[1.0] The Grumman OV-1 Mohawk

v1.1.1 / 1 of 4 / 01 jun 02 / greg goebel / public domain

* The pioneer in the field of battlefield surveillance aircraft was the Grumman "OV-1 Mohawk", a twin-turboprop machine that served with distinction with the US Army for several decades. This chapter provides a short history of the OV-1.

[1.1] MOHAWK ORIGINS: G-134 / OV-1A
[1.3] OV-1B / OV-1C

[1.1] MOHAWK ORIGINS: G-134 / OV-1A

* In late 1954, the US Army issued a requirement for a battlefield surveillance and utility aircraft, and in early 1956, the Army met with representatives of six aircraft manufacturers to consider proposals. The proposals were used to generate a final specification, which was kicked upstairs to the Pentagon for approval in the spring of 1956.

At this point, the political wrangling began. The Department of Defense wanted the Army to talk to the US Air Force (USAF) to ensure there was no duplication of missions between the two services. The US Navy, working on behalf of the Marine Corps, also became involved.

While Army officials haggled with their opposite numbers in the other armed services, in June 1956 the Army came up with a final specification for the new aircraft, indicating that it should have twin turboprop engines for combat survivability; carry a crew of a pilot and observer; be capable of short takeoffs and landings on rough airstrips; and be able to fly in bad weather conditions.

The Navy also wanted the aircraft to operate from small escort carriers. Although one of the consequences of interservice politics of the time was that the Army was restricted to operating fixed-wing aircraft with an empty weight of less than 2,270 kilograms (5,000 pounds), the limit was waivered in this case.

In March 1957, Grumman Aircraft Corporation was awarded the contract to develop the aircraft, which was given the company designation of "G-134". The contract specified development of nine prototype and evaluation aircraft, to be designated "YAO-1AF".

The relationship between the services over the G-134 was quarrelsome. The Marines wanted a simple spotter aircraft to replace their old Cessna "OE-1 Bird Dogs", and were not interested in the sophisticated sensor payloads that the Army was considering for the G-134. The Marines also wanted the G-134 to have stores pylons to carry weapons, which annoyed the Air Force, which wanted to retain control of the battlefield close-support mission.

However, the Navy decided to build a fleet tanker vessel and didn't have the money to buy the Marines a new observation aircraft. The Navy and Marines pulled out of the program in September 1957, much to the relief of the Army. The Army reserved an "OF-1" designation for a Marine version in case the issue raised its ugly head again, and got on with development.

* The first Grumman YAO-1AF flew on 14 April 1959, with Grumman chief test pilot Ralph Donnell at the controls. The flight test program went very smoothly, with few major changes required to the basic design. The aircraft proved to be extremely agile, and had a low stalling speed and very good short-field performance.

It was also powerful and fast, compared to the piston-engined aircraft previously flown by many Army aviators, and in fact set a number of performance records for its class. The type's high performance would eventually lead to a number of deadly accidents, caused by pilots who became overenthusiastic at the controls. This was not really a fault of the aircraft, which flew well and was extremely strong. Although the YAO-1F was designed for a ten-year service life, fatigue tests demonstrated that it would probably be good for twice that, a tribute to Grumman's custom of building very rugged aircraft.

The YAO-1F was originally to be named "Montauk", following the Army's tradition of naming aircraft after American native tribes, but the Montauk tribe was judged too obscure and the name "Mohawk" was used instead. The Mohawk tribe was better known, and also had a reputation as a tribe of fearsome warriors.

The YAO-1F was ordered into production as the "AO-1F", going into US Army service in Germany in 1961 and in Vietnam in September 1962. In that month, the US military services consolidated their aircraft designation schemes, and the Mohawk designations became "YOV-1A" and "OV-1A" respectively. The most distinctive difference between the evaluation and production aircraft was that the production aircraft were fitted with black rubber pneumatic de-icing boots on the leading edges of the flight surfaces.



* The OV-1A carried a KA-30 or later a KS-61 camera in a bay in the rear fuselage. The camera could pivot from horizon to horizon. Boxy photoflash flare pods could be fitted above the wing roots to provide a total capacity of 52 flares, which were ejected upward to ensure that the flash was above the camera and so did not blind them.

One unusual feature was an AN/ADR-6 radiation detector ("radiac"), fitted in the rear fuselage to allow the aircraft to map radiation on a nuclear battlefield. The detector activated a cockpit alarm if radiation levels were high enough to put the crew at risk. The Mohawk was also fitted with a comprehensive suite of radio and navigation gear, as well as an IFF transponder.

