Another Saucer That Could Not Fly
Preface
An earlier story that I wrote, ”The Avro Flying Saucer Story” was about the most infamous example of Earthlings’ attempts to build a flying saucer that would rival the performance of those reported in UFO sightings. The Avro Flying Saucer, eventually becoming a joint project of the USAF and US Army, never flew, but it did something that none of the extraterrestrial-built flying saucers have ever accomplished, it earned a display in the National Museum of the United States Air Force at Wright –Patterson, in Dayton Ohio. I’m sure that if the museum had an opportunity to display a successfully flown flying saucer built elsewhere in this world or other worlds, it would do so. But the Avro Saucer is the only flying saucer project the USAF has ever funded, and it is the only saucer to ever come into possession of the museum. This story is about another saucer-shaped aircraft that could not fly, what the Irish would call a “Folly”, or something that is not what it purports to be.
Many aviation pioneers explored a saucer shape for the configuration of their aircraft designs. They were attracted to the saucer shape because its potential for superior maneuverability. But the low aspect ratio of a saucer brings with it an adverse increase in induced drag. Induced drag is that part of the total drag that is associated with the generation of lift, the other components being profile drag and skin friction drag. These characteristics of aircraft with low aspect ratio were identified by Ludwig Prandtl, whose analytic aerodynamic theories are the foundation of the applied science of aeronautical engineering. Prandtl’s theories predict, and empirical wind tunnel data substantiate, that the coefficient of induced drag is inversely proportional to the aspect ratio. That means that a saucer with an aspect ratio of 1.0 has an induced drag that is ten times greater than that of a winged aircraft with an aspect ratio of ten. Empirical data show that it is even worse: because of the rounded edges of a saucer, it behaves as if the aspect ratio were 0.95, when compared with a rectangular wing of the same proportions.
This discussion concerns one of the saucer-shaped aircraft developments of the 1940s, one that was thought to have a solution to the induced drag problem.
Chance Vought
Chauncey (Chance) M. Vought (1890–1930) was an aeronautical engineer, educated at Pratt Institute, New York University, and the University of Pennsylvania. Chance left school in 1910 to work for Harold McCormick who was president of McCormick Reapers, and as well as an aviation enthusiast. Among the earliest aircraft designs that he was responsible for was the McCormick-Rome Cycloplane of 1911. Vought gave his name to a succession of aircraft companies in his lifetime, and after his death, to such corporations as Ling-Tempo-Vought. With his death in 1930, he did not live to see some of the most famous aircraft that carried his name. The Chance Vought Corsair was among the best fighter aircraft supplied to the US Navy in World War II, made famous by Pappy Bloomington and his Marine Corps “Black Sheep Squadron”.
Charles Zimmerman
Charles M. Zimmerman was an aerodynamicist at Chance Vought, and one of several experimenters who toyed unsuccessfully with concepts for saucer-shaped aircraft in the 1930s. As an aeronautical engineer, he was familiar with the lifting-line aerodynamic theories of Ludwig Prandtl, and understood the severe induced drag penalty of the low aspect ratio saucer shape, and its relation to the formation of wingtip vortices. But he thought he had the solution. On his own he built a radio-controlled, electrically powered model airplane that he called the “Zimmer Skimmer”, and which carried the Chance Vought designation of V-162. He submitted his ideas and documentation to the National Advisory Committee for Aeronautics (NACA), but they thought the concept was “too advanced”, in other words, not thoroughly substantiated. In 1939, the US Navy approached Zimmerman and offered to fund additional development of his concept. By 1941 he had some wind tunnel data of a model to support the feasibility of his concept, and by 1942, the US Navy had awarded a contract for construction of a “proof of concept”, aircraft with the designation V-173.
