Flexible path routing had its precursive roots in the previous century in the route planning of clipper and steam ships. Its advocates such as Sir Francis Galton (1866) for ships at sea, M.A. Giblett (1924) and Dr. John Bellamy (1940’s – 50’s) for air flights pioneered its utility. In the 20’s and 30’s it was reduced to practice by the Zeppelin airships as they sought the most favorable winds to shorten their long flights. Today, FAA’s Free Flight implements this same concept using state-of-the-art technology for air traffic.
The Graf Zeppelin design is the result of the inspired legacy of Count Ferdinand von Zeppelin, a German army officer. He became intrigued with air flight as he observed a free balloon launch in the United States. His pursuit for developing air transport became an obsession. The airship was deployed for military operations during WWI by the Germans, but proved to be a vulnerable weapon platform. The Zeppelin Company, founded by Count Zeppelin, pursued the commercialization of progressively improved rigid airships resulting in the launching of the Graf Zeppelin in 1928. In 1929, the Graf Zeppelin, 775 feet long and 100 feet high with luxurious appointments for its 20 passengers, embarked on an around‑the‑world flight. Dr. Hugo Eckner was in command of the flight. The flight was completed in 21 days 5 hours and 54 minutes of which 47 hours were ground time.
The flight was preceded by extensive flight planning. Typically, the captain and his navigators decided what the optimal flight path should be. Borrowing from the past, the route traveled would exploit the wind as the famous clipper ships did in the previous century. The airship with its long flights was the ideal flight vehicle to take advantage of the winds. The flight path would be planned to what is now known as the minimal time path. Weather forecasts from stations enroute were studiously examined. The flight plan would take advantage of the tail wind component of the wind even if it meant deviating from a great circle. Credit for the concept of the minimal flight path concept derives from the work of Sir Francis Galton in 1866 and M.A. Giblett in 1924 (both of England). During WWII and into the late 40’s and 50’s, Dr. John Bellamy, an American civil engineer and meteorologist, advanced this concept to include pressure-pattern flight planning and in-flight navigation. How the minimal flight path (minimum time path) could be prepared is shown beginning with the wind triangle and its vector components (Figure 12 ) followed by a hypothetical minimal flight path. The technique shown is a close approximation of the classical minimal flight path.
Figure 12. Wind Triangle
The Route. The
route and all its waypoints are established and the weather forecast for
key points along the route are recorded.
The navigator lays out the minimal flight path by arcing off a
radius equal to one hour’s airspeed of the airship from departure.
He then lays the regional one-hour wind vectors along the airspeed
arc (with the tail touching the arc).
It is quite evident in which direction the optimal path gen-erally
lay. Connecting the head points of these vectors establishes a
locus of the ground positions as shown in
Figure 13. Establishing the First Hour Ground Positions (Sample)
Figure 14. Establishing the Minimal Flight Path (Hypothetical)
In this hypothetical plan for the minimal flight path, the following information is given: To avoid solving the vector triangle for groundspeed depicted in Figure 12 , assume that it is approximately equal to airspeed plus the average tail wind component.
Airspeed: 70 knots
Great circle distance: 1,120 nmi
Minimal flight path: 1,190 nmi
Average tail wind along great circle: 10 knots
Average tail wind along minimal time path: 40 knots
Selecting this minimal flight path yields savings in time over the great circle (shown as a straight line) of:
The answer is:
The 10 knots tail wind component for the great circle is added to the airspeed to yield a ground speed of 80 knots. The 40 knots tail wind component of the wind for the minimal flight path is added to the airspeed to yield a ground speed of 110 knots
Airship navigation employed celestial, dead reckoning and map reading navigation. In-flight winds are computed by obtaining drift readings on three different headings. The airship maneuvers once an hour to obtain the wind calculation. The airship turns left 45 degrees away from its heading and obtains a drift reading. It then turns right 90 degrees toward its original course and obtains another drift reading. Finally, it turns 45 degrees in the opposite direction resuming its original heading and obtains its third drift reading. With three drift readings on three different headings, a wind calculation and ground speed is readily obtained when combined with the airspeed information as seen in Figure 12 . Knowing the wind, the navigator establishes his dead reckoning position using the drift, heading and projected ground speed. The new wind information is used to reassess the minimal flight path plan and make necessary changes in the flight plan.
Pressure pattern flying evolved in the 40’s and 50’s with the advent of radar which enabled a comparison of the pressure altitude and the absolute altitude (measured by the radar altimeter). This makes it possible to determine pressure altitude variation and the slope of the constant pressure surface. This information is used to determine the cross wind component of the air vehicle from its air path. The drift angle can be deter-mined using a formula derived by John Bellamy (Bellamy drift) that relies on the radar sounding information. Pressure pattern techniques are adopted by the airlines and military transports for more efficient navigation and time en route savings.