My design here is pretty much a bad idea. If you want a practical pedal-powered blimp, look at the white dwarf, which was built around 1984 and is now flying about Oregon.
Air weighs 1.2 kilos per cubic meter, so a 72-kilo person (160 pounds) (myself) weighs as much as 60 cubic meters of air. That means a slightly larger volume of H2 or near vacuum could offset my weight, and a larger volume yet (um, 120 cubic meters) could hold me and the container. That's a sphere 6.12 meters (20 feet) in diameter.
Air pressure is 15.5 pounds per square inch (1.1 kilos per square centimeter). That's 5,200,000 kilos of pressure on the sphere. If you want to use vacuum instead of some gas (H2 and CH4 are explosive, He is expensive, NH3 is ammonia), then you need an extremely light frame that can support thousands of tons of pressure. I don't think that is feasible today, although I don't really know. If you know either way, then tell me.
People want to get from here to there, and quickly. Using a dirigiped, that means going up and down, and from here to there, as quickly as possible, and regardless of the wind direction.
The dirigiped should be stable -- it shouldn't flip upside down. That means the center of weight should be under the center of lift. It should also go in a straight line no matter how fast it is going.
The dirigiped pilot should see where they are going. Note that where they are going (relative to the air) and where they are going (relative to the ground) are not quite the same direction.
The way airplanes produce lift also produces drag. However, there is nothing about a dirigible that requires drag. Dirigibles also don't need to burn (or carry) fuel to keep aloft. Only surface drag affects dirigibles, and since they should go fast, they should be as streamlined as possible.
Where would the pilot sit? They have to see where they are going, they can't produce drag, and their center of gravity has to be below the center of the balloon. If the balloon is vacuum-filled, putting the pilot through the center would require extra bracing, so that would be a bad idea.
The vehicle must be propelled and turn on command.
Going up and going down could be handled by aerodynamics, the same way airplanes work now. That's extra drag when going up or down, but not when going straight. Or the craft+pilot could be a little heavier than air, slowly sinking if the thing isn't moving.
It should be hard to lose lift. That can be done by dividing the long body into sealed segments, maybe 10 centimeters apiece. In the case of vacuum-filled, strings inside each segment can hold the sides together, preventing a punctured segment from bulging into the neighboring still-vacuum segments.
Pedalling a bicycle at 25 kilometers per hour isn't too hard, even with road friction and entirely absent streamlining. A dirigiped is much bigger, but there is no road and it is very streamlined. Then there is the efficiency of the propeller. How fast could I make a dirigiped go with the same effort it takes to pedal my bike? I don't even know how to guess.
What range of payloads can a dirigiped deal with?
If you have a multilayer skin, with air outside, then N2, then CH4, then H2, no two adjacent gases will burn, all are cheap, and none are terribly poisonous. A gas interior places much less demand on the skeleton.
To summarize: A dirigiped looks like a 2 meter x 3 meter x 20 meter horizontal ellipse, with a transparent streamlined underside holding wires (to keep the balloon from bending) and the pilot, and with a propeller a few meters ahead of the pilot's feet connected to pedals by chains. Going up or down can be managed by bending the balloon. I'd guess a dirigiped could be pedalled at 50 kph and deal with 10 kilos (22 pounds) variance in payload.
How do you park a dirigiped? You hook it onto a chain when you land. You get out, the craft rises, the chain follows a wire to the nose of the craft. There is a 2x3 meter footprint on the ground, then 20 meters flopping about in the air. Many dirigipeds could be parked next to each other, bouncing off one another, on top of automobile parking structures. You could pack them much closer than cars. But wouldn't all that bouncing put holes in them?
How do you get into a dirigiped? Good question.
What if a dirigiped gets loose, how do you fetch it out of the stratosphere? Another good question. Airplanes would hate these things. Dan Root suggests that they should have a gas release valve attached to an altimeter. If you allow future technology, the craft could monitor sensitive sites (airports, military bases) and autopilot clear of those in a pinch.
How do you repair them / inflate them / evacuate them? Dunno. If it's filled with CH4, you inflate/deflate it with a gas line. Deflating makes storage easier when not in use. CH4 is stable enough to hook to stoves and furnaces in houses. On the other hand, George Palickar points out that local governments would frown upon flying potential firebombs overhead.
What if you want to get groceries? Then you don't use a dirigiped.
Dirigipeds would be great in southern Utah, with its big tracts of nothingness and with its soil is too fragile to step on.
Will these things work? Has anyone ever seen one, or tried to build one? Anyone have any useful ideas about their design? Tell me and I'll incorporate useful comments.
For questions about designing this and related crafts, um, I've never really tried to build one. You'll have better luck with the newsgroups rec.aviation and rec.models.rc.air, or by looking at the airship site.
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Scheduled for Fall '97: The New Zeppelin