Tuesday, August 20, 2013

Slate's Strange Dirigible

The Archive recently had the opportunity to acquire seven original press photo prints of the Slate dirigible built at Glendale. In researching the history of this unusual airship, I found lots of tidbits in various places around the internet, but no one comprehensive history. I also became fascinated with Slate's mis-guided ideas of aerodynamics. All of that came together in this post...I hope its length doesn't produce too much tedious reading...

The Slate hangar under construction at Glendale, with a Curtiss JN-4 Jenny, which
probably belonged to the Wilson Brothers, whose hangar is just out of view on the
right. This image is a part of the Archive's Glendale negative collection, was probably
taken in 1927 and is believed to have been shot by prolific Glendale-based
aviation photographer Ray C. Talbott.
To say that Thomas Benton Slate was an interesting character is a bit of an understatement. Many inventors have a flair for the flamboyant, as it helps them gather backers for their improbable schemes, but Slate was one of those rare ones who had actually managed to turn one of his ideas into a commercial success.

Styling himself as "Captain" Slate for the media (what he was captain of, other than the dirigible, I can't find any record of), the inventor made his fortune by developing a commercially viable method of producing frozen carbon dioxide, and his company came up with the name that we all know it by, "Dry Ice". He then turned around and essentially lost that fortune on Slate Aircraft Corp and his dirigible project, in which he combined a host of technologically radical "better ways" together in one doomed effort. Slate received four patents for various aspects of the airship's concepts.

This photo's original caption read: "The all-metal "City of Glendale", which will
take off in a few weeks on its first flight. Revolutionary ideas have been embodied
in the building of this dirigible by Capt. T.B. Slate. It will be driven by steam, and
will not use mooring masts - elevators instead will raise and lower the passengers,
of which there will be 40." The print is date Dec 19, 1929, but the small tail surfaces
and lack of the engine in the aft end of the car indicate that it was taken several
months earlier.
What Slate envisioned was a fleet of airships that were efficient, affordable, larger, smoother, safer and more convenient than contemporary dirigibles. Slate knew how to work the media, and pandered to the local well-to-do by offering to let them invest in his project, which was sure, he promised, to change the future of transcontinental air travel. To further make nice with local civic leaders, he declared that he would name the prototype ship City of Glendale. Thinking far beyond the Glendale, Slate conceived of fleets of even larger dirigibles plying transcontinental air routes, with coast-to-coast times of 36 hours. To develop his project, he looked at other airships of that era, idenfied the various shortcomings of their designs, and came up with what he thought of as innovative solutions. Interest in his project wasn't just confined to the Glendale aviation community...representatives from at least one European cruise line visited the project, interested in seeing whether the airship could be used to transfer passengers to and from ships at sea.

Slate started the aircraft company in 1924 along with his brothers Grover C. and Frank P. Slate. He
leased land at the southeastern corner of Glendale Airport (it wasn't yet known as the Grand Central Air Terminal), dug a big trench and started to build the airframe in 1925. Twice, Santa Ana winds destroyed the partially built frame-work. He then built a large hangar (our first image, above) to better protect the project.

In an era when non-flamable helium supplies were tightly controlled and flammable hydrogen was readily available, Slate supposed that he could make the dirigible "fireproof" - keep in mind, this was 6 1/2 years before the Hindenberg disaster - by constructing the shell, or "envelope" from duralumin (the contemporary trade name for the age-hardened copper-aluminum alloy most commonly used in early airship and aircraft construction). The metal was formed into long strips that were interlinked and then riveted together to produce a gas-tight structure, one of the features he patented. To save weight, Slate designed the envelope to be a monocoque structure, with no underlying framework to carry the loads.

An early advertisement for Slate's dirigible. If you were
a snazzily-dressed high society type, would you be willing
to get into this elevator?
The cabin, or "car", was 80 feet long and could accommodate a crew of five plus 30 (some sources said 40) passengers in relatively luxurious comfort. Plans included sleeping accommodations plus a dining salon.

Another of Slate's patented "innovations" was a passenger elevator system. With it, Slate claimed that the Glendale would not have to land to off-load and take on passengers. Slate envisioned a network of hotels and "stations" across the country where his transcontinental airships would make passenger stops, the first of which was built on the roof of the Glendale Hotel.

