Vertical Airships

http://www.airship.org/

Hmmm! Vertical airships! I like where they are going with that. The big problem with airships is wind. The other one is obstacles. Occasionally bumping into big buildings would not be a too much of a problem. An airframe should be able to withstand a collision at a reasonable speed without failing. A flexible frame that can deform significantly combined with an overpressure relief and recovery system would allow the gas envelope to survive hard landings and mid-air collisions. Semi rigid ships have no gas pressure in the envelope. There is no danger of an envelope popping like a balloon, and most of the time envelope punctures can be repaired in flight.
Airships are like big boats. They need to be able to follow headings that are close to the wind without slipping leeward. Variable envelope geometry provides the ability to change drag and lift properties in flight, and could also act much like a sail while using engine thrust as a kind of air keel. This may be more efficient than just powering directly into the wind, especially at lower altitudes where air density is greater.
I could just ramble on about how this stuff might work all day, but really it needs some testing. Lots of testing. But anyway, vertical ships shaped like sails looks like a good idea if the lift dynamics are correct. I think large horizontal envelopes provide more lifting power though. Like a barge.