Most experimental aircraft never make it into production, but many feature revolutionary design elements that regularly end up on military and commercial planes – sometimes decades later.
From fanciful dragons and flying pancakes and pusher-pullers to tail-sitters, the aircraft on this list were among the boldest designs of their day.
That said, some now reside in dusty warehouses and lonely scrapyards, while others have been lovingly restored and occupy prominent positions in aviation museums around the world.
Now – four groundbreaking aircraft of yesteryear.
Dornier Do 335 “Arrow”
Though they’re not so well-known these days, Dornier Do 335 “Arrows” were the fastest piston-powered aircraft in the Luftwaffe during World War II.
Twin-engine planes were common then, but unlike nearly all of its contemporaries the Arrow had its dual power plants housed in the fuselage – one in front of the cockpit and one behind it.
This allowed it to benefit from the power of two engines while retaining the aerodynamic slipperiness associated with traditional single-engine fighters.
The Do 335 V1 first flew in October of 1943, but though performance was encouraging, a number of mechanical and handling issues came to light which were addressed in later variants with improved cockpits and strengthened undercarriages.
Keen to capitalize on the revolutionary design and in dire need of new fighters and interceptors to help turn the tide of the air war in Europe, Adolph Hitler and Herman Goering ordered the Arrow into production after a relatively short development period.
The final production variant first took to the skies in late January of 1944 after only 60 hours of flight time.
Test pilots described the Arrow as fast and reliable, and more than a match for American twin-engine P-38 Lightning in nearly every respect.
Though additional bomber, destroyer and ground attack variants were planned, none actually went into production.
It was expected that more than 2,000 units would be produced, but labor, fuel and material shortages caused by round-the-clock Allied bombardment kept production to less than 50.
But though the Do 335 made its debut during the late stages of the war, its origins dated back decades when company founder Claude Dornier developed a number of non-traditional multi-engine aircraft.
In the Do 26 flying boat he housed the plane’s four engines in two nacelles beside the fuselage in the same push-pull configuration that would later be used in the Arrow.
In addition to power and aerodynamic advantages, this layout counteracted persistent torque issues referred to as asymmetrical thrust, which was caused by multiple engines turning in the same direction.
However with push-pull aircraft, one engine’s torque negates the other’s, which makes for straighter, faster and more efficient flight.
At 45 feet (13.8 m) long and with a nearly identical wingspan, the Dornier was only slightly bigger than other single-engine fighters of the day, but it was much heavier.
The Arrow had an empty weight of more than 16,000 pounds (7,200 kg) and a gross weight of more than 21,000 pounds (9,600 kg).
Early prototypes were equipped with various engines, but later models featured two massive 44-liter Daimler-Benz DB 603E-1 inverted, liquid-cooled V-12s, each of which produced about 1,800 horsepower at takeoff and drove constant-speed propellers nearly 12 feet (3.6 m) across.
Top speed was 474 miles per hour (763 km/h) at 21,300 feet (6,500 m), and even with one engine switched off the Arrow could hit 350 miles per hour (563 km/h).
To put these numbers into perspective, the Rolls-Royce Merlin engines in Mustangs and Spitfires displaced just 27 liters and produced between 1,300 and 1,500 horsepower until the war’s end when more powerful versions were introduced.
With top speeds of 369 miles per hour (594 km/h) and 436 miles per hour (703 km/h) respectively, Dorniers could outrun their adversaries with relative ease, though they didn’t handle as well thanks to their added weight.
Armament included one centrally mounted 30 mm cannon with 70 rounds of ammunition, and two 20 mm cannons mounted in the cowl, each with 200 rounds.
2,200-pound (1,000 kg) bomb loads could be carried in an internal bay as well as from hardpoints under the wings.
In April of 1945 famed French aviator Pierre Clostermann encountered a Do 335 while leading a flight of Hawker Tempests over Germany.
With a top end of more than 430 miles per hour (695 km/h), Clostermann’s Tempest wasn’t a slouch in the speed department, but when he attempted to engage the Arrow it simply accelerated away from him before he could get off a good shot.
After the war at least one Do 335 was shipped overseas for testing and assessment.
In April of 1945 one suffered an engine fire and crashed into a schoolyard near the Royal Aircraft Establishment at Farnborough.
Another is now on display in Chantilly, Virginia at the Steven F. Udvar-Hazy Center.
Bartini Beriev VVA-14
Once envisioned as a state-of-the art Cold War sub hunter, now the sole remaining Bartini Beriev VVA-14 more resembles a long forgotten sci-fi movie prop than an actual flying machine.
