Do you know why that the Helicopters can't increase their forward speed ?

Even the speediest helicopters move slowly compared to their fixed wing equivalents.

The reasons for this are ultimately to do with the fact that a helicopter has rotating wings. It is the helicopter’s spinning rotor blades which produce lift and enable the helicopter to fly, and these only produce an equal amount of lift in a still air hover.

When there is any wind at all, or the helicopter moves forward, the advancing blade (the forward moving one) has more air blowing over it – i.e., a higher airspeed – than the retreating blade (the backward moving one), and therefore produces more lift.

To counteract this dissymmetry of lift, we allow the blades to flap up and down, and this flapping equalises the lift across the rotor disc as the blades “flap to equality.” But a side effect of this flapping is that when the cyclic is moved forward to increase speed, the rotor disc tilts forward initially, but then flaps back, and further forward cyclic movement is required in order to continue to accelerate.

This phenomenon, known as “flapback,” occurs throughout the whole speed range of the helicopter. So if we want to increase our speed, the cyclic has to be moved progressively further and further forward. There will come a point at which the cyclic is on its forward limit, and the helicopter cannot fly any more quickly.