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Bell UH-1 Iroquois

The Bell UH-1 Iroquois (nicknamed "Huey") is a utility military helicopter designed and produced by the American aerospace company Bell Helicopter. It is the first member of the prolific Huey family, as well as the first turbine-powered helicopter in service with the United States military.

Development of the Iroquois started in the early 1950s, a major impetus being a requirement issued by the United States Army for a new medical evacuation and utility helicopter. The Bell 204, first flown on 20 October 1956, was warmly received, particularly for the performance of its single turboshaft engine over piston engine-powered counterparts. An initial production contract for 100 HU-1As was issued in March 1960. In response to criticisms over the rotorcraft's power, Bell quickly developed multiple models furnished with more powerful engines; in comparison to the prototype's Lycoming YT53-L-1 (LTC1B-1) engine, producing 700 shp (520 kW), by 1966, the Lycoming T53-L-13, capable of 1400 shp, was being installed on some models. A stretched version of the Iroquois, first flown during August 1961, was also produced in response to Army demands for a version that could accommodate more troops. Further modifications would include the use of all-aluminum construction, the adoption of a rotor brake, and alternative powerplants.

The Iroquois was first used in combat operations during the Vietnam War, the first examples being deployed in March 1962. It was used for various purposes, including conducting general support, air assault, cargo transport, aeromedical evacuation, search and rescue, electronic warfare, and ground attack missions. Armed Iroquois gunships carried a variety of weapons, including rockets, grenade launchers, and machine guns, and were often modified in the field to suit specific operations. The United States Air Force deployed its Iroquois to Vietnam, using them to conduct reconnaissance operations, psychological warfare, and other support roles. Other nations' armed air services, such as the Royal Australian Air Force, also dispatched their own Iroquois to Vietnam. In total, around 7,000 Iroquois were deployed in the Vietnam theatre, over 3,300 of which were believed to be destroyed. Various other conflicts have seen combat deployments of the Iroquois, such as the Rhodesian Bush War, Falklands War, War in Afghanistan, and the 2007 Lebanon conflict.

The Iroquois was originally designated HU-1, hence the Huey nickname, which has remained in common use, despite the official redesignation to UH-1 in 1962. Various derivatives and developments of the Iroquois were produced. A dedicated attack helicopter, the Bell AH-1 Cobra, was derived from the UH-1, and retained a high degree of commonality. The Bell 204 and 205 are Iroquois versions developed for the civilian market. In response to demands from some customers, a twin-engined model, the UH-1N Twin Huey, was also developed during the late 1960s; a further updated four rotor model entered service in Canada but not the US, the Bell 412. A further updated UH-1 with twin engines and four-bladed derivative, the Bell UH-1Y Venom, was also developed during the early twenty-first century for the USMC. In US Army service, the Iroquois was gradually phased out following the introduction of the Sikorsky UH-60 Black Hawk and the Eurocopter UH-72 Lakota in the early 21st century. However, hundreds were still in use more than 50 years following the type's introduction. In excess of 16,000 Iroquois have been built since 1960. With new orders from Japan and Czech Republic, the UH-1 remains in production. Several export customers, such as Canada, Germany, Taiwan, Japan, and Italy, opted to produced the type under license. Operators have been located across the world, including the Americas, Europe, Asia, Africa, the Middle East, and the Pacific region.

Development

In 1952, the U.S. Army identified a requirement for a new helicopter to serve as medical evacuation (MEDEVAC), instrument trainer, and general utility aircraft. The Army determined that current helicopters were too large, underpowered, or too complex to maintain easily. During November 1953, revised military requirements were submitted to the Department of the Army. Twenty companies submitted designs in their bid for the contract, including Bell Helicopter with the Model 204 and Kaman Aircraft with a turbine-powered version of the H-43. On 23 February 1955, the Army announced its decision, selecting Bell to build three copies of the Model 204 for evaluation with the designation XH-40.

Model 204
Powered by a prototype Lycoming YT53-L-1 (LTC1B-1) engine producing 700 shp (520 kW), the XH-40 first flew on 20 October 1956, at Fort Worth, Texas, with Bell's chief test pilot, Floyd Carlson, at the controls. Even prior to the first flight, the Army had placed an order for six YH-40 service test helicopters. During 1957, a further two prototypes were completed. In 1959, the Army awarded Bell a production contract for 182 aircraft, which was designated "HU-1A" and officially named Iroquois after the Native American nations.

The helicopter quickly developed a nickname derived from its HU-1 designation, which came to be pronounced as "Huey". The reference became so popular that Bell began casting the name on the helicopter's anti-torque pedals. The official U.S. Army name was almost never used in practice. Even after September 1962, at which point the designation for all models was changed to UH-1 under a unified Department of Defense (DOD) designation system, yet the nickname persisted.

While glowing in praise for the helicopter's advances over piston-engined helicopters, the Army reports from the service tests of the YH-40 found it to be underpowered with the production T53-L-1A powerplant producing a maximum continuous 770 shaft horsepower (570 kilowatts). The Army indicated the need for improved follow-on models even as the first UH-1As were being delivered. In response, Bell proposed the UH-1B, equipped with the Lycoming T53-L-5 engine producing 960 shp (720 kW) and a longer cabin that could accommodate either seven passengers or four stretchers and a medical attendant. Army testing of the UH-1B started in November 1960, with the first production aircraft delivered in March 1961. Bell commenced development of the UH-1C in 1960 in order to correct aerodynamic deficiencies of the armed UH-1B. Bell fitted the UH-1C with a 1,100 shp (820 kW) T53-L-11 engine to provide the power needed to lift all weapons systems in use or under development. The Army eventually refitted all UH-1B aircraft with the same engine. A new rotor system was developed for the UH-1C to allow higher air speeds and reduce the incidence of retreating blade stall during diving engagements. The improved rotor resulted in better maneuverability and a slight speed increase. The increased power and a larger diameter rotor required Bell's engineers to design a new tail boom for the UH-1C. The longer tail boom incorporated a wider chord vertical fin on the tail rotor pylon and larger synchronized elevators.

Bell also introduced a dual hydraulic control system for redundancy as well as an improved inlet filter system for the dusty conditions found in southeast Asia. The UH-1C fuel capacity was increased to 242 US gallons (920 liters), and gross weight was raised to 9500 lb, giving a nominal useful load of 4673 lb. UH-1C production started in June 1966 with a total of 766 aircraft produced, including five for the Royal Australian Navy and five for Norway.

Model 205

While earlier short-body Hueys were a success, the Army wanted a version that could carry more troops. Bell's solution was to stretch the HU-1B fuselage by 41 in and use the extra space to fit four seats next to the transmission, facing out. Seating capacity increased to 15, including crew. The enlarged cabin could also accommodate six stretchers and a medic, two more than the earlier models. In place of the earlier model's sliding side doors with a single window, larger doors were fitted which had two windows, plus a small hinged panel with an optional window, providing enhanced access to the cabin. The doors and hinged panels were quickly removable, allowing the Huey to be flown in a doors off configuration.

The Model 205 prototype flew on 16 August 1961. Seven pre-production/prototype aircraft had been delivered for testing at Edwards AFB starting in March 1961. The 205 was initially equipped with a 44 ft main rotor and a Lycoming T53-L-9 engine with 1,100 shp (820 kW). The rotor was lengthened to 48 ft with a chord of 21 in. The tailboom was also lengthened, in order to accommodate the longer rotor blades. Altogether, the modifications resulted in a gross weight capacity of 9500 lb. The Army ordered production of the 205 in 1963, produced with a T53-L-11 engine for its multi-fuel capability. The prototypes were designated as YUH-1D and the production aircraft was designated as the UH-1D.

During 1966, Bell installed the 1400 shp Lycoming T53-L-13 engine to provide more power for the helicopter. The pitot tube was relocated from the nose to the roof of the cockpit to prevent damage during landing. Production models in this configuration were designated as the UH-1H.

Marine Corps
In 1962, the United States Marine Corps held a competition to choose an assault support helicopter to replace the Cessna O-1 fixed-wing aircraft and the Kaman OH-43D helicopter. The winner was the UH-1B, which was already in service with the Army. The helicopter was designated the UH-1E and modified to meet Marine requirements. The major changes included the use of all-aluminum construction for corrosion resistance, radios compatible with Marine Corps ground frequencies, a rotor brake for shipboard use to stop the rotor quickly on shutdown and a roof-mounted rescue hoist.

The UH-1E was first flown on 7 October 1963, and deliveries commenced on 21 February 1964; a total of 192 Iroquois of this model were completed. Due to production line realities at Bell, the UH-1E was produced in two versions, both with the same UH-1E designation. The first 34 built were essentially UH-1B airframes with the Lycoming T53-L-11 engine producing 1,100 shp (820 kW). When Bell switched production to the UH-1C, the UH-1E production benefited from the same changes. The Marine Corps later upgraded UH-1E engines to the Lycoming T53-L-13, which produced 1,400 shp (1,000 kW), after the Army introduced the UH-1M and upgraded their UH-1C helicopters to the same engine.

