Top 10: extraordinary experimental aircraft

These are an ongoing series of extraordinary aircraft existing at the extreme cutting edge of technology.

The “X-planes” are radical, often bizarre, machines that push the boundaries of what is possible to an astonishing degree, sometimes at the cost of test pilots’ lives.

We have chosen a diverse bunch of these exciting aircraft (some excellent and some terrible) to illustrate the incredible achievements of these remarkable aircraft. The order and selection are somewhat arbitrary, but we had to start somewhere…

10: Boeing X-32

Few, if any other experimental aircraft have become a popular meme, but the X-32’s air intake’s resemblance to a cheerful grin has earned the Boeing X-32 a surprise cult appeal that far outlives its flying career of 2000-2001. The X-32 was built as a concept demonstrator aircraft for the programme that eventually led to today’s Lockheed Martin F-35 Lightning II.

One of the hardest tasks the X-32 B-variant had to attempt was vertical take-off and landing. Whereas the rival X-35 used vectored thrust and a lift fan, the X-32 came with a different solution: the main exhaust vectored thrust was combined with two lift nozzles in the fuselage.

The X-32 proved a troubled design. The basic configuration was different to what a notional F-32 would be, which did not impress the Department of Defense. The aircraft was also overweight and underpowered, and vertical take-off required the prior removal of a section of the aircraft.

The rival Lockheed Martin X-35 was selected, leading to the F-35 Lighting II in service today. Though unsuccessful, the Boeing X-32 was a charismatic and intriguing design that offered a very different approach to the creation of a supersonic stealthy fighter bomber with a Short Take-Off Vertical Landing (STOVL).

9: Grumman X-29

Today, almost every aircraft that travels faster than 500 mph has a swept-back or delta wing. However, this isn’t the only solution to high-speed flight: the swept forward wing offers several advantages (for the same given wing area), among them a higher lift-to-drag ratio, better agility, higher range at subsonic speed, improved stability at high angles of attack, and a shorter take-off and landing distance.

In 1977, Thanks to the advent of stronger materials, the US Defense Advanced Research Projects Agency (DARPA) returned to this promising idea, and along with the U.S. Air Force Flight Dynamics Laboratory (now the Air Force Research Laboratory) issued proposals for a research aircraft designed to explore the forward-swept wing concept.

To save time and money, the Grumman company proposed an X-29 which upcycled many parts from other existing aircraft, and it defeated a rival bid from General Dynamics with a modified F-16. The two X-29 aircraft were flown at NASA  Ames-Dryden Flight Research Facility (now Armstrong Flight Research Center) at Edwards Air Force Base, California, from 1984 to 1992.

The forward-swept wings were mounted well back, with canards (horizontal stabilizers to control pitch) in front of the wings. The X-29 demonstrated dazzling manoeuvrability, supersonic performance and an impressively light structure. Air moving over the forward-swept flows inward toward the root of the wing instead of outward (as on a conventional aircraft). This reverse airflow kept the wing tips and their ailerons from stalling at high angles.

8: Convair X-6

It is a mind-blowing fact that America made great efforts to create a nuclear-powered aircraft strategic bomber between 1946 to 1961, spending the equivalent of well over 10 billion dollars doing it – and some estimates are much higher still. Without reliance on liquid jet fuel, it was hoped such aircraft could fly for weeks on end providing a constant state of readiness and limitless range to hit any target.

The Convair X-6 was a proposed x-plane converted from the NB-36H to mature the nuclear-powered bomber technology. The Convair NB-36H was an experimental aircraft that carried a nuclear reactor but did not use it to power the aircraft, instead it was there to test the safety of its shielding and airborne operation.

The extremely alarming NB-36H flew for a total of 215 hours over 47 missions, during 89 of which the reactor was operational. The experiments took place between September 17 1955, and March 1957 over New Mexico and Texas.

In 1961 President Kennedy cancelled the entire programme citing the immaturity of the technology and great costs involved. The Convair X-6 itself was never to fly, which many people, considering the catastrophic risks in a potential accident, may consider a good thing.

7: Ryan X-13 Vertijet

One approach to vertical take-off and landing was the ‘tail sitter’. This is an aircraft that takes off (and often lands) with its nose in the air and tail on the ground. It is a tricky technology, requiring an awkward (and potentially deadly) transition from horizontal to vertical flight. Another difficulty for a tail-sitter is the position and view of the pilot.

The delta (triangular) winged aircraft used a British Rolls-Royce Avon RA.28 turbojet. Pitch and yaw control in the hover was achieved by vectored engine thrust, whereas roll control was provided by small "puffer" jets. The aircraft first flew on December 10, 1955, and during testing achieved a rather modest top speed of 350mph.

The X-13 was more successful than its turboprop tail-sitting brethren but was championing the wrong approach. In an attempt to promote the aircraft, Ryan arranged for the X-13 to cross the Potomac River and land at the Pentagon.

The thrust-to-weight ratio of 1.48 was extremely impressive, and one of the most dramatic in history. To put this into context, today’s powerful F-22 Raptor fighter has a maximum thrust-to-weight ratio of around 1.25.

6: Bell X-22

One way to achieve vertical take-off and landing is to direct the thrust by swivelling your entire engine. Aircraft with propellers mounted in wingtip swivelling ducts have been a staple of science fiction for some time, but they are even older in reality dating back at least as far back as the X-22 of 1966.

The three-bladed propellers of the X-22 were mounted on four wings. The propellers were driven by four gas turbines mounted in pairs on the rear wings. Control was achieved by tilting the propeller blades and the use of control surfaces (elevators and ailerons), which were located downstream of the propellers.