The Mohawk had a bulged cockpit with a snub nose to provide a superlative view for its pilot and observer. Canopy side panels hinged upward to allow entrance and exit from the cockpit, with a boarding step sliding down from each side of the nose below the panels.

The two flight crew sat in armored Martin-Baker Mark 5 ejection seats, which fired through frangible canopy top panels. Ejection seats were adopted because the Mohawk had big props on either side of the cockpit that blocked a conventional bailout, and the Mohawk was intended to fly operationally at low altitudes, making a "manual" bailout impossible in any case. The Mark 5 seats could be fired from zero altitude, but required a minimum flight speed of 185 KPH (100 knots) to be used safely. Improved seats were later fitted that reduced the minimum safe flight speed to 110 KPH (60 knots).

The cockpit floor was 6.4 millimeter (1/4 inch) thick aluminum alloy plate for protection against small-arms fire; the windshield glass was bullet-resistant and 2.5 centimeters (an inch) thick; and the cockpit could be fitted with flak curtains on the forward and rear walls. Heavy-duty windshield wipers were provided to deal with wet weather.

The Mohawk had tricycle landing gear, with the nosewheel retracting backward and the main gear retracting outward into the wings. All the gear had single wheels. Low-pressure tires were fitted to heavy-duty struts for rough-field operation, and a small tailskid was fitted to protect the rear fuselage during steep short-field takeoffs.

The evaluation aircraft had been fitted with Lycoming T53-L-3 turboprops with 960 horsepower each, but in production the engines were updated to T53-L-7s with 1,005 horsepower each. The Mohawk is said to have been the first fixed-wing aircraft to be fitted with the T53, the engine already having been selected for the Bell "Huey" helicopter.

The engines were mounted on top of the wings, giving them some protection against ground fire, and "toed out" slightly to improve engine-out handling. The exhausts were on top of the nacelles, which would later give the aircraft a degree of protection against man-portable heat-seeking surface to air missiles (SAMs). The engines drove three-bladed Hamilton Standard propellers with a diameter of 3.05 meters (10 feet). The propellers were fully reversible to reduce landing roll.

The wings had a 6.5 degree dihedral, and were fitted with large-area flaps and full-span leading-edge slats to reduce takeoff roll. Interestingly, the leading-edge slats would prove ineffective in Vietnam and would be generally bolted down in that theater.

Take-off run was 358 meters (1,175 feet), in contrast to 1,160 meters (3,800 feet) for a Lockheed C-130 Hercules. Although an early G-134 mockup featured a high tee tail, this proved inconvenient for engine-out flight, and a distinctive triple-fin tail scheme was adopted instead.

A hydraulically-operated dive brake was fitted to either side of the rear fuselage behind the wing. Internal fuel capacity was 1,125 liters (297 US gallons) in a single self-sealing fuel tank in the fuselage above the wings, giving the Mohawk an endurance of 2 hours 20 minutes. The tank was part of the aircraft's frame, and it was built very strong, with panels from it subjected to machine-gun fire in tests to ensure survivability.

In operation the Mohawk generally carried a 567 liter (150 US gallon) fuel tank under each wing, giving the aircraft an endurance of 4 hours 30 minutes. Double-size external tanks could be used for ferry flights, but these were rarely used. The aircraft was designed for easy maintenance, with direct access to the majority of systems through panels that could be reached without ladders or work stands.

* The OV-1A was designed to be fitted with up to six underwing pylons, but only two were fitted in production, to allow it to carry the two external tanks. The Army, claiming that they wanted the OV-1A to be able to fire rockets to mark targets and protect itself, had Grumman refit 54 Mohawks with all six pylons and install a Mark 20 fixed reticle gunsight in the cockpit for the pilot.

These modified aircraft were redesignated "JOV-1A", and evaluated with 7 round or 19 round 70 millimeter (2.75 inch) rocket pods; SUU-12 12.7 millimeter (0.50 caliber) machine gun pods; 225 kilogram (500 pound) bombs; 140 millimeter (5 inch) Zuni rockets; and flares.

The Air Force saw the JOV-1A as a Army attempt to perform the close support mission, which they believed was their job. They objected loudly, and the Army formally changed the name back to OV-1A, though they did not remove the pylons or the gunsight. Many of these aircraft would serve in Vietnam, often carrying smoke rockets, and sometimes more lethal stores for "self-defense".

The Air Force remained very touchy about the issue, even demanding that Grumman drop company brochures that highlighted the Mohawk's attack capability. In 1965, the Pentagon handed down a directive dictating that the Army would not operate armed fixed-wing aircraft.


