Curtiss F9C Sparrowhawk (1931)

US Navy Paraside Fighter (1931), 8 built

The Curtiss F9C Sparrowhawk is a light interwar biplane designed to be carried as a parasite flighter by the United States Navy airships USS Akron and Macon. This was the first operational use of the concept in the US, for the short time both airships were in commission (before their loss in 1933 and 1935). This was a fairly small series, with perhaps 7 manufactured given each airship could onl carry 4 at any given time. Their tactical role evolved from local defensive asset to reconnaissance asset for a US battle group. They would remain the only US fighters provided with a skyhook in order to dock to an airship while in flight. The very last was still flying in 1939. However nowadays these are forgotten, just as the airships that operated them, the first and only aircraft carrier airships commissioned.

This a complement piece to the Akron class airships

Development

It's difficult to separate these aircraft from the airships that would carry them: USS Akron and Macon were the largest American airships ever built, and among the largest in existence altogether. They were planned back in 1928 after the USN tested the USS Shenandoah and Los Angeles with success, as a way to provide a long range reconnaissance asset to the fleet, that could operate its own defensive squadron to defend itself against enemy carrier's fighter aircraft. However the idea of a parasite fighter was not new, it drew from WWI experience and projects of British fighters operating from airships to save fuel and be carried at the same altitude as Zeppelins. The first flying aircraft carriers were indeed British. However the war ended before any of these new a airships could be built.

An N2Y-1 pictured about to hook on to the trapeze used for launching and recovering aircraft during evaluation of the equipment on board the rigid airship Los Angeles (ZR 3) at Naval Air Station (NAS) Lakehurst, New Jersey, on August 18, 1931

Already in the interwar, the USN tested USS Los Angeles (ZR-3), a Zeppelin built on US specifications in Germany as compensation for the war reparation airships lost. It was following USS Shenandoah by being "laid down" on July 1922, launched on August 1924 and commissioned on 25 November 1924. Until decommissioned on 30 June 1932, which was a remarkable carrer of eight years, when most airships had a life expectancy of two years at most, USS Los Angeles was instrumental in the development of the American parasite fighter program. Indeed by 1929, USS Los Angeles was used to test the trapeze system developed by the U.S. Navy to launch and recover fixed wing aircraft from rigid airships. This success consitioned the ordrer of a pair of purpose-built Akron-class airships.

Together with a new class of airship, the USN wanted a new type of parasite fighter to go with it. Curtiss, by then the primary contractor for anything that had wings, especially fighters for the interwar, both for the USAAC and USN, was awarded a contract for a modification of its proposal of a small pursuit plane. It was chosen because of its small size of 20.2 ft (6.2 m) long, 25.5 ft (7.8 m) in wingspan. It was however never designed for a reconnaissance role. It was calculated that up to five aircraft (4 in the hangar bay, 1 stored on the trapeze) was ideal, but compromised led to have on USS Akron, two structural girders obstructing the aft hangar bay so she only could carry three Sparrowhawks and a modification was due when she was lost so her sister USS macon was modified when fitted out and was the one that carried five.

This is the justification to see an overall serie of only seven, three F9Cs for Akron and five F9Cs for Macon. After the loss of the first, USS Macon had seven total serviceable, meaning three were training at her naval air station, five carried, albeit in practice the extra one was replaced by a 2-seater training model used as "taxi" with the ground. USS Macon routinely indeed carried four airplanes in service, the 5th being the "taxi" (see below).

The F9C was one of the smallers fighter developed by Curtiss in 1930-31. The first prototype BuAer number 8731, first flew on 12 February 1931 and was retained for tests, notably of an alternative version of the skyhook, perfected until production started. This model remained operational for further tests and training until scrapped in 1936. The XF9C-2 was the second prototype built (BuAer number 9264), with all the recommendations made in March 1931. Its main difference was its optional skyhook. When tests were all satisfying, the USN signed a contract for six more, three per airship given the initial capacitty of USS Akron. This made for a total of eight, prototypes included. The model produced all had the definitive skyhook and were registered under BuAer numbers 9056 to 9061.

Design of the F9C

General layout

The F9C was a modern biplane with an almost all-metal (duralumin) structure, fuselage, and wings that had a metallic structure, covered by doped fabric, as the tail and rudder. Technically a sesquiplane as the lower wing was smaller, and connected to the stagerred upper wing with "N" struts, the upper wing root was integrated into the fuselage, with a gullwing configuration so that the pilot could see between the wing's shoulders. This enabled extra rigidity, to the cost of the pilot's vision. This came back to haunt Curtiss when the Navy decided to use them for reconnaissance.

Otherwise, the F9C (the Navy acronym was composed of the "Fighter" category, followed by the model in sequence from the manufacturer, and the latter's name, Curtiss). It measured 20 ft 2.0 inches (6.147 m) long, like a WWI fighter essentially, for a wingspan of just 25 ft 6 inches (7.772 m), and an height of 10 ft 6 in (3.2 m) not including the skyhook above. Its wing area was 172.79 sq ft (16.053 m2). It needed enough to climn to the altitiude of the airship. This airfoil was of the Clark YH standard. The Curtiss F9C needed to be light enough to be carried in the airship, at just 2,089 lb (948 kg) empty for a gross weight of 2,776 lb (1,259 kg). Left Photo: Late version without undercarriage and instead belly fuel tank.

