Stinson L-5 Sentinel

The Stinson L-5 Sentinel was a World War II era liaison aircraft used by all branches of the U.S. military and by the British Royal Air Force. Along with the Stinson L-1 Vigilant, the L-5 was the only other American liaison aircraft of World War II that was purpose-built for military use and had no civilian counterpart. All other military liaison airplanes adopted during World War II were lightly modified "off-the-shelf" civilian models.

Development
The origins of the L-5, affectionately known as the "Flying Jeep", can be traced to the pre-war civilian Stinson HW-75. The 75 horsepower civilian high-wing design was built by the Stinson Aircraft Company at Wayne, Michigan and first flew in 1939. The HW-75 featured two seats up front side-by-side, and a third "jumpseat" in the rear in which a small passenger could sit sideways. The design was easy to fly, and it was difficult to stall or spin. Shortly after the introduction of the HW-75, Stinson became a subsidiary of the Vultee Aircraft corporation. Under Vultee management, the HW-75 was equipped with an 80-horsepower four-cylinder engine for the 1940 model year and the HW-75 became known as the Model 105 "Voyager", touting its 105 mph cruise speed. Fitted with a four-cylinder 90hp Franklin engine for the 1941 model year, the type became known as the Model 10A. In the post-war era, the fuselage of the Model 10A was enlarged to accommodate four seats, and the four-cylinder powerplant was replaced with a Franklin 150hp six-cylinder engine. This conversion became the Stinson Model 108 Voyager and the only civilian aircraft commercially produced by Stinson after WWII.

Six examples of the Model 105 Voyager were equipped with 100 horsepower Franklin O-200 engines and provided to the military for testing under the experimental designation YO-54. Evaluated by the U.S. Army and Air Forces in 1940 for potential use as a low-cost short range observation aircraft, it failed to meet performance requirements. The Voyager was then completely re-engineered by Stinson into a much stronger and more powerful tandem-seat airplane that met rigorous Army engineering handbook standards for the design of military aircraft. The prototype, designated as the Model V-76 by Vultee / Stinson was accepted by the military after accelerated service trials and entered into service in December 1942 as the Army O-62 ('O' for observation).

In March 1943, with the creation of the liaison category of light observation aircraft, the designation was changed to L-5. The primary purpose as a liaison aircraft was courier and communication work, artillery spotting and casualty evacuation. The fuselage of later models was redesigned so the aircraft could also be used as an air ambulance, or for cargo work. With a wider and deeper rear fuselage section and a large rear door that folded downward, a litter patient or 250 pounds of cargo could be quickly loaded aboard.

The L-5 series was manufactured between December 1942 and September 1945, during which time 3,590 of the unarmed two-seaters were built for the United States armed forces, making it the second most widely used light observation aircraft of the war behind the Piper L-4 Cub.

Construction
The fuselage was constructed using chrome-moly steel tubing covered with doped cotton fabric and the wings and empennage were constructed of spruce spars and plywood ribs and skins, also covered with fabric. The use of aluminum, which was in critically short supply and more urgently needed for other aircraft, was limited to the engine cowling, tail cone, framework for the ailerons, rudder and elevator and the landing gear fairings. The L-5 was powered by a six-cylinder 190 horsepower Lycoming O-435 engine.

Operational history
Capable of operating from short unimproved airstrips, the L-5 "Sentinel" delivered personnel, critical intelligence and needed supplies to the front line troops. On return flights, wounded soldiers were often evacuated to rear area field hospitals for medical treatment, providing a huge boost to the morale of combat troops fighting in remote areas. L-5's were used for many other important activities, such as aerial photography, controlling vehicle convoys, para-dropping food, medical supplies and ammunition, laying communication wire, distributing propaganda leaflets, spraying pesticide, transporting prisoners, and directing fighter-bombers to ground targets. The L-5 was also popular with Generals and other high-ranking officers for fast, efficient short-range transportation.

During the Battle of Okinawa, L-5 operated from an LST using the Brodie landing system which allowed a light aircraft to take off and land without a flat surface by snagging a wire hung between two booms. One of the L-5 that used the Brodie system off Okinawa is now on display at the Smithsonian Institution Aerospace Museum in Washington, DC.

Variants
Five versions of the Sentinel were produced for the U.S. Army Air Force (USAAF); the L-5, L-5B, L-5C, L-5E and L-5G. There was no official L-5A variant as is often reported because the designation was intended for a version of the aircraft that was never built. Nonetheless, many people in and out of the military still refer to the standard "observer" version of the L-5 as an L-5A. Like the L-5A, the L-5D was a planned version that was not adopted. A single L-5F was an L-5B equipped with an experimental low-noise "stealth" propeller and exhaust system for research purposes. An L-5H version was on the drawing boards at Stinson when the war ended, and it never reached the prototype stage.

The L-5 carried a pilot and observer in a tandem seating configuration, which was preferred by the military for observation work. The L-5B through L-5G models were modified to carry a litter patient or light cargo, or a rear seat passenger sitting in the normal position. The Navy and Marine version of the L-5 through L-5E were designated 0Y-1, and all these aircraft has 12-volt electrical systems. The 24-volt L-5G became the 0Y-2. Neither the L-5G nor OY-2 saw combat during World War II because production did not begin until July, 1945, just weeks before the war ended, but they were used extensively during the Korean War. The British RAF procured 40 L-5s and 60 L-5Bs, and designated them Sentinel Is and Sentinel II's respectively. These aircraft were used exclusively in the India-Burma theater of operations.