[1.3] OV-1B / OV-1C

* Even before the first flight of the OV-1A, plans were in place to develop two follow-on variants, the "OV-1B" and the "OV-1C", to exploit new sensor technologies as they became available. Despite their consecutive designations, the OV-1B and OV-1C were produced in parallel.

The prototype OV-1B was modified from one of the nine evaluation YOV-1As. The primary enhancement was the addition of of the big Motorola "AN/APS-94 Side Looking Airborne Radar (SLAR)", with the radar antenna in a 5.5 meter (18 foot) long, boxy, yaw-stabilized fiberglass pod slung under the right fuselage. The OV-1B retained the optical cameras.

The APS-94 SLAR provided an imaging reconnaissance capability in day or night, in any weather. It shot radar signals to either or both sides of the aircraft and recorded the echoes onto photographic film strips, which were automatically developed in flight. The right-side flight controls were deleted and a sensor station was installed to allow an operator to monitor and control the SLAR. The SLAR also had a "moving target indicator (MTI)" capability to highlight vehicles in motion on the imagery.

The OV-1B featured an increased wingspan of 1.79 meters (5 feet 10 inches) to help lift the SLAR. Airbrakes were deleted to save weight, and only two stores pylons were fitted. An autopilot and Doppler navigation radar were added to help the aircraft fly nice neat radar mapping patterns, and a new VHF data link was installed to allow real-time relay of SLAR data to a ground station.

Late OV-1B production featured uprated Lycoming T53-L-15 turboprops with 1,150 horsepower each. Oddly, this was not done to improve flight performance. While the bugeyed canopy provided excellent visibility, it also turned the cockpit into a greenhouse in sunny weather, and flight crews could return to base sopping with sweat. Operations in the hot Vietnamese climate demanded a hefty air conditioning system, and the additional horsepower was to drive the air conditioning! Older aircraft were retrofitted with the T53-L-15 as well. 90 OV-1Bs were built.

* Initial OV-1C production examples simply amounted to an OV-1A with a "UAS-4 Red Haze" infrared (IR) sensor system in addition to its optical cameras. The UAS-4 could spot fires, hot truck engines, and other evidence of enemy activities at night, in poor weather, or under jungle canopy. The UAS-4 was originally mounted in the rear fuselage, but was later put in a blister on the belly just behind the wing, along with a panoramic camera and an anti-collision light.

As with the OV-1B, the right-side flight controls were deleted and replaced with a sensor control station, and the IR data was recorded on film strips. The UAS-4 was later replaced with the more sensitive "UAS-14", which included a data link to allow reconnaissance data to be transferred to battlefield commanders in real time. The OV-1C was also said to have featured a chute to allow the flightcrew to drop messages to field units. It is unclear if other Mohawk variants had this little feature.

After the delivery of a few initial OV-1Cs, production was changed to feature the wider span of the OV-1B, and the airbrakes were deleted as well. Late production OV-1Cs had a nose panel for a KA-60 panoramic camera that could take 180 degree pictures in front of the aircraft, and this feature was also retrofitted to older aircraft.

Late production OV-1Cs were also fitted with the uprated T53-L-15 engines, and these aircraft were sometimes called "Super Cs". As with the OV-1B, many older OV-1Cs were retrofitted with the uprated engines. 133 OV-1Cs were built, with final delivery in 1969.



* The SLAR and IR sensor systems greatly enhanced the Mohawk's capabilities, and it was logical to develop a Mohawk variant that could carry either, though not both, of them. The result was the definitive "OV-1D".

The OV-1D featured three fast-access compartments that allowed the IR sensor and the APS-94 SLAR electronics systems to be easily swapped out. The canoe pod for the SLAR could be bolted on or removed quickly. A full configuration change took no more than an hour. Both sensor systems used the same cockpit display and control panel.

The OV-1D had the wider wingspan of the OV-1B, but also was fitted with airbrakes. It featured an updated camera suite, including the panoramic camera in the nose, a panoramic camera in the fuselage, and a "serial frame" camera under the fuselage to provide along-track imagery.

Night imagery could be obtained using an electronic photoflash unit carried in a wing pod. Flares had proven troublesome to handle in the field, with improperly loaded flares said to have caused accidents in which the entire flare load lit off in the launcher pack. The photoflash pod was also used by older Mohawk variants.

Avionics improvements were added as well, particularly in the form of the "ASN-86 inertial navigation system", which provided accurate all-weather flight guidance. In addition, updated defensive countermeasures systems were added, and the aircraft could carry an "ALQ-147 Hot Brick" heat-seeking missile jammer on an underwing pylon.

