Aviation history is littered with hi-tech experiments that took the art of aerodynamics, aircraft size, structure and propulsion to the limit. Most passenger aircraft today have 2 or 4 engines, but in the past aircraft designers were sometimes prepared to experiment with much more complex configurations for insufficient power and/or performance of conventional designs. Some of these planes had strange outlines because they were required to have odd shapes, others were distinctive due to a conscious redistribution of the engines to positions away from where they were normally placed. The outcome was an intriguing array of aircraft, that defied common notions regarding its appearance and performance.

The term “bizarre” has a fascinating origin, having been introduced into the English language sometime between 1640 and 1650, and it may have originated in French and Italian, and ultimately in the Basque language for “beard.” In aviation, however, the term applies to any aircraft that has an unusual appearance, structural features or engine configuration, or simply seems like something that could never have been made up. These designs were not made to be different. They were all associated with some real engineering problem; such as the carriage of huge loads, lowering the drag caused by the air, making an aircraft more visible, or enabling it to fly very slowly.

Some of these experimental aircraft proved to be significant testbeds in the bounds of aircraft design. Some did manage to fly their ideas, some others became prototypes or limited production aircraft. They encompass both the large seven-engine transports and the small rear-mounted propeller fighters, and they all illustrate a way of solving a problem when the traditional aircraft design was no longer possible. What has also been revealed by these weird devices is that technology in the aviation industry often has to be created from the ground up, and require a leap of faith from innovators to consider things that at first might appear impractical.

1. The Seven-Engine Kalinin K-7

The Kalinin K-7 is one of the most interesting cases of extreme multi-engine aircraft construction. The huge plane was built in the 1930s in the Soviet Union and was nicknamed the “Russian Flying Fortress” due to its size and strength. It’s wingspan was over 170 feet, making it large compared to the conventional aircraft of the time. To get such a big airframe off the ground was an extraordinary amount of propulsion.

Propulsion and Operational Characteristics:

• Extreme Multi-Engine Design
• Seven-Engine Propulsion Layout
• Massive Aircraft Capabilities
• Crew and Flight Demands

It is equipped with six tractor engines originally placed in the front section of the wing structure. That massive configuration was still insufficient to deliver the necessary overall power to the design goals of the aircraft, and a seventh engine was added at the rear of the plane in a pusher configuration. This resulted in one of the most unusual propulsion layouts of its time for the K-7. The aircraft spread its engines throughout its body, instead of having them in a single line.

It was a massive operation to operate the K-7, which needed a flight crew of at least 11. It was only about 140 mph, but that’s not what it was known for. It was to be able to accommodate up to 120 passengers, about 15,000 pounds of mail or 112 fully-equipped paratroopers. Though it was quite radical in appearance and with its unusual propulsion setup, test flights confirmed that the seven-engine giant would fly and made an impressive display of bold multi-engine design.

2. The Wooden Giant: Hughes H-4 Hercules 

Hughes’ H-4 Hercules proved another remarkable answer to the challenge of designing for mass production. It was built by the Hughes Aircraft Company during World War II, and is often referred to as the “Spruce Goose.” During the war there was a shortage of aluminium, making manufacturing a big challenge, so wood was used extensively in its construction. The outcome was an immense flying boat whose construction technique was as bizarre as its size.

Construction and Transportation Capabilities:

• Wartime Material Challenges
• Massive Transport Ambitions
• Alternative Construction Approach
• Historic First Flight

The plane would be able to carry many people and heavy military equipment. It was designed for up to 700 passengers or a large load like two 30-ton tanks of M4 Sherman. Its strength was up to 150,000 pounds, and its wingspan grew longer than a football field. The scale of the aircraft presented an opportunity for designers to investigate how they could work with other materials where they were unable to use traditional aircraft manufacturing materials.

The H-4 never flew again, until 1947 when it made a single flight, which turned into one of the most celebrated events in experimental aviation! The aircraft was not put into regular service but it showed the potential of building an aircraft that was very large and of materials not normally used for aircraft. The historic aircraft has been preserved in the Evergreen Aviation Museum, McMinnville, OR, and is still a testament to the vision of that ambitious engineering.

3. The Pregnant Guppy and Oversized Cargo 

As America’s space program expanded, transportation issues arose that were not readily addressed by traditional planes. The components of rockets were very large, fragile and sometimes produced at sites distant from where they were to be launched. The Pregnant Guppy was a plane that was quite a bit different from nearly any other in the air at the time, and Aero Spacelines responded with it. It had a very large body, for which it was built to carry cargo not big enough for standard transport planes.

