X-Planes and Test Aircraft
XF5U The ‘Flying Pancake’
The proposed armament for the XF5U was six 0.5-in (12.7-mm) machineguns, to be fitted three in each wing root between the cockpit pod and engine. A later proposal called for the replacement of the six machine guns with four 20-mm cannon. In the event, no armament was ever fitted to the type.
Vought’s XF5U-1 was a singularly adventurous essay in aeronautical design. It was a daringly innovative aircraft which, had it been flown and successfully tested, may have provided the US Navy with an aircraft that had not only the high top speed of the first generation of jet fighters, but the slow-flying capabilities of a helicopter – a perfect combination for use aboard aircraft-carriers. It was not to be, however, and the type was cancelled in 1947.
The XF5U-1 was fitted with Pratt & Whitney radial air-cooled engines buried within the aircraft’s circular body. Lying in a horizontal position, each engine drove a four-bladed propeller through a complicated system of shafting and gearboxes. The two props were counter-rotating, each turning inwards towards the cockpit. It was essential that both propellers would continue to turn with equal power should one engine fail; the resulting common driveshaft proved problematic during development.
Would it have worked?
Although the XF5U programme looked as though it may yield a potentially valuable aircraft for the US Navy, the development problems it would have had to overcome may have led to disappointment. The XF5U-1’s powerplant configuration, in which the drive train was required to turn 90º twice between engine and propeller, was complex, inefficient and by Zimmerman’s own admission, likely to fail after relatively little use. The original ‘discoid’ aerofoil concept was proved sound aerodynamically by the V-173, but it may have been difficult to turn into a front-line fighting machine.
Zimmerman’s original flying models of his concept – ‘Zimmer’s Skimmers’ – had no tailplane, the entire rear of the body hinging as a control surface. Both the V-173 and the XF5U-1, however, were fitted with conventional tailplanes.
Painted in a standard overall midnight blue US Navy colour scheme, the XF5U-1 also had matt black sheets taped on walking areas in order to provide some degree of protection for the Metalite surface. These black areas gave the type a somewhat patchwork appearance in plan view.
The XF5U-1 used Metalite throughout its structure; this was a Vought-developed material comprising balsa wood sandwiched between aluminium. It was immensely strong and when the XF5U-1 had to be broken up after its cancellation in 1947, a metal wrecking ball dropped from a crane had to be used repeatedly to dismantle the aircraft’s main body.
SPECIFICATION Vought XF5U-1 Type: short take-off and vertical landing experimental fighter Powerplant: two Pratt & Whitney R-2000-7 Twin Wasp turbo-supercharged 14-cylinder two-row radial air-cooled engines, each rated at 1,350 hp (1008 kW) for take-off and war emergency, and driving 16-ft (4.88-m) four-bladed articulated airscrews
The props initially fitted to the XF5U-1 were conventional Hamilton Standard Hydromatic examples, as fitted to the F4U-4 Corsair. Later a pair of speciallydesigned propellers with offset ‘flapping’ blades, as used on helicopters, were fitted, as seen in this illustration.
Performance: (estimated at weight of 16,802 lb/7620 kg) maximum speed at military rated power 482 mph (775 km/h) at 30,700 ft (9357 m), at war emergency power 504 mph (811 km/h) at 28,900 ft (8808 m); initial climb rate at normal rated power, 2,200 ft/min (11.18 m/sec), service ceiling, 32,000 ft (9754 m); max range at 1,000 ft (305 m), 910 mls (1465 km) at average speed of 236 mph (380 km/h); take-off distance (calm), 930 ft (283 m); stalling speed (full load with military rated power), 46 mph (74 km/h) Weights: normal loaded, 16,802 lb (7620 kg); max overload, 18,917 lb (8581 kg); landing weight 15,542 lb (7050 kg) Dimensions: overall width (across tailplane) 32 ft 6 in (9.90 m); overall width across airscrews (diagonal) 31 ft 9 in (9.68 m), (square) 36 ft 5 in (11.10 m); length 28 ft 7½ in (8.72 m); height 14 ft 9 in (4.50 m) Armament: (proposed) six 0.5-in (12,7-mm) Browning MG 53-2 machine guns with 400 rounds per gun and provision for two 1,000-lb (453,6-kg) bombs on ventral racks
X-Planes and Test Aircraft
Vought XF5U The ‘Flying Pancake’ Without question one of the most peculiar – but promising – designs in aviation history, the XF5U-1 was developed to explore the possibilities of a blended wing and fuselage, which, if matched with sufficiently powerful engines, would enable both very fast and very slow flight. Its career may not have been long had it entered production, but it could have been a useful stopgap.
From the cockpit
Top speed Had it flown, the XF5U-1 would probably have shown similar performance to the new generation of jet fighters such as the FH-1 Phantom and would have been faster than the last piston fighters. However, the XF5U-1 would have had the added advantage of being capable of very slow flight and would have been virtually impossible to stall, an immensely valuable asset for a carrierborne aircraft.