A total of 37 OV-1Ds were built, and 108 older Mohawk variants were upgraded to the OV-1D standard, including four OV-1Cs rebuilt as the "YOV-1D" pre-production prototypes. Two OV-1Ds of this total were provided to Israel in 1974, though they were returned to US service in 1976.

   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                14.63 meters        48 feet
   length (no SLAR)        12.5 meters         41 feet
   length (with SLAR)      13.69 meters        44 feet 11 inches
   height                  3.86 meters         12 feet 8 inches

   empty weight            5,330 kilograms     11,760 pounds
   max loaded weight       8,215 kilograms     18,110 pounds

   maximum speed           490 KPH             305 MPH / 265 KT
   service ceiling         7,620 meters        25,000 feet
   range                   1,520 kilometers    945 MI / 822 NMI
   _____________________   _________________   _______________________

* Apparently the Army had some misgivings about the Mohawk at first, as the fussing over the type with the Marines and the Navy had led to an aircraft that was more complicated and expensive than the Army had originally wanted. However, once the machine went into combat the Army decided they'd made the right choice after all.

The Mohawk performed outstanding service in the Vietnam war, providing excellent intelligence on enemy positions and activities. It operated over South Vietnam, Laos, Cambodia, and occasionally even North Vietnam. The Mohawk often flew in cooperative missions with US Air Force, Navy, or Marine forces.

The Mohawk's ability to operate from forward airstrips and provide real-time intelligence was a great benefit to Army field commanders, permitting immediate artillery or air strikes to be called in on enemy troop movements. It was also occasionally used to parachute supply canisters carried on underwing pylons to combat units in isolated locations.

Its agility, ability to fly low, and its relatively quiet turboprop engines allowed it to sneak up on the enemy unannounced, and the enemy is said to have referred to it as the "Whispering Death". Its combat survivability was very good, and its reliability and maintainability were outstanding, with the highest availability rate of any Army aircraft.

The lack of serious offensive armament was troublesome to field commanders, however, because in many cases the Mohawks found concentrations of enemy troops who would be gone by the time strikes could be called in. The field commanders argued at length but in vain against the Air Force restriction on armed Army fixed-wing aircraft. In many cases, the Mohawks were armed anyway.

In 1966, one Mohawk reputedly shot down a North Vietnamese MiG-17 fighter with 70 millimeter unguided rockets. The North Vietnamese pilot made a pass at one of a pair of Mohawks, the other loosed a salvo of rockets at the fighter, and to the surprise of all, managed to hit the MiG. However, one Mohawk was shot down by a North Vietnamese MiG in 1969, evening the score. This was the only Army fixed-wing aircraft loss in air-to-air combat during the war.

A total of 27 Mohawks were lost in combat action in Vietnam, including one destroyed on the ground, and a further 36 were lost in accidents. There is a story that the crew of one shot-up Mohawk ejected, and the aircraft then obligingly crashed into a scrapyard.

In 1972, Mohawks began to be transferred to the Army National Guard, but the type still remained in first-line regular Army service for two decades longer.



* Several Mohawk modifications also saw service. Since it was designed for the reconnaissance role and could carry sophisticated sensors, it was well suited for modification to the "electronic intelligence (ELINT)" role.

The exact history of the use of the Mohawk as an ELINT platform is a little confusing, particularly because in some cases the modifications were secret. Apparently, in the early 1960s, one OV-1B had its SLAR removed and replaced by an "AN/ALQ-133 emitter location system", with antennas in boxy underwing pods and an antenna in place of the SLAR boom, to locate and target adversary radar and communications centers. Targeting data was relayed in real time back to ground commanders over a data link.

This aircraft was originally given the designation "EV-1D", though the designation was later changed to "RV-1B". This led to a formal program named "QUICK LOOK I" to update a number of OV-1Cs to a similar ELINT configuration based on the ALQ-133. Several dozen OV-1Bs were later updated to the OV-1B airframe configuration, given an upgraded ELINT suite under the "QUICK LOOK II" program, and redesignated "RV-1D". There were also some classified ELINT modifications of the Mohawk whose details remain unclear.

In the late 1980s, a few OV-1Ds were modified as prototypes for an advanced "OV-1E" variant with a "glass cockpit", "Global Positioning System (GPS)" satellite receiver, and other modernized kit, but the military didn't buy off on the idea and it went no farther.

