How much cruising speed did the Airspeed Courier gain by retracting its undercarriage?

In the early 1930s a retractable undercarriage was a rarity in British light aviation. When conceiving the Courier, Hessell Tiltman championed a fast, advanced aeroplane, and some saw in a retracting undercarriage better aerodynamic performance and a cleaner, more elegant exterior. But in the country's aeronautical circles there were also critics who doubted it would pay off, given the more complex mechanisms needed to raise and lower the wheels. The doubt reached Airspeed's own board: Lord Grimthorpe was pessimistic, apparently influenced by those detractors. Tiltman defended the solution firmly, several directors backed him, and the undercarriage stayed.

The published balance sheet is clear. The raising and lowering mechanism was estimated to add about 30 lb of weight, while the reduced drag contributed about 20 mph of cruising speed, some 32 km/h. The Spanish-language Enciclopedia ilustrada de la aviación adds that Tiltman carried on without entering into controversy and, in due course, was able to demonstrate that improvement with the main undercarriage retracted. The pilot operated the system with a manual hydraulic pump. According to British Civil Aircraft since 1919, the Courier with the Lynx IVC engine cruised at 132 mph (212 km/h), so the gain was not a minor one.

The undercarriage was an Airspeed patent, and the company later earned income when it was adopted on other aircraft, among them the twin-engined Airspeed Oxford. It also had a curious safety net: the wheels protruded slightly even when retracted, so if the pilot forgot to lower them the landing usually ended with a bent propeller and an otherwise intact aeroplane.