China Civil UAV Theory Questions — Questions 759–768

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.

Factors affecting lift include aircraft size or area, flight speed, air density, and lift coefficient.

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.

In constant-altitude flight, lift must balance weight. When airspeed is high, lift at the same angle of attack increases, so the angle of attack must be decreased to keep lift unchanged. Option A is wrong because when airspeed is low, the angle of attack should be increased; option C is wrong because increasing the angle of attack at high airspeed would make lift even greater and cannot maintain altitude.

The stagnation point is where air meets the leading edge of the wing.

Induced drag (vortex drag) decreases as speed increases, while profile drag (form drag, friction drag) is proportional to the square of speed V and increases with speed. Therefore, when induced drag equals the sum of other drags, total drag is exactly at its minimum.

Study note: for load factor, look for 'load'. Aircraft load means the sum of all forces acting on the aircraft other than gravity (thrust, aerodynamic forces), denoted R, and it is commonly expressed by the load factor (also called load factor or g factor). The load factor is the ratio of the aircraft's external load (R) to the aircraft's own weight (W). The load factor is a dimensionless vector.

An aircraft's center of gravity is the equivalent point of application of all of the aircraft's weight, while the center of pressure is the equivalent point of application of the aircraft's lift. To maintain longitudinal stability, the center of gravity usually needs to lie ahead of the center of pressure. Then, when the aircraft is disturbed upward, the layout with the center of gravity ahead of the center of pressure produces a downward moment that helps the aircraft return to its original flight attitude. Conversely, if the center of gravity is aft, longitudinal stability weakens. A memory example: a shuttlecock or dart, heavy at the front and light at the tail, has good longitudinal stability.

An aircraft's center of gravity is the equivalent point of application of all of the aircraft's weight, while the center of pressure is the equivalent point of application of the aircraft's lift. To maintain longitudinal stability, the center of gravity usually needs to lie ahead of the center of pressure. Then, when the aircraft is disturbed upward, the layout with the center of gravity ahead of the center of pressure produces a downward moment that helps the aircraft return to its original flight attitude. A memory example: a shuttlecock or dart, heavy at the front and light at the tail, has good longitudinal stability.

In a turn, the vertical component of lift decreases, so the angle of attack must be increased to maintain altitude; at the same time, drag increases, so throttle must be increased to maintain airspeed. The control combinations in options B and C cannot maintain both altitude and airspeed.