In the old zeppelins, the lifting gas was contained in separate cells; in the new, the helium fills the whole envelope, except for two large bags called ballonets, which are filled with air. The pilot can trim the ship by changing the amounts of air in the two bags, or by shifting fuel from the aft tank to the midship tank, or vice versa.
The three engines are 200-horsepower Lycoming IO-360Cs, the same series that powers the Piper Arrow light airplane. The variable-pitch props are 106 inches, more than half again as long as the standard Piper Arrow propeller. As a result, they operate at a lower rpm than the props of conventional aircraft.
Even with three engines, the zeppelin NT is remarkably stingy with fuel. At cruise speed, it uses about 15 gallons an hour, only a little more than the single-engine Arrow.
Every morning before the first flight, a crew chief brings a large aluminum suitcase filled with electronic sensors to the airship and, standing outside the envelope, hooks it up to through-hull connectors. The device reads and records the air volume in the ballonets; the temperature, pressure, purity, and dew point of the helium; and the temperature and dew point of the outside air. It then comes up with an equation describing the buoyancy of the airship. That measure is e-mailed to the pilot, who can then decide, given the load factors of the mission, whether to add helium. The same data is available during flight on a display in the instrument panel.
"I can use the information to do my weight and balance and preflight activity, which is computerized," says Jim Dexter, Eureka’s director of flight operations. "Then it helps me to manage my trim, where I need to put my fuel."
Dexter came to Airship Ventures with 27 years of commercial lighter-than-air experience, all in blimps. Compared with airplanes, Dexter says, blimps, which have no internal framework, "are much harder to manage and operate. The zeppelin, because of its high degree of maneuverability, is a joy to fly. The pilot has direct control throughout the entire operation, from launch to recovery."
Kate Board, presently the only woman zeppelin pilot in the world, observes: "Flying by feel in a blimp, you have wheels and pedals and you’re kicking it around the sky, really. It is very physically demanding. The zeppelin is more mentally demanding to fly."
A takeoff demonstrates the degree of control the pilot enjoys. The airship backs slowly away from the mast by reversing the pitch of the side engine propellers. "Once we get our clearance," says Dexter, "we swivel the main engines up." At this point, each side joystick is controlling the prop pitch and engine vector. The airship ascends vertically. Once clear of ground obstacles, the engines are swiveled to level-flight position, and the joystick input goes directly to the control surfaces on the fins—"barn doors," Dexter calls them.
Strictly speaking, the zeppelin is not lighter than air. "We’re normally operating about 380 kilos [838 pounds] heavy," he says. "As we vector the engines up for landing, we lose aerodynamic lift over the bag, so we start to descend. That’s the ideal configuration for the ship to bring itself down." In a pinch, descent can also be controlled by releasing water ballast, helium, or both.
One day Dexter invited me to sit in the right seat on a flight out of Long Beach airport in southern California. As we cruised at 1,000 feet over the Pacific a half-mile off the breakwater, I told him that one of the things that impressed me was the way he could turn the ship on its vertical axis. He smoothly vectored the engines up to hover and, using the thrust of the aft engine’s side prop, began a long gentle turn to the right. It’s a maneuver the pilots often use for sightseeing passengers who want to linger over a certain patch of landscape—say, their own neighborhood.