China Civil UAV Theory Questions — Questions 809–818

By Bernoulli's principle, ρv²/2 + p = constant, where ρ is fluid density, v is fluid velocity, and p is fluid pressure (static pressure). Therefore dynamic pressure is related to density ρ and velocity v.

Friction drag is mainly produced by the friction between air flowing over the aircraft surface and the aircraft surface. This friction depends on air viscosity, aircraft surface roughness, and the area of aircraft surface in contact with the air. Option A: a smooth aircraft surface means low surface roughness, effectively reducing air friction. Option B: reducing frontal area reduces compression of air ahead of the aircraft, lowering pressure-difference drag. Option C: increasing the sweep angle is mainly to reduce shock-wave drag in transonic and supersonic flight.

The windward surface of an aircraft (usually the nose) experiences higher pressure, while the leeward surface (the tail) experiences lower pressure. Reducing frontal area reduces this pressure difference, thereby reducing pressure-difference 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. By rationally laying out the positions of aircraft parts, airflow paths can be optimized, unnecessary vortices and airflow separation reduced, thereby lowering interference drag. Option A: a smooth wing surface reduces friction drag. Option B: fairing measures reduce interference drag. Option C: a streamlined design effectively reduces compression of the airflow ahead during flight, reducing pressure-difference drag.

Study note: sweep gives roll, dihedral gives yaw. When an aircraft sideslips, the wing on the sideslip side has greater effective speed and lift than the other side, producing a rolling moment. Further reading: sweep angle gives an aircraft lateral stability. When an external disturbance tilts the aircraft to the right, lift tilts with it, and the aircraft sideslips in the direction of the resultant force. Because of the sweep angle, the right wing's effective speed v1 is greater than the left wing's effective speed v3. The right wing produces greater lift than the left wing, forming a rolling moment that tends to reduce or eliminate the tilt.

Study note: sweep gives roll, dihedral gives yaw. When an aircraft sideslips, the wing on the sideslip side has greater effective angle of attack and drag than the other side, producing a yawing moment. Further reading: during level flight, if a gust strikes the left wing, lifting it and dropping the right wing, the aircraft is disturbed into a rightward tilt, causing it to sideslip downward to the right along the direction of the resultant force. Air then comes from the front right; because of the dihedral angle, the right wing's effective angle of attack increases and drag increases, while the left wing's effective angle of attack and drag decrease. This drag difference between the two wings gives the longitudinal axis a tendency to yaw toward the direction of motion — the nose chasing the airflow — eliminating the sideslip. At the same time, the sideslip-forward wing (here the right wing) has a large angle of attack and large lift, while the sideslip-aft wing (the left wing) has a small angle of attack and small lift, and the lift difference between the left and right wings forms a rolling moment that tends to reduce or eliminate the tilt, eliminating the sideslip, so the aircraft has a tendency to automatically restore lateral balance. Therefore, the yawing moment produced by an aircraft's dihedral angle is actually a restoring moment, helping the aircraft automatically return to its original flight attitude after a disturbance. This restoring moment is achieved by changing the effective angles of attack and drag of the two wings, giving the aircraft better flight stability.

In measuring aircraft aerodynamic parameters, a wind tunnel is often used. Although the aircraft is stationary in the wind tunnel, setting the wind speed simulates the aircraft's flight state in the air. In this way, the wind can be regarded as moving fluid relative to the stationary aircraft, allowing measurement of the aircraft's aerodynamic performance under this relative motion.

Study note: when you see speed squared, choose it. By Bernoulli's principle, ρv²/2 + p = constant, where ρ is fluid density, v is fluid velocity, and p is fluid pressure (static pressure). Therefore dynamic pressure is proportional to ρ and to v².

Study note: ideal, incompressible, no energy exchange. The three conditions of Bernoulli's equation are: (1) ideal fluid (no viscosity); (2) the fluid must be incompressible; (3) no energy exchange with the outside during flow. For a viscous fluid, because of viscous forces, energy is lost during flow, so Bernoulli's theorem cannot simply be applied directly.

Study note: of two short options and one long one, choose the longest. The continuity equation applies to any fluid in steady flow.