Zimmerman’s concept was to counteract the wingtip vortices with giant propellers rotating in the opposite sense. By reducing the strength of wingtip vortices, he expected to reduce the spanwise airflow of the low aspect ratio lifting surface, and thereby reduce the energy lost in induced drag. With his model airplane, he had already learned that in order to accommodate the very large diameter propellers, the saucer had to sit on its tail with a very large angle of attack in order to prevent the propeller tips from striking the ground, and that some control surface appendages were required to provide control and stability in yaw, pitch and roll, characteristics that the saucer shape alone did not provide. The large angle of attack, necessitated very long undercarriage, and placement of the pilot’s cockpit as high and as far forward as possible. These characteristics carried over to the design of the V-173.
Distinguishing Features of V-173
It is worthwhile to examine some of the design features that distinguish the V-173 saucer from the conventional winged aircraft. The “proof of concept” aircraft constructed by Chance Vought was given the nickname, “the “Flying Pancake”. Its lifting surface was based on the NACA 0015 airfoil. The NACA designation indicates that the airfoil has no camber (00), that its maximum thickness is 15°, and that the maximum thickness occurs at 30% if the chord length. The V-173’s airframe was constructed of wood, and covered with canvas. The large wooden propellers were powered by two 80 hp reciprocating aircraft engines. Because of the extremely large angle of attack required for take-off and landing, long, non-retractable undercarriage struts were required. In flight, the undercarriage drag contributed to a nose down moment that had to be balanced by horizontal control surfaces mounted near the tail, and to minimize undercarriage drag, pants and wheel spats were installed. Wheel spats! An outdated feature from the days before retractable undercarriage were developed, and as outdated as the spats worn by a gentleman of fashion. Even Adolph Menjou had given up the wearing of spats by 1940. For yaw control and stability, twin vertical control surfaces were mounted at the trailing edge, with conventional balanced rudders for yaw control and stability. Why twin rudders? Twin rudders needed to be located away from the centerline, where a single rudder would otherwise be located, since the centerline location would be engulfed in the turbulent wake of the high profile cockpit when flying at a high angle of attack. Roll control and stability were provided by all-moving horizontal control surfaces at the trailing edge. Note that trim tabs and counterbalances forward of the leading edges were necessary to reduce control forces on these appendages. Pitch control surfaces (elevators) were located on the trailing edge, between the rudders.
Implications of High Angle of Attack.
The NACA 0015 airfoil has its maximum lift coefficient at an angle of attack of 16°. Ideally, the pilot would want to affect a stall just prior to touchdown in order to “stick ” the landing with the minimum roll after touch down. That means that, even for the relatively shallow approach angle of 10°, the saucer would have to maintain a 26°angle relative to the ground in order to stay close to the angle for maximum lift, and would have to exceed that angle to stall prior to touchdown. Because of the high angle of attack, the pilot’s cockpit was mounted as high and as far forward as possible. To see the ground while flying at an angle of 26°, the V-173 pilot relied on the view through the window located at his feet.
Carrier Capability
The US Navy in 1942 was looking for a fighter aircraft with carrier landing capability for the war in the Pacific.. The slow-flying capability of the saucer might have been an attractive feature, but practically speaking, they needed a carrier-based aircraft with high speed and maneuverability that would be superior to a Japanese Zero (over 300 mph at 16,000 feet). The V-173 did not address the problems of carrier landings, nor did it possess any of the aerodynamic features that would have given it high speed. The aircraft carriers of 1942 were quite different from the modern nuclear–powered carriers built for jet aircraft and equipped with steam catapults, angled decks, modern arrestor systems, and hydraulic energy absorbers. The use of cables to snare the tail hooks on carrier-based aircraft goes all the way back to the first shipboard landing of an aircraft in 1911. The carriers of 1942 were not much more sophisticated, but relied on means other than sandbags attached to the arresting cables to slow the aircraft. Note that the V-173 was not equipped with a hook, just a tail wheel, and its wooden structure was not designed to withstand abrupt arresting loads introduced at the tail.
