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Name: Sophia
Status: student
Grade: 6-8
Country: Canada
Date: N/A 


Question:
Why are hot air balloons shaped the way they are, and why are they not shaped like aircraft?



Replies:
If a hot air balloon were not carrying its carriage (where the people are) its shape would be spherical. This is the shape of a common toy balloon. However, there needs to be room for the people and cargo, which, being heavier than the mass of the balloon volume becomes a distorted sphere. In principle, the buoyant force is the mass of the volume of displaced air. But there are structural constraints. If the balloon was expanded too much, the fabric would rip. A balloon could be another shape but its buoyancy would be affected negatively.

Vince Calder


The goal of a hot air balloon is to go up into the air. To do so it is necessary to have a lightweight "envelope" to hold the buoyant hot air, and have load tapes or "strings" to hold the basket. The best envelope has a shape that holds the most air with the least amount of material and stress. The upside-down teardrop shape is just about the best. Some people make envelopes in different and artful shapes, but this requires more work to build, and extra parts to keep the envelope in its special shape, like an animal or a castle. One could make a balloon in the shape of an airplane, and it would be special to look at, but costly and heavy. Actual airplanes have a shape that works well for their task of flying along at high speed. Balloons don't fly along at high speeds. Airships that actually propel themselves are called airships, and the cigar shape works well for the kind of flying that they do.

Robert Erck


Sophia,

Hot air balloons are one type of aircraft, as are kites and helicopters and gliders and jets. I assume you mean 'airplanes', not the more general term 'aircraft'. If I am mistaken, please let me know!

One major difference between hot air balloons and airplanes is the source of 'lift' -- which is the force that holds the aircraft up in the air. Hot air balloons generate lift through buoyancy (the hot air has lower density than the cold air around the balloon, so it literally 'floats' in the cold air like a cork floating in water). In contrast, airplanes use air flowing over wings to generate lift. Airplanes get air to flow over their wings by propelling themselves forward, and allow the flowing air to generate lift over their wings.

Airplanes have a streamlined appearance because their shape reduces 'drag'. Drag is resistance force of the air pushing back on a moving object. It counters forward motion. As a plane flies through the air, the air resists its movement (if you stick your hand out a car window while it is driving, you can feel the air drag). Because a hot air balloon isn't moving very fast, the importance of drag relative to its lift is very small, so a hot air balloon does not need to be streamlined.

Hope this helps,
Burr Zimmerman


Hi Sophia,

I assume you are referring to large balloons that carry heavy loads or people. Balloons are shaped the way they are because of the natural pressure in the contents inside (helium, or hot air). Their shape is due only to their internal gas pressure, and is not caused by the design of the balloon itself.

Balloons are not shaped like an airplane because airplanes are designed the way they are because they must fly through the air at high spedp, whereas a balloon does not travel at any speed through the air. It simply floats along with the wind.

Regards,

Bob Wilson


Hi Sophia,

Hot air balloons are not shaped like airplanes because they get their lift in a very different way, and they move in a very different way. Airplanes are heavier-than-air aircraft and generate lift dynamically -- that is, they need to move quickly through the air so that the wind over the wings generate lift. The lift is produced by the wing pushing the air downwards, and by Newton's 3rd law the air pushes back on the wings in the up direction. Wings do this by creating a low pressure zone above the wing and a high pressure zone below, and you can calculate lift if you know the pressure to good enough accuracy.

Hot air balloons in contrast are lighter-than-air aircraft, and they get their lift from buoyancy. Archimedes' Principle says that the buoyancy force of an object is equal to the weight of the fluid (air) that it displaces. Also, the Ideal Gas Law tells us that hot air is less dense and cold air is more dense. So if you calculate the weight of cool, dense air that would have filled a floating hot air balloon, this will be approximately equal to the weight of the hot less dense air filling the balloon plus the weight of the basket, equipment, fabric, and pilot and passengers. Hot air balloons don't need to move quickly and in fact they move with the wind, so their shape is usually just designed to hold in a lot of air. Often you'll find them in very fancy shapes as well, just for the fun of it. :)

John Strong



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