McDonnell FH-1 Phantom
Vought XF5U-1 ‘Flying Pancake’
Vought F4U-5 Corsair SECTION
480 mph (773 km/h)
479 mph (771 km/h)
“…I felt elated and had the impulse to yell ‘Charlie, she flies!’…” Test pilot Boone Guyton (seen below) joined Vought in 1939 and took part in the testing of many of the company’s wartime and post-war aircraft, including the F4U Corsair and jet-powered F6U Pirate and F7U Cutlass. Boone also made the maiden flight of the V-173 on 23 November 1942 from the Vought factory in Stratford, Connecticut. Here he describes making the first flight of the ‘Flying Flapjack’:
470 mph (756 km/h)
Multi-role ‘Pancake’ missions 1
lthough one of the oddestlooking flying machines ever built, Vought’s XF5U could have been an invaluable stopgap resource during the US Navy’s transition from the piston era to first-generation jets. Designed by former NACA aerodynamicist Charles H. Zimmerman, the XF5U was the final part in an ambitious programme to prove the ‘disc-wing’ concept, in which large propellers mounted in the forward tips of a circular aerofoil blew huge amounts of air over a wing of extremely low-aspect-ratio, providing lift and cancelling any draginducing tip vortices. In April 1941 Zimmerman persuaded the US Navy to let him build a prototype, the V-173, which
first flew on 23 November 1942. It performed well and two prototypes of a larger, more powerful version, the XF5U, were completed just after the end of World War II. Sadly, neither flew, the project being cancelled in 1947 in favour of the development of jet fighters.
Having experimented with low-aspect-ratio aircraft designs for NACA and Vought since the early 1930s, Charles H. Zimmerman proposed a fighter design of ‘discoid’ configuration for the US Navy in February 1939. Construction of a prototype, the V-173, began in mid-1939.
Right: The XF5U, also known as the ‘Flying Pancake’ or ‘Flying Flapjack’, was the brainchild of Charles H. Zimmerman (third from right). Zimmerman started work on the circular aerofoil concept in 1933, his various test models being referred to by colleagues as ‘Zimmer’s Skimmers’.
Two prototype XF5U-1s were built, one for static testing, and the other, BuNo 33958 (illustrated), to fly. It would have made its first flight from Muroc Dry Lake (later Edwards AFB) in California, but the project was cancelled before it could get there.
Powered by a pair of 80-hp (60-kW) Continental engines, each driving a large three-bladed prop, the V-173 made its first flight, from Bridgeport Airport, Connecticut, on 23 November 1942, in the hands of test pilot Boone Guyton. Its handling was unusual, but showed great promise.
Rapid-response carrier air defence
V-173 test mishaps
The V-173 quickly accumulated over 130 hours of flying time during 1943, in the hands of several pilots, including Charles Lindbergh. Nicknamed the ‘Flying Pancake’, it suffered two forced landings, but its immense strength meant it was soon ready to fly again.
The high lift generated by the F5Us’ wings minimises their approach and landing speeds and therefore the landing run on the carrier deck. In a given time, this allows more F5Us to be recovered than conventional aircraft.
s soon as we were airborne and away from the ground effect, “A which was large on this type of air-
Fighter and attack missions
F5U attacks enemy convoy with gunfire and bombs.
Very short deck landing Mother carrier steams into wind to reduce over-deck wind and minimise landing speed.
F5Us return to carrier after mission.
Enter the XF5U-1
These F5Us are shown operating from large ‘Essex’class fleet aircraft-carriers.
Able to take off in a very short distance, the F5Us may be launched at minimal intervals.
Here, the F5U demonstrates its ability to carry out multiple tasks in the same mission. While one F5U attacks a North Korean Lavochkin La-9 piston fighter, another F5U attacks an enemy convoy.
These diagrams show typical operations with the ‘F5U’ had the type entered service with the US Navy in the immediate post-war era. Here, F5Us are launched in rapid succession and in strength to meet enemy airborne threats.
In September 1942, before the V-173 had flown, the US Navy instructed Vought to build two prototypes (one for static testing only) of an upscaled, more powerful version, to be designated XF5U-1. The first prototype was rolled out of the factory on 20 August 1945.
F5Us make maximum flare landing in very short space.
Difficulties with the extremely intricate gearing of the propellers – initially a pair of standard F4U-4 Corsair four-bladed props (as seen above) – delayed progress on the XF5U-1, as did a wartime lack of funds and engineering expertise. This eased when the war finished.
craft. I was quickly distressed by the extreme heaviness and sluggishness of the controls. There was a question as to whether I could obtain enough control to bank the aircraft sufficiently to complete a turn back to the runway. ”Slowly the aircraft moved back towards the airport. I felt elated and had the impulse to yell ‘Charlie, she flies!’. ”The landing was not at all normal. I managed to get the stick full aft and the aircraft settled so quickly on the ground from its last few feet of descent that it was both startling and pleasing. The V-173 rolled about 50 ft (15 m); landing speed was about 50 mph (80 km/h). “I noted that this aircraft, designed to be a VTOL machine, showed every promise of filling its design mission.”
End of the line
By early 1947 the XF5U-1, fitted with specially-developed articulated propellers, had finished its engine test programme and was ready to be sent to Muroc Dry Lake in California to fly. On 17 March 1947, however, the entire XF5U project was cancelled.