Hybrid electric does not make sense for current, existing aircraft. The turbofans are capable of getting up to 60% thermodynamic efficiency while flying at -50 C, 30,000 ft. A hybrid electric cannot even match that, fuel cells systems can do only marginally better, but at the expense of much more weight, price and maintenance.
When we're talking hybrid electric to replace piston ICE for GA air-planes, it does not make sense either. If you use a smaller ICE for the hybrid system it will consume about the same quantity of fuel for the range, but at slower Carson speed. So you save maybe 30% of weight on the ICE, but the rest of the hybrid system weights a lot more and you get there later. More about
Carson speed.
Pure electric aircraft makes sense only in certain cases:
1. The aircraft mission is short range and duration, up to about 1h. In this case the electric system can even be
lighter than the comparable ICE. If the air-frame is built around the electric system, and not a conversion, the resulting aircraft will have less drag.
2. Self launching and motor-gliders. Is much easier to fit an electric system for this purpose.
3. The electrical system enables new functionality to the aircraft - e.g. VTOL. The plethora of multi-copters, manned and not, demonstrates plentily this point of view. I'm not a big fan of multi-copters, yes they are simpler, but aerodynamically they are the worst performing aircraft out there. Considering the low specific energy (gravimetric energy density) of current batteries, aerodynamics plays a big role in aircraft range and endurance.
Electric single rotor helicopter outperforms any similarly sized multi-copter, due to disk loading and Reynolds number. Cruise performance is mostly dictated by the aircraft's L/D, helicopters are not so good performers, simple multi-rotors as Ehang184 are even worse. Flying is the most efficient when using wings, due to the high L/D wings have, but historical VTOL aircraft design failures since 1950 shows this is not an easy path to take. This is due to conflicting parameters of cruise and hovering performance: L/D drives the performance in cruise and that means large aspect ratio wings, low wetted area and small propeller size. Disk loading and no flow interference drives the hovering performance and that means large rotors and no wings.