Engine Failure
The V-173 had two engines, each driving a propeller. What happens in the event of a failure of one of the two engines or one of the two propellers? The failure could be a mechanical part failure, a fire in the exhaust manifold, or failure to receive sufficient fuel because of a leak. Or the propeller and its pitch control mechanism could fail, or suffer a bird impact. If the pilot is fighting a crosswind on landing, and is executing an intentional sideslip in order to stay aligned with the runway, and while rolled in the direction of the crosswind, he must take care to not let one of those extra large props to hit ground and become damaged. For an aircraft with only minimal pitch and roll capability, it may take an excessive roll angle to oppose even a moderate 20 mph crosswind.
The result of an engine or prop failure on one side would be a sudden loss of lift that would roll the saucer to the failed side. The twin elevators which are intended to act in concert would suddenly contribute to the roll torque, and due to loss of effectiveness on the failed side, a nose-up pitch. The predominant unbalance, however, would be due to the continued thrust on one wingtip while there is a sudden increase in drag on the other wingtip, due to the failure to provide vortex cancelling by the propeller. The result would be a sudden yaw torque. In other words there would be strong and sudden unbalance torques occurring simultaneously about all three axes of rotation. In 1942, aircraft designers did not have available the computers, software logic, sensors, actuators, and autopilots that would detect, make a decision, and take corrective action in less time that it takes the pilot to act. The saucer shape has no inherent stability on its own. Those who doubt the unstable aerodynamics of a saucer should try throwing a paper plate across the room while not allowing it to spin. The saucer will begin to tumble as soon as it leaves your hand.
Engine Redundancy
One cure for engine failure in a twin-engine aircraft is to link the engines together with drive shafts, gearboxes, and clutches so that each engine can drive either or both propellers.. That approach introduces new multiple single-point failure points, and failure chains, considerations such as to drive crazy the unfortunate engineer tasked with reliability analysis. It is for that reason that the Vertol V-22 Osprey is not one of my favorite aircraft/
Design Evolution of the F5U-1
The US Navy was apparently satisfied with the “proof of concept”, because they contracted with Chance Vought to proceed with a combat fighter version. Remember that this was wartime: funding was readily available, Chance Vought the man had been dead for 13 years, but Chance Vought the company was thriving, and building one of most highly-respected of US Navy Fights, the Corsair.
Consider now, the task facing Chance Vought engineers to transform this “proof of concept” aircraft to a combat-ready, fighting machine. First, the canvas covering would not have sufficient strength to resist the high aerodynamic forces of high speed, nor would the wooden airframe have sufficient strength to withstand the high acceleration forces at the extremes of the flight envelope, typically, for a fighter aircraft, +12g positive acceleration, and -6g negative acceleration. Therefore, an all-metal, stressed-skin, semi-monocoque airframe construction would be required. Secondly, to achieve a credible airspeed, in view of the high induced drag not compensated by the reversal of the wingtip vortices, as much as five times greater propulsive thrust would be requires, or engines with five times the horsepower, five times the mass. A carrier-based fighter must have a range of at least 1000 miles to fly a useful combat role. That means hundreds of gallons of fuel at eight pounds per gallon, or another 1000 lb of weight that the V-173 did not carry for its short hops. To carry armament of four cannons, as some Spitfires did, would add another 400 lb, plus another 100 lb for ammunition. Engaged in a dogfight or against flak, you would like to have another hundred pounds of armor to protect the pilot, and when a fighter aircraft performs as fighter-bomber role against ground targets, it would be expected to carry one or more 500-lb bombs.
All of this added weight means more lift, more drag, more thrust, bigger engines, more weight, more lift, etc., in a vicious design circle, a circle that never closes, but spirals upward.
When the design cycle was eventually ended, Chance Vought had a saucer-shaped aircraft that was hopelessly overweight, under-powered, and did not meet any of the US Navy requirements for carrier operation, speed, maneuverability, range, pilot armor protection, or armament. But most seriously, it did not meet the fundamental purpose of all aircraft: it could not fly.
Although Ludwig would certainly not have approved, I am going to coin the phrase, “Prandtl’s Curse”. Remember, you heard it here first.