Air Force
The United States Air Force (USAF) held a competition for a helicopter to be used for support on missile bases included a specific requirement to mandate the use of the General Electric T58 turboshaft as a powerplant. The Air Force had a large inventory of these engines on hand for its fleet of HH-3 Jolly Green Giant rescue helicopters and using the same engine for both helicopters would save costs. In response, Bell proposed an upgraded version of the 204B with the T58 engine. Because the T58 output shaft is at the rear, and was thus mounted in front of the transmission on the HH-3, it had to have a separate offset gearbox (SDG or speed decreaser gearbox) at the rear, and shafting to couple to the UH-1 transmission.

Twin-engine variants
The single-engine UH-1 variants were followed by the twin-engine UH-1N Twin Huey (Bell 212) and years later the UH-1Y Venom. Bell began development of the UH-1N for Canada in 1968. It changed to the more powerful Pratt & Whitney Canada PT6T twin-engine set. The US also ordered the helicopter with the USAF receiving it in 1970. Canada's military, the U.S. Marine Corps, and the U.S. Navy first received the model in 1971.

In 1996, the USMC launched the H-1 upgrade program via the award of a contract to Bell Helicopter for development of the improved UH-1Y and AH-1Zs variants. The UH-1Y includes a lengthened cabin, four-blade rotor, and two more powerful GE T700 engines. The UH-1Y entered service with the USMC in 2008.

The Bell 412 is an improved 4-rotor version of the twin-engined Bell 212, and while not adopted by the US, was chosen by Canada (CH-146 Griffon), license produced in Italy as the Augusta-Bell AB412, and is going to be made for the Japan Self Defense Force in Japan by Subaru. It is a popular commercial and military helicopter, and it competed in the early 2000s Army Light Utility Helicopter program, but lost to what would be the UH-72 Lakota. That program replaced aging UH-1 and OH-58A/C models being used by the National Guard.

Design
The Bell UH-1 Iroquois is a utility helicopter designed for military use. It has a metal fuselage of semi-monocoque construction with tubular landing skids and two rotor blades on the main rotor. Early UH-1 models featured a single Lycoming T53 turboshaft engine in versions with power ratings from 700 to 1400 shp. Later UH-1 and related models often featured twin engines and four-blade rotors. All members of the UH-1 family have similar construction. The UH-1H is the most-produced version, and is representative of all types. The main structure consists of two longitudinal main beams that run under the passenger cabin to the nose and back to the tail boom attachment point. The main beams are separated by transverse bulkheads and provide the supporting structure for the cabin, landing gear, under-floor fuel tanks, transmission, engine and tail boom. The main beams are joined at the lift beam, a short aluminum girder structure that is attached to the transmission via a lift link on the top and the cargo hook on the bottom and is located at the aircraft's center of gravity. The lift beams were changed to steel later in the UH-1H's life, due to cracking on high-time airframes. The semi-monocoque tail boom attaches to the fuselage with four bolts.

The UH-1H's dynamic components include the engine, transmission, rotor mast, main rotor blades, tail rotor driveshaft, and the 42-degree and 90-degree gearboxes of the tail rotor. The main rotor transmission consists of a 90 degree bevel gear assembly with a reduction ratio of 2.14:1, followed by a 2-stage planetary gearset with a ratio of 9.53:1 (two stages of 3.087:1 each). This is in addition to the output gearbox of the T53 engine with a reduction ratio of 3.19:1. This combined reduction results in 324 rpm at the main rotor. The two-bladed, semi-rigid rotor design, with pre-coned and underslung blades, is a development of early Bell model designs, such as the Bell 47 with which it shares common design features, including a damped stabilizer bar. The two-bladed system reduces storage space required for the aircraft, but at a cost of higher vibration levels. The two-bladed design is also responsible for the characteristic 'Huey thump' sound when the aircraft is in flight, which is particularly evident during descent and in turning flight. The tail rotor is driven from the main transmission, via the two directional gearboxes which provide a tail rotor speed approximately six times that of the main rotor to increase tail rotor effectiveness.

The UH-1H also features a synchronized elevator on the tail boom, which is linked to the cyclic control and allows a wider center of gravity range. The standard fuel system consists of five interconnected fuel tanks, three of which are mounted behind the transmission and two of which are under the cabin floor. The landing gear consists of two arched cross tubes joining the skid tubes. The skids have replaceable sacrificial skid shoes to prevent wear of the skid tubes themselves. Skis and inflatable floats may be fitted. While the five main fuel tanks are self-sealing, the UH-1H was not equipped with factory armor, although armored pilot seats were available.

Internal seating is made up of two pilot seats and additional seating for up to 13 passengers or crew in the cabin. The maximum seating arrangement consists of a four-man bench seat facing rearwards behind the pilot seats, facing a five-man bench seat in front of the transmission structure, with two, two-man bench seats facing outwards from the transmission structure on either side of the aircraft. All passenger seats are constructed of aluminum tube frames with canvas material seats, and are quickly removable and reconfigurable. The cabin may also be configured with up to six stretchers, an internal rescue hoist, auxiliary fuel tanks, spotlights, or many other mission kits. Access to the cabin is via two aft-sliding doors and two small, forward-hinged panels. The doors and hinged panels may be removed for flight or the doors may be pinned open. Pilot access is via individual hinged doors.

The UH-1H's dual controls are conventional for a helicopter and consist of a single hydraulic system boosting the cyclic stick, collective lever and anti-torque pedals. The collective levers have integral throttles, although these are not used to control rotor rpm, which is automatically governed, but are used for starting and shutting down the engine. The cyclic and collective control the main rotor pitch through push-pull tube linkages to the swashplate, while the anti-torque pedals change the pitch of the tail rotor via a tensioned cable arrangement. Some UH-1Hs have been modified to replace the tail rotor control cables with push-pull tubes similar to the UH-1N Twin Huey.

U.S. Army

The HU-1A (later redesignated UH-1A) first entered service with the 101st Airborne Division at Fort Campbell, Kentucky, the 82nd Airborne Division, and the 57th Medical Detachment. Although intended for evaluation only, the Army quickly pressed the new helicopter into operational service, and Hueys with the 57th Medical Detachment arrived in Vietnam in March 1962. The UH-1 has long been a symbol of US involvement in Southeast Asia in general and Vietnam in particular, and as a result of that conflict, has become one of the world's most recognized helicopters. In Vietnam primary missions included general support, air assault, cargo transport, aeromedical evacuation, search and rescue, electronic warfare, and later, ground attack. During the conflict, the craft was upgraded, notably to a larger version based on the Model 205. This version was initially designated the UH-1D and flew operationally from 1963.

During its Vietnam War service, the UH-1 was used for various purposes and various terms for each task abounded. UH-1s tasked with ground attack or armed escort were outfitted with rocket launchers, grenade launchers, and machine guns. As early as 1962, UH-1s were modified locally by the companies themselves, who fabricated their own mounting systems. These gunship UH-1s were commonly referred to as "Frogs" or "Hogs" if they carried rockets, and "Cobras" or simply "Guns" if they had guns. UH-1s tasked and configured for troop transport were often called "Slicks" due to an absence of weapons pods. Slicks did have door gunners, but were generally employed in the troop transport and medevac roles.

UH-1s also flew hunter-killer teams with observation helicopters, namely the Bell OH-58A Kiowa and the Hughes OH-6 Cayuse (Loach). Towards the end of the conflict, the UH-1 was tested with TOW missiles, and two UH-1B helicopters equipped with the XM26 Armament Subsystem were deployed to help counter the 1972 Easter Invasion. USAF Lieutenant James P. Fleming piloted a UH-1F on a 26 November 1968 mission that earned him the Medal of Honor.

During the course of the conflict, the UH-1 went through several upgrades. The UH-1A, B, and C models (short fuselage, Bell 204) and the UH-1D and H models (stretched-fuselage, Bell 205) each had improved performance and load-carrying capabilities. The UH-1B and C performed the gunship, and some of the transport, duties in the early years of the Vietnam War. UH-1B/C gunships were replaced by the new AH-1 Cobra attack helicopter from 1967 to late 1968. The increasing intensity and sophistication of NVA anti-aircraft defenses made continued use of UH-1 gunships impractical, and after Vietnam the Cobra was adopted as the Army's main attack helicopter. Devotees of the UH-1 in the gunship role cite its ability to act as an impromptu Dustoff if the need arose, as well as the superior observational capabilities of the larger Huey cockpit, which allowed return fire from door gunners to the rear and sides of the aircraft. In air cavalry troops (i.e., companies) UH-1s were combined with infantry scouts, OH-6 and OH-58 aero-scout helicopters, and AH-1 attack helicopters to form several color-coded teams (viz., blue, white, red, purple, and pink) to perform various reconnaissance, security, and economy of force missions in fulfilling the traditional cavalry battlefield role.