To get the passengers up and down, an "anchor", which doubled as a reserve fuel tank, would be lowered from the car on a cable. At the same time, the ship could be refueled while floating high overhead. Once the anchor and cable were secure on the roof of the station, a small, one-person "elevator" would then descend, attached to the anchor cable so that it wouldn't be blown in the wind. Slate's advertisements showed only a single elevator, while the actual patent featured a more complex dual elevator system, with both running up and down independently.

The original caption is missing from the Archive's copy of this press photo,
which is date stamped Jan 10, 1929.
Slate designed the airship's dimensions very specifically, building it in a teardrop shape (as opposed to ovoid) and making the diameter much larger in proportion to its length than standard, because of how he perceived the aerodynamics would perform in conjunction with his propulsion scheme. The length of his airship was 212 feet and the diameter was 58 feet. In comparison, the Hindenberg was 804 feet long and 135 feet in diameter, and for a more modern comparison, the current GZ-20 class of Goodyear blimp is 192 feet long and 50 feet wide. The structure weighed about 14,000 pounds (Slate claimed his design weighed up to half as much as that in other comparable airships), and when filled with hydrogen, it had a useful load of 7,000 pounds. The shape, Slate believed, would prevent a vacuum from forming behind the airship as it moved through the air.

The Glendale outside on a test float. The photo is date stamped Oct. 8, 1929.
The only a fragment of the original caption remains with the print. Presuming
that this photo was actually taken sometime during early October, note that the
airship, at this date, still retains the small tail surfaces.
But it was the "air displacement" propulsion system that Slate believed set his airship apart from the rest. The inventor came up with the idea of mounting a high-speed centrifugal blower at the nose of the dirigible which would pull air from in front of the airship and direct it out perpendicular to the axis of the ship. He believed that if the blower spun fast enough (Slate claimed that the 4 foot 10 inch blower fan would run at 4,000 to 6,000 rpm), it would produce a propulsive force through two effects: one, the air pulled into the blower would create a low pressure area in front of it and the ship would move forward to fill it, and second, the airflow exiting the blower would move along the surface of the airship at initial speeds of up to 300 miles per hour, with the velocity creating a lower pressure on the front skin than felt at the aft of the ship, so again it would move forward much in the same way that a wing experiences pressure differentials which create lift. Slate claimed that wind tunnel tests performed at New York University had proven that this would work, and that the airflow would somehow "stick" to the surface of the envelope all the way back to the tail, retaining enough velocity that the tail surfaces could be of minimal size and still have control authority. Slate believed that this system would give the Glendale an 80 mph cruise speed and a 100 mph top speed. (We'll look at the aerodynamic realities that this scheme faced a bit later in the article.)

Orginal caption: "The boiler, at left, which will furnish steam to  dirve the 5-bladed
"blower" propeller held by Chief Engineer L.R. Hawkins. The blower will be installed
in the nose of the ship, and is expected to push away the air in front of the liner,
causing a vacuum which will allow a speed of 100 miles an hours."
To power the blower, Slate first considered using a 400 hp steam turbine, claiming (ridiculously) that it would be so efficient that only six gallons of water would be needed for a transcontinental flight. Later, he switched to a petroleum-powered engine. Another of his patents dealt with how the fuel was stored...some of it in liquid state, some of it in lighter-than-air gaseous state, so that the net "weight" of the fuel could be adjusted positive or negative, depending on the flight needs at the moment (this, among other things, eliminated the requirement for ballast). Eventually, though, Slate abandoned the steam idea and settled on a 150 hp gasoline engine for the nose blower, plus a 90 hp engine driving a standard propeller in the back of the car. Slate's reasoning for the cabin-mounted engine was to overcome the asymetric drag created by the car.

This photo is dated 12/20/29, a day before the formal first flight. The caption
reads: "The new all-metal airship, designed by Captain Thomas W. [sic] Slate
at Glendale, Calif., being walked from its hangar at the Grand Central Air Terminal
preparatory to a series of test flights after a final adjustement of motors and
controls. The dirigible is propelled by a blower in the nose, which, according
[to] its designer, will create a vacuum into which the ship will be pulled. The
auxiliary motor seen at the rear of gondola is used to overcome the down-ward
drag of the cabin when the ship is in flight. An important feature of the tests
will be an effort to determine definitely whether the small tail members are of
sufficient area to control a dirigible of this size."

