The Second World War is mostly associated with the roar of fighter aircraft and the destructive impact of strategic bombing raids. All of these facts were recorded in numerous books and documents due to their fast action and immediate impact on the formation of air superiority on each front line. But behind all the chaos and fighting, there was the quiet revolution that happened in the field of aviation. Rotorcraft, or helicopters, were already becoming real aircraft instead of experimental drawings and projects. At that time, helicopters had not yet been widely recognized as practical instruments of military warfare. Nevertheless, engineers solved all the necessary tasks and invented new methods of helicopter construction, which included stabilizing the helicopter during flight and distributing the engine power properly.

Still, the urgency of the Second World War forced experts to experiment in the field of helicopter building. Therefore, such countries as Germany, the USA, and the Soviet Union started developing prototypes of helicopters with various capabilities, including the purpose for transport, reconnaissance, and rescue missions.

This era is important as it can be regarded as the origin of helicopters. Even though those helicopters could be fragile, have very limited flight range, or be unable to complete their missions due to some technical mistakes, they provided valuable data on different rotorcraft design solutions. Thus, this period can be considered as the beginning of rotorcraft evolution, the results of which we can see in current military aviation today.

1. Doblhoff WNF 342

One of the most interesting Austrian wartime helicopter projects was the Doblhoff WNF 342 project. This helicopter prototype did not use the gearbox and drive shaft but relied on rotor tips for creating a lift during the flight with the help of combustion process in them. Such an innovative project aimed to create a new, efficient helicopter that would have much simpler rotor mechanics and would not require tail rotor installation.

Tip-Jet Propulsion Development Features:

• Rotor tip jet powered rotation system.
• No mechanical gearbox or transmission.
• Eliminated need for tail rotor design.
• Experimental wartime Austrian prototype.
• Focus on simplified rotor mechanics.

However, such an alternative method of creating a lift had a number of disadvantages. First, it consumed large amounts of fuel and created a lot of noise and heat due to the engine. Still, experiments with this helicopter conducted from 1943 till 1945 proved the possibility of hovering and flying in a controlled way using this type of rotor. Therefore, it became clear that new helicopter rotor technologies can work and be implemented in practice in the future. Even though the WNF 342 project did not get into the mass production and operational use stage, its design became the starting point for further studies in rotorcraft technologies.

2. Flettner Fl 265

Another interesting wartime German helicopter prototype was the Flettner Fl 265. Anton Flettner created this project to overcome stability difficulties related to using only one rotor. He decided to introduce a double rotor system into his helicopter to solve this issue. Both rotors were rotating in opposite directions but could overlap without causing any damages due to counter-rotations of their torques. This helicopter became a breakthrough in helicopter engineering at the times when it was created.

Synchropter Design Innovation Points:

• Intermeshing dual rotor configuration.
• Counter-rotating torque balancing system.
• No tail rotor required for stability.
• Compact single pilot cockpit design.
• BMW radial engine powered operation.

Tests of this helicopter showed its outstanding stability and hovering capabilities. Moreover, landing the helicopter was relatively easy due to the stability of flight and controllability. Therefore, it attracted the attention of military authorities who were ready to evaluate the helicopter as a potential combat tool. Though the Fl 265 project did not go to mass production, its results provided valuable information that could be used in further rotorcraft design.

3. Rotabuggy

The Rotabuggy was one of the most unusual military projects of the Second World War that consisted in converting a regular jeep into a rotor aircraft. As its name says, the project used a regular car with additional rotor equipment installed on it. It aimed at the creation of simple aircraft that would perform aerial transport missions. However, unlike other projects discussed below, the Rotabuggy did not rely on a special motorized rotor system but used the concept of a rotor kite vehicle.

Rotary Glider Concept Features:

• Jeep converted into rotor kite vehicle.
• No onboard engine or propulsion system.
• Towed launch system for takeoff.
• Free-spinning rotor generated lift.
• Experimental airborne mobility concept.

Initial testing of the prototype was hard since it was quite difficult to stabilize and control the vehicle when it was being towed from the ground. Still, the further trials revealed that such aircraft could fly in the air easily. During airborne tests, it was revealed that this helicopter was capable of controlled gliding after the takeoff. The results of this experiment were not promising, and the developers of the Rotabuggy had to abandon the project and concentrate on the creation of conventional gliders instead.

4. Focke-Achgelis Fa 330 “Bachstelze”

The Focke-Achgelis Fa 330, nicknamed “Bachstelze,” was a German invention used in naval warfare for reconnaissance purposes. It was not a conventional helicopter with a power system but the towed rotor kite. It meant that the helicopter lifted one person high into the air in order to extend visual range for submarines that carried this helicopter. The compact nature of this vehicle made it possible to install it on the board of a submarine and use it at sea.

Submarine Reconnaissance Features:

• Towed rotor kite observation system.
• Extended visual range for submarines.
• Compact foldable onboard storage.
• Single observer lightweight cockpit.
• Used in naval reconnaissance missions.

Once it was launched into the air, the rotor would start spinning due to the forward motion of the submarine itself. Such a helicopter allowed a person inside to have a wide perspective on the surroundings. Still, deploying the helicopter could be done only under favorable weather conditions as well as retrieving it. Nevertheless, this helicopter prototype was successfully used in some missions as it could detect approaching ships in advance. The Fa 330 helped to confirm that helicopters could be used for conducting reconnaissance missions during naval battles.