The Army tested a great variety of experimental weapons on the UH-1; nearly anything that could be carried. The Army desired weapons with large calibers and high rates of fire, which led to the testing of a 20 mm cannon on a large mount bolted to the cabin floor. The size of the weapon allowed very little room for movement. The Army further tested a full-size Vulcan cannon firing out the door of a UH-1. It was capable of firing 2400 rounds per minute, or about 40 rounds per second. Despite this being a significant reduction from the nearly 100 rounds per second fired by a standard Vulcan cannon, the installation proved too kinetic for the UH-1. Podded versions of the M24 20 mm cannon were tested in combat over Vietnam. There was a wide variety of 7.62 mm automatic weapons tested, including different installations of the M60 machine gun. AS-10 and SS-11 missiles were tested in several different configurations. High-capacity rocket launchers were also tested, such as the XM3 launcher, which had 24 launching tubes. Press photos were taken with the XM5 and XM3 installed on the same aircraft, but this arrangement could not be used because it was more than the gross take-off weight of the aircraft.

During the Easter Offensive of 1972 by North Vietnam, experimental models of the TOW-firing XM26 were taken out of storage and sent to South Vietnam in response to the onslaught. The pilots had never fired a TOW missile before, and were given brief crash courses. Despite having little training with the units, the pilots managed to hit targets with 151 of the 162 missiles fired in combat, including a pair of tanks. The airborne TOW launchers were known as "Hawks Claws" and were based at Camp Holloway. During the conflict, 7,013 UH-1s served in Vietnam and of these 3,305 were destroyed. In total, 1,151 pilots were killed, along with 1,231 other crew members (these figures are not including Army of the Republic of Vietnam losses).

Post Vietnam, the US Army continued to operate large numbers of Iroquois; they would see further combat during the US invasion of Grenada in 1983, the US invasion of Panama in 1989, and the Gulf War in 1991. In the latter conflict, in excess of 400 Iroquois performed a variety of missions in the region; over a nine-month period, the fleet cumulatively reached 31,000 flight hours and achieved a stable fully mission capable rate of 70%. The type comprised more than 20% of all rotorcraft across the coalition and recorded 21% of the overall flying hours. Even after the Gulf War, the US Army had more than 2,800 Iroquois in its inventory; in particular, 389 UH-1Vs comprised 76% of the Army's medevac aircraft. Nevertheless, plans were mooted as early as 1992 to undertake a slow withdrawal of the aging type in favor of larger and more technologically advanced rotorcraft.

The US Army began phasing out the UH-1 with the introduction of the Sikorsky UH-60 Black Hawk starting in the 1980s, although the Army UH-1 Residual Fleet had around 700 UH-1s that were to be retained until 2015, primarily in support of Army Aviation training at Fort Rucker and in selected Army National Guard units. Army support for the craft was intended to end in 2004; The UH-1 was retired from active Army service in early 2005. During 2009, Army National Guard retirements of the UH-1 accelerated with the introduction of the Eurocopter UH-72 Lakota. In 2009 the Army National Guard marked 50 years of service of the Huey in the Army, and 40 with the National Guard and had a ceremonial end to its service at that time; in the coming years many final flights of the Huey in State National Guard took place. The Washington, D.C. Army National Guard had its last flight in 2011, and Pennsylvania in 2010. The final U.S. Army UH-1 was retired in late 2016, and transferred to a State police department. In context, the Huey is still in service with Armed Forces overall, and the Air Force UH-1N were still in service as of 2024, but scheduled for replacement.

U.S. Air Force

In October 1965, the United States Air Force (USAF) 20th Helicopter Squadron was formed at Tan Son Nhut Air Base in South Vietnam, equipped initially with CH-3C helicopters. By June 1967, the UH-1F and UH-1P were also added to the unit's inventory and, by the end of the year, the entire unit had shifted from Tan Son Nhut to Nakhon Phanom Royal Thai Air Force Base, with the CH-3s transferring to the 21st Helicopter Squadron. On 1 August 1968, the unit was redesignated the 20th Special Operations Squadron. The 20th SOS's UH-1s were known as the Green Hornets, stemming from their color, a primarily green two-tone camouflage (green and tan) was carried, and radio call-sign "Hornet". The main role of these helicopters were to insert and extract reconnaissance teams, provide cover for such operations, conduct psychological warfare, and other support roles for covert operations especially in Laos and Cambodia during the so-called Secret War.

USAF UH-1s were often equipped with automatic grenade launchers in place of the door guns. The XM-94 grenade launcher had been tested on Army rotorcraft prior to its use by the USAF. The unit was capable of firing 400 grenades per minute, up to 1,500 yards effective range.

Into the twenty-first century, the USAF operates the UH-1N for support of intercontinental ballistic missile (ICBM) sites, including transport of security personnel and distinguished visitors. On 24 September 2018, the USAF announced that the Boeing/Leonardo MH-139 (an AW-139 variant) had won a competition to replace the UH-1N fleet. The UH-1N fleet is in service as of 2024, as production of the new type is ramped up.

U.S. Navy
The US Navy acquired a number of surplus UH-1B helicopters from the U.S. Army, these rotorcraft were modified into gunships, outfitted with special gun mounts and radar altimeters. They were known as Seawolves in service with Navy Helicopter Attack (Light) (HA(L)-3). UH-1C helicopters were also acquired during the 1970s. The Seawolves worked as a team with Navy river patrol operations.

Four years after the disestablishment of HA(L)-3, the Navy determined that it still had a need for gunships, establishing two new Naval Reserve Helicopter Attack (Light) Squadrons as part of the newly formed Commander, Helicopter Wing Reserve (COMHELWINGRES) in 1976. Helicopter Attack Squadron (Light) Five (HA(L)-5), nicknamed the "Blue Hawks", was established at Naval Air Station Point Mugu, California on 11 June 1977 and its sister squadron, Helicopter Attack Squadron (Light) Four (HA(L)-4), known as the Red Wolves, was formed at Naval Air Station Norfolk, Virginia on 1 July 1976.

The last regular Navy Huey's, HH-1's for search and rescue were retired in 2009 and replaced by H-60 Seahawks, and the last Marine Corp UH-1N were retired in 2014.

Drug Enforcement Administration
The UH-1H has been used on multiple occasions by the American Drug Enforcement Administration (DEA); initially, these were usually borrowed from the U.S. Army to support planned missions, such for Operation Snowcap, a large multi-year counter-narcotics action covering nine countries across Latin America.

During the War in Afghanistan, the DEA made use of a number of UH-1s stationed in the country for the purpose of conducting counter-narcotics raids. Operated by contractors, these Hueys provide transportation, surveillance, and air support for DEA FAST teams. During July 2009, four UH-1Hs and two Mi-17s were used in a raid that led to the arrest of an Afghan Border Police commander on corruption charges.

Argentina
Nine Argentine Army Aviation UH-1Hs and two Argentine Air Force Bell 212 were included with the aircraft deployed during the Falklands War. They performed general transport and SAR missions and were based at Port Stanley (BAM Puerto Argentino). Two of the Hueys were destroyed and, after the hostilities had ended, the remainder were captured by the British military. Three captured aircraft survive as museum pieces in England and Falklands.

Australia

The Royal Australian Air Force employed the UH-1H until 1989. Iroquois helicopters of No. 9 Squadron RAAF were deployed to South Vietnam in mid 1966 in support of the 1st Australian Task Force. In this role they were armed with single M60 doorguns. In 1969 four of No. 9 Squadron's helicopters were converted to gunships (known as 'Bushrangers'), armed with two fixed forward firing M134 7.62 mm minigun (one each side) and a 7-round rocket pod on each side. Aircrew were armed with twin M60 flexible mounts in each door. UH-1 helicopters were used in many roles including troop transport, medevac and Bushranger gunships for armed support. No. 35 Squadron and No. 5 Squadron also operated the Iroquois in various roles through the 1970s and 1980s. Between 1982 and 1986, the squadron contributed aircraft and aircrew to the Australian helicopter detachment which formed part of the Multinational Force and Observers peacekeeping force in the Sinai Peninsula, Egypt. During 1988, the RAAF began to re-equip with S-70A Blackhawks.

In 1989 and 1990, the RAAF's UH-1H Iroquois were subsequently transferred to the 171st Aviation Squadron in Darwin, Northern Territory and the 5th Aviation Regiment based in Townsville, Queensland following the decision that all battlefield helicopters would be operated by the Australian Army. On 21 September 2007, the Australian Army retired the last of their Bell UH-1s. The last flight occurred in Brisbane on that day with the aircraft replaced by MRH-90 medium helicopters and Tiger armed reconnaissance helicopters.

