Helos are a nightmare.

/r/AskEngineers is your best bet unless you want to ask over at /r/engineering or even worse /r/AerospaceEngineering

Helicopters have more moving parts, greater complexity, and far greater power requirements when compared to the equivalent fixed wing airplane. Helicopters weigh more because of all these parts, and then need more powerful engines, and that led to some early helicopters simply not having the power to leave the ground, even when the mechanics had been worked out. Science had to progress a little further for rotary wing to literally get moving. Even after helicopters flew, they were limited by their cargo capacity to very small loads, while fixed wing airplanes were orders of magnitude more capable. Fixed wing planes went through a similar period of non-recognition, and it took Billy Mitchell sinking a number of captured naval ships after WWI for a more widespread understanding of what flight could accomplish. Military helicopters were used for medical evacuation in the 1950s, and became iconic in the 1960s and 1970s; for instance Apocalypse Now.

Some specific issues helicopters have with flying are: -Rotor Downwash: Helicopter rotors have a positive camber, more so towards the end of the blades, and a positive angle compared to the direction of rotor movement. When the rotors create lift, they also induce a downwash, air moves from the top of the aircraft to the bottom of the aircraft. Beyond blowing objects below the helicopter around, this down flow of air lowers the angle of attack of the rotor blades, reducing the amount of lift they have. However, the physical angle of the rotor blade remains the same and still increases the induced drag of the rotor. This phenomenon is worse in a hover, and decreases as the helicopter moves faster. Above a 'translational lift' speed, each rotor blade has moved into new air that is not flowing down relative to the helicopter, which reduces power requirements.

-Asymmetrical Lift: When a helicopter flies forward, the rotor blades are moving at different speeds relative to the airflow. The advancing blade, as it's moving at the same speed as the helicopter, sees faster airflow and greater lift. The retreating blade, moving in the opposite direction of flight, sees a slower airspeed. This imposes an absolute limit on helicopter speed, as the aircraft speed, plus the very tip of the rotor can not exceed the speed of sound without greatly increasing drag and losing control. Helicopters compensate by Flapping and Hunting.

Flapping rotors physically move UP and DOWN as they rotate, with the retreating blades, which normally produce less lift, falling, which increases the angle of attack of that blade relative to the wind and increases lift. The advancing blade rises, which lowers the angle of attack, which lowers lift.

Hunting is the forward and aft motion of the helicopter blade. The advancing blade bends backwards, effectively slowing its speed, and the retreating blade bends forward, speeding it up. Both of these actions combined aim to balance lift laterally on the helicopter.

High amplitude vibrations can cause physical damage to components. Vibrations can be lateral, caused by a mass imbalance of the rotor blades. If the center of mass of the blades is offset from the center of rotation, everything bounces around like a cheap ceiling fan.

Vertical Vibrations are caused by an imbalance in lift between individual rotor blades. While there are differences in the same rotor blade depending on it's phase (advancing, retreating etc.), there can be differences in the multiple rotors themselves. A difference in construction, camber, physical twist, causes rotors to fly differently, and bounce the aircraft up and down.

The solution is that everything has to be delicately balanced within grams. That takes fine manufacturing tolerances, and fine instrumentation to detect anomalies, and correct them, not only at the factory, but in routine operation.

Method of control: While Longitudinal and Lateral control in an airplane vary the angle of ailerons and elevators, which a single control influencing a single axis, with engine power driving the plane forward it is the rotors of a helicopter that influence both axis of control as well as power, and do so by different the amount of lift provided by each rotor blade throughout the phase of rotation.

When you integrate a trig function, it shifts 90 degrees. Likewise, the input to helicopter controls must happen 90 degrees prior to their greatest influence. Control is maintained by moving the rotor disk's direction of lift, and the 'Cyclic' controls have to be twisted 90 degrees to make that happen.

Simultaneously, the power controls raise of lower the lift of the main rotor equally at all points of rotation. So the total input on each rotor blade depends on where it is in rotation, the collective/power controls, as well as the cyclic controls.

The speed the rotors spin at is important. Too fast causes damage; too slow causes a lack of control and an eventual crash. There needs to be a system to maintain proper speed. The simplest system is a pilot controlled throttle, but more powerful, faster control systems reduce pilot workload and increase safety and flight envelope.

Controls: It's more difficult to fly a helicopter because it's unstable. Each control input influences each other control input. To take off, you raise the collective, increasing the pitch of each rotor blade which increases lift and increases drag. As drag increases, the blades slow down, requiring increasing the throttle on the engines to maintain rotor speed. As the torque from the engines increase, the airframe twists in the opposite direction, requiring an anti-torque input from the tail rotor. Increasing the lift from the tail rotor provides anti-torque, stabilizing the un-commanded yaw from the engine, but also increases drag requiring an additional engine output. The anti-torque also has a lateral component, pushing the aircraft sideways. This unintentional sideways thrust from the tail rotor is balanced by on opposite input from the cyclic. As each rotor blade passes 90 degrees away from the direction of intended motion, the twist of the rotor decreases in order to decrease lift. 90 degrees afterwards, while the lift of that rotor in that position has now increased, the blade's deflection is greatest, and the entire lift vector of the rotor disk has tilted, providing a horizontal component which counters out the tail rotor's unintended lateral thrust. However, this horizontal lift from the main rotor has reduced the vertical lift, requiring an increase in collective, and...start all over again. There are a constant set of forces in balance, each of which influence the other.

-Anti-Torque: As the engines move the rotors faster, the body of the helicopter twists in the opposite direction with equal force. This needs to be corrected; the primary methods are an aerodynamic force on a moment arm, like a tail rotor, fenestron, or rotating cylinder, or two main rotors spinning in opposite directions. Each have their own engineering issues. Tail rotors require a significant amount of power, and add a great amount of complexity to transmissions and controls. Multiple main rotors (See CH-46 and CH-47) need to be synchronized to ensure the Hunting, Flapping Main Rotors can't strike each other or the airframe.

-Reliability: All of these combined mean that helicopters require a great deal more effort to produce, fly, and maintain, than an equivalent Fixed Wing aircraft that doesn't have the same number of moving parts, controls, and tolerances, and cost is always a huge driving factor for technology adoption.

Fixed wing airplanes require lightweight engines, and the ability to influence the lift of a single wing. Rotary wing require the same knowledge, as well as another enormous body of conceptual information, manufacturing ability, and so forth.

Source: http://www.airvectors.net/avheli_2.html Also Fly helicopters.

This might be better suited to an engineering specialised subreddit - I'm no expert, but historians draw from evidence left by the past. There probably isn't much evidence for helicopters not existing and why. I doubt many engineers wrote down how frustrated they were with their inability to invent the helicopter.