[V.Narayan]

Originally Posted by Random_Alien

As a person with a lot of interest in aircrafts, i always had a question in mind- why don't open fan aircrafts use electric motor that is powered by an efficient turbine (similar to APU) like the way honda civic hybrid works?

Wouldn't it be better since the turbine can run at max efficiency and electric motors being torque-y and quick to spool up and down would add to performance and efficiency? Sure, the weight of aircraft would increase but the thrust can more than compensate for that isnt it?

@Random_alien, thank you for your question. I have added a point from@ads11's post to provide more context as the two are related.

All,

The suggestion made would involve conversion of jet fuel's latent energy to kinetic energy of a gas turbine to electric energy of a generator then transferred to a motor and finally to rotary kinetic energy of a rotor. We will have energy loss at each stage resulting in lower eventual thrust by the multi-blade propfan rotor as measured by the calorific value of the jet fuel pumped in. This would also add significant weight of a generator and motor. The generator and motor, in my uneducated guess, would weigh as much or more than the gas turbine and its gearing. Basically in this design the generator+motor are replacing the gearing that converts the turbine's 20,000 rpm* to the rotors say 2000 to 4000 rpm*. The gearing will be lighter and occupy less space.

To your other point what an aircraft at take-off needs is maximum thrust rather than torque. So the torque advantage of a electric motor does not come into play. Also modern jet engines spool up in seconds and can be at full thrust before brakes release. I hope this helps address your query.

*figures are only indicative

Originally Posted by ads11

Looks like the Swedish firm Heart Aerospace have kinda gone this way with their 4 prop airframe, with the two inboard props being electric whilst the outer pair are conventional turboprops.
https://www.Youtube.com/watch?v=zeYySXlo8js

Thank you for sharing this, @ads11.

All,

It is both interesting and encouraging that research is being made on electric aviation propulsion. However for now the whole story is summed up in one tiny para --

Energy density of aviation fuel is several times greater than the best battery – ~250 to 300 watt-hours per kg versus ~12800 watt-hours/kg. A flying device is super sensitive to every ounce of weight. As of now due to this 40X to 50X gap it is not possible to design a flying machine powered by batteries that has a superior performance than a turbine powered machine.

When flying in electric mode the aircraft has to carry the dead load of the jet fuel. While flying in hybrid mode it must carry the significantly extra load of super low energy density fuel aka the batteries. Think of this as an aircraft with two fuels - one with the energy density of 12,800 watt-hours/kg {jet fuel} and the other with 300 watt-hours/kg {batteries}. It simply makes no sense. The super low density fuel is basically deadweight ballast. And as all aviation enthusiasts know every kilo of weight saved is like gold in aviation.

Hydrogen with its very high energy density could be an answer but for the volumes occupied and the cryogenic tanks needed not to mention the Hindenburg.