Beechcraft AT-11 over the West Texas prairies, circa 1944.
Private Beech H18 with the optional tricycle undercarriage visiting Lannion, France
The Beechcraft Model 18, or "Twin Beech", as it is better known, is a six- to 11-seat, twin-engined, low-wing, tailwheel aircraft manufactured by the Beech Aircraft Corporation of Wichita, Kansas. This model saw military service during and after World War II in a number of versions, including the United States Army Air Forces (USAAF) C-45 Expeditor, AT-7 Navigator, AT-11 Kansan; and for the United States Navy (USN), UC-45J Navigator and the SNB-1 Kansan.
By the late 1930s, Beechcraft management speculated that a demand would exist for a new design dubbed the Model 18, which would have a military application, and increased the main production facilities. The design was mainly conventional for the time, including twin radial engines, all-metal semimonocoque construction with fabric-covered control surfaces and tailwheel undercarriage. Less conventional was the twin-tailfin configuration. The Model 18 can be mistaken for the larger Lockheed Electra series of airliners which closely resemble it. Early production aircraft were powered either by two 330-hp (250-kW) Jacobs L-6s or 350-hp (260-kW) Wright R-760Es. The 450-hp (336-kW) Pratt & Whitney R-985 became the definitive engine from the prewar C18S onwards. The Beech 18 prototype first flew on 15 January 1937.
Prior to the Pearl Harbor attack, the Beech 18 was outsold by the Lockheed 12 by two-to-one. However, war priorities forced Lockheed to concentrate on its heavier aircraft, and Beechcraft received a major boost through wartime contracts.
The aircraft has used a variety of engines and has had a number of airframe modifications to increase gross weight and speed. At least one aircraft was modified to a 600-hp (447-kW) Pratt & Whitney R-1340 powerplant configuration. With the added weight of about 200 lb (91 kg) per engine, the concept of a Model 18 fitted with R-1340 engines was deemed unsatisfactory due to the weakest structural area of the aircraft being the engine mounts. Nearly every airframe component has been modified.
In 1955, deliveries of the Model E18S commenced; the E18S featured a fuselage that was extended 6 in (150 mm) higher for more headroom in the passenger cabin. All later Beech 18s (sometimes called Super 18s) featured this taller fuselage, and some earlier models (including one AT-11) have been modified to this larger fuselage. The Model H18, introduced in 1963, featured optional tricycle undercarriage. Unusually, the undercarriage was developed for earlier-model aircraft under an STC by Volpar, and installed in H18s at the factory during manufacture. A total of 109 H18s were built with tricycle undercarriage, and another 240 earlier-model aircraft were modified with this.
Construction of the Beechcraft Model 18 ended in 1970 with a final Model H18 going to Japan Airlines. Beechcraft set a record that still stands today for the longest continuous production of a piston-engine aircraft. Through the years, 32 variations of the basic design had flown, over 200 improvement modification kits were developed, and almost 8,000 aircraft were built. In one case, the aircraft was modified to a triple tail, trigear, humpbacked configuration and appeared similar to a miniature Lockheed Constellation. Another distinctive conversion was carried out by PacAero as the Tradewind. This featured a lengthened nose to accommodate the tricycle nosewheel, and the Model 18's twin tailfins were replaced by a single fin.
Beechcraft 18 on floats
Production got an early boost when Nationalist China paid the company US$750,000 for six M18R light bombers, but by the time of the U.S. entry into World War II, only 39 Model 18s had been sold, of which 29 were for civilian customers. Work began in earnest on a variant specifically for training military pilots, bombardiers, and navigators. The effort resulted in the Army AT-7 and Navy SNB. Further development led to the AT-11 and SNB-2 navigation trainers and the C-45 military transport. The United States Air ForceStrategic Air Command had Beechcraft Model 18 (AT-11 Kansans, C-45 Expeditors, F-2 Expeditors (the "F" standing for "Fotorecon"), and UC-45 Expeditors) from 1946 until 1951. From 1951 to 1955, the USAF had many of its aircraft remanufactured with new fuselages, wing center sections, and undercarriages to take advantage of the improvements to the civil models since the end of World War II. Eventually, 900 aircraft were remanufactured to be similar to the then-current Model D18S and given new designations, constructor's numbers and Air Force serial numbers. The USN had many of its surviving aircraft remanufactured, as well, these being redesignated as SNB-5s and SNB-5Ps. The C-45 flew in U.S. Air Force service until 1963, the USN retired its last SNB in 1972, while the U.S. Army flew its C-45s through 1976. In later years, the military called these aircraft "bug smashers" in reference to their extensive use supplying mandatory flight hours for desk-bound aviators in the Pentagon.