Space Transportation and Specialized Design:

• Space Program Transportation Needs
• Oversized Cargo Capacity
• Specialized Fuselage Design
• Super Guppy Evolution

The Pregnant Guppy was used from 1962 to 1977, and was capable of transporting very large components required for the needs of America’s space program. The unusual shape was not designed to enhance traditional comfort for passengers or for the appearance of the aircraft. Rather, the large fuselage was used to carry large aerospace equipment. The aircraft’s top speed was around 320 mph and its maximum load capacity was 141,000 pounds.

The idea eventually grew into the Super Guppy which extended the capabilities of its predecessor. The Super Guppy was a specialized workhorse – with a wingspan of over 150 feet and a maximum takeoff weight of over 170,000 pounds – for America’s space industry. It was the smallest aircraft available that was large enough to carry the entire S-IVB stage, the third part of the Saturn V. This peculiar shape was therefore not just a garnish of novelty, but a real engineering necessity.

4. The Divided Cabin of the Bleriot 125 

Another solution to design problems of the aircraft was offered by the French Bleriot 125. The plane was exhibited at the Paris Salon de l’Aeronautique in 1930 and was an unusual design for a passenger aircraft at the time: wooden construction. The Bleriot 125 was similar, but carried its 12 passengers in two very large pontoon-like pods, one on either side of the plane.

Passenger Layout and Flight Configuration:

• Unconventional Passenger Arrangement
• Central Nacelle Configuration
• Push-Pull Engine Concept
• Experimental Flight Performance

Between the two passenger compartments there was a central nacelle, which was the home of the flight crew. This odd central section also featured a push-pull engine arrangement, creating a unique mix of separated “passenger” compartments and unorthodox engine layout. The design was an experiment in the distribution of weight and power throughout the structure of the aircraft, whilst retaining practical passenger-carrying capability.

The Bleriot 125 was intended to have a cruising speed of about 137 mph, but did not perform as well as it was expected to during testing. A single prototype was built, and the aircraft was a curious, but unsuccessful endeavor. Although the aircraft was a never-to-be-flyer, the split seating and midship engine layout demonstrated early aviation designers’ tolerance for unconventional arrangements.

5. Push-Pull Propulsion Takes Shape 

When aircraft with traditional designs were reaching their performance limits, designers started experimenting with different placement of engines and propellers. Power plant placement off the normal wing position may offer aerodynamic drag reduction, better forward visibility or other benefits that standard power plants might not. One of the most interesting ideas was push-pull propulsion, which featured engines and propellers at the opposite ends of the aircraft and operated together.

Engineering Principles and Advantages:

• Rearranging Traditional Powerplants
• Aerodynamic Efficiency Advantages
• Improved Forward Visibility
• Mechanical Design Challenges

This would be a way to have more than one engine in the same plane and not having to have the large engine nacelles side by side along the wings. It was an especially appealing idea for aircraft in which aerodynamic efficiency and the visibility for the pilot were key. Placing one propeller at the front and another at the back allowed designers to have a streamlined fuselage in the middle with a good amount of propulsion.

Push-pull aircraft were some of the most recognisable experimental aircraft ever designed. While the arrangement presented new mechanical and safety issues, it proved to be a demonstration of the idea that the placement of the engine could affect an aircraft’s performance. A number of aircraft did use this concept, including the Dornier Do 335.

6. The High-Speed Dornier Do 335 

One of the most successful push-pull applications was the Dornier Do 335, the “Pfeil” or “Arrow”. The Dornier aircraft was a German heavy fighter developed during World War II, with one propeller at the front and the other at the rear, also known as an inline propeller. This layout was helpful to minimise the aerodynamic drag caused by mounting conventional engines and propellers in front of the fuselage.

Performance and Pilot Safety:

• Streamlined Push-Pull Configuration
• Exceptional Piston-Engine Speed
• Reversing the engine’s propeller to ensure safety.
• Innovative Pilot Escape System

The clean layout helped to assure outstanding performance for an aircraft of its time powered by a piston engine. The Do 335 attained a level flight speed of about 474 mph, and was the fastest German piston-engined aircraft in World War II service. The system of propulsion was therefore an odd one and not just an engineering curiosity. It directly contributed to the aircraft’s objective of high speed, but with the power of two engines.

An important safety issue was also posed by the back propellors. If a pilot were to bail out of the aircraft, he or she might run into the spinning back propeller, which would be a very dangerous way to exit the aircraft. To overcome this challenge, an ejector seat was installed in the aircraft. The Do 335 thus had integrated in one aircraft unusual propulsion, aerodynamic efficiency, and pioneering pilot safety engineering.

