China Civil UAV Theory Questions — Questions 829–838

By energy conservation, total pressure (total energy) = static pressure (potential energy) + dynamic pressure (kinetic energy).

At maximum lift coefficient, the aircraft is at the critical angle of attack, where drag is greater than the minimum drag value. At maximum lift-to-drag ratio, drag is smallest, and the angle of attack at that point is less than the critical angle of attack. Option A is incorrect. The lift-to-drag ratio is the ratio of aircraft lift to drag, reflecting the drag encountered when the aircraft produces a given lift. This ratio changes with angle of attack, because both lift and drag are functions of angle of attack. And the lift-to-drag ratio is a nonlinear function of parameters such as speed and angle of attack, showing complex changes in different flight phases, not simple linear growth.

Study note: pressure-difference drag goes with body shape and frontal area; interference drag goes with fairings, relative positions of components, and rational layout; induced drag goes with aspect ratio and winglets; friction drag goes with surface finish. Pressure-difference drag on a body shape is a force produced by the pressure difference around the object acting on it. This drag is affected by the object's shape, size, and airflow pattern. When an aircraft flies through the air, air ahead of the fuselage is compressed and pressure increases, while air behind the fuselage forms a low-pressure region. This pressure difference produces a backward drag, i.e., pressure-difference drag. The larger the frontal area, the more air ahead of the fuselage is compressed, and the larger the pressure-difference drag. The closer to streamlined, the more easily air flows along the surface, reducing compression, and the smaller the pressure-difference drag.

Study note: pressure-difference drag goes with body shape and frontal area; interference drag goes with fairings, relative positions of components, and rational layout; induced drag goes with aspect ratio and winglets; friction drag goes with surface finish. Adding winglets can reduce this drag; it is related to wing planform shape, airfoil section shape, and aspect ratio. Induced drag is drag produced by vortices trailing from the wingtips or any other plane. Wing planform shape is an important factor affecting induced drag. A wing's aspect ratio, i.e., the ratio of wingspan to mean chord, is directly related to induced drag: the larger the aspect ratio, the smaller the induced drag usually is. Option B, streamlined design letting air flow better over surfaces and reducing compression of air ahead of the aircraft, is a way to reduce pressure-difference drag. Option C, adding fairings to streamline airflow interference over the fuselage surface, is a way to reduce interference drag.

In flight, different parts of an aircraft (such as wing, fuselage, tail) and other protrusions (such as engine nacelles, landing gear) interfere with each other's airflow. These interferences produce additional drag, called interference drag. Option A is wrong: downwash airflow produces induced drag (vortex drag). Option B is wrong: adding fairings streamlines airflow interference over the fuselage surface, which is a way to reduce interference drag.

From the drag formula D = 1/2 × ρ × v² × S × Cd, drag is proportional to air density ρ, velocity squared v², wing area S, and drag coefficient Cd. Therefore every item in option A affects drag magnitude, so A is excluded. Airfoil and planform shape affect the aircraft's aerodynamic shape, affecting the drag coefficient and thus drag magnitude, so B is excluded. An aircraft's angle of incidence and dihedral angle, although they affect flight characteristics, relate more to aircraft stability and controllability than to directly affecting drag magnitude. The angle of incidence is the tilt angle of the wing relative to the fuselage, mainly affecting lift distribution. The dihedral angle is the upward tilt angle of the wing, mainly providing roll stability.

Study note: shape and induced drag are unrelated. Induced drag is drag produced by vortices trailing from the wingtips or any other plane. Wing planform shape is an important factor affecting induced drag. A wing's aspect ratio, i.e., the ratio of wingspan to mean chord, is directly related to induced drag: the larger the aspect ratio, the smaller the induced drag usually is. The wing taper ratio (root-to-tip ratio) affects the spanwise lift distribution. A larger taper ratio means a longer root chord, which usually makes the root carry more lift and the tip carry less lift. This uneven lift distribution affects the strength and formation of wingtip vortices. When a wing produces lift, vortices form at the wingtips, i.e., wingtip vortices. These vortices carry away some energy, producing induced drag. The wing taper ratio indirectly affects wingtip vortex strength by affecting lift distribution. Generally, a more uniform lift distribution helps reduce wingtip vortex strength, thereby lowering induced drag.

A and B are descriptions of Bernoulli's principle; C is a description of the continuity equation.

The lift-to-drag ratio is the ratio of aircraft lift to drag, reflecting the drag encountered when the aircraft produces a given lift, denoted L/D. The higher the lift-to-drag ratio, the less drag the aircraft encounters when producing the same lift, and the better the aircraft's aerodynamic characteristics.

Study note: subsonic flow through a converging duct speeds up and pressure drops. A simple memory aid: in fluid flow, low-speed regions have high pressure and high-speed regions have low pressure. When a small parcel of air approaches a low-pressure region, the pressure ahead of the parcel is low and the pressure behind it is high, so a pressure difference toward the low-pressure side is produced, pushing the parcel to accelerate.