The earliest helicopters had very limited performance; the type referred to in that article was the Sikorsky YR-4 operated by the 1st Air Commando Group who managed to obtain four prototypes before they had completed full evaluation.

Orde Wingate's long-range penetration unit, known as the Chindits, launched Operation Longcloth in 1943 to strike at Japanese communications and supply lines in Burma. They moved on foot but were supplied from the air, and results were mixed. Casualties were high, supplies received were barely enough, damage inflicted was not great, but with all the other setbacks in South-East Asia any offensive action had a morale-boosting effect so Operation Thursday was authorised as a second, larger expedition. One of the problems with Longcloth was that casualties who could not walk had to be abandoned, so Wingate sought the support of light liaison aircraft that could land on the most rudimentary of airstrips for casualty evacuation. The USAAF officers put in charge of the project, Philip Cochran and John Alison, envisaged a more ambitious operation: transporting the Chindit force by air to save weeks of marching, keeping them supplied in the field, and providing close air support. The result was the 1st Air Commando Group, a composite unit flying C-47 and C-64 transports, Waco gliders, L-1 and L-5 light aircraft, P-51A Mustang fighters and B-25H bombers, as well as the YR-4 helicopters. Part of the unit was the 900th Airborne Engineer Company, equipped with air transportable tractors and bulldozers; they (with the support of the Chindits) were landed by glider and constructed air strips in the forest, allowing two Chindit brigades to be flown in (a third marched on foot).

The L-1 and L-5 could land on very short rudimentary airstrips and performed sterling work flying supplies and casualties; it was hoped the helicopters would allow for similar missions to even more inaccessible locations. The hot and humid conditions were less than ideal, though, the YR-4 could barely carry its pilot and a single passenger for short flights and the engine was prone to overheating. Two were lost in crashes, one being transported and one on a training flight, before the opportunity to test it in action. On 21st April 1944 an L-1 carrying three casualties crashed in a Japanese-controlled area without a suitable landing spot. A YR-4 was despatched; it could only carry one passenger at a time, and overheated after two trips so the operation lasted two days. As R D Van Wagner puts it in Any Place, Any Time, Any Where: The 1st Air Commandos in World War II: "In the 23 combat sorties performed, the concept of the helicopter was proved; however, the YR-4 was grossly under-powered and maintenance-intensive. Eventually it was withdrawn after the engine failed due to overheat."

The small numbers of US and German helicopters employed during the Second World War were useful for testing and limited missions, but until performance improved into the 1950s they were not particularly practical.

People are often surprised at the differences in complexity between helicopters and airplanes so I’ll add this for any technical junkies. One thing to keep in mind is a helicopter lift distribution is incredibly complex and dynamic.

On one side the rotors are traveling into the flight direction. The other side is traveling opposite of flight direction.

So while hovering with no wind, the lift generated by the rotor disk is just a concentric circle of the rotor disk. Imagine a paper plate as the rotating rotor disk and that smaller, center disk flat area in the middle - that’s a helicopter rotor lift distribution in perfect conditions - no wind, hovering.

As soon as the helicopter starts to move forward the lift distribution changes dramatically. The rotors on one side see an effective air speed of their rotational velocity PLUS vehicle velocity. The other side, rotating away from flight path, sees effective air speed of rotational velocity MINUS vehicle velocity. Wind and any vehicle maneuvering exacerbates this. With V being vehicle and R being rotor velocities that means effective blade speeds on each of the rotor disk are: V-R V+R

This means the lift distribution of a rotor disk is constantly, and dramatically, changing all the time. When you see that lift distribution while the vehicle is in motion it’s constantly changing into all sorts of distorted lumps, arcs, and bulges. How they modeled or understood that before computers is crazy to me. Kind of like variably squeezing a balloon really fast. I was less excited to fly in heli’s than planes when I first learned rotor theory. In many ways planes are simple, helicopters are not. But we all know this from growing up - kids make paper airplanes, not paper helicopters.

This is far less forgiving, harder to control, and has more single points of failure than an airplane. The response and details about WWII are fascinating. Those engine failures highlight some particular weaknesses.

An airplane/jet can glide and generally have somewhat controlled landings with an engine out. They’re usually designed to fly/glide, if not translate, engine out. Helicopters can auto-rotate but that’s subject to ideal contexts, conditions and not easy. Military pilots do practice autorotating maneuvers now. Any emergency landing is undesirable but helicopter vulnerability is notable.