The Royal Australian Navy's 723 Squadron also operated seven UH-1B from 1964 to 1989, with three of these aircraft lost in accidents during that time. 723 Squadron deployed Iroquois aircraft and personnel as part of the Experimental Military Unit during the Vietnam War.

El Salvador
Numerous UH-1s were operated by the Salvadoran Air Force; during the 1980s, it became the biggest and most experienced combat helicopter force in Central and South America, fighting for over a decade during the Salvadoran Civil War and having been trained by US Army in tactics developed during the Vietnam War. By the start of 1985, El Salvador had 33 UH-1s in its inventory, some configured as gunships and others as transports; furthermore, in the following years, the country expanded its UH-1 fleet further with assistance from the US government. Several Salvadorean UH-1M and UH-1H helicopters used were modified to carry bombs instead of rocket pods. The UH-1s enabled the military to avoid ground routes vulnerable to guerilla ambushes; the gunships were typically used to suppress hostile forces ahead of troops being inserted by UH-1 transports.

Germany
The German aerospace company Dornier constructed 352 UH-1Ds under license between 1967 and 1981 for the West German Bundeswehr. These saw service with both the German Army and German Air Force as utility helicopters, they were also commonly used for search and rescue (SAR) missions. After being replaced by newer twin-engine Eurocopter EC145s, the last UH-1Ds in German service were withdrawn on 12 April 2021.

Israel
Israel withdrew its UH-1s from service in 2002, after 33 years of operation. They were replaced by Sikorsky UH-60 Blackhawk helicopters with an initial batch of 10 delivered during 1994. While some were passed on to pro-Israeli militias in Lebanon, eleven other UH-1Ds were reportedly sold to a Singapore-based logging company but were, instead, delivered in October 1978 to the Royal Rhodesian Air Force to skirt a United Nations-endorsed embargo imposed on the country during the Rhodesian Bush War.

Japan
In 1960 Subaru made agreement to produce the HU-1B for Japan, which was renamed the UH-1B in 1992, along with the HU-1H, renamed UH-1H. Bell and Suburu developed the UH-1J.

Entering service starting in 1993, 130 UH-1J were produced by Fuji for the Japanese Self-Defense Force.

In 2005, a pair of Japan Ground Self-Defense Force (JGSDF) UH-1 helicopters were deployed to Pakistan for earthquake disaster relief. During 2010, after floods in Pakistan, UH-1s were again deployed to the country to aid in disaster relief. Japanese UH-1s have also been periodically used to conduct water bombing against fires.

In the aftermath of the 2011 Tōhoku earthquake and tsunami, Japan's UH-1 fleet was extensively deployed across the country for disaster relief purposes; they also conducted reconnaissance flights over the stricken Fukushima Daiichi Nuclear Power Plant while carrying radiation detection equipment to help inform planners of the plant's condition.

By the early 2020s, Japan's Acquisition, Technology & Logistics Agency was in the process of recapitalising much of the JGSDF's rotarywing capability; it is planned for a locally-built model of the twin-engined multirole Bell 412 helicopter to replace the remaining older UH-1s in Japanese service; the new helicopter will be called the H-2 in JSDF service. The first flight of the Subaru UH-2 took place in 2022.

Lebanon
During the early 1990s, the Lebanese Air Force (LAF) inducted their first UH-1 helicopters. During the 2007 Lebanon conflict, at the Battle of Nahr el-Bared in North Lebanon, the Lebanese Army, lacking fixed-wing aircraft, modified several UH-1Hs to carry 500 lb Mark 82 bombs, enabling it to perform helicopter bombing, and used it to strike militant-held positions. Specifically, special mounting points were installed along the sides of each Huey for the carriage of these high explosive bombs. In the aftermath of the 2020 Beirut explosion, UH-1s participated in the disaster response, and were used to extinguish fires.

Typically, the fleet is tasked with performing search and rescue, troop transport, aerial firefighting and utility missions. In the late 2010s, specially modified UH-1Ds participated in the first LIDAR mapping exercise in the country. During February 2021, an additional three Bell UH-1H-IIs were delivered to the LAF by Bell to augment their existing fleet.

New Zealand

The Royal New Zealand Air Force had an active fleet of 13 Iroquois serving with No. 3 Squadron RNZAF. The first delivery was five UH-1D in 1966 followed in 1970 by nine UH-1H and one more UH-1H in 1976. All of the UH-1D aircraft were upgraded to 1H specification during the 1970s. Two ex-U.S. Army UH-1H attrition airframes were purchased in 1996. Three aircraft have been lost in accidents.

The RNZAF has retired the Iroquois, with the NHIndustries NH90 as its replacement. Eight active NH90 helicopters plus one spare have been procured. This process was initially expected to be completed by the end of 2013, but was delayed until 2016. Individual aircraft were retired as they reach their next major group servicing intervals; the UH-1H was retired as the NH90 fleet stood up. On 21 May 2015, the remaining UH-1H fleet of six helicopters conducted a final tour of the country ahead of its planned retirement on 1 July. During 49 years of service the type had seen service in areas including the U.K., Southeast Asia, Timor, the Solomon Islands, various South Pacific nations, and the Antarctic.

One was preserved in the New Zealand National museum after its retirement from service in 2013, delivered to the museum in 2015; it had had been in service many decades entering RNZAF service in 1966.

Philippines

The Philippine Air Force (PAF) has a long history of acquiring United States Air Force assets, including the Bell UH-1. In PAF service, the type was regularly used to combat local insurgents as well as to conduct disaster relief operations after several earthquakes and typhoons hit the nation. Francis Ford Coppola filmed Apocalypse Now in the Philippines primarily because President Ferdinand Marcos agreed to let Coppola use Philippine Hueys to film the iconic scene with Robert Duvall as Lt. Colonel Kilgore.

During 2013, the PAF was pursuing the acquisition of 21 used UH-1H helicopters; the deal reportedly included their refurbishment prior to delivery. Furthermore, during October 2019, the Philippines made a deal with Japan to acquire some of its spare parts inventory; this reportedly was to facilitate the restoration of seven stored UH-1s to flightworthy condition. By January 2021, the PAF had 13 UH-1H and 10 UH-1D helicopters in an operational condition.

On 23 January 2021, President Rodrigo Duterte announced a plan to retire all of the PAF's remaining UH-1 helicopters, following a series of crashes involving the type. The latest crash occurred on 16 January 2021, killing seven passengers and prompting the grounding of all Hueys for further inspection. On 14 October 2021, the PAF officially decommissioned the remainder of its UH-1D fleet, despite some having only been acquired seven years prior; the retired rotorcraft were stored at Clark Air Base. The role of the UH-1 is to be performed by recently delivered UH-60 Black Hawk helicopters instead; in January 2022, a deal to buy 32 new-build Black Hawks was announced.

In 2022, the Philippines said that some of the retired Huey's will be reactivated with the help of Turkish company ASFAT. Also, in 2022 Japan planned to donate UH-1 leaving its service to the Philippine Army. (Japan is introducing new Huey's as replacement)

Rhodesia
Very late in the Rhodesian Bush War, the Rhodesian Air Force obtained 11 former Israeli Agusta-Bell 205As in violation of sanctions on the nation, allegedly having used a series of intermediaries to evade them. Locally known as Cheetahs, these helicopters were returned to a flightworthy condition and then assigned to No. 8 Squadron, after which they participated in the counter-insurgency effort, usually functioning as armed gunships as well as troop transports. During September 1979, one Cheetah was lost in combat after being hit by an RPG while flying over Mozambique. At least another three other UH-1s were also lost. The surviving helicopters were put up for sale in 1990.

Yemen
In July 2009, Yemen received four UH-1Hs. These remained grounded for almost all the time they were in Yemen; at least one helicopter was heavily damaged during Saudi-led airstrikes on Al Daylami and Al Anad Air Bases.

