How do blimps/rigid airships actually land when filled with gasses lighter than air?
In: Engineering
~~Thy have ballast chambers called ballonets which, attached to an air pump, can be pressurized or depressurized. This changes the overall buoyancy of the ship. If you pack the ballonets full of pressurized air, the airship will sink. If you vent them, it will rise.~~ a person with flying experience tells me this is incorrect, please disregard!
The gas is lighter then air but the airship is heavier then air. So together they are just as heavy as air. The exact weight of the airship can be adjusted by either releasing gas, refilling the gas from bottles or dropping ballast. But most of the time the altitude can easily be controlled by changing the pitch of the airship using the elevators in the tail and then use the engines to go up or down depending on the pitch. This is very similar to how an aircraft changes altitude.
I’m afraid there are a lot of half-right answers here.
I’ve actually [flown a Zeppelin](http://www.efalk.org/Eureka/), and that included a day of ground school.
The ship is filled with ballast so that it’s slightly heavier than air. During the flight, fuel is burned off and it becomes lighter. (The great airships of the past actually had ways to recapture the water from the exhaust to use it for ballast to replace the burned fuel.)
The goal is to start out heavy enough when you take off that you’re not too light to land later. Calculating weight and balance is ridiculously complicated. They even account for how much sunlight they expect to hit the envelope and warm up the gas during the flight.
Ballast can be released (it’s just water) to make the ship lighter, but if you do this you’ve probably screwed up your weight and balance calculations. Lifting gas can be released to go down, but it’s crazy expensive so you only do it in an emergency.
While moving at speed the elevator can control up and down, just like an airplane. In fact, flying it is just like flying an airplane except that the controls are really really sluggish.
At low speeds, the elevator doesn’t do very much. Instead, the engines can be tilted up or down to be used as thrusters. A typical approach to landing involves balancing the throttles and the angle of the engines so that the engines push the ship forward and down at just the right rates. Helicopter pilots actually do this better than airplane pilots. If you’ve ever played the old “Lunar Lander” video game, it’s a lot like that.
Once you get in close enough, the ground crew grabs the mooring line, feeds it into the mooring mast attached to the ground or a big heavy truck, and pulls you in the rest of the way. If needed, they can lob some sandbags into the back of the gondola to make it heavy enough to sit on the ground so the passengers can get out.
The ballonets have nothing to do with lift and there’s no practical way to compress air or lifting gas. That would require a big heavy compressor and big heavy storage tanks. A ballonet would burst if you tried to pressurize it. The ballonets are just there to keep the main envelope fully inflated. On the ground, the envelope is not 100% filled with helium. Helium expands as the airship climbs, and you can’t have it venting out from overpressure. So instead, you fill the envelope with just enough helium to make the airship fly, then inflate the ballonets to make the difference. As the ship climbs, you vent air out of the ballonets instead of venting helium. On the way back down, you re-inflate the ballonets. Note that once the ship has climbed high enough that the ballonets are completely empty, that’s your service ceiling and you cannot go any higher. On the NT, that was about 8000 feet.
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