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Why do blimps have fins? Can't the two (or so) engines already control direction?

Image of blimp with rear fins

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up vote 26 down vote accepted

What is when the engines fail? In early airships this happened frequently, and many ships limped home on a reduced number of engines.

Note that all airships were both vertically and laterally unstable. The helmsman had to continuously adjust the rudder angle to keep the ship on course. In NACA TN 204, Frank Rizzo concluded that enlarging the fins would be beneficial, and reducing them would be unwise. They were just as large as needed to control the ship, but not sufficiently large to give it static stability.

Static stability in aircraft means any attitude change creates forces which return the craft to its old attitude. In airships, several types of static stability must be observed:

  • Buoyancy: If the ship rises or sinks, the rate of cooling of the atmosphere and the gas in the hull might differ. If the cooling rate of the atmosphere (lapse rate) is above the adiabatic rate, the ship is unstable.
  • Pitch attitude: If the ship pitches up or down, the lifting gas will shift towards the highest point, and the heavier gas in the ballonets will shift to the lower point, creating an unstable pitch condition.
  • Directional stability: When moving forward, any heading change will cause a side force which is centered at the forward hull and pushes the ship further off course. The fins create a counteracting force, but due to their size this force is too small to turn the ship back. Only by adding rudder the helmsman can keep the ship stable.
  • Longitudinal stability: In a similar way, pitching the ship up or down while in motion will create a destabilizing force on the forward hull which is mitigated, but not eliminated, by the horizontal fins. Elevator deflection is necessary to keep the ship at the desired pitch angle.
  • Roll stability: Since the center of gravity is below the center of buoyancy, the mass of the gondola will pull an airship right side up. This is also valid when flying turns: Then the gondola will roll the ship into the turn such that a rudder command into the turn will also cause a nose-down pitching moment. Now all control surfaces need to be coordinated to keep the nose of the airship up.

Note that only the first and last type of static stability are ensured, and the first only for special atmospheric conditions! The other three are lacking!

Zeppelin airships used two helmsmen, one for directional and one for pitch control. Due to the size of the ships any movement was very slow, so the task of keeping them on course was easier than it might sound. However, taking any of the fins away or even reducing their size would had made the ships uncontrollable.

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You mentioned pitch stability twice but didn't mention roll stability at all. – ratchet freak Feb 2 at 16:43
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@ratchetfreak: In roll airships are stable. That's boring. – Peter Kämpf Feb 2 at 17:11

Fins, which are also known as stabilizers, increase the horizontal and vertical stability of the blimp.

Without the fins the blimp would porpoise and sway, especially at low speeds.

For example, if the blimp encounters an updraft, the nose would want to rise up, and the tail down. The horizontal stabilizers resist the pitching moment, the same way as the feathers on an arrow.

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The fins that are present in the blimps serve two purposes:

  • They help in controlling the aircraft, much like the normal fixed wing aircraft- the movement of control surfaces in fins (the vertical fins and horizontal stabilizers) helps in rotation of the airship about cg while the engine (i.e. propellers) are used solely for providing thrust in the forward direction.

  • They also aid in stability- for example, if the airship pointed into the wind turns in one direction due to a disturbance, the wind on the fins creates an opposing moment about the cg and tends to return the craft to its original position. This is especially important in case of tethered blimps or aerostats, which usually lack engines.

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No, the fins on airships are too small to return them into the original direction! -1! – Peter Kämpf Feb 2 at 14:41
    
@PeterKämpf - "tends to return", not "will return". Perhaps "tends to return" is a very small bit overstated, but "creates a force that contributes to returning the craft to its original direction" is perhaps a bit wordy. But, of course, you are correct. – Kevin Fegan Feb 2 at 21:17
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@KevinFegan: I tend to possibly agree, some conditions pending. Padding answers with weasel words maybe doesn't sometimes enhance potentially the desired clarity, sort of. – Peter Kämpf Feb 2 at 21:20

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