U.S. military variants

 * XH-40: The initial Bell 204 prototype. Three prototypes were built, equipped with the Lycoming XT-53-L-1 engine of 700 shp.
 * YH-40: Six aircraft for evaluation, as XH-40 with 12 in cabin stretch and other modifications.
 * Bell Model 533: One YH-40-BF rebuilt as a flight test bed with turbojet engines and wings.
 * HU-1A: Initial Bell 204 production model, redesignated as the UH-1A in 1962. 182 built.
 * TH-1A: UH-1A with dual controls and blind-flying instruments, 14 conversions.
 * XH-1A: A single UH-1A was redesignated for grenade launcher testing in 1960.
 * HU-1B: Upgraded HU-1A, various external and rotor improvements. Redesignated UH-1B in 1962. 1014 built plus four prototypes designated YUH-1B.
 * NUH-1B: a single test aircraft, serial number 64–18261.
 * UH-1C: The UH-1B gunship lacked the power necessary to carry weapons and ammunition and keep up with transport Hueys. So Bell designed yet another variant, the UH-1C, intended strictly for the gunship role. It is an UH-1B with improved engine, modified blades and rotor-head for better performance in the gunship role. 767 built.
 * YUH-1D: Seven pre-production prototypes of the UH-1D.
 * UH-1D Iroquois: Initial Bell 205 production model (long fuselage version of the 204). Designed as a troop carrier to replace the CH-34 then in US Army service. 2008 built; many later converted to UH-1H standard.
 * HH-1D: Army crash rescue variant of UH-1D.
 * UH-1E: UH-1B/C for USMC with different avionics and equipment. 192 built.
 * NUH-1E: UH-1E configured for testing.
 * TH-1E: UH-1C configured for Marine Corps training. Twenty were built in 1965.
 * UH-1F: UH-1B/C for USAF with General Electric T58-GE-3 engine of 1325 shp. 120 built.
 * TH-1F: Instrument and Rescue Trainer based on the UH-1F for the USAF. 26 built.
 * UH-1H: Improved UH-1D with a Lycoming T53-L-13 engine of 1400 shp. 5435 built.
 * CUH-1H: Canadian Forces designation for the UH-1H utility transport helicopter. Redesignated CH-118. A total of 10 built.
 * EH-1H: Twenty-two aircraft converted by installation of AN/ARQ-33 radio intercept and jamming equipment for Project Quick Fix.
 * HH-1H: Search and rescue (SAR) variant for the USAF with rescue hoist. A total of 30 built.
 * JUH-1: Five UH-1Hs converted to SOTAS battlefield surveillance configuration with belly-mounted airborne radar.
 * TH-1H: Recently modified UH-1Hs for use as basic helicopter flight trainers by the USAF.
 * HH-1K: Purpose-built SAR variant of the Model 204 for the US Navy with USN avionics and equipment. 27 built.
 * TH-1L: Helicopter flight trainer based on the HH-1K for the USN. A total of 45 were built.
 * UH-1L: Utility variant of the TH-1L. Eight were built.
 * UH-1M: Gunship specific UH-1C upgrade with Lycoming T53-L-13 engine of 1400 shp.
 * UH-1N: Initial Bell 212 production model, the Bell "Twin Pac" twin-engined Huey powered by Pratt & Whitney Canada T400-CP-400.
 * UH-1P: UH-1F variant for USAF for special operations use and attack operations used solely by the USAF 20th Special Operations Squadron, "the Green Hornets".
 * EH-1U: No more than two UH-1H aircraft modified for Multiple Target Electronic Warfare System (MULTEWS).
 * UH-1V: Aeromedical evacuation, rescue version for the US Army.
 * EH-1X: Ten Electronic warfare UH-1Hs converted under "Quick Fix IIA".
 * UH-1Y: Upgraded variant developed from existing upgraded late model UH-1Ns, with additional emphasis on commonality with the AH-1Z.

Note: In U.S. service, the G, J, Q, R, S, T, W and Z model designations are used by the AH-1. The UH-1 and AH-1 are considered members of the same H-1 series. The military does not use I (India) or O (Oscar) for aircraft designations to avoid confusion with "one" and "zero" respectively.

Other military variants

 * Bell 204: Bell Helicopters company designation, covering aircraft from the XH-40, YH-40 prototypes to the UH-1A, UH-1B, UH-1C, UH-1E, UH-1F, HH-1K, UH-1L, UH-1P and UH-1M production aircraft.
 * Agusta-Bell AB 204: Military utility transport helicopter. Built under license in Italy by Agusta.
 * Agusta-Bell AB 204AS: Anti-submarine warfare, anti-shipping version of the AB 204 helicopter.
 * Fuji-Bell HU-1B/HU-1H: Military utility transport helicopter for the Japan Ground Self-Defense Force. Built under license in Japan by Fuji Heavy Industries.
 * Bell 205: Bell Helicopters company designation of the UH-1D and UH-1H helicopters.
 * Bell 205A-1: Military utility transport helicopter version, initial version based on the UH-1H.
 * Bell 205A-1A: As 205A-1, but with armament hardpoints and military avionics. Produced specifically for Israeli contract.
 * Agusta-Bell 205: Military utility transport helicopter. Built under license in Italy by Agusta.
 * AIDC UH-1H: Military utility transport helicopter. Built under license in Taiwan by Aerospace Industrial Development Corporation.
 * Dornier UH-1D: Military utility transport helicopter. Built under license in Germany by Dornier Flugzeugwerke.
 * UH-1G: Unofficial name applied locally to at least one armed UH-1H by the Khmer Air Force in Cambodia.
 * Fuji-Bell UH-1J: An improved Japanese version of the UH-1H built under license in Japan by Fuji Heavy Industries was locally given the designation UH-1J. Among improvements were an Allison T53-L-703 turboshaft engine providing 1,343 kW (1,800 shp), a vibration-reduction system, infrared countermeasures, and a night-vision-goggle (NVG) compatible cockpit.
 * Bell 211 Huey Tug With up-rated dynamic system and larger wide chord blades, the Bell 211 was offered for use as the US Army's prime artillery mover, but not taken up.
 * Bell Huey II: A modified and re-engined UH-1H, improvements were an Allison T53-L-703 turboshaft engine providing 1,343 kW (1,800 shp), a vibration-reduction system, infrared countermeasures and a night-vision-goggle (NVG) compatible cockpit. This significantly improves performance and cost-effectiveness. Currently offered by Bell to all current military users of the type.
 * UH-1/T700 Ultra Huey: Upgraded commercial version, fitted with a 1,400-kW (1900-shp) General Electric T700-GE-701C turboshaft engine.
 * Bell 412 a four rotor version of the Bell 212 used by Canada as the CH-146 Griffon, license produced in Italy as the Augusta-Bell 412, and is going to be made in Japan.
 * UH-1H II is a modernized version of the UH-1H that mixes in Bell 212 components.

Accidents

 * On 11 April 2024, a UH-1 crashed after take-off in Elgeyo Marakwet County, Kenya, killing Kenya's Chief of Defence Forces General Francis Ogolla and nine other military officers.
 * On 11 April 2024, a UH-1 crashed after take-off in Elgeyo Marakwet County, Kenya, killing Kenya's Chief of Defence Forces General Francis Ogolla and nine other military officers.
 * On 11 April 2024, a UH-1 crashed after take-off in Elgeyo Marakwet County, Kenya, killing Kenya's Chief of Defence Forces General Francis Ogolla and nine other military officers.

Notable appearances in media
The image of American troops disembarking from a Huey has become an iconic image of the Vietnam War, and can be seen in many films, video games and television shows on the subject, as well as more modern settings. The UH-1 is seen in many films about the Vietnam War, including The Green Berets, The Deer Hunter, Platoon, Hamburger Hill, Apocalypse Now, Casualties of War, and Born on the Fourth of July. It is prominently featured in We Were Soldiers as the main helicopter used by the Air Cavalry in the Battle of Ia Drang. Author Robert Mason recounts his career as a UH-1 "Slick" pilot in his memoir, Chickenhawk.

The 2002 journey of Huey 091, displayed in the Smithsonian National Museum of American History, is outlined in the 2004 documentary In the Shadow of the Blade.

title: Bell UH-1 Iroquois


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Heliox

Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing.

Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas. Heliox is also used in saturation diving and sometimes during the deep phase of technical dives.

Medical uses
In medicine, heliox may refer to a mixture of 21% O2 (the same as air) and 79% He, although other combinations are available (70/30 and 60/40).

Heliox generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs. "Work of breathing" (WOB) is reduced by two mechanisms: Heliox 20/80 diffuses 1.8 times faster than oxygen, and the flow of heliox 20/80 from an oxygen flowmeter is 1.8 times the normal flow for oxygen.
 * 1) increased tendency to laminar flow;
 * 2) reduced resistance in turbulent flow due to lower density.

Heliox has a similar viscosity to air but a significantly lower density (0.5 g/L versus 1.25 g/L at STP). Flow of gas through the airway comprises laminar flow, transitional flow and turbulent flow. The tendency for each type of flow is described by the Reynolds number. Heliox's low density produces a lower Reynolds number and hence higher probability of laminar flow for any given airway. Laminar flow tends to generate less resistance than turbulent flow.

In the small airways where flow is laminar, resistance is proportional to gas viscosity and is not related to density and so heliox has little effect. The Hagen–Poiseuille equation describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so heliox has a significant effect.

There is also some use of heliox in conditions of the medium airways (croup, asthma and chronic obstructive pulmonary disease). A recent trial has suggested that lower fractions of helium (below 40%) – thus allowing a higher fraction of oxygen –  might also have the same beneficial effect on upper airway obstruction.

