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Why do hot-air balloons float

I want to know Why do hot-air balloons float

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  • 1 0 年前
    最佳解答

    Construction and theory of operation

    A hot air balloon for manned flight uses a single layered, fabric gas bag (lifting "envelope"), with an opening at the bottom called the mouth. Attached to the envelope is a basket, or gondola, for carrying the passengers. Mounted above the basket and centered in the mouth is the "burner" which injects a flame into the envelope, heating the air within. Raising the air temperature inside the envelope makes it lighter than the surrounding (ambient) air. This causes the balloon and its payload to rise.

    Modern hot air balloons are usually made of synthetic fabrics such as ripstop nylon, a light weight fabric of high strength. During the manufacturing process, the material is cut into panels and sewn together, along with structural load tapes (webbing) that carry the weight of the gondola or basket. The heater or burner is fueled by propane, a liquefied gas stored in pressure vessels, similar to high pressure forklift cylinders.

    The amount of lift (or buoyancy) provided by a hot air balloon depends primarily upon the difference between the temperature of the air inside the envelope and the temperature of the air outside the envelope. For most envelopes made of nylon fabric, the maximum internal temperature is limited to approximately 120 °C (250 °F). It should be noted that the melting point of nylon is significantly higher than these maximum operating temperature -- about 230 °C (450 °F). However the lower temperatures are generally used because the higher the temperature, the more quickly the strength of the nylon fabric degrades over time. With a maximum operating temperature of 120 °C, balloon envelopes can generally be flown for between 400 and 500 hours before the fabric needs to be replaced. Many balloon pilots operate their envelopes at temperatures significantly below the maximum in order to extend the longevity of their envelope fabric.

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