus made steeper. Once the bank angle exceeds a given type-specific amount (say, 30°), the nose falls into the turn, the speed increases as the roll increases, and the aircraft enters into a spiral dive. Usually the rate of divergence in the spiral motion is so gradual the pilot can control the tendency without any difficulty. Sweepback is an addition to the dihedral that increases the lift created when a wing drops from the level position. A sweptback wing is one in which the leading edge slopes backward. When a disturbance causes an aircraft with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises, and the aircraft is restored to its original flight attitude. Sweepback also contributes to directional stability. When turbulence or rudder application causes the aircraft to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the right wing increases and it acquires more drag than the left wing. The additional drag on the right wing pulls it back, turning the aircraft back to its original path. Keel effect. An aircraft always has the tendency to turn the longitudinal axis of the aircraft into the relative wind. This “weather vane” tendency is similar to the keel of a ship and exerts a steadying influence on the aircraft laterally about the longitudinal axis. When the aircraft is disturbed and one wing dips, the fuselage weight acts like a pendulum returning the airplane to its original attitude. Laterally stable aircraft are constructed so that the greater portion of the keel area is above and behind the CG. Thus, when the aircraft slips to one side, the combination of the aircraft’s weight and the pressure of the airflow against the upper portion of the keel area (both acting about the CG) tends to roll the aircraft back to wings-level flight. Shock wave. When flow velocities reach sonic speeds at some location on an aircraft (such as the area of maximum camber on the wing), further acceleration results in the onset of compressibility effects such as shock wave formation, drag increase, buffeting, stability, and control difficulties. |