It's interesting to note the reference to the size of the tail surfaces, since by this
point in the ship's development, they have clearly been redesigned and enlarged.
Also note that the ground has been filled in, compared to the third photo above.
A number of times during 1929, the Glendale was pulled out of its hangar for "test floats", starting in January, and it never failed to draw a crowd. The media followed the progress breathlessly, and the reputable magazine Flight published an article on the Glendale  in February 1929. As such testing continued through the year, the final touches were put on the ship. One of the design elements that changed were the tail control surfaces, which were enlarged, but clearly still not large enough.

Finally, on December 19, 1929, the Glendale was pulled from the hangar for final checks, and on the 20th, a crowd of several thousand gathered to watch the pride of the city take flight for the first time. It was a rather warm first day of winter in Southern California, and the sun on the aluminum shell quickly began to heat the hydrogen, which naturally expanded. Slate had expected this, and designed pressure relief valves, but on this day, they stuck. As the giant airship finally began to rise, the pressure inside exceeded the structural strength of the envelope, and rivets began to pop, sounding to some like gunfire; the crowds scattered. As the hydrogen escaped, the Glendale ingloriously settled back to the ground. One aspect of Slate's goals was achieved, however: despite the rupture, the craft did not catch fire.

It's December 21, 1929, and the first day of winter in Southern California clearly
isn't very cold. Note the men ballasting the ship by hanging from the bottom of
the car. Also note the relative smallness of the blower in proportion to the size
of the airship.

The original caption reads: "Slate Dirigible Not Yet Off Ground --
The new all-metal type airship designed at Glendale, Calif., by Captain Thomas
Slate has been beset by minor mishaps, previous to actual flight tests under its
own power. The latest, the bursting of a seam in the gas bag, will require nearly
a month for repairs. Photo shows the dirigible just after it was pulled from its
hangar for what was to have been its maiden flight. Note small blower on nose
of ship, by which  Capt. Slate believes the ship successfully will be propelled by
creation of a vacuum ahead of it. The craft is equipped with no other means of
propulsion, except a small auxillary motor at the rar [sic] of the gondola, designed
chiefly to overcome the downward drag of the cabin and keep the ship on an even
Initially, Slate told the media that it would take at least a month to repair the problem. A few weeks later, Slate's team had looked over the damage and came to the conclusion that because of how the individual duralumin strips were interlinked with each other, there was no practical way to access the damaged area in order to make repairs. The hull was a total loss. Slate met with his investors to see if he could get them to pony up the cash needed to rebuild the shell. While I can find no mention on how much was initiall spent for the ruined airship, one tidbit that came from Slate's son was that an airship equivalent to the Glendale would cost $1 million in 1932 dollars. Unfortunately for the inventor, the incident occurred only two months after Black Tuesday, the worst of the 1929 stock market crashes. And while the market had actually rebounded a bit, investors of all stripes were becoming extra cautious with their portfolios, and the mood for bankrolling such a risky endeavor, no matter how 'glamourous', just wasn't there. Undetered, Slate continued to try to resurrect the project. An article in the July 4, 1930 edition of the Berkeley Gazette (buried on page eleven, and full of other errors, so I don't consider it terribly reliable) indicates that some rebuilding was attempted and that a subsequent test flight was to be tried. However, there are no records that this actually came to fruition.

A comparison of the tail surfaces from January 1929 (left) to December 1929.
Two years after the City of Glendale's disappointing debut, its scrap value, and that of the hangar, were all the assets left to Slate. The company filed for bankrupcy in 1931 and the assets sold off; the buyer (not mentioned in any references I could find) supposedly employd some of the Slate staff and family members to continue some of the engineering work and to promote the idea of a metal airship with air-displacement propulsion. It came to nothing, though. Finally, with members of the media watching, Slate stood on a catwalk above the airship, and ceremoniously dropped a 50-lb sandbag onto the airship's shell. Without the pressure of hydrogen inside, the shell crumpled and collapsed in upon itself. Later, the hangar was disassembled in sections and shipped to Arizona, to become hay barns. Several years later, Slate's son Claude was still trying to generate interest in the concept, as described in this proposal he wrote to Congress. Slate himself moved to Oregon where he continued to come up with new inventions (for instance, he received a patent for a peculiar flying boat in 1946), until he passed away in 1980 at the life-well-lived age of 99.

