China Civil UAV Theory Questions — Questions 839–848

Study note: with volume given, choose proportional; without volume, choose inversely proportional; otherwise choose proportional; with absolute temperature, choose absolute temperature. From P = RρT, at fixed volume, pressure is proportional to the product of density and absolute temperature (using absolute temperature).

Study note: thickness goes with thickness. The ratio of an airfoil's maximum thickness to its chord is called relative thickness. The ratio of a wing's maximum camber to its chord is called relative camber.

Study note: camber goes with camber. The ratio of an airfoil's maximum thickness to its chord is called relative thickness. The ratio of a wing's maximum camber to its chord is called relative camber.

Study note: when you see camber and thickness, choose them in most cases. Wing efficiency is greatly affected by airfoil shape, to a certain extent by airfoil camber and thickness.

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. By changing the airflow pattern at the wingtips, winglets help reduce the strength and extent of vortices, improving flight efficiency.

Study note: when you see transition point, choose forward movement. Simple to remember: the faster the speed, the further forward the transition point from laminar to turbulent boundary layer moves.

A dirty surface or leading-edge icing on a wing causes air flowing over the wing surface to experience greater friction, increasing friction drag and the drag coefficient. A dirty surface or leading-edge icing, because of increased surface roughness, causes more turbulence and separation in the airflow over the wing surface, reducing the wing's effective area and lift coefficient. At the same speed and angle of attack, the lift coefficient is lower, so wing lift is less than the basic airfoil lift.

When vortices are produced at the rear of an airfoil, this is usually caused by airflow separation on the airfoil surface. Airflow separation creates a low-pressure region at the rear of the airfoil, producing vortices. These vortices carry away energy and exert a backward force on the airfoil; this force is called 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. When a wing produces lift, vortices are produced at the wingtips; these vortices tilt lift outward toward the wingtips and produce a drag component opposite to the flight direction, i.e., induced drag. Increasing the aspect ratio increases the wing's effective area, making lift distribution more uniform, thereby reducing wingtip vortex formation.

Study note: when you see camber and thickness, choose them in most cases. Wing efficiency is greatly affected by airfoil shape, to a certain extent by airfoil camber and thickness, which affect airflow over the wing surface. Wingspan and wing area are unrelated to airfoil shape, so options A and B are excluded.