The most fuel-efficient aircraft possible would be an electric one (for the sake of argument lets assume batteries of high enough Wh / kg would exist) flying at much higher altitudes (20km+) than the current fuel and oxygen fired engines.
That definitely involves both engine and aircraft design.
Even for jet engines: an engine from the 50's is not going to cut it in terms of fuel efficiency on ANY airframe compared to current flying machines. Even if you had unlimited budget and time to design the perfect airframe for those engines.
> (for the sake of argument lets assume batteries of high enough Wh / kg would exist)
Highly unlikely, it's not even close right now, and nothing in recent progress indicates that soon we might have batteries that are denser and lighter by a factor of 10 any time soon.
The most fuel-efficient aircraft possible would be an electric one (for the sake of argument lets assume batteries of high enough Wh / kg would exist) flying at much higher altitudes (20km+) than the current fuel and oxygen fired engines.
That definitely involves both engine and aircraft design.
Even for jet engines: an engine from the 50's is not going to cut it in terms of fuel efficiency on ANY airframe compared to current flying machines. Even if you had unlimited budget and time to design the perfect airframe for those engines.