In an ironic twist, ten months after the failure of the Glendale to achieve her maiden flight, one of Slate's engineers, A. H. Watkins (who filed Slate's UK patents) was serving as a crew member on the British airship R101 when it crashed and burned; Watkins was one of 48 killed and his body was never identified.

But if the Glendale hadn't come apart at the seams, would it have actually worked? For the rest of this article, we'll dive a little deeper into the technical and engineering aspects of Slates designs, as described in the actual patents (so if you're not an engineer, you just might find the rest a bit boring...you've been warned!).

The patent for the propulsion system, which Slate filed in July, 1925, includes a couple of interesting drawings of how he envisioned the "Air Displacement" propulsion system to work. To introduce his idea, Slate wrote, "It is...my object to provide in this type of propulsion a novel and efficient means of applying the power directly to the displacement of air completely over the forward end of the ship and replacing it from the point of largest diameter of the ship back to the tip or tail end of the ship without producing a vacuum at the rear of the ship. With this system of propulsion the fan does not function in the manner of the usual propeller and does not pull on the ship directly by its shaft, and [thus] power is not concentrated at one point on the ship but the propelling means or force is applied completely over the exterior surface of the ship's hull from one end to the other."

Slate said the blower would take "a great volume of air in at its forward open end and discharge it in a solid radial sheet at the periphery of the fan...so that the rear surface of the of the current of air...comes in contact with the surface of the nose of the ship. The fan is open at both ends, and a suction from the rear end of the fan pulls the radial flow of air tightly against the surface...at the point of contact and thus seals the passage, the current of air following the contour of the ship."

Slate went on to describe his aerodynamic propulsion concept by stating, "The tendency of a great volume of air discharged from the fan at high speed is to cause a less than atmospheric pressure between the surface of the ship and the air flow, causing the ship to move toward the radial air stream at a speed goverened by the velocity given to the radial air stream by the fan." Low pressure in front, normal pressure in back, the ship should move forward, as Slate saw it.

To aid in this effect, Slate believed his idea would reduce the ship's form drag, which he described as the "vacuum" behind the craft. He said that "The great volume of air thrown around the nose of the ship and past its largest diameter at high speed, then loses its velocity and begins to replace immediately behind the ship the space occupied by that portion of the ship, thus allowing the ship to travel on without producing a partial vacuum behind the ship. This result relieves the power plant of the burden of pulling a volume of air behind the ship for replacement."

Close-up view of the impeller as it was seen in January, 1929.

Slate believed that the "solid radial sheet of air" would bend back along the surface of the envelope, and besides the direct result of producing propulsion, would also have some additional benefits. The first of these was that the airflow would eliminate the problem of parasite drag (the drag imposed on an object as it encounters and pushes out of the way the static air ahead of it). "The remaining volume of air in front of the ship that does not flow through the fan will be entrained into the stream of air flowing from the fan at high velocity and will follow the contour of the ship without building up pressure on the nose of the ship, and will pass the volume of air for the ship's displacement past its largest diameter at high speed. Consequently, pressure on the ship's hull is relieved."

The impeller configuration in January 1929 is shown on the left, while the final
configuration in December is shown on the right. Slate had to abandon his idea
of a steam turbine, and settled on a gasoline engine, and from this photo, it
appears that he used a radial engine, which resulted in the impeller being mounted
much farther forward than what Slate's concept called for in his patent.
Another aspect of this method was large in Slate's mind, protecting the airship from turbulence, and in the patent narrative, was actually listed as the first benefit of this method: "It is the objective of my invention to provide a novel means of propulsion...adapted to relieve the frame of the ship of air pressures due to a high velocity through the air and due to storm conditions in bad weather." He went on to say, "It is also an object...to increase the factor of safety in storm conditions through wrenching cross currents of air and which will also provide a heavy covering of air flowing completely around the ship and longitudinally with the ship to protect it from cross currents of air...which otherwise would tend to wrench the ship in storm conditions [but] would not be able to strike the ship until after they have first come in contact with the flowing air [which] has a cushioning effect on cross air currents."