Patients with these conditions may develop a range of symptoms including dyspnea (breathlessness), hypoxemia (below-normal oxygen content in the arterial blood) and eventually a weakening of the respiratory muscles due to exhaustion, which can lead to respiratory failure and require intubation and mechanical ventilation. Heliox may reduce all these effects, making it easier for the patient to breathe. Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly. Research has also indicated advantages in using helium–oxygen mixtures in delivery of anaesthesia.

History
Heliox has been used medically since the early 1930s. It was the mainstay of treatment in acute asthma before the advent of bronchodilators. Currently, heliox is mainly used in conditions of large airway narrowing (upper airway obstruction from tumors or foreign bodies and vocal cord dysfunction).

Usage in diving
Helium diluted breathing gases are used to eliminate or reduce the effects of inert gas narcosis, and to reduce work of breathing due to increased gas density at depth. From the 1960s saturation diving physiology studies were conducted with helium from 45 to 610 m over several decades by a Hyperbaric Experimental Centre operated by the French company COMEX specializing in engineering and deep diving operations. Owing to the expense of helium, heliox is most likely to be used in deep saturation diving. It is also sometimes used by technical divers, particularly those using rebreathers, which conserve the breathing gas at depth much better than open circuit scuba.

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan, but it is often hypoxic and may be less than 10%. Each mix is custom made using gas blending techniques, which often involve the use of booster pumps to achieve typical diving cylinder pressures of 200 to 300 bar from lower pressure banks of oxygen and helium cylinders.

Because sound travels faster in heliox than in air, voice formants are raised, making divers' speech very high-pitched and hard to understand to people not used to it. Surface personnel often employ a piece of communications equipment called a "helium de-scrambler", which electronically lowers the pitch of the diver's voice as it is relayed through the communications gear, making it easier to understand.

Trimix is a less expensive alternative to heliox for deep diving, which uses only enough helium to limit narcosis and gas density to tolerable levels for the planned depth. Trimix is often used in technical diving, and is also sometimes used in professional diving.

In 2015, the United States Navy Experimental Diving Unit showed that decompression from bounce dives using trimix is not more efficient than dives on heliox.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Heliox


text:
Squid

A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida. Though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

Squid diverged from other cephalopods during the Jurassic and occupy a similar role to teleost fish as open water predators of similar size and behaviour. They play an important role in the open water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed hunting cooperatively. They are preyed on by sharks, other fish, sea birds, seals and cetaceans, particularly sperm whales.

Squid can change colour for camouflage and signalling. Some species are bioluminescent, using their light for counter-illumination camouflage, while many species can eject a cloud of ink to distract predators.

Squid are used for human consumption with commercial fisheries in Japan, the Mediterranean, the southwestern Atlantic, the eastern Pacific and elsewhere. They are used in cuisines around the world, often known as "calamari". Squid have featured in literature since classical times, especially in tales of giant squid and sea monsters.

Taxonomy and phylogeny
Squid are members of the class Cephalopoda, subclass Coleoidea. The squid orders Myopsida and Oegopsida are in the superorder Decapodiformes (from the Greek for "ten-legged"). Two other orders of decapodiform cephalopods are also called squid, although they are taxonomically distinct from squids and differ recognizably in their gross anatomical features. They are the bobtail squid of order Sepiolida and the ram's horn squid of the monotypic order Spirulida. The vampire squid (Vampyroteuthis infernalis), however, is more closely related to the octopus than to any squid.

The cladogram, not fully resolved, is based on Sanchez et al., 2018. Their molecular phylogeny used mitochondrial and nuclear DNA marker sequences; they comment that a robust phylogeny "has proven very challenging to obtain". If it is accepted that Sepiidae cuttlefish are a kind of squid, then the squids, excluding the vampire squid, form a clade as illustrated. Orders are shown in boldface; all the families not included in those orders are in the paraphyletic order "Oegopsida", except Sepiadariidae and Sepiidae that are in the paraphyletic order "Sepiida",

Evolution
Crown coleoids (the common ancestor of octopuses and squid) diverged in the late Paleozoic (Mississippian), according to fossils of Syllipsimopodi, an early relative of vampire squids and octopuses. True squid diverged during the Jurassic, but many squid families appeared in or after the Cretaceous. Both the coleoids and the teleost fish were involved in much adaptive radiation at this time, and the two modern groups resemble each other in size, ecology, habitat, morphology and behaviour, however some fish moved into fresh water while the coleoids remained in marine environments.

The ancestral coleoid was probably nautiloid-like with a strait septate shell that became immersed in the mantle and was used for buoyancy control. Four lines diverged from this, Spirulida (with one living member), the cuttlefishes, the squids and the octopuses. Squid have differentiated from the ancestral mollusc such that the body plan has been condensed antero-posteriorly and extended dorso-ventrally. What may have been the foot of the ancestor is modified into a complex set of appendages around the mouth. The sense organs are highly developed and include advanced eyes similar to those of vertebrates.

The ancestral shell has been lost, with only an internal gladius, or pen, remaining. The pen, made of a chitin-like material, is a feather-shaped internal structure that supports the squid's mantle and serves as a site for muscle attachment. The cuttlebone or sepion of the Sepiidae is calcareous and appears to have evolved afresh in the Tertiary.

Description

Squid are soft-bodied molluscs whose forms evolved to adopt an active predatory lifestyle. The head and foot of the squid are at one end of a long body, and this end is functionally anterior, leading the animal as it moves through the water. A set of eight arms and two distinctive tentacles surround the mouth; each appendage takes the form of a muscular hydrostat and is flexible and prehensile, usually bearing disc-like suckers.

The suckers may lie directly on the arm or be stalked. Their rims are stiffened with chitin and may contain minute toothlike denticles. These features, as well as strong musculature, and a small ganglion beneath each sucker to allow individual control, provide a very powerful adhesion to grip prey. Hooks are present on the arms and tentacles in some species, but their function is unclear. The two tentacles are much longer than the arms and are retractile. Suckers are limited to the spatulate tip of the tentacle, known as the manus.

In the mature male, the outer half of one of the left arms is hectocotylised – and ends in a copulatory pad rather than suckers. This is used for depositing a spermatophore inside the mantle cavity of a female. A ventral part of the foot has been converted into a funnel through which water exits the mantle cavity.

The main body mass is enclosed in the mantle, which has a swimming fin along each side. These fins are not the main source of locomotion in most species. The mantle wall is heavily muscled and internal. The visceral mass, which is covered by a thin, membranous epidermis, forms a cone-shaped posterior region known as the "visceral hump". The mollusc shell is reduced to an internal, longitudinal chitinous "pen" in the functionally dorsal part of the animal; the pen acts to stiffen the squid and provides attachments for muscles.

On the functionally ventral part of the body is an opening to the mantle cavity, which contains the gills (ctenidia) and openings from the excretory, digestive and reproductive systems. An inhalant siphon behind the funnel draws water into the mantle cavity via a valve. The squid uses the funnel for locomotion via precise jet propulsion. In this form of locomotion, water is sucked into the mantle cavity and expelled out of the funnel in a fast, strong jet. The direction of travel is varied by the orientation of the funnel. Squid are strong swimmers and certain species can "fly" for short distances out of the water.

Camouflage
Squid make use of different kinds of camouflage, namely active camouflage for background matching (in shallow water) and counter-illumination. This helps to protect them from their predators and allows them to approach their prey.

The skin is covered in controllable chromatophores of different colours, enabling the squid to match its coloration to its surroundings. The play of colours may in addition distract prey from the squid's approaching tentacles. The skin also contains light reflectors called iridophores and leucophores that, when activated, in milliseconds create changeable skin patterns of polarized light. Such skin camouflage may serve various functions, such as communication with nearby squid, prey detection, navigation, and orientation during hunting or seeking shelter. Neural control of the iridophores enabling rapid changes in skin iridescence appears to be regulated by a cholinergic process affecting reflectin proteins.

Some mesopelagic squid such as the firefly squid (Watasenia scintillans) and the midwater squid (Abralia veranyi) use counter-illumination camouflage, generating light to match the downwelling light from the ocean surface. This creates the effect of countershading, making the underside lighter than the upperside.

Counter-illumination is also used by the Hawaiian bobtail squid (Euprymna scolopes), which has symbiotic bacteria (Aliivibrio fischeri) that produce light to help the squid avoid nocturnal predators. This light shines through the squid's skin on its underside and is generated by a large and complex two-lobed light organ inside the squid's mantle cavity. From there, it escapes downwards, some of it travelling directly, some coming off a reflector at the top of the organ (dorsal side). Below there is a kind of iris, which has branches (diverticula) of its ink sac, with a lens below that; both the reflector and lens are derived from mesoderm. The squid controls light production by changing the shape of its iris or adjusting the strength of yellow filters on its underside, which presumably change the balance of wavelengths emitted. Light production shows a correlation with intensity of down-welling light, but it is about one third as bright; the squid can track repeated changes in brightness. Because the Hawaiian bobtail squid hides in sand during the day to avoid predators, it does not use counter-illumination during daylight hours.

