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Remote Sensing: Blimp

Your blimp system relies on the interplay of basic physical concents such as density, pressure, and buoyancy. The density, temperature, and pressure of the helium inside the blimp compared to the atmosphere outside the blimp determines whether your blimp will fly and how much it can carry.

  • Density
  • Helium
  • Pressure
  • Buoyancy
  • Blimp

DensityDensity is the amount of matter in a certain volume of space. Specifically, it is the mass of the object divided by the volume of the object, so it usually has units of grams (or kilograms) per cubic centimeter (or cubic meter, or liter). Consider the cylinder-shaped regions of space at right. The right volume contains four times as many particles as the one on the left, so it has four times the mass, and four times the density.

An object that is very dense is usually very heavy for its size. For example, a bowling ball is more dense than a beach ball. Sometimes, the density of an object can change, like when you pack a snowball - the snow starts out light and fluffy and then you compress it to a higher density. Essentially, the more mass you pack in to a smaller volume, the higher the density.

While density it calculated as mass per volume, it can depend on temperature and pressure. This is mostly the case for gases, such as our atmosphere. Higher temperatures cause the gas particles to move around more quickly, making larger spaces between molecules, and therefore the same mass takes up a larger volume of space, causing a lower density. Higher pressures can push molecules together, making the space bewteen particles smaller, and therefore the same mass takes up a smaller volume of space, causing a higher density.

Density of the atmosphere and of the helium gas inside your blimp play a huge part in determining how much your blimp can carry. And since density is modified by temperature and pressure, you may notice that you need different amounts of helium on hot days compared to colder days or at higher altitudes compared to lower altitudes.

Helium Tank

A blimp, unlike an airplane, does not require fuel to fly, but relies on the fact that it is filled with a helium gas. Helium is lighter than air, so it simply "floats" in air through the buoyant force. Hydrogen is even lighter than helium, so it would also be a good lifter, and, in fact, blimps were once built with hydrogen. But hydrogen is very flammable and, therefore, very dangerous. Even though helium is more expensive than hydrogen, we now use it for blimps because it is so much safer than hydrogen.

The lifting power of pure helium is 0.064 lbs/ft^3. This means that for every cubic foot of volume of helium, it can carry 0.064 pounds. Your blimp can hold 255 cubic feet of helium, corresponding to an ideal lifting power of 255 ft^3 multiplied by 0.064 lbs/ft^3, which is 16.32 pounds of lift. This is much more than your payload weighs, for several reasons. First of all, the weight of the balloon itself must also be accounted for. This includes the polyurethane case, the net, rings and tethers. Second, the helium tanks you use are not exactly pure helium. While over 99% of the tank may be just helium, trace amounts of other gases are bound to contamintae at least a small fraction of a percentage of the tank. And third, the lifting power of helium is only relative to the surrounding air pressure. Close to sea level, the air is thicker (more dense), so helium "floats" on it better. Many places in Montana, however, are well above sea level. At high elevations, atmospheric pressure is lower and the air is less dense, so helium doesn't "float" as easily, meaning it can carry less weight.

Given these factors, the carrying power of your helium blimp is closer to 8 pounds.

Pressure is the force in a given area. Imagine trying to push a door open with your finger instead of the whole palm of your hand – the same amount of force opens the door, but you would have to apply more pressure because your finger touches less area than the palm of your hand.

Atmospheric Pressure

Gases can exert pressure as well. The atoms in the air are like little particles randomly flying around. When they hit each other or the walls of a container, they create pressure. This occurs in our atmosphere in different amounts. While air may seem to weigh very little, all the gas above us in our atmosphere weighs down on the gas at lower altitudes, causing it to be more dense and to exert more pressure. This effect is called atmospheric pressure. At sea level atmospheric pressure is strongest, and we define it to have units of 1 atmosphere (1 atm). Many places in Montana have high elevation, meaning we have less of the atmosphere weighing down on the air we breath, causing it to be less dense and exert less pressure. For example, Bozeman is at an elevation of just over 4800 feet, causing the atmospheric pressure to be about 0.85 atm (85% of the atmospheric pressure at sea level). Pressure

The helium in your blimp exerts a pressure on the polyurethane shell, causing it to inflate as you add more helium. There is a constant balance between the pressure from the helium inside and the atmospheric pressure outside. Your blimp will stay close enough to the ground so that the atmospheric pressure will be relatively constant as you send it up, but some remote sensing systems such as high altitude balloons need to under-fill the balloons on the ground so that when they encounter lower atmospheric pressures on their journey up, they don't pop from the inward pressure overpowering the outer pressure.

Particles in liquid exert pressure similarly to air particles. They can also weigh down on lower levels of liquid, which is why you feel more pressure when you dive deeper in a lake or pool.

You can read more about atmospheric pressure here.

Buoyant Force

Buoyancy:

Buoyancy arises from variations in pressure on an object. Consider a solid box submerged in water. The fluid pressure on each side of the box cancels out. However, the top of the box is being pushed on by less water than the bottom of the box, because it is deeper. This leads to a net upward force on the box, which is called the buoyant force. Gases can also be considered a fluid. In the case of your blimp, the atmospheric pressure surrounding the blimp is pushing slightly more on the bottom of the blimp than the top.

 

Archimedes' Principle

 

Archimedes' Principle:

Archimedes' Priciple states that the Buoyant Force is equal to the weight of the fluid displaced by the object. So the buoyant force on an object actually doesn't depend at all on the weight or shape of the object. It just depends on the weight of the fluid that is displaced by that object. Larger objects displace more fluid, so they usually experience a stronger buoyant force than smaller objects of the same material. The bouyant force on your blimp is the weight of the air it displaces (the volume of the air displaced would be the same volume of the blimp).

Sinking and FloatingSinking and Floating:

Whether an object will float or sink depends on how its weight force compares with its buoyant force. If the weight force of the object is stronger than the buoyant force (weight of the fluid displaced), the object will sink. If the weight force of the object is weaker than the buoyant force, the object will float up until the forces balance. Another way of thinking of it is comparing the density of the object to the density of the fluid. If the density of the object is larger than the fluid's then it will sink, if it's the same it will stay where it is placed, and if it is less it will float up until the forces balance. In the case of your blimp system, the helium in the blimp has a lower density than the air around it, so it floats. It would float up very high to a place where the atmospheric pressure balances the helium pressure, but your tether system holds it back.

Kingfisher blimp

Remote sensing apparatus does not require speed or agility, but rather stability. So instead of an airplane with a motor to move quickly through the air, a helium balloon is much better suited for hovering in one spot for an extendd period of time. This gives the camera system a stable vantage point for taking clean, crisp images. Your blimp is a Kingfisher Aerostat. It is designed to military specifications, so it fits the stabilitiy and reliability requirements for remote sensing.

The blimp lining is made from 4 millimeter thick polyurethane, which is both lightweight and non-porous. This material is strong and durable, but you will want to be sure to use your protective tarp to prevent rips and tears. Ultraviolet light (UV) inhibitors have been added to the polyurethane to prevent fading and degradation of the fabric by the breaking of chemical bonds that hold it together. Hydrolysis inhibitors have also been added to prevent the degrading chemical reaction that occurs when moisture sits on the fabric too long.

The mesh wing that hangs from one side gathers wind and positions the balloon so all forces are distributed to the tether line.