Review of Military Technology: History and Development Prospects of Convertiplanes

A convertiplane is a craft with swiveling engines (usually propellers) that function as lift during takeoff and landing, and as thrust during horizontal flight.

20/11/2023 - 11:53

Aircraft with vertical takeoff and landing capabilities, achieving speeds akin to airplanes, have always fascinated writers, designers, and particularly the military. The prospect of flying faster than a helicopter, with all the resulting advantages, and the ability to airdrop anywhere, irrespective of runway availability, led to the creation of the convertiplane – a hybrid aircraft that's part airplane, part helicopter. Find out more in's coverage.

Pros and Cons

Firstly, it's crucial to precisely define what qualifies as a convertiplane and what doesn't. These machines are craft with swiveling engines (usually propellers) that function as lift during takeoff and landing, and as thrust during horizontal flight; lift is typically provided by an airplane-type wing. Usually, the engines pivot with the propellers, but sometimes only the propellers rotate.

However, aircraft with vertical takeoff don't belong to this class as they use different engines for takeoff and horizontal flight.

Clearly, a hybrid that combines the best qualities of its 'parents' should yield to airplanes in terms of flying characteristics, maneuverability, and the ability to hover, similar to a helicopter. Yet, under certain conditions, the convertiplane is indispensable: when landing in regions lacking an airbase or runway, such as developing oil fields, disaster zones, or 'hot' spots.

An experimental comparison was conducted between the characteristics of the Bell V-22 Osprey convertiplane and one of Sikorsky's best helicopters, the S-46. It turned out that the V-22 boasted double the cruising speed, could carry three times the payload, and outstripped the S-46 fivefold in terms of flight range. The tactical radius of the V-22 is 690 km, which means it can be based quite a distance from the front line.

So, the utility of the convertiplane is evident. However, there are several downsides. Firstly, its structure tends to be heavier, and in aviation, every kilogram counts. Additionally, the existence of a critically important complex unit increases the risk of breakdowns and, consequently, accidents. Thirdly, complexity in control. Piloting a convertiplane requires versatile skills, akin to the technology itself, demanding proficiency in both airplane and helicopter handling. Such specialists are not easily trained.

Half a Century of Failures

The idea of creating a convertiplane isn't as new as it might seem. In the 1920s and 1930s, aircraft designers from developed countries started work in this direction.

In 1936, the Moscow Aviation Institute defended the "Sokol" project – an aircraft with a rotating wing. The project's author managed to predict the development of convertiplanes 30 years before the first prototype was built in 1964 when American companies Vought, Ryan, and Hiller developed the military transport tiltrotor XC-142A.

Nevertheless, the most detailed project of a hybrid is considered to be the P.1003 from Weserflug, developed in Germany in 1938. German designers planned to create an aircraft with two wings, the ends of which could rotate to switch between airplane and helicopter modes. However, the idea wasn't realized due to the outbreak of war.

The history of hybrid development also included the work of British designers on the Rotodyne – a convertible helicopter. In 1958, this craft was presented at the Farnborough Airshow, reaching a record speed of 400 km/h for rotorcraft.

Moving further than a simple test program, Canadian company Canadair, in 1965, produced its own hybrid – the CL-84 Dynavert. The aircraft, accommodating 12 people (plus a crew of 2), had a fuselage of traditional round cross-section.

The remarkable feature of the CL-84 was its wings' ability to rotate up to 100 degrees, allowing it not only to hover in place but also to fly backward at a speed of 56 kilometers per hour. Going forward, the craft reached speeds of 500 km/h.

However, the Canadian development was memorable not only for this achievement. Despite crashing after only 145 flight hours, the defense ministry ordered three units of the improved CL-84-1 and even designated it as the CX-84. The military variant was equipped with a decent arsenal: a 20mm cannon, a 7.62mm machine gun, and 19 rockets. Ultimately, this unique hybrid was not put into service.

