Materials and Thermal Emissivity
Why are hot air balloons shaped the way they are, and why are
they not shaped like aircraft?
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.
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
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
Hope this helps,
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.
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. :)
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Update: June 2012