The Bell X-22 was created to explore the plausibility of a fast VTOL troop transport, and controllability issues of wingtip swivelling ducts. It first flew on 17 March 1966. It achieved a top speed of 310 mph (515 km/h) which is far faster than a conventional helicopter but slightly shy of the target speed of 326 mph (525km/h).

Today the militaries of the US and Japan operate the Bell Boeing V-22 Osprey. The Osprey is not a tilt-wing design, but a tilt-rotor design in which only the engine moves. There were several reasons why the tilt-rotor has been more successful than the tilt-wing one being the ‘sail effect’ of the tilted wing during take-off and landing is very hard to control.

5: Douglas X-3 ‘Stiletto’

On looks alone, the extraordinary sleek Douglas X-3 appears to be the fastest aeroplane ever made; its elongated needle-like nose and minute wing suggest an astonishing swiftness. Indeed, it was built to be one of the fastest aircraft in the world, studying prolonged flight at Mach 2+.

Looks, however, can be deceptive, and the X-3 was able to achieve less than half its intended top speed. Rather than Mach 2+ it failed to even reach Mach 1 in level flight, largely due to it being chronically underpowered. Unlike the earlier Bell X-1 and X-2, the X-3 was to use jet engines, but this would cause problems.

Engine developmental issues resulted in the X-3 having 18 per cent less power than required. The original planned engine type, the Westinghouse J46, grew too large in diameter across its development and was replaced with a smaller weak engine type, the Westinghouse XJ34-WE-17.

Though it failed in its goals, the X-3 did at least pioneer the use of titanium in major aircraft structures – and, less laudably, the achievement of one the fastest take-off speeds of 260mph. A useful spinoff of this high-speed take-off was that it inspired a new generation of tyres of greater heat resistance!

4: Hiller X-18 ‘Propelloplane’

Eliminating the need for a runway gives an aircraft an enormous degree of flexibility, allowing operations wherever required. While a helicopter can take off and land vertically it is far slower than an aeroplane. The search for a faster form of vertical take-off and landing (VTOL) has been approached in many different ways.

One way to affect vertical flight is to rotate the entire wing, including its engines, 45 degrees, a concept known as a ‘tilt wing’. To save money, many parts were scrounged from other aircraft and projects, its massive coupled turbofan engines were taken from a cancelled VTOL fighter project. Control in pitch came from  a Westinghouse J34 jet engine.

4: Hiller X-18

The X-18 first flew 24 November 1959 and two issues with the concept were soon abundantly clear. A large vertical wing acts like a boat sail, affecting controllability in the landing and take-off cycle, and if one set of coupled engines were to fail, the result would be disasterous. The solution to the latter problem would be cross-linking both sets of engines to both propellers, a complex solution used today the the V-22 Osprey.

The X-14 was not the first attempt at a tilting, with the German Weserflug P.1003 concept predating the aircraft by some 21 years. The X-14 would also not be the last, the tiltwing concept returned in the 2018 Airbus A³ Vahana, a technology demonstrator for semi-automated flying cars.

3: North American X-15

The US series of ever faster-crewed X-planes reached its zenith with the spectacular North American X-15, an unparalleled aircraft that remains the fastest human-crewed aircraft ever made, despite the record being made in 1967.

Carried to height nestled beneath the wing of its NB-52 mothership, it was released at 45,000 feet altitude at a speed of around 500mph. It was then that its rocket propulsion would push the aircraft to speeds above Mach 6 and altitudes greater than 100 kilometres.

The XLR-99 rocket engine used in most X-15 flights generated 57,000 pounds of thrust, more than two Eurofighter Typhoon engines in full afterburner. It ran on anhydrous ammonia and liquid oxygen.

The rocket only fired for 80 to 120 seconds, after that there was an 8- to 12-minute flight without power ending in a 200-mph glide landing. The X-15 was a hypersonic research programme and lessons learned from it aided the development of the Mercury, Gemini, Apollo and Space Shuttle programmes.

2: X-62 VISTA (Variable Stability In-flight Simulator Test Aircraft)

The versatile F-16 fighter has often found itself working in the realm of research. Over the years, F-16s have been fitted with an odd assortment of wings, control services, inlets, nozzles and ‘black boxes’. The X-62 is one of the latest in this long line of research F-16s.

Starting as the NF-16D conversion of the F-16, it had a multi-axis thrust vectoring (MATV) engine nozzle that allowed a high degree of active control of the aircraft in a post-stall situation. Put in simpler terms, the aircraft’s thrust could be steered to enable controlled flight in seemingly impossible directions (or attitudes to the direction of flight). This could be used for a decisive edge in a dogfight but has other interesting uses.

2: X-62 VISTA

The aircraft could even be programmed to fly in similar ways to many other types, a great training tool. By obeying different control laws the aircraft can be made to have the manoeuvring characteristics of a variety of types, including yet unflown or enemy types.  

Later in its career, it was rechristened the X-62 to fully acknowledge its contribution to cutting-edge research. It returned to explore the autonomous piloting of an aircraft by artificial intelligence. In a historic moment the X-62, piloted by artificial intelligence ‘fought’ other US fighters in a simulated dogfight, and did extremely well.

1: Bell X-1

Second world war piston-engined aircraft could be dived at speeds approaching the speed of sound (around 760mph at sea level) but found some odd unpredictable, and dangerous, characteristics at such speeds. A media misunderstanding of this created a popular idea of a seemingly untraversable speed, the so-called ‘Sound Barrier’.

The new technologies of jet and rocket propulsion made supersonic flight possible, indeed German V2 rockets had flown at 3580mph during the war. What was needed was a crewed aircraft to prove that supersonic flight was safe for a person, and to fully understand the aerodynamic peculiarities.