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Douglas M-1 / M-2 / M-3 / M-4

 

douglasm2

M-2


The US Post Office Department had been responsible for US internal air mail routes from 1918 onwards, and by 1925 the various types of DH-4 biplane which had been primary equipment since inception of the service were worn out. A decision was thus made to order a conversion of the Douglas O-2 observation biplane, which had been ordered into quantity production for the US Army.


The Douglas DAM-1 (Douglas Air Mail-One), quickly shortened to M-1, was test flown during the spring of 1925. It had twice the payload of the DH-4, but made use of the same Liberty engine, of which large numbers were in store and readily available. The M-1 was a straightforward conversion of the O-2, with the forward cockpit covered in sheet aluminium to form a reinforced mail compartment with access through two deck hatches, the pilot being located in what was formerly the rear (observer's) cockpit in the O-2. During tests, extended exhaust piping was installed to keep fumes away from the pilot. The M-1 was adjudged successful, but no production order was received by Douglas.

 

douglasm3
M-3


With the introduction of Contract Air Mail (CAM) routes, however, the newly formed Western Air Express Company (later Western Airlines) ordered six Douglas mailplanes. Designated M-2, they differed from the M-1 mainly by replacement of the original tunnel radiator with a frontal type. Provision was also made for quick conversion of the freight section to permit carriage of a passenger in place of mail. A month before Western Air Express inaugurated its Los Angeles-Salt Lake City service in April 1926, the US Post Office ordered 50 of the M-3 version for its major routes. The M-3s differed only in detail from the M-2s, sporting an overall aluminium finish with US Air Mail emblazoned in black on the fuselage sides and on the undersurfaces of the lower wing. Western's machines had a red and silver paint scheme.


The Douglas Company's chief engineer, J. H. 'Dutch' Kindelberger, then redesigned the M-3 with the aim of doubling its payload. The main change in this new M-4 was an entirely new 'stretched' wing which spanned 1.47m more than the 12.09m of the earlier types, and lacked the cut-out in the trailing edge of the upper wing inherited from the US Army O-2s. The Post Office was sufficiently impressed to arrange for 40 of the 50-plane M-3 order to be delivered in M-4 configuration. A single M-4 bought by Western Air Express was designated M-4A by Douglas to differentiate it from the Post Office order. With the leasing of route CAM-3 (Chicago-Dallas) to National Air Transport (NAT) in October 1925, a need arose for more mailplanes. NAT at first used the Curtiss Carrier Pigeon and then, having acquired the important Chicago-New York route, bought at auction all 10 M-3s and eight M-4s from the Post Office when, during July 1926, that department relinquished all its routes to private operators.
The Douglas mailplanes were introduced by NAT on 1 September 1927, and were phased out during 1930 in favour of three-engined Ford tri-motors. In their three years' service they performed admirably in all weathers and in the most difficult flying conditions. NAT had bought other M-4s from a variety of sources and at one stage had as many as 24 Douglas mailplanes in operation. Among them was a privately owned aircraft which had been confiscated by the US Treasury while illegally smuggling liquor from Cuba to Florida during Prohibition; it became known as the 'Booze Ship'. NAT M-3s were flown with new long-span wings from the spring of 1928 onwards; for economic reasons these had been designed and constructed by the company's own engineering department. One M-4 was converted by NAT to take a 391kW Pratt & Whitney Hornet radial engine.


A total of 57 Douglas mailplanes was built, but with the advent of the Ford and other three-engined types they were soon withdrawn from air mail services. A few were sold to private owners but the majority were scrapped.

M-4
Engine: 1 x Liberty 12 V-12 piston engine, 298kW
Max take-off weight: 2223 kg / 4901 lb
Loaded weight: 1544 kg / 3404 lb
Wingspan: 13.56 m / 44 ft 6 in
Length: 8.81 m / 28 ft 11 in
Height: 3.07 m / 10 ft 1 in
Wing area: 38.18 sq.m / 410.97 sq ft
Max. speed: 225 km/h / 140 mph
Cruise speed: 177 km/h / 110 mph
Ceiling: 5030 m / 16500 ft
Range: 1127 km / 700 miles

 


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