7. Sweden’s Rear-Engine SAAB 21 

The Swedish SAAB 21 was a single-seat monoplane fighter that was developed in the early 1940s under a similar concept of propulsion. It was fitted with a distinctive twin fuselage design with the engine behind the pilot and was powered by a Daimler-Benz DB 605B engine. Rather than mounting the powerplant in front of the cockpit, engineers relocated it towards the tail and used a pusher propeller.

Visibility and Emergency Design:

• Rear-Engine Fighter Configuration
• Improved Forward Pilot Visibility
• Pusher Propeller Arrangement
• Emergency Ejection Solution

This provided a great advantage to a fighter aircraft: good forward vision. This would allow the cockpit to offer a much better view in front of the aircraft, without the engine in the way. That visibility might have been especially important during combat when seeing and tracking other aircraft in a hurry was essential.

The propeller was placed at the back, causing the same basic emergency-exit problem that was faced by other pusher type aircraft. The designers solved the issue by fitting the SAAB 21 with an ejector seat so that the pilot would be expelled from the spinning discs and safely into the air. The aircraft proved to be yet another example of the potential that could be offered by an unusual engine configuration, and the solutions to any potential safety issues were just as unusual.

8. Japan’s Radical Kyushu J7W Shinden 

The Kyushu J7W “Shinden” (roughly “Magnificent Lightning”) was an even more radical design by Japanese engineers. The aircraft was the land based interceptor to counter the high altitude B-29 bombers. It really did look like a bold design, as the major wings were set high at the rear, and small canard wings were set forward.

Aerodynamics and Armament:

• Radical Aerodynamic Geometry
• Rear-Mounted Propulsion System
• Concentrated Forward Firepower
• Limited Prototype Development

Another unusual aspect of the design was the propulsion system. A rear mounted pusher engine powered the propeller, the frontsection of the aircraft was designed to accommodate four 30 mm cannons. This configuration meant that the interceptor had a lot of firepower up front, in the nose.

The Shinden was not put into operational service due to the production of only two prototypes before the end of World War II. Despite this, this aircraft is an important example of radical aerodynamic thinking. It was an odd mix of wing surfaces, both forward and backward, canard forward and powered tail surfaces, and concentrated nose-mounted armament, showing the ingenuity of designers who dared to think outside the box in their approach to almost every major aspect of a fighter’s conventional design.

9. The Flying Pancake: Vought XF5U 

The Vought XF5U, commonly known as the “Flying Pancake” or “Flying Flapjack”, brought unconventional aircraft design to a new level! H. Zimmerman, its developer, took the innovative approach of replacing the traditional long, narrow wings with a flat, disc-shaped body. It was quite an unusual sight, so it became one of the most recognisable experimental aircraft concepts ever.

Lifting Concept and Propulsion:

• Disc-Shaped Lifting Body
• Internally Mounted Engine Arrangement
• Wingtip Propeller Configuration
• Experimental Performance Goals

The two Pratt & Whitney WR-2000-2 radial engines were installed within the aircraft’s body, rather than in the traditional external nacelles. The propellers were mounted close to the wing tips leading edges and the engines were used to power them. The design was supposed to allow for fluid flow over the plane and to provide ample strength to the plane. It was a groundbreaking thinking of propulsion and lift.

The XF5U reportedly hit speeds of up to 452 mph, proving that its unnatural form was linked to a viable engineering idea. It was not created for show. This was an experiment in designers to determine if a small, sturdy airframe and wingtip propellers carrying engines would offer any benefit. The project was never made into an actual airplane, but it is a notable case of experimental aviation engineering.

10. The Jet-Powered PZL M-15 Belphegor 

Another outstanding departure from the traditional aircraft design was the aircraft of the Polish PZL M-15 “Belphegor”. It is known as one of the very few production biplanes with a production jet powered aircraft. The M-15 was designed by WSK PZL-Mielec, in response to the requirements of the Soviet Union in the replacement of Antonov An-2 crop-dusting aircraft.

Agricultural Design and Performance:

• Unusual Jet Biplane Design
• Agricultural Aviation Requirements
• High-Mounted Engine Configuration
• Specialized Low-Speed Performance

The plane had its jet engine mounted high over the crew cabin, between the biplane wings and twin tail booms. The layout was a bit atypical, but the size of the wing was useful. In addition, agricultural aircraft had to fly at very low speed when performing their tasks, and the biplane configuration afforded the M-15 greater lift-ability without stalling so readily at low speeds.

The aircraft was nicknamed “Belphegor” for its jet engine’s unique sound. Some 175 aircraft were made until operating costs finally made production stop. Even though the M-15 is not on the market anymore, it is still an amazing case of work done in specialized engineering. This lopsided, multi-winged aircraft with a ‘farm’ plane is an example of one of those oddly-designed creations that results from a very specific operational need.