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Name: Caitlin
Status: student
Grade: 6-8
Country: United Kingdom
Date: Fall 2012


Question:
What reduces the drag on a hot air balloon?



Replies:
Caitlin

Here is an article in Wikipedia that goes into detail about drag on vehicles (Hot Air Balloon) traveling through a fluid (Air).

http://en.wikipedia.org/wiki/Drag_coefficient

Drag is the force of resistance on a body as it moves through a fluid, Like when you stick your hand out of a moving car window, you feel the wind pushing back on it. Well that push is the force of drag.

In this article, there is a formula that shows that drag is proportional to Fd=1/2p(v squared)CdA

One-half The density of the fluid. (how thick it is (air or cold maple syrup)) The velocity (speed) of the vehicle through the fluid (squared) The drag coefficient which is based on the shape of the vehicle facing the wind stream (see the URL article for a table of drag coefficients and shapes) The area of the vehicle facing the wind stream.

Hot air balloons essentially have zero velocity in the wind stream as they drift along with the windstream more than travel through it. While a hot air balloon experiences some velocity in the windstream, that velocity in the windstream is very small and therefore the drag operating on a hot air balloon is very small. That is one reason why a hot air balloon does not need an internal support structure to help it keep its shape. It is essentially a bubble of hot air that rises, and it drifts horizontally in the air mass as the air mass drifts over the surface of the earth.

Here is a cool image of the forces operating on an airplane in flight. I found it by going to http://www.google.com and searching the word ?drag?.

http://www.grc.naSA.GOV/www/k-12/airplane/images/drag1.jpg

Sincere regards, Mike Stewart


Hi Caitlin,

Drag is a force that happens because the air is pushing back on any object moving through the air in the direction opposite its path. Drag becomes especially important for things that move very quickly -- for examples, airplane, rockets, race cars. This is because the force of drag increases with the square of the speed. For example, if a race car goes twice as fast, the force of drag will be four times as big.

A hot air balloon drifts with the winds, and so it moves very slowly with respect to the air. But aerodynamic drag does play a role, because the force of the wind on the balloon is what propels it along. This force from the wind is also a drag force. This force is going to depend on the size and shape of the balloon, the square of the relative speed of the balloon (speed of the balloon minus speed of the wind), and the density of the air. A larger balloon will have more drag. A balloon with a rounder shape (as opposed to some of the other fancy shapes you see) will have less drag. When a balloon is sitting stationary on the ground before lift off, it will be experiencing the most drag force because the relative wind speed will be the greatest at this point. Here it becomes very important to make sure the anchor ropes are very secure so the balloon doesn't get blown away by the wind. However, after a balloon has lifted off and has begun drifting with the wind, the drag force on the balloon becomes small since the relative speed of the balloon has become small, regardless of its size or shape.

John C. Strong


Hi Caitlin,

If the "drag" on a hot air balloon you are referring to is aerodynamic drag as it moves with the wind, then there is nothing one can do to reduce this aerodynamic drag, simply because a balloon has no drag of this type in the first place. Because balloons float along with the air stream, their speed, relative to the air around them, is zero. For example, in still air, a balloon remains motionless. Similarly, in a 10 km/h breeze, a balloon is also moving a the same 10 km/h. Either way, the balloon travels with the air and its speed relative to the air around it is zero, and hence it experiences no drag at all.

Alternatively, if you are referring to the drag a balloon experiences as it rises, the answer is a little different. A hot air balloon's upward speed is indeed limited by drag. Hot air balloons must be large for the weight they carry, because hot air is only very slightly lighter than the surrounding cool air. Therefore a lot of hot air is needed (hence a large balloon) to be able to develop enough buoyancy to lift a given load, and a large object will encounter greater drag as it moves through the air.

If the balloon could be made into a narrow, very long teardrop shape, it would encounter lower upward drag. But because a balloon must be made of very light (and therefore very thin) material, the balloon has no rigidity and naturally tends to take on a roughly spherical shape, which will result in greater upward drag that impedes its upward movement. Any attempt to use a more rigid material, to force the balloon to be more teardrop in shape, would result in so much extra weight that the balloon would too heavy to rise.

Regards, Bob Wilson


Hi Caitlin,

Thanks for the question. The drag on a hot air balloon can be reduced by making the surface smooth so that air can flow smoothly over it. What this means is that the surface should be free of sharp edges and smooth to the touch.

Jeff Grell


Hi Caitlin,

Perhaps when you say 'drag', do you mean 'what allows the balloon to rise'? If that is what you mean, the answer is that the air inside the balloon is less dense than the air outside the balloon, and the balloon is floating in the sky (just like a ball floats in water). When you heat up air, it becomes less dense -- meaning a gallon (or liter, or any volume you choose) of hot air weighs less than the same volume of cold air. The total weight of the balloon (including the warm air, the basket, the people, etc.) is actually less than the weight of the same volume of cold air.

You may wonder now, how come the balloon does not float all the way up to space?!?!? It turns out, the air gets less dense as you gain altitude (if you have ever been up on a tall mountain, you might have noticed how thin the air seems -- it really is!). Just like a ball stops floating when it reaches the top of the water, so too the balloon floats up to a point where the air is thin enough so that its weight *equals* the weight of the air around it. If you heat up the air, or have a larger balloon, you can go higher. And, to float back down to earth, you just need to release some of the hot air, or allow it to cool.

Hope this helps, Burr Zimmerman



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