Predator distraction with ink

Squid distract attacking predators by ejecting a cloud of ink, giving themselves an opportunity to escape. The ink gland and its associated ink sac empties into the rectum close to the anus, allowing the squid to rapidly discharge black ink into the mantle cavity and surrounding water. The ink is a suspension of melanin particles and quickly disperses to form a dark cloud that obscures the escape manoeuvres of the squid. Predatory fish may also be deterred by the alkaloid nature of the discharge which may interfere with their chemoreceptors.

Nervous system and sense organs
Cephalopods have the most highly developed nervous systems among invertebrates. Squids have a complex brain in the form of a nerve ring encircling the oesophagus, enclosed in a cartilaginous cranium. Paired cerebral ganglia above the oesophagus receive sensory information from the eyes and statocysts, and further ganglia below control the muscles of the mouth, foot, mantle and viscera. Giant axons up to 1 mm in diameter convey nerve messages with great rapidity to the circular muscles of the mantle wall, allowing a synchronous, powerful contraction and maximum speed in the jet propulsion system.

The paired eyes, on either side of the head, are housed in capsules fused to the cranium. Their structure is very similar to that of a fish eye, with a globular lens that has a depth of focus from 3 cm to infinity. The image is focused by changing the position of the lens, as in a camera or telescope, rather than changing the shape of the lens, as in the human eye. Squid adjust to changes in light intensity by expanding and contracting the slit-shaped pupil. Deep sea squids in the family Histioteuthidae have eyes of two different types and orientation. The large left eye is tubular in shape and looks upwards, presumably searching for the silhouettes of animals higher in the water column. The normally-shaped right eye points forwards and downwards to detect prey.

The statocysts are involved in maintaining balance and are analogous to the inner ear of fish. They are housed in cartilaginous capsules on either side of the cranium. They provide the squid with information on its body position in relation to gravity, its orientation, acceleration and rotation, and are able to perceive incoming vibrations. Without the statocysts, the squid cannot maintain equilibrium. Squid appear to have limited hearing, but the head and arms bear lines of hair-cells that are weakly sensitive to water movements and changes in pressure, and are analogous in function to the lateral line system of fish.

Reproductive system

The sexes are separate in squid, with a single gonad in the posterior part of the body. Fertilisation is external and usually takes place in the mantle cavity of the female. The male has a testis from which sperm pass into a single gonoduct where they are rolled together into a long bundle, or spermatophore. The gonoduct is elongated into a "penis" that extends into the mantle cavity and through which spermatophores are ejected. In shallow water species, the penis is short, and the spermatophore is removed from the mantle cavity by a tentacle of the male, which is specially adapted for the purpose and known as a hectocotylus, and placed inside the mantle cavity of the female during mating.

The female has a large translucent ovary, situated towards the posterior of the visceral mass. From here, eggs travel along the gonocoel, where there are a pair of white nidamental glands, which lie anterior to the gills. Also present are red-spotted accessory nidamental glands containing symbiotic bacteria; both organs are associated with nutrient manufacture and forming shells for the eggs. The gonocoel enters the mantle cavity at the gonopore, and in some species, receptacles for storing spermatophores are located nearby, in the mantle wall.

In shallow-water species of the continental shelf and epipelagic or mesopelagic zones, it is frequently one or both of arm pair IV of males that are modified into hectocotyli. However, most deep-sea squid lack hectocotyl arms and have longer penises; Ancistrocheiridae and Cranchiinae are exceptions. Giant squid of the genus Architeuthis are unusual in that they possess both a large penis and modified arm tips, although whether the latter are used for spermatophore transfer is uncertain. Penis elongation has been observed in the deep-water species Onykia ingens; when erect, the penis may be as long as the mantle, head, and arms combined. As such, deep-water squid have the greatest known penis length relative to body size of all mobile animals, second in the entire animal kingdom only to certain sessile barnacles.

Digestive system

Like all cephalopods, squids are predators and have complex digestive systems. The mouth is equipped with a sharp, horny beak mainly made of chitin and cross-linked proteins, which is used to kill and tear prey into manageable pieces. The beak is very robust, but does not contain minerals, unlike the teeth and jaws of many other organisms; the cross-linked proteins are histidine- and glycine-rich and give the beak a stiffness and hardness greater than most equivalent synthetic organic materials. The stomachs of captured whales often have indigestible squid beaks inside. The mouth contains the radula, the rough tongue common to all molluscs except bivalvia, which is equipped with multiple rows of teeth. In some species, toxic saliva helps to control large prey; when subdued, the food can be torn in pieces by the beak, moved to the oesophagus by the radula, and swallowed.

The food bolus is moved along the gut by waves of muscular contractions (peristalsis). The long oesophagus leads to a muscular stomach roughly in the middle of the visceral mass. The digestive gland, which is equivalent to a vertebrate liver, diverticulates here, as does the pancreas, and both of these empty into the caecum, a pouch-shaped sac where most of the absorption of nutrients takes place. Indigestible food can be passed directly from the stomach to the rectum where it joins the flow from the caecum and is voided through the anus into the mantle cavity. Cephalopods are short-lived, and in mature squid, priority is given to reproduction; the female Onychoteuthis banksii for example, sheds its feeding tentacles on reaching maturity, and becomes flaccid and weak after spawning.

Cardiovascular and excretory systems
The squid mantle cavity is a seawater-filled sac containing three hearts and other organs supporting circulation, respiration, and excretion. Squid have a main systemic heart that pumps blood around the body as part of the general circulatory system, and two branchial hearts. The systemic heart consists of three chambers, a lower ventricle and two upper atria, all of which can contract to propel the blood. The branchial hearts pump blood specifically to the gills for oxygenation, before returning it to the systemic heart. The blood contains the copper-rich protein hemocyanin, which is used for oxygen transport at low ocean temperatures and low oxygen concentrations, and makes the oxygenated blood a deep, blue color. As systemic blood returns via two vena cavae to the branchial hearts, excretion of urine, carbon dioxide, and waste solutes occurs through outpockets (called nephridial appendages) in the vena cavae walls that enable gas exchange and excretion via the mantle cavity seawater.

Buoyancy

Unlike nautiloids and cuttlefish which have gas-filled chambers inside their shells which provide buoyancy, and octopuses which live near and rest on the seabed and do not require to be buoyant, many squid have a fluid-filled receptacle, equivalent to the swim bladder of a fish, in the coelom or connective tissue. This reservoir acts as a chemical buoyancy chamber, with the heavy metallic cations typical of seawater replaced by low molecular-weight ammonium ions, a product of excretion. The small difference in density provides a small contribution to buoyancy per unit volume, so the mechanism requires a large buoyancy chamber to be effective. Since the chamber is filled with liquid, it has the advantage over a swim bladder of not changing significantly in volume with pressure. Glass squids in the family Cranchiidae for example, have an enormous transparent coelom containing ammonium ions and occupying about two-thirds the volume of the animal, allowing it to float at the required depth. About half of the 28 families of squid use this mechanism to solve their buoyancy issues. The family Bathyteuthidae get their buoyancy from an oily substance found in their liver and around their mantle and head.

Largest and smallest

The majority of squid are no more than 60 cm long, although the giant squid may reach 13 m. The smallest species are probably the benthic pygmy squids Idiosepius, which grow to a mantle length of 10 to 18 mm, and have short bodies and stubby arms.

In 1978, sharp, curved claws on the suction cups of squid tentacles cut up the rubber coating on the hull of the USS Stein. The size suggested the largest squid known at the time.

In 2003, a large specimen of an abundant but poorly understood species, Mesonychoteuthis hamiltoni (the colossal squid), was discovered. This species may grow to 10 m in length, making it the largest invertebrate. In February 2007, a New Zealand fishing vessel caught the largest squid ever documented, weighing 495 kg and measuring around 10 m off the coast of Antarctica. Dissection showed that the eyes, used to detect prey in the deep Southern Ocean, exceeded the size of footballs; these may be among the largest eyes ever to exist in the animal kingdom.

Development
The eggs of squid are large for a mollusc, containing a large amount of yolk to nourish the embryo as it develops directly, without an intervening veliger larval stage. The embryo grows as a disc of cells on top of the yolk. During the gastrulation stage, the margins of the disc grow to surround the yolk, forming a yolk sac, which eventually forms part of the animal's gut. The dorsal side of the disc grows upwards and forms the embryo, with a shell gland on its dorsal surface, gills, mantle and eyes. The arms and funnel develop as part of the foot on the ventral side of the disc. The arms later migrate upwards, coming to form a ring around the funnel and mouth. The yolk is gradually absorbed as the embryo grows. Some juvenile squid live higher in the water column than do adults. Squids tend to be short-lived; Loligo for example lives from one to three years according to species, typically dying soon after spawning.

