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At higher speeds, advancing helicopter blades will operate at velocities approaching mach 1, reducing efficiency. But what about lower speeds?
A helicopter is basically a wingless plane with a giant propeller. Why is it more efficient to move around using propellers to provide forward trust while using separate fixed wings to provide lift?
The answer lies in the difference between energy and momentum. Both have to be maintained, but energy goes up as the square of velocity, while momentum goes up linearly. To impart a given momentum, you can either move a lot of air slowly, or a little air quickly -- but as you move the air more quickly, it requires disproportionately more energy.
In other words, the bigger the wing, the more efficient (from an energy/momentum perspective, anyway).
A rotor's size is limited, both because its fast rotation means it needs to be light (and thus can't be very reinforced, etc) and because the larger its radius, the faster the end spins. That means it has to touch less air, but move it more -- which requires more energy.
There's more detail at this question on Physics StackExchange (disclaimer: I wrote that question).
A helicopter is not exactly wingless - the rotor is the helicopter's wings.
A wing's drag is proportional to $W\cdot(L/D)$, where $W$ is weight and $L/D$ is lift/drag. The energy spent overcoming this drag each second is $V_{wing}\cdot W\cdot(L/D)$, where $V_{wing}$ is the wing's airspeed.
The amount of fuel required to travel distance $S$ is then $SFOC\cdot S/V_{aircraft}\cdot V_{wing}\cdot W\cdot(L/D)$, where $V_{aircraft}$ is the aircraft's ground speed, where $SFOC$ is specific fuel consumption.
For a fixed wing, $V_{wing}$ and $V_{aircraft}$, ignoring wind, are the same. In a rotary wing (helicopter), $V_{wing}$ is higher than $V_{aircraft}$ - the wing travels a longer path through the air, with rotation.
For this reason, a helicopter will always be less efficient than a fixed wing aircraft with the same lift/drag ratio. If the ratio is 3x, that's 3 times more fuel for the same distance. This is of course a vast simplification, ignoring all the details and just describing the principle.
To put it in even simpler terms, a helicopter is like a plane with its wings flying a path full of loops.
the wing on a small plane moves through the air at around 100MPH, whereas the main rotor blade on a small helo moves through the air at around 400MPH. the blade is smaller than the wing to be sure, but the drag forces acting at 400MPH are still big, and soak up a lot of horsepower.
Try this comparison: imagine a Cessna 150 with two people in it at cruise conditions and compare it with a Robison R22 with two people in it, same altitude, etc. Look at their respective fuel burn rates.
As pointed out in this answer:
The picture from J. Gordon Leishman, Principles of Helicopter Aerodynamics, illustrates the complexity of the multitude of aerodynamic drags that can be found in a helicopter, and that decrease efficiency.
