China Civil UAV Theory Questions — Questions 819–828

By the continuity equation, the mass of fluid flowing into and out of the duct in the same time is the same, so when the cross-section narrows, the flow speed at that section must increase.

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. A non-smooth aircraft surface means a rough aircraft surface, increasing friction drag.

Study note: for downwash, look for pressure difference. Downwash, also called downwash airflow, is an aviation term for the downward motion of airflow passing the wing caused when the wing produces lift; this downward velocity component is called downwash. When a wing produces lift, there is a pressure difference between its upper and lower surfaces: high pressure below, low pressure above. Air flows from the lower surface around the wingtip to the upper surface, forming vortices at the wingtips. At the same time, because of the pressure difference between the upper and lower surfaces, the airflow behind the wing tends to deflect downward, producing downwash.

Simple to remember: disturbance of the airflow by the wing surface turns laminar flow into turbulent flow. As airflow passes over the wing surface, it may first form a laminar boundary layer, in which air flows in layers without interference between layers. But as the airflow moves forward, especially when it encounters an adverse pressure gradient or other disturbance, the laminar boundary layer may transition into a turbulent boundary layer. In a turbulent boundary layer, the speed and direction of airflow change irregularly. Example: a stall is laminar flow turning into turbulent flow.

The factors affecting the change from laminar to turbulent flow are the Reynolds number, including fluid density (air density), fluid velocity (airflow speed), characteristic length (length of flow over the wing surface), and fluid dynamic viscosity.

Bernoulli's equation describes how, for an incompressible fluid in steady flow, along a streamline, various forms of mechanical energy (including pressure energy, gravitational potential energy, and kinetic energy) can convert into one another, but their sum remains unchanged — i.e., energy conservation.

The continuity equation is a mathematical expression of the law of conservation of mass in fluid flow: the mass of fluid passing any effective section of a duct or stream tube per unit time is constant.

The ideal gas equation of state is P = RρT (P is pressure, ρ is density, T is absolute temperature, R is a constant).

As speed increases, induced drag decreases, parasite drag increases, and skin friction drag increases. Parasite drag is all drag other than drag directly related to lift production. It is usually divided into form drag, friction drag, and interference drag. Induced drag is drag produced by vortices trailing from the wingtips or any other plane. Parasite drag increases rapidly with speed, while induced drag decreases with speed.

During flight, although a larger lift coefficient and angle of attack can provide greater lift, letting the aircraft climb faster or turn more steeply, they also increase the risk of a stall. When an aircraft's angle of attack approaches or exceeds the critical angle of attack, the lift coefficient drops rapidly, causing the aircraft to stall and possibly enter an uncontrollable descent. Therefore, to ensure flight safety, an aircraft's lift coefficient and angle of attack are usually limited to two limits, both below the maximum lift coefficient and the critical angle of attack. This ensures the aircraft has enough lift while reducing stall risk and ensuring stable, safe flight.