How a shortened Boeing 707 derivative rewrote the rules of short-runway flight
In short: Announced in 1957, the Boeing 720 was a clever evolution of the 707. By lightening the airframe and modifying the wings, Boeing created a jet capable of operating from shorter runways, offering a profitable, low-development alternative for medium-range routes.
The Boeing 720 was designed as a derivative of the 707-120, specifically to handle shorter flights from shorter runways. To achieve this, Boeing engineers reduced the aircraft's length by 8 feet 4 inches (2.54 m), removing four frames from the front of the wing and one from the aft. The airframe was also lightened through the use of thinner fuselage skins and lighter forged metal parts, resulting in a lower maximum takeoff weight.
A critical component of the 720's performance was its modified wing. Engineers added a wing root glove to reduce drag by decambering the root, which increased the effective local wing sweep and raised the drag divergence Mach number by Mach 0.02. Additionally, the inclusion of Krueger flaps outboard of the outboard engines allowed for lower takeoff and landing speeds, further reducing the required runway length.
The aircraft saw two primary configurations. The original version, which first flew on November 23, 1959, was powered by four Pratt & Whitney JT3C turbojets. The reconfigured 720B, which first flew on October 6, 1960, utilized more powerful JT3D turbofans. These engines provided higher thrust and lower fuel consumption, increasing the 720B's range to 3,200 nautical miles (5,900 km) while seating 156 passengers in a single class.
While the 720 was a commercial success for airlines like United and American, it also served diverse roles. During the late 1960s, US military forces employed it for transporting soldiers to the Far East. Later, the aircraft found life as a private charter jet for rock bands like Led Zeppelin, and even as a flying engine testbed for Pratt & Whitney Canada until 2010.
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Sources
- Summary by Intellect · source: Boeing 720
- Also: Wikidata — Boeing 720 · CC0
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