A rotorcraft with a regenerative drive system (201, 301, 501) for an aircraft, comprising: a drive system configured to generate power for the rotorcraft having: a first engine (207, 307a, 507a) for generating power; a transmission (205, 305) coupled to the first engine, the transmission configured to receive power from the engine and drive a rotor system (103, 161); a first driveshaft (213, 315) coupled between the first engine and the transmission for transferring power; a first power augmentation device (209, 309, 509a) being in selective communication with the drive system and configured to selectively engage and disengage from the drive shaft, the first power augmentation device (209, 309, 509a) being configured to selectively store energy produced by the first engine (207, 307a, 507a) and provide supplemental power to the aircraft during flight; the first power augmentation device having at least three operating ways: 1. store up power in a spin-up mode; 2. produce a controlled power output in a power burst mode; 3. being disengaged in a drag-reduction mode so as to reduce drag on the first engine; characterized in that the first power augmentation device (209, 309, 509a) is installed between the first engine (207, 307a, 507a) and the transmission (205, 305); and in that in the spin-up mode of the first power augmentaion device (209, 309, 509a) the power is stored in springs or flywheels. a drive system configured to generate power for the rotorcraft having: a first engine (207, 307a, 507a) for generating power; a transmission (205, 305) coupled to the first engine, the transmission configured to receive power from the engine and drive a rotor system (103, 161); a first driveshaft (213, 315) coupled between the first engine and the transmission for transferring power; a first power augmentation device (209, 309, 509a) being in selective communication with the drive system and configured to selectively engage and disengage from the drive shaft, the first power augmentation device (209, 309, 509a) being configured to selectively store energy produced by the first engine (207, 307a, 507a) and provide supplemental power to the aircraft during flight; the first power augmentation device having at least three operating ways: 1. store up power in a spin-up mode; 2. produce a controlled power output in a power burst mode; 3. being disengaged in a drag-reduction mode so as to reduce drag on the first engine; a first engine (207, 307a, 507a) for generating power; a transmission (205, 305) coupled to the first engine, the transmission configured to receive power from the engine and drive a rotor system (103, 161); a first driveshaft (213, 315) coupled between the first engine and the transmission for transferring power; a first power augmentation device (209, 309, 509a) being in selective communication with the drive system and configured to selectively engage and disengage from the drive shaft, the first power augmentation device (209, 309, 509a) being configured to selectively store energy produced by the first engine (207, 307a, 507a) and provide supplemental power to the aircraft during flight; the first power augmentation device having at least three operating ways: 1. store up power in a spin-up mode; 2. produce a controlled power output in a power burst mode; 3. being disengaged in a drag-reduction mode so as to reduce drag on the first engine; 1. store up power in a spin-up mode; 2. produce a controlled power output in a power burst mode; 3. being disengaged in a drag-reduction mode so as to reduce drag on the first engine;