Helicopter manufactures publish a chart in the flight manual that depicts combinations of airspeed and altitude that should be avoided. It is commonly referred to as the H-V curve or, technically, the height-velocity diagram. Typically it is located in the performance section of the flight manual, not the limitations section, so the pilot is not prohibited from flying in these areas. The chart shows shaded areas that should be avoided because in the event of a power failure the helicopter might not be able to perform a successful autorotation.
The instant that a helicopter’s engine quits, it has stored energy in the form of altitude, airspeed and rotor rpm. A successful autorotation is the effective use of that energy to safely land the helicopter. It is worth noting that this same energy, if not used properly, can destroy the helicopter and its occupants. The shaded area on the left side of the chart shows low airspeeds and altitudes where the helicopter does not contain enough stored energy to perform a successful autorotation. The bottom of the graph also shows a shaded area. This area of low altitude, high speed flight should also be avoided because it does not allow the pilot sufficient reaction time to establish a level attitude and may require an aggressive flare that could result in the tail rotor striking the ground.
The chart shown here is from an R44 and depicts a shaded area for sea level and 8,500 feet density altitude. It also shows a recommended take off profile that favors airspeed over altitude until about 50 kts. Other factors such as high power settings (more pitch in the main rotor blades will cause a faster decay of rotor rpm due to drag), high gross weight and pilot experience (the chart is based on the reactions of an experienced pilot) can affect the outcome as well. Due to the nature of helicopter operations like confined area take offs, sometimes pilots need to operate in the shaded area. Knowing the H-V diagram for the model helicopter you are flying is important for understanding when recovering from an engine failure might be difficult or even impossible.