China Civil UAV Theory Questions — Questions 749–758

Pulling the stick: the elevator deflects upward, the nose pitches up, and the aircraft climbs. Pushing the throttle: pushing the throttle increases engine thrust, making the aircraft accelerate. Pushing the stick: the elevator deflects downward, the nose pitches down, and the aircraft descends.

The favorable glide speed is the speed at which an aircraft in a glide obtains its maximum glide distance. At the favorable glide speed, aircraft glide drag is smallest and the lift-to-drag ratio is greatest. This speed is calculated from aircraft performance, weight, atmospheric conditions, and other factors, aiming to let the aircraft cover the greatest distance safely and economically before landing.

Study note: where it flows does not matter. Temperature, air pressure, humidity, air composition, and flow speed all affect air viscosity, with temperature having the greatest effect. The air viscosity coefficient is a physical quantity describing fluid viscosity, and can be regarded as the combined result of factors such as temperature, air pressure, humidity, air composition, and flow speed.

Form drag, or pressure-difference drag: drag caused by differing pressure distribution around an object as airflow passes it. Skin friction drag, or viscous drag: produced by contact between air and the aircraft surface. Induced drag: drag produced by vortices trailing from the wingtips or any surface.

This question mainly tests the order of Newton's three laws: the description in option A is actually Newton's second law of motion, and the description in option B is actually Newton's third law.

There are three situations in which an aircraft may exceed the critical angle of attack: (1) in low-speed flight, as airspeed decreases, the angle of attack must increase to maintain lift, which may exceed the critical angle of attack; (2) in high-speed flight, such as pulling the elevator sharply in a high-speed dive, the angle of attack may suddenly increase beyond the critical angle of attack; (3) in turning flight, the angle of attack must increase to provide centripetal force, which may exceed the critical angle of attack. Therefore option C is correct.

Static stability margin: in aviation, the static stability margin is defined as the distance between an aircraft's center of gravity and its focus, and it is an important parameter for evaluating aircraft stability. Aircraft with similar static stability margins have comparable stability, and the larger the margin, the stronger the stability. Movement of the center of gravity changes the static stability margin. Therefore option A is correct and option B is wrong. When an aircraft's center of gravity moves forward, additional weight at the rear is needed to restore balance, which can be achieved by deflecting the elevator downward to increase rear lift and achieve balance. Conversely, when the center of gravity moves aft, rear weight must be reduced to restore balance, which can be achieved by deflecting the elevator upward to decrease rear lift and achieve balance. Therefore option C is correct.

Every part of an aircraft, including the wing, tail, fuselage, and every part exposed to the air, produces drag. Factors affecting drag include flight speed, air density, aerodynamic shape, and size. The drag coefficient combines all of an aircraft's characteristics and is an abstract coefficient measuring the aerodynamic 'cleanliness' of the aircraft. Drag includes profile drag (form drag + friction drag) and induced drag (vortex drag).

The glide angle is the angle between the flight path and the horizontal plane. A larger glide angle leads to a large gravity component pulling the aircraft along its flight path. In the glide state, the engine is at low throttle, idling, or shut down. Gravity acts vertically downward, but can be resolved into a component along the flight direction and a component perpendicular to the flight direction. The resultant of the air's reaction force can be approximately resolved into lift perpendicular to the flight direction and drag opposite to the flight direction. Option A is an excerpt from the textbook text; the gravity component here means the gravity component along the aircraft's flight path, which is large. Option B is an excerpt from the textbook text. In the dive state, in some extreme cases, the flight path is completely vertical downward. Option C is wrong: the thrust needed to climb steeper and faster is provided by the engine, and the wing's role is secondary at that point.

From the drag formula, drag is proportional to the square of speed. If flight speed doubles, i.e., the current flight speed is 2 times the original, drag increases to 4 times its original value.