02 February 2007

Flightdeck Friday - Vought V-173/XF5U-1 "Flying Flapjack"

(ed: BTW, huge props to Vought for their heritage site from whence most of these photos were sourced and double props since they are refurbishing the V-173 at the Vought facility in Gran Prarie, Tx. (Thanks for the heads-up Tommy!) )

June, 1947. Navy Day. Bathers along the beaches lining Long Island Sound look aloft to an incredulous sight – a silver and yellow disc that is speeding along, looking unlike any other flying object seen in the area. They didn’t know it at the time, but those folks were witnesses to the one and only public flight of the Vought V-137/XF5U, nicknamed the “Flying Flapjack.” At onetime a cutting edge aerodynamic design, it was now not much more than a curiosity as the age of the jet was ascendant. Soon, it would be sent to the breakers to be broken up along with the one and only prototype and assume its place in the annals of aeronautical fantastical or just plain weird designs.

During the war years, considerable energy was expended in making fighters go faster (“speed is life” didn’t originate with the movie ‘Top Gun’). Much thought was spent in streamlining and reducing drag. The effort took many paths – some more conventional than others. Flush riveting, new airfoils (like the Mustang’s laminar flow), reduced frontal area via in-line, liquid cooled engines represented most of the mainstream efforts. Some, like Jack Northrop and the engineers at Vought, thought that reduced drag could be accomplished by eliminating entire structural components, like the tail. Northrop’s work evolved through the N9MB into the familiar flying wing. Over at Vought, it was if anything, a little more unconventional.

Beginning in 1933, Charles Zimmerman, an aeronautical engineer with the National Advisory Committee for Aeronautics (NACA) at Langley Field, Virginia began to promote a tailless “pancake” design. He filed for a design patent on April 30, 1935 and was granted patent #2,108,093 on February 14, 1938. With the concurrence of NACA, Zimmerman approached United Aircraft Corporation with his novel design in 1937 and joined United’s Chance Vought Aircraft Division in that year as project engineer. By August 15, 1939, drafting, engineering design, and aerodynamic studies were far enough along for Vought to submit a proposal to the U.S. Navy for a full-scale prototype of the V-173. The U.S. Navy placed a contract for one V-173 on May 4, 1940. First flight of the airplane was on November 23, 1942.

To say the V-173 was unconventional would be an understatement. Powered by two Continental A-80 engines, rated at 80-horsepower each, turning two 16.5-foot three-bladed propellers, the V-173 used a semicircular planform (wide at front, narrow in back) with two vertical tails for stabilization and two “ailerators” for control. The aircraft had long fixed main landing gear and a 22-degree nose-high static ground angle. The cockpit had a windowed leading edge ahead of the pilot for downward visibility. Four segmented leading edge inlets (left and right) provided air to the engine. Built light, the V-173 structure was made of wood with fabric covering. The upside was with a wing loading of only 5 lbs/sq ft; the V-173 could lift off in 200 feet in no wind conditions, and with a zero run against a 25-knot headwind. The implications for a carrier-based fighter were readily apparent. However, with a mere 80-hp driving the prototype, max speed was only 138 mph. Clearly a proof of concept prototype should be built, designed from the keel up as a fighter.

The letter of intent for the Vought VS-315 (XF5U-1) was issued September 17, 1942. Designed for land- or carrier-based operations (the latter minus a tailhhook), the XF5U-1 was a twin-engine, single-seat, low aspect ratio flying wing type of airplane, manufactured by the Chance Vought Division, United Aircraft Corporation, Stratford, Connecticut. The basic wing area (~472 sq. ft) and planform of the XF5U-1 was the same as the V-173. Power was substantially upped with the use of two embedded Pratt & Whitney R-2800-7 radial engines rated at 1350 hp each. These were in turn, connected via shafts to two hydraulically operated, fast-acting, electro-mechanically governed propellers. Each engine was cross-connected to the opposite prop such that if an engine was lost on take-off/recovery, both props would continue turning. (ed: And you thought an engine out on a Cessna 310 could be a handful…imagine what the possibilities might have been here.) Continuing the unconventional theme, the props themselves consisted of four Pregwood blades and load-relieving hubs which differed from the conventional four-way hub in that the blades were free to “flap” in pairs about the shaft axis. Low pitch stop was 15 degrees; high pitch stop was 70 degrees. The cockpit was a monocoque shell with a bubble canopy for better visibility and space set aside (but never utilized) for six 50-calibre guns and ammo boxes. Unlike the V-173’s fixed gear, the landing gear on the XF5U-1 would retract.

The first XF5U-1 airplane (Bureau Number 33958) was used for static tests; proof loads, extended to ultimate, largely confirmed structural design predictions. The second XF5U-1 airplane (Bureau Number 33959) was used for experimental flight test and concept validation. Early in the test series though, significant problems were encountered with vibrations from the props that translated to the shafts, gearboxes and airframe structure and considered excessive. By this point (1947) the war was over and the budgetary long-knives were out. With proven prop-driven fighters in its inventory already (notably the F4U) and intent on moving into jets (recall McDonnell’s Phantom and Banshee, Grumman’s Panther and Vought’s own Pirate were in various stages in the pipeline), the decision was made to axe the XFU5 on 17 March 1947. Both prototypes were subsequently destroyed, although the V-173 was saved and is now in the National Air and Space Museum’s extended storage facility awaiting restoration.

Specifications (XF5U-1):
Wingspan: 32.50 ft
Overall Length: 28.62 ft
Height: 16.96 ft
Aspect ratio of basic wing: 1.275
Wing airfoil section (NACA): 0016
Wing area less ailavators (48) sq ft: 427
Weights and Capacities
Empty Weight: 13107 lb
Gross Weight: * 16758 lb
Useful Load
Fuel Capacity: 261 gal
Oil Capacity
Power plant Characteristics
Type: Two Pratt & Whitney R-2800-7
Rating: 1350 hp

Maximum Speed, Sea Level: ** 425 mph
Landing Speed, Sea Level: 490 mph
Initial Rate-of-Climb: *** 3000 ft/min
Range at Cruise Speed: **** 1152 miles
Absolute Ceiling: 30700 ft
Crew: 1
Armament: Provisions for six 50-caliber Browning Machine Guns


* Empty weight plus 261-gallons internal fuel, pilot, ammo, two 150-gallon
drop tanks. Overload gross weight equals 18,931 pounds with two
1,000-pound bombs.

** Max speed, 425 mph at sea level (501 mph with advanced engine)

*** Rate of Climb
fpm at mph EAS at alt, ft
3,000 175 Sea Level
2,500 175 10,000
1,000 165 20,000

**** Max range, 597 statute miles at 10,000 ft altitude with 261 gallons of internal fuel (less 50 gallons for warm-up, take-off and climb) with high blower, 1700 engine rpm, 31 inches Hg M.P., auto lean mixture, 280 mph TAS, prop gear ratio 0.1763:1. With two 150-gallon drop tanks, max range, 1,152 statute miles.

Postscript: While the V-173 flew some 130+ flights, not all ended in a clean landing...