A spy basket (also known as a cloud car) is a type of airship appurtenance consisting of a streamlined one-(usually)-person gondola attached to its host airship by a long cable; to use it, the host airship hides in or above a cloud layer, and the spy basket is then lowered, cum crewmember,1 from the airship down through the base of the cloud deck, and serves as an observation station in the (hopefully) clearer lower-altitude air, allowing the airship to know what happens to be below it without exposing its gigantic soft underbelly beneath the clouds.2

It was pioneered by the German armed forces during World War I, who used (or at least tried to use) it to drop bombs on Londoners3 without getting shot down; they eventually discarded the whole concept, but it was picked back up, five seconds over a decade later, by the United States Navy (the only post-1918 military user of rigid airships),4 when the U.S.S. Akron (ZRS-4) was built with provisions for the fitting and use of a spy basket. The Akron’s spy basket was first deployed in April 1932.

It did not go well:

Soon after returning to Lakehurst to disembark her distinguished passengers, Akron took off again to conduct a test of the "spy basket"—something like a small airplane fuselage suspended beneath the airship that would enable an observer to serve as the ship's "eyes" below the clouds while the ship herself remained out of sight above them. The first time the basket was tried (with sandbags aboard instead of a man), it oscillated so violently that it put the whole ship in danger. The basket proved "frighteningly unstable", swooping from one side of the airship to the other before the startled gaze of Akron's officers and men and reaching as high as the ship's equator.[18] Though later improved by adding a ventral stabilizing fin, the spybasket was never used again.[26]

This was not, so far as I can tell, a problem faced by the German airship bomber crews of World War I (who were blessed with spy baskets that dangled placidly from their mounts), and I’m having trouble seeing why the Akron’s spy basket would exhibit such remarkably unstable behaviour, unless the first (and only) trial of her spy basket was held in severe turbulence (which, given the purpose and method of use of the spy basket, I’m having great trouble believing that her commanding officers would have allowed). Can someone clarify this for me?

1: Presumably the one with the most favourable combination of good vision, non-acrophobia, and low rank.

2: It can also, if necessary, be used to determine if the cloud deck does, in fact, have a base, or if it extends all the way to ground level. Obviously, due to the likelihood of eventual ground/water/tree/automobile/watership/Paul Bunyan contact in the event that the cloud layer is continuous to ground level, this is only advisable when the airship is in a stationary hover, unless you really don’t like the person manning the spy basket.

3: And probably some other people, too.

4: The German military, having lost World War I fair and square, was forbidden to build, use, possess, rent, touch, or think dreamily about any sort of aircraft at all, lighter-than-air or heavier-than-air, while the British military, in order to save money, scrapped its fleet of rigid airships at the end of the war and cancelled all those not yet delivered (with the sole exceptions of one they were going to sell to the Americans but which, due to poor judgement on both the designers’ and the pilot’s parts, crashed while they were showing it off for its prospective customers and one which was hastily reconfigured for passenger service but proved too small for the role).


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Very interesting bit of physics going on here with the cable car swinging "up to the equator" of the airship. Perhaps something similar to what happened with the "Galloping Gertie" Tacoma Narrows suspension bridge in 1940.

The suspension cable could be hundreds of feet long. In a cross wind, if the mass to surface area (wind pressure) of the airship and the cable car did not match, the cable car would swing to the side and then back. If the wind and the swing motion became harmonicly re-inforceing, each swing becomes a little higher, eventually reaching a full 180 degree sweep.

The elasticity of a cable several hundred feet long may have contributed, stretching a little at the bottom of the arc and contracting a little at the top of the swing, helping propell the object back in the other direction.

Another factor have been a very slight difference in airspeed as the car swung away from the wind, pulling the airship with it, as compared with the return swing back into the wind. Each swing adds more energy to the system.

Without "dampers", the cable and car are wildly out of control. The fix, amazingly, may have been to fashion adjustable "sails" for the cable car. Some wind tunnel testing may have been helpful as well.

  • $\begingroup$ But what's driving the resonance? For it to be the wind, the wind would have to be gusting with about the same frequency as the pendulum, which seems unlikely. $\endgroup$ May 26, 2019 at 18:10
  • $\begingroup$ What drives the resonance is some form of energy storage like a spring. These cables were up to 1/2 mile long, enough to stretch a little. Very surprised they did not study this and try again. I would trace out the exact arcs and see if the movement down wind of the airship added energy as well. A piloted version could have used speed brakes on the return swing to slow it. $\endgroup$ May 26, 2019 at 21:32
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    $\begingroup$ @DavidRicherby Mode coupling in a free hanging pendulum means that a perturbation in any two non orthogonal directions will begin to precess. If one axis is damped (say the one in direction of wind/airflow) then the motion will "accumulate " in the undamped mode. The spring of the pendulum will exacerbate this coupling but is not required for this to happen , only a fee hanging pendulum with minimal damping. As robert points out a damping moment in the form of speed brakes would solve the issue. Intuitevly this behavior can be seen in a foucault pendulum. $\endgroup$
    – crasic
    May 26, 2019 at 21:40
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    $\begingroup$ @crasic thank you so much. I was seeing the relative motion of the pendulum and the airship as slightly different when the pendulum swung back upwind, and the airship continued downwind. I believe the energy is added when the pendulum reaches the top of the upwind arc and gets "pulled" by the airship back into its downwind arc. This would not happen if the top of the pendulum was fixed (unless the fixture started to sway in the pendulum's harmonic frequency). $\endgroup$ May 27, 2019 at 12:35

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