I’m currently working on a scene for a book where the heroes stumble across a seemingly abandoned Age of sail airship. After a brief investigation they find that the entire crew seems to have mysteriously dropped dead. How and why I am keeping close to the vest, but what I want to know is, if all the crew are dead, how long could such a ship remain airborne? I want to know weather or not the ship will be flying or crashed by the time my heroes stumble across it.
Rigid Airships require constant adjustment - ie. likely not very long at all (minutes only)
Unfortunately, rigid airships ('Airships' implies rigid, which are motored and different from 'blimps') require constant operator input to stay afloat. This is because rigid airships can only have one 'buoyancy setting', the gas inside the gas bags are inflated, and lift is altered only by:
- adjusting ballast
- adjusting pitch of propellers
- adjusting pitch and speed of airship as a function of wind over control surfaces
Initial buoyancy settings are set to allow the airship to be controlled in normal flight and allow it to land (landing being one of the most dangerous requirements of airship flight).
Unless the initial buoyancy setting is quite high, by the release of all ballast, the airship will succumb to either:
- common downdrafts forcing the airship to lose altitude (faster than you may think, this was the cause of many airship crashes)
- pitch adrift, causing loss of control and propellers to orient in a way that could cause the airship to pitch down
- wind movement over incorrect control surface settings, causing pitch down and loss of altitude
As they require constant adjustment, an estimate of time with a complete loss of control would mean the airship really only has minutes to survive, maximum perhaps an hour, unless all ballast is released suddenly, causing lift upwards.
However even if this happened the airship is still susceptible to downdrafts or wind which can only be countered by pitch up propellers and elevators in normal flight, so in these events no control would still cause an imminent crash and loss of the airship.
It seriously depends on the airship. For example some
airships blimps can stay up for as little as 24 hours. Some may stay up for a few days to a week. Basically until the lifting gas escapes, which depends on the gas. But lets assume the lifting gas does not escape, then until the first storm comes and breaks the airship, since they are quite fragile (Most airship disasters were because of this. Such as the USS Shenandoah) But if it was made with age of sail manufacturing, I'd give it a week at most, since the would gas to escape, but certainly not over a hundred, since the fabric of the shell would rot assuming the airship magically avoided all storms.
Absolute maximum: half a day
The buoyancy of an airship changed constantly from many factors, but the greatest of these is the simple thermal heating of the gas from sunlight warming. Both direct (on the surface of the blimp) and via atmosphere heating in the daylight.
As the air around the blimp warm or cools, and as the gas in the blimp warms and cools at different rates, the buoyancy of the blimp gets upset.
Unattended, it will either sink down until it hits the unforgiving surface of the Earth, or worse it will rise up unchecked until it hits the even less forgiving limit of altitude where the pressure safety valves of the gas bladders are forced to vent gas to prevent rupturing.
Once vented, and especially with no crew to replenish (if there even is a way to replenish, which is very uncommon!!) the airship will start descending very rapidly, and is due for another meeting with the hard surface of the Earth.
Some blimps, and all dirigible airships like Zeppelins, controlled their altitude dynamically, by literally "flying" up or down using control surfaces. They could also drop ballast to lighten themselves, or vent gas to make themselves heavier, but these were undesirable as they consumed very limited resources, whereas steering via control surfaces just required a tiny amount of fuel to keep the dirigible moving to enable the control surfaces.
Like the ocean, the atmosphere can have temperature and density layering. If it happens that the ship's buoyancy was trimmed correctly to float on a denser air layer (cold air trapped in a valley or canyon, for instance) the ship might remain aloft until leakage of the lifting gas brings it down.
Alternatively, if the ship happens to be trimmed "light" when control is lost (or close to neutral with engines running, which will make the ship light as fuel burns off), it will climb, ultimately to "pressure altitude" where the gas in the lift cells cannot expand further (because of limited space or elasticity in the cell material); if the cells are strong enough, an airship might remain at that altitude (much too high for crew survival without oxygen) until, once again, lifting gas leakage brings it down. In fact, if control of the gas vents was lost, this might be what killed the crew -- a combination of hypoxia and hypothermia as the ship ascends uncontrollably to the tropopause.
Aside from the ships cat, there is a survivor on this age of sail airship - a boy. He is a foreigner of some sort - and a deaf mute and possibly a simpleton. But he has kept the boilers fired on the ship and kept other things running. He cannot steer the ship or land it and he does not know how to signal for help, but he knows enough to keep the ship aloft and he knows he does not want to crash.
He has covered the bodies of the dead with sails but otherwise left them alone. He probably saw what happened. Or so one would think.
The cat will have nothing to do with this boy.