Lockheed 3 Air Express / 4 Air Express / 7 Air Express
The success and reliability of the Vega made this aircraft of interest to many airlines, but Western Air Express required some of its own ideas incorporated. With a fuselage, landing gear and tail unit generally similar to the Vega, this aircraft differed primarily by having an increased-span parasol wing, a cabin seating four passengers or carrying 2.83cu.m of mail, with the pilot's open cockpit moved to the rear of the cabin, as the Lockheed 3 Air Express.
Power was provided as standard by a 306kW Pratt & Whitney Wasp, but at least one was flown with a Wasp engine of 391kW, and some of these engines were enclosed by the NACA-developed low-drag cowling.
A total of seven of these aircraft was built, plus one Air Express Special with which Laura Ingalls intended to make a non-stop transatlantic flight in 1931. Western Air Express, which had inspired this development of the Vega, acquired only a single example.
The Lockheed 4 Explorer variant of the Air Express/Vega series with low-set monoplane wing, fixed landing gear and 336kW Pratt & Whitney Wasp, had a wingspan was 14.78m and maximum take-off weight 4086kg. Designed for a non-stop trans-Pacific flight to Japan, only two were built. The first crashed during take-off for the record attempt in July 1929, and the replacement aircraft, with jettisonable landing gear, crashed during trials in September 1929. The theoretical range was 8850km.
The Lockheed 7 Explorer variant was an improved version of the Model 4 with a 336kW Wasp C. The first aircraft crashed during trials in May 1930, and second made some moderately successful flights before being written off
Lockheed 3 Air Express
Max take-off weight: 1984 kg / 4374 lb
Wingspan: 12.95 m / 42 ft 6 in
Max. speed: 269 km/h / 167 mph
Lockheed 4 Explorer
Engine: 336kW Pratt & Whitney Wasp
Maximum take-off weight: 4086kg
Lockheed 7 Explorer
Engine: 336kW Wasp C