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I have read about how airships work, especially about how they control their altitude. Because what I understand from readings about the Zeppelin airships is that they were able to perform long range trips, with several stop-and-go. Also, some airships were subject to several low and high flying variations.

So my question is: How is altitude controlled aboard Zeppelins?

My understanding is that the Zeppelins weights W, and is made of X ballonets that creates a S strength equals to W. All altitude manoeuvers are then performed using vertical motors and the lift created by the movement of the fuselage. At higher altitude, higher external pression allows the ballonets to expand and thus to creates enough lift, despite surrounding air being lighter. Am I correct?

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A zeppelin leaves the ground by dropping ballast. It can then take a nose-up attitude and climb on buoyancy and engine power. As it climbs, the gas in its cells expands and when they are full, climbing further causes them to go overpressure which vents gas overboard. Then, when the zeppelin descends, the gas contracts and the cells contract and, displacing less volume, produce less lift. Then the zeppelin must dump ballast to maintain altitude. This altitude cycling consumes ballast and when all the ballast is gone, the zeppelin must come in to land.

The last of the zeppelin designs used condensers to extract the water content of the engine exhaust and store it as ballast, so as to maximize range before replentishing ballast became necessary.

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    $\begingroup$ I'm sure you didn't mean it: Air does not enter the cells, only the envelope, when the Zeppelin descends. Buoyancy is already lost when the overpressure lets the cells vent hydrogen (above the Prallhöhe). Also using fuel made the ship lighter, so constant adjustment was needed. Dynamic lift could help, but only a bit (at top speed maybe 20% of mass could be carried by dynamic lift), but woe betide the ship that tries to land (and stop!) when flying too heavy. $\endgroup$ Commented May 12, 2023 at 6:25
  • $\begingroup$ @PeterKämpf,will edit! -NN $\endgroup$ Commented May 12, 2023 at 16:11
  • $\begingroup$ A minor point, but surely the bouancy of hydrogen in air is independent of the pressure (and therefore altitude); one mol of hydrogen has a mass of 2g and displaces around 28g of air if both gases are at the same pressure. $\endgroup$
    – Frog
    Commented May 12, 2023 at 20:40
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Your question is about blimps, but in the text you ask about Zeppelins. Altitude control is similar, but there are distinct differences between both types.

Blimps first: Here the envelope is pressurized (just a bit) to keep it taut when flying. Actually, the pressure is detemined by the blimps top speed: Internal pressure must be higher than the total pressure at the bow so the envelope would not be pushed in. This is done by pumping air into bags, called ballonets, inside the envelope which is otherwise filled with lifting gas. The amount of lifting gas is determined by the weight of the ship and adjusted such that the ship is balanced. All remaining volume of the envelope that is not taken up by lifting gas is now filled by the ballonets.

Climbing can be done in two ways: Either by pointing the ship up and opening the throttles, or by dropping some ballast such that balance is lost and lift exceeds the remaining weight of the ship. As the ship ascends into thinner air, less pressure is needed to keep the envelope taut and the ballonets are deflated so the lifting gas can take up a larger proportion of the envelope. Pressure is adjusted such that the pressure difference between outside and the envelope stays constant. On the way down the reverse happens and now air needs to be pumped into the ballonets to keep the internal pressure up.

Since one mol of lifting gas will displace the same mol of air, lift does not change as the ship ascends (given that no temperature difference builds up between outside air and the gas in the envelope). Climbing by imbalance means that the ship will accelerate upwards since lift is continually larger than weight. At some point lifting gas has to be vented or balance is restored by seepage. At the latest this happens when the lifting gas has expanded so much that all ballonets are empty. Safety valves will then open as the airship climbs further in order to avoid excessive pressure which would rupture the envelope.

The Zeppelin NT is a blimp, despite its name. Here is one rare case where the propellers can be swiveled up- or downwards. Most blimps and all "real" Zeppelins used fixed propeller installations, so "vertical motors" are rare. They help to make the airship more responsive but altitude control is not done exclusively by them. Instead, the traditional technique of releasing either ballast or lifting gas is used to balance the ship and dynamic lift to control vertical speed, so you are right about "lift created by the movement of the fuselage". At higher altitude, lower external pressure allows the lifting gas to expand (shrinking the ballonets) and thus to create enough lift.

Now over to Zeppelins: Here the lifting gas is held in a number of gas bags which are taking up the upper part of the envelope. It in turn is held taut by the internal structure. Pressure in the gas bags is equal to static air pressure and altitude is controled much as in blimps: Either by dropping ballast (see here how USS Akron drops water to unsuccessfully prevent the lower fin to hit the ground) or by pointing its engines upwards. Raising the nose will also create dynamic lift, so both thrust and dynamic lift will make the airship climb. As the ship ascends, the gas bags expand and push air out of the envelope. Also here, the reverse happens on the way down, only in Zeppelins the air will flow into the envelope all by itself when the gas bags contract as ambient pressure rises.

Note that altitude control in airships will be unstable below a critical speed: In order to lift the nose, the elevator needs to push the tail down, creating a downforce. At low speed, this downforce is higher than the dynamic lift which can be created from the pitch attitude of the hull, so commanding a nose-up maneuver will make the airship sink. Experienced Zeppelin commanders used this effect to make precise landings: The ship would come in heavy (static lift being a bit less than weight) and just at the right height and distance from the desired stoppig point, the engines were reversed and the tail raised by the elevator. Now the elevator lift would help to stop the descent, helped by the upward-pointing thrust component of the propellers which at the same time decelerated the ship such that it came to rest just where the commander wanted. Being a bit heavy, only a small ground crew was needed to bring the ship in. Performing this maneuver, however, required an experienced crew onboard.

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