The devices that can fly off the ground are collectively called aircraft. Flight is the main feature of aviation model. Aircraft can be divided into outer space aircraft and atmospheric aircraft. Spacecraft in outer space are called spacecraft, such as man-made satellites, spacecraft, etc. The aircraft in the atmosphere is called aircraft, which includes light aircraft and heavy aircraft.

轻航空器和重航空器虽然都可以在大气层内飞行，但是它们的飞行历史截然不同的。

Although both light aircraft and heavy aircraft can fly in the atmosphere, their flight history is very different.

轻航空器

Light aircraft

轻航空器是指它的重量比同体积空气轻的航空器。它是依靠空气的浮力而升空的。根据阿基米德定律，任何物体在空气中都会受到向上的浮力，这个浮力的大小等于被物体排开的空气的重量。如果航空器的重量等于它所排开的空气的重量，它所受到的浮力就会大于重力，航空器就会像上升起，正像放在水底的木块回向上浮起一样。

Light aircraft refers to an aircraft whose weight is lighter than air of the same volume. It is lifted up by the buoyancy of the air. According to Archimedes' law, any object in the air will be subject to upward buoyancy, and the magnitude of this buoyancy is equal to the weight of the air displaced by the object. If the weight of the aircraft is equal to the weight of the air it displaces, the buoyancy it receives will be greater than gravity, and the aircraft will rise like a wooden block on the bottom of the water.

常见的轻航空器有气球和飞艇。气球和飞艇都充入比空气轻的气体，如氢气和氦气。有些气球还充入热空气。气球是没有动力装置的，靠自然风运动。飞艇使用发动机做动力，发动机带动螺旋桨，推动飞艇前进。飞艇一般造成流线形，以减少阻力。飞艇还装有尾翼，以保证它前进时的稳定性，并且通过尾翼操纵飞艇的飞行方向。

Common light aircraft are balloons and airships. Balloons and airships are filled with lighter gases than air, such as hydrogen and helium. Some balloons are also filled with hot air. Balloons have no power plant and move by natural wind. The airship uses the engine as power, and the engine drives the propeller to push the airship forward. Airships are generally streamlined to reduce resistance. The airship is also equipped with a tail wing to ensure its stability when moving forward, and controls the flight direction of the airship through the tail wing.

气球的球囊一般都用不透气的布，而模型气球则用纸。

Balloons are usually made of airtight cloth, while model balloons are made of paper.

轻航空器的升空条件。要设计和制作一个轻航空器，必须要考虑它所受的浮力和重力。只有当浮力大于重力的时候，轻航空器才能升空。为了计算方便，我们引入比重这个概念。比重是指某种物质在单位体积内的重量。下面以热气球为例，介绍计算浮力和重力的方法。

Lift off conditions of light aircraft. To design and make a light aircraft, we must consider its buoyancy and gravity. Only when the buoyancy is greater than gravity can a light aircraft take off. For the convenience of calculation, we introduce the concept of specific gravity. Specific gravity is the weight of a substance per unit volume. Taking the hot air balloon as an example, the method of calculating buoyancy and gravity is introduced.

重航空器

Heavy aircraft

重航空器是指它的质量比同体积空气重的航空器。飞机、火箭、导弹等都属于重航空器。显然，重航空器所受到的浮力比重力小得多，不可能依靠浮力升空。飞机可以利用空气动力升空。火箭和导弹直接利用反作用力升空。重航空器的飞行原理要比轻航空器复杂得多。

Heavy aircraft means an aircraft whose mass is heavier than air of the same volume. Aircraft, rockets and missiles are heavy aircraft. Obviously, the buoyancy of heavy aircraft is much smaller than gravity, and it is impossible to lift off by buoyancy. The plane can take off by aerodynamic force. Rockets and missiles take off directly by reaction. The flight principle of heavy aircraft is much more complex than that of light aircraft.