Among the most notable cooling air and exhaust modifications were those engineered by Benjamin Israel while employed by Conrad Conversions. His modifications were based largely on creating a more efficient use of cooling air to reduce drag, a major detriment to cruise performance. Cruise performance was improved 10% or more at the same power settings as before the modifications. These modifications were largely copied on the factory-produced G and H models. Beech 18s were used extensively by Air America during the Vietnam War; initially more-or-less standard ex-military C-45 examples were used, but then the airline had 12 aircraft modified by Conrad Conversions in 1963 and 1964 to increase performance and load-carrying capacity. The modified aircraft were known as Conrad Ten-Twos, as the maximum takeoff weight (MTOW) was increased to 10,200 lb (4,600 kg). The increase was achieved by several airframe modifications, including increased horizontal stabilizer angle-of-incidence, redesigned undercarriage doors, and aerodynamically improved wingtips. Air America then had Volpar convert 14 aircraft to turboprop power, fitted with Garrett AiResearch TPE-331 engines; modified aircraft were called Volpar Turbo Beeches, and also had a further increase in MTOW to 10,286 lb (4,666 kg).
Engineless Hamilton Westwind conversion at an airfield in Tennessee
A factory option at one point was the addition of JATO bottles on each engine nacelle which added the equivalent of 200 hp (150 kW) per engine for about 12 seconds. The most successful powerplant upgrade was that of the Pratt & Whitney Canada PT6 turbine engine and Hartzell propeller. This conversion was carried out by Hamilton Aircraft in the 1960s and 70s as the Hamilton Westwind, successfully extending the commercial life of the aging aircraft. The Westwind II added a fuselage stretch to provide seating for 17 passengers, the Westwind III seated eight and used the remainder of the extra room for cargo, and the Westwind IV added an extra stretch and a large cargo door.
The wing spar of the Model 18 was fabricated by welding an assembly of tubular steel. The configuration of the tubes in combination with drilled holes from aftermarket STC modifications on some of these aircraft have allowed the spar to become susceptible to corrosion and cracking while in service. This prompted the FAA to issue an Airworthiness Directive in 1975, mandating the fitting of a spar strap to some Model 18s. This led, in turn, to the retirement of a large number of STC-modified Model 18s when owners determined the aircraft were worth less than the cost of the modifications. The corrosion on unmodified spars was not a problem, and occurred due to the additional exposed surface area created through the STC hole-drilling process. Further requirements have been mandated by the FAA and other national airworthiness authorities, including regular removal of the spar strap to allow the strap to be checked for cracks and corrosion and the spar to be X-rayed. In Australia, the airworthiness authority has placed a life limit on the airframe, beyond which aircraft are not allowed to fly.
First production model with seating for two pilots and seven or eight passengers, fitted with Wright R-760E-2 engines of 350 horsepower (260 kW), MTOW: 6,700 lb (3,000 kg)
Version of Model 18A capable of being fitted with skis or Edo 55-7170 floats; MTOW: 7,200 lb (3,300 kg)
Improved model with increased range and useful load, fitted with 285 hp (213 kW) Jacobs L-5 engines
Version of Model 18B capable of being fitted with skis or floats.
Variant with seating for two pilots and nine passengers, fitted with Jacobs L-6 engines of 330 horsepower (250 kW), MTOW: 7,200 lb (3,300 kg).
Version of Model 18D capable of being fitted with skis or Edo 55-7170 floats, MTOW: 7,170 lb (3,250 kg)
Variant of 18D with MTOW increased by 300 lb (140 kg) to 7,500 lb (3,400 kg), fitted with Pratt and Whitney R-985 engines with 450 hp each
Seaplane version of Model A18D, but same MTOW as S18D, fitted with Edo 55-7170 floats
Version fitted with Pratt and Whitney R-985 engines of 450 horsepower (340 kW), MTOW: 7,500 lb (3,400 kg)
Seaplane version of Model A18A, fitted with Edo 55-7170 floats, MTOW: 7,170 lb (3,250 kg)
Model with Pratt and Whitney R-985-A1 engines with dual-stage blower for increased power at higher operating altitudes, 450 horsepower (340 kW), seven built, one to Sweden as an air ambulance, six to Nationalist China as M18R light bombers
Nine-passenger pre-World War II civil variant, served as basis for USAAF C-45C
Nine-passenger pre-World War II civil variant, served as basis for USAAF F-2
Variant of B18S with seating for eight passengers, and equipment and minor structural changes
First post-World War II variant introduced in 1945, with seating for eight passengers and MTOW of 8,750 lb (3,970 kg), 1,035 built
Two hundred and eighty 280 D Models were made for the Royal Canadian Air Force (RCAF), and they were delivered between 1951 and 1952. (Their serial numbers ranged from CA-1 to CA-280.) Seating for RCAF was for five passengers, or two RCAF Navigator students and one RCAF navigator instructor. MTOW for RCAF was 9300 lbs.