Even in its heyday (if it ever had one), the VVA-14 was often referred to as Zmei Gorynich after a fearsome dragon in a Russian folk story, and with three “heads,” stubby wings and two bulbous haunches, it looked the part.
Conceived by eccentric Italian expat Robert Bartini who’d lived in the Soviet Union since the early 1920s, the VVA-14 was a flying boat designed to takeoff and land vertically like a helicopter, then streak across the sea at high subsonic speeds on its quest to find and destroy enemy nuclear ballistic missile submarines.
The letters in its name are an acronym for “vertical take-off amphibious aircraft,” and to achieve such incredible feats the revolutionary aircraft would need 14 engines.
Not surprisingly, this is why it was so expensive, so impractical, and ultimately relegated to the scrapyard in favor of cheaper, easier to produce and more effective traditional aircraft.
Meant to be little more than a preliminary testbed in a long research and development process, the first Beriev had a crew of three, was approximately 85 feet (26 m) long, had a wingspan of nearly 100 feet (31 m), and weighed about 52,000 pounds (23,300 kg) empty.
However, due to huge fuel and ordinance loads and sophisticated sub hunting equipment, later models were expected to be at least twice as heavy
To get the big birds airborne vertically, each would be fitted with 12 turbofan lift engines producing 10,000 pounds of thrust a piece.
Then once aloft, forward thrust for horizontal flight would be provided by two larger Soloviev D-30M turbofan engines producing 15,000 pounds of thrust each.
Top speed was expected to be in the neighborhood of 475 miles per hour, (755 km/h) with a relatively efficient cruising speed of about 400 miles per hour (640 km/h) and a range of approaching 1,500 miles (2,400 km).
Designers tinkered with rigid and inflatable pontoons, folding wings and multiple power plant configurations, but when the first prototype flew in September of 1972 it took off from a conventional runway because its lift engines hadn’t been installed.
In fact, between 1972 and 1975 only 107 test flights were recorded totalling just over 100 hours, none of which came anywhere close to achieving the aforementioned performance goals.
In the end only the first prototype ever actually flew, and when Bartini died in 1974 his dream for a high-speed VTOL sub hunter/killer perished with him.
Additional testing was performed immediately after his death, but Soviet military brass realized that the VVA-14’s effectiveness would be limited, that it’s payload and performance projections were hopelessly unrealistic, and that the cost to produce it would be prohibitive.
Of the two remaining examples one was sent to the Central Air Force Museum near Moscow in the late ‘80s, and the other was salvaged for scrap.
The former can still be seen on the grounds outside the Central Air Force Museum in a state of advanced decay, ironically emblazoned with Aeroflot numbers and logos, though it thankfully never flew in a civil aviation role.
Vought XF5U “Flying Pancake”
During the Second World War the Vought company produced an odd shaped experimental fighter for the US Navy called the XF5U “Flying Pancake.”
Whereas most aircraft consist of distinct fuselages, wings, cockpits and tail surfaces, the XF5U’s designers sought to incorporate these elements into one relatively unified shape.
It was a big gamble during the war when resources were scarce and time was of the essence, but the design promised a multitude of benefits that were nothing short of groundbreaking for their day.
The disc-shaped aircraft was what was referred to as a lifting surface, or in contemporary jargon, a lifting body.
In traditional airplanes, wings provide nearly all lift, but in lifting bodies the entire underside of the aircraft does, which means that wings can be shorter and narrower and have smaller surface areas.
Another benefit of this design was that the XF5U was more robust than traditional aircraft which meant that it could withstand higher G loads – a huge bonus in dogfighting and ground attack.
The XF5U was derived from an earlier smaller prototype, but the promise of low stall speed, high angles of attack, good visibility, and enough lift and power to carry a hefty weapons load remained.
Two 1,400+ horsepower Pratt & Whitney XR-2000-2 radial engines buried in the stubby round wings drove large propellers at the wingtips.
At 28 ft 7 in (8.73 m) long and with a wingspan of 32 ft 6 in (9.91 m) the plane was narrow, but with a maximum takeoff weight of 18,700 pounds (8,500 kg) it wasn’t light.
The pancake’s top speed was estimated to be 452 mph (727 km/h) at 28,000 feet (8,530 m), but its 710 mi (1,142 km) range and 34,500 ft (10,516 m) service ceiling weren’t particularly impressive considering twin engine P-38s had actual top speeds of about 420 miles per hour (670 km/h) and could climb to 46,000 feet (14,000 m).