In a well-studied bioluminescent species, the Hawaiian bobtail squid, a special light organ in the squid's mantle is rapidly colonized with Aliivibrio fischeri bacteria within hours of hatching. This light-organ colonization requires this particular bacterial species for a symbiotic relationship; no colonization occurs in the absence of A. fischeri. Colonization occurs in a horizontal manner, such that the hosts acquires its bacterial partners from the environment. The symbiosis is obligate for the squid, but facultative for the bacteria. Once the bacteria enter the squid, they colonize interior epithelial cells in the light organ, living in crypts with complex microvilli protrusions. The bacteria also interact with hemocytes, macrophage-like blood cells that migrate between epithelial cells, but the mechanism and function of this process is not well understood. Bioluminescence reaches its highest levels during the early evening hours and bottoms out before dawn; this occurs because at the end of each day, the contents of the squid's crypts are expelled into the surrounding environment. Approximately 95% of the bacteria are voided each morning before the bacterial population builds up again by nightfall.

Locomotion

Squid can move about in several different ways. Slow movement is achieved by a gentle undulation of the muscular lateral fins on either side of the trunk which drives the animal forward. A more common means of locomotion providing sustained movement is achieved using jetting, during which contraction of the muscular wall of the mantle cavity provides jet propulsion.

Slow jetting is used for ordinary locomotion, and ventilation of the gills is achieved at the same time. The circular muscles in the mantle wall contract; this causes the inhalant valve to close, the exhalant valve to open and the mantle edge to lock tightly around the head. Water is forced out through the funnel which is pointed in the opposite direction to the required direction of travel. The inhalant phase is initiated by the relaxation of the circular muscles causes them to stretch, the connective tissue in the mantle wall recoils elastically, the mantle cavity expands causing the inhalant valve to open, the exhalant valve to close and water to flow into the cavity. This cycle of exhalation and inhalation is repeated to provide continuous locomotion.

Fast jetting is an escape response. In this form of locomotion, radial muscles in the mantle wall are involved as well as circular ones, making it possible to hyper-inflate the mantle cavity with a larger volume of water than during slow jetting. On contraction, water flows out with great force, the funnel always being pointed anteriorly, and travel is backwards. During this means of locomotion, some squid exit the water in a similar way to flying fish, gliding through the air for up to 50 m, and occasionally ending up on the decks of ships.

Feeding
Squid are carnivores, and, with their strong arms and suckers, can overwhelm relatively large animals efficiently. Prey is identified by sight or by touch, grabbed by the tentacles which can be shot out with great rapidity, brought back to within reach of the arms, and held by the hooks and suckers on their surface. In some species, the squid's saliva contains toxins which act to subdue the prey. These are injected into its bloodstream when the prey is bitten, along with vasodilators and chemicals to stimulate the heart, and quickly circulate to all parts of its body. The deep sea squid Taningia danae has been filmed releasing blinding flashes of light from large photophores on its arms to illuminate and disorientate potential prey.

Although squid can catch large prey, the mouth is relatively small, and the food must be cut into pieces by the chitinous beak with its powerful muscles before being swallowed. The radula is located in the buccal cavity and has multiple rows of tiny teeth that draw the food backwards and grind it in pieces. The deep sea squid Mastigoteuthis has the whole length of its whip-like tentacles covered with tiny suckers; it probably catches small organisms in the same way that flypaper traps flies. The tentacles of some bathypelagic squids bear photophores which may bring food within its reach by attracting prey.

Squid are among the most intelligent invertebrates. For example, groups of Humboldt squid hunt cooperatively, spiralling up through the water at night and coordinating their vertical and horizontal movements while foraging.

Reproduction

Courtship in squid takes place in the open water and involves the male selecting a female, the female responding, and the transfer by the male of spermatophores to the female. In many instances, the male may display to identify himself to the female and drive off any potential competitors. Elaborate changes in body patterning take place in some species in both agonistic and courtship behaviour. The Caribbean reef squid (Sepioteuthis sepioidea), for example, employs a complex array of colour changes during courtship and social interactions and has a range of about 16 body patterns in its repertoire.

The pair adopt a head-to-head position, and "jaw locking" may take place, in a similar manner to that adopted by some cichlid fish. The heterodactylus of the male is used to transfer the spermatophore and deposit it in the female's mantle cavity in the position appropriate for the species; this may be adjacent to the gonopore or in a seminal receptacle.

The sperm may be used immediately or may be stored. As the eggs pass down the oviduct, they are wrapped in a gelatinous coating, before continuing to the mantle cavity, where they are fertilised. In Loligo, further coatings are added by the nidimental glands in the walls of the cavity and the eggs leave through a funnel formed by the arms. The female attaches them to the substrate in strings or groups, the coating layers swelling and hardening after contact with sea water. Loligo sometimes forms breeding aggregations which may create a "community pile" of egg strings. Some pelagic and deep sea squid do not attach their egg masses, which float freely.

Ecology
Squid mostly have an annual life cycle, growing fast and dying soon after spawning. The diet changes as they grow but mostly consists of large zooplankton and small nekton. In Antarctica for example, krill is the main constituent of the diet, with other food items being amphipods, other small crustaceans, and large arrow worms. Fish are also eaten, and some squid are cannibalistic.

As well as occupying a key role in the food chain, squid are an important prey for predators including sharks, sea birds, seals and whales. Juvenile squid provide part of the diet for worms and small fish. When researchers studied the contents of the stomachs of elephant seals in South Georgia, they found 96% squid by weight. In a single day, a sperm whale can eat 700 to 800 squid, and a Risso's dolphin entangled in a net in the Mediterranean was found to have eaten angel clubhook squid, umbrella squid, reverse jewel squid and European flying squid, all identifiable from their indigestible beaks. Ornithoteuthis volatilis, a common squid from the tropical Indo-Pacific, is predated by yellowfin tuna, longnose lancetfish, common dolphinfish and swordfish, the tiger shark, the scalloped hammerhead shark and the smooth hammerhead shark. Sperm whales also hunt this species extensively as does the brown fur seal. In the Southern Ocean, penguins and wandering albatrosses are major predators of Gonatus antarcticus.

In literature and art

Giant squid have featured as monsters of the deep since classical times. Giant squid were described by Aristotle (4th century BC) in his History of Animals and Pliny the Elder (1st century AD) in his Natural History. The Gorgon of Greek mythology may have been inspired by squid or octopus, the animal itself representing the severed head of Medusa, the beak as the protruding tongue and fangs, and its tentacles as the snakes. The six-headed sea monster of the Odyssey, Scylla, may have had a similar origin. The Nordic legend of the kraken may also have derived from sightings of large cephalopods.

In literature, H. G. Wells' short story "The Sea Raiders" featured a man-eating squid species Haploteuthis ferox. The science fiction writer Jules Verne told a tale of a kraken-like monster in his 1870 novel Twenty Thousand Leagues Under the Seas.

As food

Squid form a major food resource and are used in cuisines around the world, notably in Japan where it is eaten as ika sōmen, sliced into vermicelli-like strips; as sashimi; and as tempura. Three species of Loligo are used in large quantities: L. vulgaris in the Mediterranean (known as Calamar in Spanish, Calamaro in Italian); L. forbesii in the Northeast Atlantic; and L. pealei on the American East Coast. Among the Ommastrephidae, Todarodes pacificus is the main commercial species, harvested in large quantities across the North Pacific in Canada, Japan and China.

In English-speaking countries, squid as food is often called calamari, adopted from Italian into English in the 17th century. Squid are found abundantly in certain areas, and provide large catches for fisheries. The body can be stuffed whole, cut into flat pieces, or sliced into rings. The arms, tentacles, and ink are also edible; the only parts not eaten are the beak and gladius (pen). Squid is a good food source for zinc and manganese, and high in copper, selenium, vitamin B12, and riboflavin.

Commercial fishing
According to the FAO, the cephalopod catch for 2002 was 3173272 t. Of this, 2,189,206 tonnes, or 75.8 percent, was squid. The following table lists squid species fishery catches that exceeded 10000 t in 2002.

In biomimicry

Prototype chromatophores that mimic the squid's adaptive camouflage have been made by Bristol University researchers using an electroactive dielectric elastomer, a flexible "smart" material that changes its colour and texture in response to electrical signals. The researchers state that their goal is to create an artificial skin that provides rapid active camouflage.

The squid giant axon inspired Otto Schmitt to develop a comparator circuit with hysteresis now called the Schmitt trigger, replicating the axon's propagation of nerve impulses.

title: Squid