3N: Fitted as a navigation trainer with astrodome and two trainee stations in the cabin; 88 built
3NM: Fitted primarily as a navigational trainer, and is fitted with floor lugs to accept transport seats on removal of navigation equipment; 59 built
3NMT: Basically a 3NM, converted to a transport aircraft; 67 built
3NMT(Special): Fitted as a navigation trainer personnel transport. First navigation training position retained, slightly modified ie: removal of air position indicator and replaced with radio compass and indicator from removed second navigation position; in addition, three reclining-type chairs fitted; 19 built
3TM: Normally fitted with transport type seats, but has the necessary wiring, plumbing, and fittings for conversion to a navigation trainer, including provisions for fitting an astrodome; 44 built
3TM(Special): Specifically modified RCAF Expeditor under Project WPB6, and refers specifically to overseas Expeditors; three built
Variant with Continental R9-A engines of 525 horsepower (391 kW) and MTOW of 9,000 lb (4,100 kg), introduced in 1947, 31 built.
Variant with redesigned wing and MTOW of 9,300 lb (4,200 kg); 403 built
Variant of E18S with MTOW of 9,700 lb (4,400 kg); 57 built
1959-built G18S in 2011
Superseded E18S, MTOW of 9,700 lb (4,400 kg); 155 built
Lightweight version of G18, MTOW of 9,150 lb (4,150 kg); one built
Last production version, fitted with optional tricycle undercarriage developed by Volpar and MTOW of 9,900 lb (4,500 kg); 149 built, of which 109 were manufactured with tricycle undercarriage
Six-seat staff transport based on C18S; 11 built
Eight-seat utility transport based on C18S; 20 built
Redesignation of all surviving F-2, F-2A, and F-2B aircraft by the USAF in 1948
Based on C18S, but with modified internal layout; 223 ordered, redesignated UC-45B in 1943
Expeditor I: Some C-45Bs were supplied to the RAF under Lend-Lease.
Two Model 18S aircraft impressed into the USAAF, redesignated UC-45C in January 1943
Designation given to two AT-7 aircraft converted as passenger transports during manufacture, redesignated UC-45D in January 1943
Designation given to two AT-7 and four AT-7B aircraft converted as passenger transports during manufacture, redesignated UC-45E in January 1943
Standardized seven-seat version based on C18S, with longer nose than preceding models; 1,137 ordered, redesignated UC-45F
Expeditor II: C-45Fs supplied to the RAF and Royal Navy under Lend-Lease
Expeditor III: C-45Fs supplied to the RCAF under Lend-Lease
AT-7s and AT-11s remanufactured in early 1950s for the USAF to similar standard as civil D18S with autopilot and R-985-AN-3 engines; 372 aircraft rebuilt
Multiengine crew trainer variant of C-45G; AT-7s and AT-11s remanufactured in early 1950s for the USAF to similar standard as civil D18S, 96 aircraft rebuilt
Based on C18S with R-985-AN3 engines; 549 built
Bombing and gunnery trainer for USAAF derived from AT-7, fuselage had small, circular cabin windows, bombardier position in nose, and bomb bay; gunnery trainers were also fitted with two or three .30-caliber machine guns, early models (the first 150 built) had a single .30-cal AN-M2 in a Beechcraft-manufactured top turret, later models used a Crocker Wheeler twin .30-cal top turret, a bottom tunnel gun was used for tail gunner training, 1,582 built for USAAF orders, with 24 ordered by Netherlands repossessed by USAAF and used by the Royal Netherlands Military Flying School at Jackson, Mississippi.
Conversion of AT-11 as navigation trainer; 36 converted
Conversion of UC-45F, modified to act as drone control aircraft, redesignated as DC-45F in June 1948
Conversion of Model 18 with nosewheel undercarriage
Volpar (Beechcraft) Super 18
Volpar (Beechcraft) Turbo 18
Beech Model 18s fitted with the Volpar MkIV tricycle undercarriage and powered by two 705-hp Garrett TPE331-1-101B turboprop engines, flat-rated to 605 hp (451 kW), driving Hartzell HC-B3TN-5 three-bladed, reversible-pitch, constant-speed feathering propellers