航空器和重航空器雖然都可以在大氣層內飛行，但是它們的飛行歷史截然不同的。

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

1、輕航空器

1. Light aircraft

輕航空器是指它的重量比同體積空氣輕的航空器。它是依靠空氣的浮力而升空的。根據阿基米德定律，任何物體在空氣中都會受到向上的浮力，這個浮力的大小等于被物體排開的空氣的重量。如果航空器的重量等于它所排開的空氣的重量，它所受到的浮力就會大于重力，航空器就會像上升起，正像放在水底的木塊回向上浮起一樣。

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.

常見的輕航空器有氣球和飛艇。氣球和飛艇都充入比空氣輕的氣體，如氫氣和氦氣。有些氣球還充入熱空氣。氣球是沒有動力裝置的，靠自然風運動。飛艇使用發動機做動力，發動機帶動螺旋槳，推動飛艇前進。飛艇一般造成流線形，以減少阻力。飛艇還裝有尾翼，以保證它前進時的穩定性，并且通過尾翼操縱飛艇的飛行方向。

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.

Figure 1-1 balloon and airship

氣球的球囊一般都用不透氣的布，而模型氣球則用紙。

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

輕航空器的升空條件。要設計和制作一個輕航空器，必須要考慮它所受的浮力和重力。只有當浮力大于重力的時候，輕航空器才能升空。為了計算方便，我們引入比重這個概念。比重是指某種物質在單位體積內的重量。下面以熱氣球為例，介紹計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.

2、重航空器

2. Heavy aircraft

重航空器是指它的質量比同體積空氣重的航空器。飛機、火箭、導彈等都屬于重航空器。顯然，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。飛機可以利用空氣動力升空。火箭和導彈直接利用反作用力升空。重航空器的飛行原理要比輕航空器復雜得多。

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.