The Forces Acting On a
Balloon
A balloon rises due to its
buoyant force overcoming its weight.
Drag further acts on a balloon to
keep it climbing at a constant rate.
Once a balloon is airborne, it’s a
captive of the wind.
The effects of these forces are
simple to understand, although
determining the amount of drag
acting on an object can be tricky, as
you’ll read. The effects of these
forces make understanding a
balloon’s flight behavior relatively
simple and its flight easy to predict.
So, let’s spend a little time discussing
the forces acting on a balloon, and
then applying that understanding to
the prediction of a balloon’s flight
profile.
Balloons experience four forces
in flight: buoyancy, weight, drag, and
winds. Buoyancy is the sole one
acting in the upward direction, while
winds act primarily in the horizontal
direction. Both weight and drag act in
the downward direction (during
ascent). Let’s start with buoyancy.
Buoyancy
Buoyancy occurs when an object
(like a balloon) displaces more fluid
weight than its own weight. In this
case, the fluid is the air. Sometimes
we forget to treat the air as if it has
weight. Its weight comes from the
earth’s gravity acting on the
molecules of nitrogen and oxygen
and atoms of argon in the
atmosphere. There are other gases,
but they make up so little of the
atmosphere that their contribution is
negligible to the three mentioned
above.
How can we calculate the weight
of the atmosphere? If we make the
assumption that we’re flying a
balloon in an atmosphere under
standard temperature and pressure,
Unless one is going to
get their weather
balloon back, there’s
no sense launching a
near space flight for a
school. All that tracking
equipment is too
expensive for a single
flight, and students
want to get their data
after all the work they
put into creating their
BalloonSats. One way
to insure a near
spacecraft’s recovery is
to make predictions of
the balloon’s flight
prior to launch. The
factors affecting a
balloon flight are
pretty simple to
understand and they
make for a great
science project itself.
This month’s article
discusses some of the
factors and physics
affecting a balloon’s
flight, and then shows
how anyone can
predict a balloon flight.
■ BY L. PAUL VERHAGE NEAR SPACE
Factors Controlling Balloon Flights
and Predicting Balloon Landing Zones
12 August 2017
paul@nearsys.com