Engine and performances

The Sparrowhawk had a Wright R-975-E3 9-cylinder air-cooled radial piston engine rated for a modest 438 hp (327 kW). Performance were as follows: Top speed 176.5 mph (284 km/h, 153.4 kn), service ceiling was 19,200 ft (5,900 m), rate of climb was 1,700 ft/min (8.6 m/s) for a wing loading of 16 lb/sq ft (78 kg/m2) and power/mass ratio of 0.086 hp/lb (0.259 kW/kg). Range was limited to 297 mi (478 km, 258 nmi), that offered in practice a radius of action, from the airship and back, of c230 km around the airship in case of reconnaissance, which was quite appreciable, but only if fitted with an extra belly fuel tank. Indeed, when it was decided to use the F9C as a reconnaissance aircraft, a small radio set was installed and the landing gear, not in use when jooked up in the aiorship, was replaced by a ventral fuel tank. Otherwise, the F9C was armed with just two .30 in (7.62 mm) Browning machine guns in the nose.

⚙ F9C specs.
Gross Weight	2,776 lb (1,259 kg)
Length	20 ft 2.0 inches (6.147 m)
Wingspan	25 ft 6 inches (7.772 m)
Height	10 ft 6 in (3.2 m)
Wing Area	172.79 sq ft (16.053 m2)
Engine	Wright R-975-E3 9-cyl. ACR: 438 hp (327 kW)
Top Speed, sea level	176.5 mph (284 km/h, 153.4 kn)
Range	297 mi (478 km, 258 nmi)
Climb Rate	1,700 ft/min (8.6 m/s)
Ceiling	19,200 ft (5,900 m)
Armament	2x Browning 0.3 in M1919
Crew	1 pilot

Operational History

To achieve launching and recovery in flight a skyhook system was developed by Curtiss, but the two prototype were tested without. Performances were excellent, but with the a hook mounted above its top wing, they degraded due to the drag, and the point of bala,ce needed to be changed with "useful ballast". Whe trying to catch the trapeze, the pilot needed adjust its speed on the underbelly, stay stable despite turbulences, then engage its cross-bar. For launching, its hook was engaged already on the trapze internally and the latter was then lowered clear of the hull and into the moving airship's slipstream. The engine needed to run at full throttle. The pilot then disengaged the hook and free fell for a short while when catching speed. The width of the trapeze cross-bar allowed a certain lateral lee-way in approach fortunately but despite this, the manoeuver was not for the faint-harted.

The hook mounting had a guide rail to provide protection for the turning propeller as well, its engagement was automatic on positive contact between the hook and trapeze. More than one attempt could be necessary for a successful engagement, especially in gusty conditions. Once the Sparrowhawk was securely caught it was hoisted by the trapeze back within the hangar and its engine cut. Pilots soon learned techniques and when cmpared to their colleagues at sea, it was described as easier than landing on a moving and pitching, rolling aircraft carrier. "The men on the Flying Trapeze" as they named thesmeves on USS Akron were soon proud of their little group and designed an emblem for the squadron, reflecting this and its acrobatic nature.

Once the system was fully developed on USS Akron, there was a change in doctrine and it was decided to use these for scouting. The ussue was endurance so their landing gear was removed and replaced by a fuel tank. The landing gear were still carried on board to be replaced while in the hangar in case they needed to land for maintenance once back over land. Basic maintenance was possible in the airship, but due to weight issues, there was not much spare gasoline not spare parts and certainly not spare engined, or even ammunition. In the end, their effectiveness was hampered by their poor radio equipment. Ot was so short range they needed to be kept within sight of the airship.

In 1934, after the loss of USS Akron and transfer of the entire squadron on USS Macon, commissioned in between, new direction-finding sets and new voice radios were fitted. These allowed operations beyond visual range and best used the belly fuel tanks. Successful exercises were performed with the Navy (see USS Macon's career). The Navy also tested the concept of 'flying ballast'. The airship took off with additional ballast or fuel aboard instead of its airplanes and once cruising, the aircraft would be flown aboard, with theit additional weight supported by dynamic lift until the motherships lightened.

So aircraft gasoline was carried above all else. However after a few exercises that proved the validity of the concept in 1935 USS Macon was lost in turn, putting an end of such experimentations. The squadron was not dissolved. The skyhooks were removed, and they continued operations fropm NAS Lakehurst as regular local defence fighters and for training until end of service in 1937. They were then declared "surplus standard".

Fortunately there is one intact Sparrowhawk still around today. This was the BuAer 9056 pending write-off at NAS Hampton Roads in 1939, the last still used for training. It was transferred to the Smithsonian Institution. and over the years, had been rebuilt, using parts from the surviving F9C-2 BuAer 9057 also kept, and from XF9C-2 (9264) from Curtiss. It was exhibited for a time at the National Museum of Naval Aviation at NAS Pensacola, but is now ay the Steven F. Udvar-Hazy Center of the Smithsonian's National Air and Space Museum collection, to the colors of the USS Macon's squadron. The remains of four aircraft are still underwater on the wreck-site of the same USS Macon, trapped in the hangar when she crashed. This was documented in 1990 and 1991.