Refer to figures.
1. Best Rate of Climb Airspeed (V_y) is the airspeed at which the helicopter has the best performance in ascent, where the helicopter has the biggest difference between power required and power available and gives the maximum altitude gained in the shortest time.

The greatest difference between power required and power available (maximum power margin) occurs at the lowest point (bottom) of the power required curve (Point 1), where the corresponding speed is the Minimum Power Airspeed (V_MP).

Also, at the lowest point of the power required curve, the fuel consumption is at its minimum. Thus, the maximum endurance can be achieved and the corresponding speed is the V_{MAX END}.

Therefore, the V_Y is also V_MP and V_{MAX END}.

2. At the tangent of the origin to the power required curve (Point 3), the maximum range speed, V_MR, is found, which is also the minimum angle of descent speed and the minimum drag speed, V_MD, (Best Rotor thrust / Rotor drad ratio).

3. On the power required curve, at the tangent from the power available line at the vertical axis (0 kt) (Point 4), the best angle of climb speed, V_X, is found. It is the speed which gives the steepest climb angle to clear obstructions.

NOTE:
Vy must not be mixed up with V_X (Best Angle of Climb Airspeed). V_X is defined as the airspeed at which the helicopter will achieve the best angle of climb. Vx speed is maintained during climb to clear obstacles and gives the greatest altitude gained in the shortest horizontal distance.