Pressure Pattern Navigation
Up until now, we have assumed that you will play the odds, that is, plan your route with the expectation that the actual winds you encounter will more or less correspond to the winds that prevail during the current month over the stretch of land or water you are flying over.
However, with the right resources, you can make ad hoc adjustments to your flight plan to take advantage of the winds that are actually blowing at the time of your flight.
The lower atmosphere is characterized areas of high and low pressure; these appear, intensify, move around, stay in one place for a while, fade, or disappear altogether. In low pressure areas, the air rises, and clouds are formed. In high pressure areas, the air subsides, and is dried out. Air flows (wind blows) out from highs and into lows. However, because of the coriolis force caused by the rotation of the earth, the near-surface wind is deflected. In the northern hemisphere, the winds spiral clockwise out of highs and counter-clockwise into lows.
So, that means that if a storm system is crossing the North Atlantic at the time of your flight, you can take advantage of the winds around it by staying south of the storm when flying to the east, and north of the storm when flying to the west.
At a minimum, this pressure pattern flying requires that you have reliable meteorological information for the area you are crossing. The reports could come from ground stations, or from ships or aircraft. Preferably, you have access to reliable meteorological forecasts, because, by the time you fly from point A to point B, the winds at point B have probably changed as a result of the movement of that storm. On the Graf Zeppelin (1928-37), the navigator picked route segments in one hour flight time increments, based on weather reports and forecasts.
Unfortunately, it will be some years before there's a good network of weather stations across the shipping routes of interest, and it will be even longer before we have reliable weather forecasting.
The next stage in the evolution of pressure pattern flying was made possible by the invention of the radar altimeter. Previously, the aircraft altimeter inferred the altitude on the basis of the barometric pressure. Since the radar altimeter measured altitude directly, that meant that a barometer could be used to measure the air pressure at the known altitude. Winds, especially at high altitude, tend to follow the curves of constant pressure (isobars) so you could use the barometer (pressure altimeter) , adjusted for your altitude, to follow the wind. If you found that your pressure was dropping, it would warn you that you were getting closer to the center of the storm.
Predicting when the 1632 universe will develop radar altimeters is well beyond my area of competence, but I would not expect them until the 1640s at the earliest.
The Graf Zeppelin determined height "by firing a shotgun and calculating altitude from the time delay of the returning echo." (Miller; Dick 61). The Hindenburg had a fancier sonic altimeter, using "a compressed air whistle at station 228, whose sound bounced off the ground and was picked up by a receiver at station 188. (Dick 88; airships.net). Yet another trick, used on both the Graf Zeppelin and the Hindenburg, was to drop a water bottle overboard, and time its fall with a stopwatch (Dick 61-62).
The Graf Zeppelin also is said to have dropped smudge pots and observed the smoke to determine wind direction and speed (Miller) , but that would only speak to surface wind velocity.
****
It's important not to expect too much of pressure pattern navigation. Even if you know exactly where the lows and highs, and the associated winds, are, it doesn't guarantee that it's worth detouring to exploit them. It all depends on the size and intensity of the pressure system, its location relative to that of the airship at a given time, and its own speed and direction of movement relative to the airship's intended course.
During World War II, pressure pattern navigation typically reduced transatlantic flying time "an average of 10% compared to a great-circle track, with occasional savings exceeding 25%. . . ." (Kayton 12).
Conclusion
"The Bozo people, who live on the southern fringes of the Sahara, believe that Wind wrestled with Water, and Water lost. . . ." (DeVilliers 12). If the new airships of the 1632 Universe, fight the wind, instead of exploiting it, they too will lose.
Author's Note: Appendices, bibliography and the spreadsheet will be posted to www.1632.org in the Gazette Extras section.
****
A Trans-Atlantic Airship, Hurrah
This is a report of the process I followed as I tried to put together design specifications for a Spanish airship for the Cadiz-Havana trade. It is provided "for your information," in case anybody might be interested in the process. Other than the hard science, nothing in this article should be considered canon.
I decided on a Spanish airship because someone has to make it, and it might as well be the Spanish. Not the government of course-they'd never be able to put together the will and the money for the project. Instead, I've picked on Don Juan Manuel Perez de Guzman y Silva (1579-1636), the eighth duke of Medina Sidonia. Who, as probably the premier duke of the Spains, has the added benefit of being the father-in-law of the Duke of Braganza-the man who became John IV, the first king of Portugal of the House of Braganza, in 1640 OTL. Together, these two command the significant resources that such a project will require. Currently the airship is running under the name Sao Martinho-named for the seventh duke's flagship in the 1588 Armada-but I'm open to alternative suggestions.
The two ports were selected because Cadiz is the main Atlantic port of Spain, while Havana is the forming-up port in the New World for the Spanish treasure fleets. The Great Circle (shortest) distance between the ports is 7,326 km (4,552 miles), and the only possible landfall on that route is the Azores, which are Portuguese (technically under Spanish control) and are some 1,934 km (1,202 miles) out from Cadiz.
Getting started
Where to begin?
First we need to set some basic parameters. I started with a desired payload of five thousand kg (5,000 kg)-about a quarter of the Hindenburg (LZ-129) payload, which suggests a final airship less than a quarter her size (or, less than 50,000 m^3).
Now we need a structure to carry our payload. Examination of data for historical rigid airships finds that the "weight empty" (deadweight) takes up something like forty to sixty percent of the gross lift (See Appendix 1). The Hindenburg, the last of a long line of Zeppelin designs, and a commercial rather than military airship, is probably the best rigid airship design we can base our calculations on, had a deadweight that was 60% of the gross lift, so we'll use that value for the Sao Martinho. Thus, with a payload of 5000 kg, we have a deadweight of 7500 kg, and a total gross lift required of 12,500 kg.
Using lift from hydrogen of 1.09 kg/m^3 (at 15 degrees C and 760mmHG: Woodhouse, p.209; Brooks, p.202), we need ~11,468 m^3 of hydrogen to lift the Sao Martinho off the ground.
If that was all it took, we'd have a pretty small airship. However, we also need to move the Sao Martinho. To calculate the power we need to move the Sao Martinho we have two choices. Either we use brute force to follow the shortest route, powering through unfavorable winds approach, or we make use of the prevailing winds just like the sailors of the period do.