 * O-62
 * Observation, artillery spotting and liaison aircraft, powered by a Lycoming 0-435-1 piston engine; 275 built.


 * L-5
 * Observation, artillery spotting and liaison aircraft; 1,538 built, 79 transferred to USN/USMC as 0Y-1.


 * L-5A
 * Cancelled conversions of L-5 with 24V electrical system and 200 hp ranger engine.


 * L-5B
 * 729 aircraft with rear fuselage hatch to permit loading of a stretcher or cargo; twin-float capability; 60 transferred to RAF as Sentinel Mk II, 40 transferred to USN/USMC as 0Y-1.


 * L-5C
 * 200 L5-B were equipped K-20 reconnaissance cameras.


 * L-5D: Not adopted. No prototype built.
 * L-5E
 * 750 STOL variants with larger tires and brakes and manually ailerons drooping ailerons allowing shorter takeoff and landing; 152 transferred to USN/USMC as OY-1. Thirty later converted to 24 volt electrical systems and re-designated 0Y-2.


 * L-5G
 * L-5E airframe powered by 190-hp (142-kW) Lycoming 0-435-11 piston engines with improved cylinders and carburetor and fitted with controllable pitch propellers; 115 built, by end of the war and 785 others cancelled. Electrical system 24-volts. Final production model redesignated U-19B in 1962.


 * XL-5
 * One test and evaluation aircraft, powered by a Lycoming 0-435-2 piston engine.


 * U-19A
 * L-5 variants still in service redesignated U-19A by the USAF in 1962.


 * U-19B
 * One L-5E used as a glider tug at the United States Air Force Academy in 1962.


 * 0Y-1
 * 306 L-5 and L-5Bs transferred to the United States Marine Corps and United States Navy.


 * 0Y-2
 * 152 transfers of L-5E to USN/USMC; 30 0Y-1 conversions to 24V electrical system.


 * Sentinel Mk I
 * 40 L-5s supplied to the RAF under Lend-Lease.


 * Sentinel Mk II
 * 60 L-5Bs supplied to the RAF under Lend-Lease.


 * L5/235
 * variant powered by Lycoming O-540-B, 235 hp, used for glider towing.

Aircraft markings
Standard camouflage as delivered from the factory was non-specular "Bulletin 41" medium gray #43 undersides with olive drab #41 above, broken around the edges of the wing and tail surfaces with medium green #41. Stars, or "stars and bars" were applied to both sides of the fuselage and on the upper left and lower right wingtips. The USAAF number appeared on both sides of the vertical stabilizer in either yellow or black. The USMC number appeared in smaller size in black only. Most aircraft were repainted silver during the post-World War II period. Some Marine Corps and Navy aircraft painted overall non-specular Sea Blue. A variety of unit identification markings including nose art were applied in the field. Interior surfaces were generally finished in yellow-green chromate primer and slightly darker ANA 611 "interior green". Instrument panels were "raw" phenolic sheet naturally matte black in color.

Units using this aircraft
The USAAF, US Marines, and US Navy used this aircraft in the European, Pacific, and Far East theaters during World War II, and in Korea during the Korean War. The Royal Air Force operated 100 Sentinels in India and Burma.

After World War II, the Philippine Army Air Corps used this aircraft from 1945 to 1947. This aircraft remained in service after 1 July 1947 when the PAAC was renamed the Philippine Air Force. The Italian Air Force operated approximately 100 L-5s from 1946 into the 1950s. Many other countries also received L-5s after the war, particularly India which received 200. A number of these went to Pakistan after the partition of India in 1948.

Postwar use and current status
After World War II, the L-5 was widely used by the Civil Air Patrol for search and rescue work. Today there are about 300 known examples left world wide and less than half are in flying condition. A restored, flying example of the 0Y-1 (L-5E) variant (VH-NOY) is located in Coolangatta, Queensland, Australia. This example was built for the USAAF but was delivered directly to the US Navy instead, serving until 1949. A group called the Sentinel Owners and Pilots Association is dedicated to the preservation and enjoyment of this aircraft type. Post 1950, In India a lot of L-5s were used by Flying Clubs for hobby flying and to teach Student Civilian pilots. These were used till about 1973 when due to lack of spare wings the DGCA withdrew their C of A all over India.

Operators

 * Royal Australian Air Force - The RAAF operated one L-5 Sentinel, on loan from the USAAF. The aircraft was in service with the RAAF from 1944 to 1946.
 * 🇬🇷 Greece
 * Royal Hellenic Air Force
 * Italian Air Force
 * National Security Force
 * Japan Ground Self-Defense Force
 * Republic of Korea Air Force
 * $\phi$
 * Philippine Army Air Corps
 * Philippine Air Force
 * Polish Air Force - The fuselage of a single L-5 used in Poland after 1945 is currently displayed within the Polish Aviation Museum
 * ROC Air Force
 * PLAAF
 * Royal Thai Air Force
 * Royal Air Force
 * No. 27 Squadron RAF
 * No. 117 Squadron RAF
 * No. 194 Squadron RAF
 * Civil Air Patrol
 * United States Army Air Forces
 * United States Air Force
 * United States Marine Corps
 * United States Navy
 * Royal Air Force
 * No. 27 Squadron RAF
 * No. 117 Squadron RAF
 * No. 194 Squadron RAF
 * Civil Air Patrol
 * United States Army Air Forces
 * United States Air Force
 * United States Marine Corps
 * United States Navy
 * United States Navy

Survivors

 * Jimmy Doolittle Air & Space Museum, Travis Air Force Base, Fairfield, California,Udvar-Hazy Center National Air & Space Museum