Proposed armament included six-.50 caliber machine guns and a bomb load of about 2,000 pounds (450 kg).
Though initial signs were encouraging, test flights at Vought’s facility in Connecticut revealed a number of inherent problems, the most troubling of which were rumbling vibrations that frequently and unexpectedly coursed through the airframe.
To counteract wingtip vortices that created drag and torque, engineers replaced the original propellers with ones taken from the company’s F4U-4 fighter which resolved a number of issues.
An ejection seat and more powerful engines were added later, but timing wasn’t on Vought’s side.
Of the two prototypes built, only one – XF5U-1 – actually flew, and its future was bleak.
In short, the writing was on the wall.
The design would require years of additional development at huge cost, and even more importantly, everyone knew that jet engines would power the warbirds of the future.
By 1947, over budget and way off schedule the project was axed and the prototype was sent to the Smithsonian in Washington DC.
The second wasn’t so lucky.
It was demolished with a wrecking ball and sold for scrap.
Convair XFY “Pogo”
Known as a “tail sitter” for the way it took off and landed vertically on its rudder and elevators, and “Pogo” for the way it hopped up and down on initial test flights, the Convair XFY was one of the boldest experimental aircraft of the 1950s.
Featuring delta wings and massive three-bladed propellers driven by a powerful turboprop engine, in many ways it promised to be the specialized fighter of the future.
After World War II developing VTOL aircraft was a top priority for many of the world’s navies and air forces.
As such, the Pogo was designed to protect fleets and convoys that didn’t have the benefit of carrier protection.
Originally intended as a fighter, interceptor and reconnaissance platform capable of operating from small warships, development began in early 1951 when the US Navy awarded contracts to Lockheed and Convair.
Though both companies were required to build two prototypes each, due to cost restraints and engineering difficulties they produced only one each.
The single-seat Pogo was 32 ft 3 in (9.83 m) long and had a wingspan of 27 ft 8 in (8.43 m).
Though heavy with a maximum takeoff weight of 16,250 pounds (7,371 kg), power was more than adequate thanks to its Allison turboprop that generated more than 5,000 horsepower, or nearly three times as much as the average piston-engine fighter of World War II.
And future variants were slated for even more muscle – some of which may have had new engines with 8,000 horsepower.
Power surged through the driveshaft to two contra-rotating six-bladed Curtiss-Wright propellers that measured 15 feet (4.8 m) across.
This combination counteracted the engine’s torque and moved enough air to lift the stubby machine skyward slowly before it would transition to horizontal flight and accelerate to a top speed of nearly 475 mph (763 km/h) at 15,000 ft (4,572 m).
Range would be about 500 mi (800 km), and at about 1,000 feet per minute (50 m/s) rate of climb was expected to be exceptional by propeller aircraft standards.
Armament would include four 20mm cannons and up to four dozen 70 mm rockets.
But like most experimental aircraft, the Pogo had a few major problems that couldn’t be overcome.
Takeoff wasn’t particularly difficult under controlled conditions, but landing was.
Pilots had to look over their shoulders while judging the distance to the ground and feathering the throttle to ensure a controlled descent.
This proved to be a hair-raising task even under the best of circumstances, and since the Pogo was destined to operate from tiny pads on rolling ships susceptible to strong crosswinds, it was eventually deemed nearly impossible.
The first tethered test flight was conducted in April of 1945 inside of a hangar at the Navy’s Moffett Field facility in Mountain View, California.
Never before had such a large, powerful manned aircraft attempted to takeoff and land vertically, and tensions ran high.
The plane was secured with multiple tethers to prevent it from surging upwards, falling backwards or teetering over on its side, all of which were likely since the design was largely untested.
The Pogo did manage to get a few meters off the ground without incident, and over the following month test pilots logged almost 60 hours of air time before additional outdoor tests were undertaken.
The most successful of these took place in early August of 1954 when a test pilot reached the staggering height of 150 feet (50m).
Four days later the first transition to horizontal flight was achieved, but it was determined that once the powerful aircraft got moving it just didn’t want to slow down.
In the end however, it was the treacherous nature of vertical landings and its lackluster flight performance that doomed the Pogo.
In the age of fighter jets capable of exceeding Mach 2, its subsonic top speed was a huge disadvantage, and it would’ve cost a fortune to develop.
The aircraft was flown to an altitude of almost 10,000 feet (3,000 m) in February of 1955, but the program was officially cancelled the following year.