5. Vought-Sikorsky VS-300

Perhaps, the most important contribution to the field of helicopter construction was the VS-300 project developed by Igor Sikorsky. This helicopter was the foundation for current helicopter technologies. First of all, it consisted of a classic configuration with a single main rotor and a tail rotor for overcoming torque effects on the helicopter. The latter issue was a significant obstacle to the widespread helicopter usage that was overcome by Sikorsky.

Foundational Helicopter Design Features:

• Single main rotor lift system.
• Tail rotor torque control mechanism.
• Stable hovering and forward flight.
• Multiple experimental design iterations.
• Basis for modern helicopter layout.

Finally, Sikorsky was able to develop a helicopter model that allowed for stable hovering and forward flight after several experiments with its design. It means that the helicopter became a viable flying aircraft at that time. Even though it could not be used on a massive scale due to the war, this project was crucial in helicopter designing. The VS-300 became the predecessor of the majority of current helicopter designs due to the creation of such an innovative system for rotor flight control.

6. Kamov A-7

Another Soviet Union rotorcraft project worth considering was the A-7 helicopter. It was the first Soviet attempt to construct a helicopter. Still, it was not a helicopter per se but an autogyro with a rotor whose movements were induced by air currents rather than a motor system. Thus, the autogyro had the same disadvantages as other autogyros, namely, the impossibility to hover in the air. In addition to that, it had a very small payload capacity.

Early Military Rotorcraft Features:

• Autogyro based rotor lift system.
• Used for reconnaissance missions.
• Short takeoff and landing capability.
• Light armament integration tested.
• Early Soviet rotorcraft development.

Despite all those disadvantages, this helicopter model saw its first military usage in the 1941. It was used for reconnaissance flights over the battlefield and artillery spotting. Moreover, the helicopter was equipped with some light guns to test whether it could be used for performing military combat missions as well. The experience gathered while using the Kamov A-7 in military missions was the starting point for further helicopter design.

7. Focke-Achgelis Fa 223 Drache

One of the best helicopters of the Second World War was the Fa 223. It was a twin-rotor helicopter that had a lot of unique features distinguishing it from other helicopters that were constructed at that time. First, it could carry substantial cargo, which could not fit into helicopters with one rotor. Also, it was able to operate in extreme environments as its construction provided such an opportunity.

Advanced Twin-Rotor Capabilities:

• Twin rotor lift and stability system.
• Heavy load transport capability.
• Mountain and rescue mission operations.
• Versatile military support roles.
• Advanced wartime helicopter design.

Nevertheless, this helicopter became one of the first victims of the war as a significant part of the plant responsible for its production was bombed. As a result, the Fa 223 could not become widely-used and was produced in small numbers. However, the few helicopters of this type operated efficiently performing various military missions, including transportation and rescue operations. It is believed that one of these helicopters performed the historic flight across the English Channel. Thus, this helicopter project can be considered as an example of the perfect helicopter design.

8. Flettner Fl 282 Kolibri

Another German project of the Second World War worth mentioning was the Fl 282 Kolibri. As we mentioned above, this helicopter inherited the most distinctive feature of previous helicopters developed by Flettner the dual rotor system, which made it easy to operate. The most impressive thing about this rotorcraft was that it could operate from a ship deck.

Shipboard Helicopter Innovation Features:

• Intermeshing rotor synchropter design.
• Compact naval operation capability.
• Stable flight in rough conditions.
• Designed for reconnaissance missions.
• Early ship-based helicopter use.

It is believed that this helicopter was developed for naval missions, and some models even managed to prove this assumption while serving. They could operate in adverse weather conditions and perform the assigned reconnaissance missions perfectly. Therefore, we should state that this rotorcraft project opened a new era in helicopter aviation, namely, naval missions.

9. Sikorsky R-5 / H-5

The helicopter designed by Sikorsky can be described as a successor of the previous Sikorsky helicopter the VS-300. This helicopter model is distinguished by increased engine power and the payload capability that made it more suitable for various missions. Moreover, this helicopter had multiple roles, and therefore, it was useful in a number of situations.

Improved Utility Helicopter Features:

• Increased engine power output.
• Higher payload carrying capacity.
• Multi-role operational flexibility.
• Rescue and medical evacuation use.
• Improved flight stability systems.

As this helicopter was developed after the Second World War, it saw its usage in rescue and military transport missions. Thus, it successfully completed a number of missions in remote areas where traditional aircraft could not land. This helicopter was the predecessor of other utility helicopters used in various missions, including the Korean War.

10. Sikorsky R-4

The first operational helicopter ever produced can be named the R-4 developed by Sikorsky. It represents a simple design and is not as complicated as modern helicopters. Nonetheless, it can be named as a starting point for the entire helicopter aviation as this model was successfully used during the Second World War.

First Operational Helicopter Milestones:

• First mass-produced helicopter design.
• Wartime rescue and transport missions.
• Medical evacuation capability introduction.
• Supply delivery in remote locations.
• Proven real-world operational reliability.

Among the major achievements of this helicopter, there was the first combat-zone medical evacuation and resupply missions accomplished using this helicopter. These missions were highly important since they were performed in difficult conditions and showed that helicopters can be used in real combat missions.