Translate the following sentences into Chinese. 1. Within limits, lift can be increased by increasing the angle of attack, the wing area, the free-stream velocity, or the density of the air, or by changing the shape of the airfoil. 2. Pilots must control the total weight that an aircraft is permitted to carry ( passengers, fuel, and freight) both in amount and in location. The distribution of weight (i.e. , the control of the center of gravity of the aircraft) is as important aerodynamically as the amount of weight being carried. 3. Thrust is obtained by accelerating a mass of ambient air to a velocity greater than the speed of the aircraft. 4. In reciprocating or turboprop-powered aircraft, thrust derives from the propulsive force caused by the rotation of the propeller, with residual thrust provided by the exhaust. 5. In a jet engine, thrust derives from the propulsive force of the rotating blades of a turbine compressing air, which then expanded by the combustion of introduced fuel and exhausted from the engine. 6. The critical Mach number for an aircraft has been defined as that at which on some point of the aircraft the airflow has reached the speed of sound.