He was obsessed with the ship's stability in "cross currents". "Complete replacement [ie, complete elimination of the turbulence created by form drag] of the air following the passage of the ship tends to hold the rear end of the ship steady and will not allow it to swing around from one side to the other of the area displaced by the ship. If a ship is forced through the air by the ordinary means and has to draw its displacement from the surrounding atmosphere it will bring it from the course of least resistance and if the atmosphere is at all disturbed by wind or storm conditions this course of least resistance is liable to be from any direction, causing cross currents over the rear of the ship where the rudders are located."

Granted, in the era when Slate was working, the sciences of aerodynamics and fluid dynamics were still very immature, so it's not surprising that concepts which seem obvious to engineers today were relatively unknown then. Slate had shown his knowlegebility over the years for engineering refridgeration concepts (besides the commercial dry ice manufacturing process, Slate held numerous patents for refridgeration-related inventions), and while these deal in manipulating fluid pressures, he certainly does not seem to have had any formal training or professional experience in aerodynamics.

Thus, it seems that Slate made some fundamental mistakes in his understanding of airflow, thinking that the blower would produce a "solid sheet" of air that would produce the pressure drops and the cushioning effects that he imagined. With the impeller completely unducted, all that would be produced in front of it would have been turbulent, circular airflow as the output swirled around and compensated for any low pressure that might be present from air being pulled into the fan. In addition, Slate never seemed to account for the turbulent interaction between his supposed sheet of air and the ambient air, an interaction that would all but eliminate any flow all the way to the widest part of the hull, much less all the way to the tail. Finally, while the concept of laminar flow hadn't been enumerated in Slate's time, the hull of the ship, with protruding-head rivets, would not have been conducive to the laminar flow needed to produce the kind of pressure differences that Slate imagined.

In a way, then, the structural failure due to the faulty valves was a really a blessing in disguise, as it saved Slate from the embarrassment of the ship making lots of noise but going nowhere, and publicly finding out that the propulsion concept was a complete dud.


  1. Terrific read! Thank you for compiling all of this information.

  2. Interesting, indeed! Of course, aside from all the other flaws in that propulsion idea, it's preposterous that a 150hp engine would be sufficient to propel something that large at those sorts of speeds.

    The basic propulsion idea did look awfully familiar, though.

    I have memories, as a child, of visiting the university research lab of one of my dad's coworkers, where they were working on testing out this contraption: https://www.google.com/patents/US5054713. In short, the basic idea was to replace the impeller by a jet engine and appropriate nozzle, turn the whole thing 90 degrees so the engine pointed upwards, and leave off the back half -- thereby producing a VTOL airplane that looked very much like an upside-down salad bowl. As with Slate's design, the idea was that very small control surfaces in the flow could be used to effectively control the craft. The patent has a sketch of the static test apparatus, which IIRC was about five feet in diameter.

    Obviously the idea never went anywhere -- but then, what 1980s VTOL concept did? My impression is that the basic ideas were reasonably sound: the airflow did indeed stay attached to the surface except where the control surfaces intentionally detached it, and the small control surfaces were reasonably effective. Where it completely failed, of course, was on practicality, and on being any better than more conventional options.

  3. Excellent piece of writing and found knowledge. I wonder just how much pressure the shell of the ship could take, we do not know how high it rose which is rather ashame. The way i see it, a metal airship should be able to fly higher than conventional built airships thus being fast and economical.

  4. The bow blower concept was based on solid wind tunnel work from before WW1. Janes had some sketchy details on a blow up and crash on test flight version called the Pax. Further work was done on these airflows in the 1920's. Buckminister Fuller Incorparated the passive profile,(no blower) version on his first Dymaxion car. This had too be changed as unlike wind tunnel and aircraft testing-ice and water buildup on the wind shield didn't shed on its own. The efficiency gains from these blunt profile clinging flows are reported too be substantial ,but only at speeds under 100 MPH. Thus with the big money-military aircraft ,hitting 200 MPH speeds by the mid 1920's- it wasn't followed up on with the attention it deserves.
    Todd Millions

  5. is not Slate's idea similar to the coanda effect"? Is not the bow of his ship similar to the bowl of a coanda style "saucer" VTOL craft?