* There were a number of proposals for Mohawk variants that never happened. In 1960, Grumman proposed an "AO-1EF (OV-1E)", with the forward fuselage stretched 71 centimeters (28 inches) to accommodate a third seat for a sensor systems operator. The additional space could also be used for cargo. The OV-1E never went beyond the mockup stage.

One particularly adventurous proposal was the "Model 134E" Mohawk, which was to be a "tiltwing" vertical takeoff aircraft with four turboprops, a horizontal tail rotor, and a stretched fuselage to allow it to carry cargo or 11 troops. The Army wasn't interested, and the project never got beyond the paper stage.

Both Germany and France were interested in the Mohawk and performed flight evaluations with it in 1962 and 1963. The French Breguet company actually obtained a license from Grumman to build it, fitted with more powerful de Havilland Gnome turboprops, but the deal fell through.

Grumman also came up with a design for an extensively modified Mohawk "Model 134R" with an armored tandem cockpit and built-in gun armament as a "counter insurgency (COIN)" aircraft for the Army's "Light Armed Reconnaissance Aircraft (LARA)" competition, but lost to Rockwell International's OV-10A Bronco.

The Philippines later came up with a request for a COIN aircraft, and Grumman responded with a more conservative proposal to update 20 OV-1Bs to a fully armed standard, while Rockwell proposed the OV-10A. However, the Philippine government came up short on money and the sale never happened.

One OV-1B was experimentally fitted with a midair refueling probe for ferry flights, but this scheme was not adopted. Snow ski landing gear was successfully evaluated for the Mohawk, but it was never used operationally.

There was even an early scheme to fit the Mohawk with water ski landing gear, in response to a Marine request, to allow it to land on calm waters and taxi up to a beach. The concept was successfully tested, but it wasn't used operationally. Apparently pilots were not comfortable with the idea of setting an aircraft down on water with the prospect of sinking if taxi speed fell too low.



* In the post-Vietnam era, the Mohawk generally operated with the US Army in Germany and South Korea, though it did see limited service in Central America as well.

It was also employed by civilian organizations. Army Mohawks were flown in missions to support the US Forest Service to spot forest fires and obtain survey data on the spread of tree diseases. In 1980, Oregon Army National Guard Mohawks flew surveillance missions to monitor the eruption of Mount Saint Helens in southwestern Washington state. Army Mohawks also assisted the US Coast Guard, the Civil Air Patrol, and state and local public-safety organizations in search and rescue operations, with the SLAR and IR sensors proving particularly useful in hunting for crashed aircraft in mountainous terrain.

In the early 1970s, the US Geological Survey (USGS) performed large-area mapping missions with an OV-1B fitted with a modified SLAR. The flights were part of a study to determine state water resources, and the aircraft was later used to perform survey missions in Alaska. During the Alaska missions, the aircraft carried emergency floatation gear on the underwing pylons to allow the flightcrew to ditch at sea in an emergency, since ejecting into the frigid Arctic environment would have given little chance of survival. The primary pilots for the USGS OV-1B were a pair of grandmothers who had tired of being secretaries, taken flight lessons, and became topnotch pilots.

The US Environmental Protection Agency (EPA) used a specially modified OV-1C on loan from the Army to perform environmental surveys of the areas around nuclear power plants in the early 1970s, and a civilian engineering firm working for the Atomic Energy Commission also obtained an OV-1C to monitor underground atomic tests during 1972. The US National Aeronautics & Space Administration (NASA) used a few Mohawks for aviation technology experiments, including one that was fitted with a small turbojet engine for noise tests.

In a particularly interesting application, in 1971 the US Customs Service received four Army surplus OV-1C Mohawks, along with two ex-Navy Grumman S-2 Tracker ocean patrol aircraft, to hunt for drug trafficers. The Mohawks were fitted with a Texas Instruments forward looking infrared (FLIR) camera in a turret under an extended nose.

As the Customs OV-1Cs were sometimes fired on by drug runners, there was some thought of arming the Mohawks so they could shoot back, but despite the fact that Customs officers often carry some impressive personal firepower arming a Customs aircraft was judged against the rules. The Mohawks were phased out in favor of more modern aircraft in 1986.

At least three Mohawks ended up in private hands, and occasionally starred in TV series such as "AIRWOLF", with their unusual looks making them well suited to playing the "bad guy.

The last Mohawks in US Army service were withdrawn in the mid-1990s. These aircraft were used to observe North Korean military activities along the demilitarized zone, and were replaced by De Havilland Canada DASH-7 / Airborne Reconnaissance Low aircraft, the subject of the next chapter. 34 US Army surplus Mohawks were provided to Argentina in the early 1990s, and at last report these aircraft were still in service.


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