In the neighboring United States, work on convertiplane projects was in full swing because the country's armed forces promised to purchase anything that passed even preliminary tests. A successful development was the Bell X-22A, equipped not with two but with four engines totaling 1250 horsepower. This convertiplane was the first where propellers, within the short history of such machines,

Flying Solo 

The only development that has made its way into the army's arsenal so far is the American convertiplane Bell V-22 Osprey. Three modifications have been designed: assault-transport, search and rescue, and anti-submarine.

The existence of this unique aircraft came at a high cost for the Americans: the development spanned 25 years, around 30 people perished during testing, and the cost of one machine amounts to approximately $120 million. Within just ten years of operation, six accidents occurred, claiming the lives of seven individuals.

Equipped with two Rolls-Royce T406 engines located at the wingtips in nacelles, capable of rotating almost 98 degrees, the aircraft's propellers with three trapezoidal blades are interconnected by a synchronizing shaft running within the wing. This shaft also enables the aircraft to land on a single engine. To reduce the construction's mass by around 70 percent (5700 kg), the aircraft is made from composite materials based on carbon and fiberglass with an epoxy binder, making it a quarter lighter than its metallic counterpart.

However, the numerous downsides of the convertiplane, such as the complexity of creation, unreliability, high cost, etc., did not escape public notice. Criticism poured in for these hybrids, and Senator John McCain aptly characterized the machine: "The V-22 looks great… when it’s not in the repair shop."

The convertiplane became memorable when, on May 3, 2011, it transported the body of Osama bin Laden, killed during a secret operation, from Bagram Air Base to the aircraft carrier "Carl Vinson."

Today, over 200 aircraft have been produced for the American army. Soon, the "Ospreys" will grace the skies of Japan – the Ministry of Defense of the "Land of the Rising Sun" ordered 21 convertiplanes. Furthermore, hybrids may appear in Israel, the UAE, and Saudi Arabia. These countries are actively interested in American convertiplanes.

Civil Hybrids and UAVs

Currently, American designers are developing both UAV convertiplanes, including strike ones, and civil passenger hybrids.

This year was supposed to see the certification of the first civil convertiplane, the Bell/Agusta BA609. It's a business jet capable of comfortably carrying 10 passengers. However, tests were halted due to the crash of the second flight prototype in 2015. Just a month ago, developers resumed flight tests.

XTI Aircraft has designed the commercial six-seater (5 + pilot) convertiplane TriFan 600. It's positioned as a means of transportation for business people, top-level managers, and those valuing their time. Speed – 640 km/h, flight range – up to 2,575 km. Flight tests for the TriFan 600 are planned for next summer. The projected cost of the aircraft is $10-12 million.

It's safe to assume that the race in convertiplane construction, which started not so long ago, will continue. This year marked the end of the American monopoly in hybrid production. The American UAV Eagle Eye is already facing competition from Russian and Korean developments.

The first flight of a Russian unmanned convertiplane produced by the "Russian Helicopters" holding took place in February. The company's near-term plans include creating a device with a hybrid power unit capable of reaching speeds of up to 500 km/h. The next step was supposed to be creating a convertiplane weighing up to 2 tons. In the future, they envisage developing a fully manned convertiplane capable of transporting people and various cargoes.

Developers from South Korea are not lagging behind. The device named TR60 will take off like a helicopter and fly at a speed of 250 km/h. This high-speed UAV has unique properties: it can carry up to 30 kilograms of payload and fly at an altitude of 4.5 kilometers, distancing itself from the command center by approximately 200 kilometers.

Clearly, the evolution of convertiplanes, like aircraft in general, is moving towards UAVs. It's conceivable that in the distant future, all military aviation will transition to fully autonomous robots, UAV scouts, kamikaze drones, etc. The question remains: will there be a place for convertiplanes in all of this? Will the designers manage to overlook the bitter experience of past trials and overcome the technological complexities in creating airplane-helicopter hybrids, the high cost of machines, reinforce reliability, and establish mass production?



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