China Civil UAV Theory Questions — Questions 879–888

When an aircraft flies, its balance state is determined jointly by the external forces and external moments acting on it. Force balance: when an aircraft is in balance, the external forces in three directions (longitudinal, lateral, and vertical) must remain balanced. This means the aircraft's thrust (or pull) must equal drag, lift must equal weight, and any lateral force must also remain balanced. Only then can the aircraft maintain a stable flight speed and altitude without accelerating, decelerating, climbing, or descending.

Study note: required speed goes with required speed. Level-flight required speed is the speed that produces enough lift to balance the aircraft's weight.

At takeoff, enough lift must be obtained quickly; with a smaller flap deflection angle, the effect of increasing the lift coefficient is greater than the effect of increasing the drag coefficient, which helps acceleration for takeoff; at landing, greater drag is needed to slow down, so the flap deflection angle is larger. Option A is wrong in saying the drag coefficient does not increase; option B is wrong in saying the lift coefficient does not increase.

Study note: lateral goes with the longitudinal axis, longitudinal goes with the lateral axis, directional goes with the vertical axis. Lateral stability is stability in rolling motion about the longitudinal axis. Longitudinal stability is stability in pitching motion about the lateral axis. Directional (heading) stability is stability in yawing motion about the vertical axis.

Lateral stability is stability in rolling motion about the longitudinal axis. Longitudinal stability is stability in pitching motion about the lateral axis. Directional (heading) stability is stability in yawing motion about the vertical axis.

Study note: lateral goes with the longitudinal axis, longitudinal goes with the lateral axis, directional goes with the vertical axis. Lateral stability is stability in rolling motion about the longitudinal axis. Longitudinal stability is stability in pitching motion about the lateral axis. Directional (heading) stability is stability in yawing motion about the vertical axis.

Glide distance: the horizontal distance an aircraft covers from starting a glide at a certain altitude to touchdown. This distance is affected by several factors, the most important being glide altitude and glide angle. The greater the glide altitude, the longer the horizontal distance the aircraft usually covers during the glide, because the aircraft has more vertical space to 'convert' into horizontal movement. The glide angle is the angle between the aircraft's glide path and the vertical line. The larger the glide angle, the longer the horizontal distance the aircraft covers when gliding from the same altitude, because the aircraft is closer to flying horizontally than descending vertically.

Study note: y goes with lift over weight. The ratio of external load acting on an aircraft in a given direction, excluding gravity, to the aircraft's weight is called the aircraft's center-of-gravity g load in that direction. When this ratio is greater than 1, the force on the aircraft exceeds its weight; when less than 1, the force is less than its weight. An aircraft's g load in the y-axis direction equals the ratio of aircraft lift to weight.

The Mach number is the ratio of an object's speed to the speed of sound. In the troposphere, the speed of sound decreases as altitude increases; when an aircraft climbs at constant speed, its speed is fixed, the speed of sound decreases, and the Mach number increases.

Study note: for a stall, look for the critical angle of attack. An aircraft stall is usually caused by too low a speed or an attitude that makes the angle of attack too large. When the angle of attack reaches the critical angle of attack, most of the boundary layer over the upper wing surface separates, causing lift to drop sharply and a stall to occur. Option A, wingtip vortices producing induced drag, is not the direct cause of a stall; option C, a rough wing surface changing the boundary layer from laminar to turbulent, mainly affects drag, not the cause of a stall.