Nuts and Volts - August 2017

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.