China Civil UAV Theory Questions — Questions 799–808

Key point — Bernoulli's principle. Static pressure acts in any direction; dynamic pressure acts in the direction of flow.

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. Option C is correct. 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. When an aircraft moves in the flight state corresponding to the maximum lift-to-drag ratio, its aerodynamic efficiency is highest. The flight angle of attack corresponding to maximum lift-to-drag ratio is generally called the favorable angle of attack. From the zero-lift angle of attack to the favorable angle of attack, lift increases faster and drag increases slowly, so the lift-to-drag ratio increases. From the favorable angle of attack to the critical angle of attack, lift increases slowly and drag increases faster, so the lift-to-drag ratio is smaller. Beyond the critical angle of attack, pressure-difference drag increases sharply and the lift-to-drag ratio decreases sharply.

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. 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. For example, in aircraft design, engine nacelles may be placed under the wing to reduce engine interference with wing airflow, or landing gear may be retracted into the fuselage to reduce the drag it produces in flight. These measures all help reduce interference drag and improve flight efficiency.

Study note: angle of incidence goes with the longitudinal axis; quarter-chord goes with sweep angle; sideslip angle goes with the plane of symmetry; climb angle goes with the climb path. The sweep angle is the angle between the wing quarter-chord line and the line perpendicular to the fuselage centerline, describing how far the wing sweeps back. Option A, the angle of incidence, is the angle between the chord and the fuselage longitudinal axis; option B, the dihedral angle, is the angle between the wing plane and the horizontal plane.

Study note: angle of incidence goes with the longitudinal axis; quarter-chord goes with sweep angle; sideslip angle goes with the plane of symmetry; climb angle goes with the climb path. A wing's angle of incidence is the acute angle between the chord and the fuselage longitudinal axis, i.e., the wing's mounting angle relative to the fuselage. Option A is the definition of angle of attack; option C does not match the definition of angle of incidence.

The angle of attack is the angle between the wing's direction of advance (airflow direction) and the chord.

A wing's center of pressure is the intersection of the chord with the line of action of the wing's aerodynamic force. Simple to remember as stated.

The aspect ratio is defined as the ratio of span to mean geometric chord, an important parameter measuring wing aerodynamic characteristics. Options A and B do not match the definition of aspect ratio.

When an aircraft moves forward, the relative motion between the wing and the air produces a flow-speed difference. Because of the streamlined design of the upper surface of the wing, airflow over the upper surface is faster than over the lower surface. By Bernoulli's principle, faster airflow means lower pressure, so a negative-pressure region forms on the upper surface of the wing.

To simplify calculation, aerodynamicists combine all the very complex factors of wing shape and trim into the lift coefficient, to describe the lift produced by an aircraft or any of its parts. The larger the lift coefficient, the greater the lift it can produce. The lift coefficient is a dimensionless quantity, the ratio of the lift on an object to the product of dynamic pressure and reference area. Different airfoils produce different lift coefficients at the same angle of attack, and likewise the same airfoil has different lift coefficients at different angles of attack.