It's part of the tradeoff between momentum and energy that you should aim to move as high of a mass of air at as low of a speed as possible for efficiency.

When you put energy into a mass of air you impart energy of 1/2 MV^2, the kinetic energy equation, which you can think of as the energy you're leaving in the air as it's accelerated to a given velocity on exhaust from the engine. The V^2 part is a killer. This does not translate directly into momentum at all and the most energy efficient way to gain momentum is with a large mass that's accelerated to a low velocity. You can actually see this with the wings which keep the plane itself up. The wings impart enough momentum to hold the weight of the aircraft up by moving a lot of air at relatively low velocity which sacrifices very little energy for the upwards momentum gained.

So engines in aircraft have been getting bigger and bigger as well as slower and slower. It's basic physics, aiming to move as high of a mass at as low of a practical velocity as possible. The 737 max issues were an example of adding giant engines to an airframe not originally built for them due to the drive to move as much air at as low of a velocity as possible while still keeping the plane moving forwards. Passenger aircraft have been getting slower over the years, the 747 was faster than the newer 787's because we're looking for efficiency above all else these days. Going open bladed makes a lot of sense as we go further down this path.

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aceofspades19 6 hours ago | parent | next [-]

This isn't true though, the 747's cruise speed is the same as the 787's at 0.85 mach. The 747 has a slightly faster max speed but that's not relevant for actual travel. The 777 has a slower max speed and cruise speed than the 787 despite being older. I don't think you can realistically draw a correlation between older/newer being faster or slower on wide body aircraft.

	▲	CGMthrowaway 3 hours ago \| parent [-]
		IDK if "bigger and slower aircraft" is what he meant, but rather "bigger and slower engines." Jets cruise @ mach .85 because that's the economic optimum set by wave drag, compressibility and passenger time costs. Hasn't changed in 50 years. The relevant metrics are amount of air moved and speed the air is accelerated to, aka efficiency gains from propulsive efficiency- e.g. increasing bypass ratio, larger fan diameter, lower jet exhaust velocity

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enopod_ 6 hours ago | parent | prev | next [-]

Sounds like a helicopter is not very efficient?

	▲	AnotherGoodName 5 hours ago \| parent [-]
		Less efficient than an aircrafts wings over a long distance but very efficient for an aircraft with engines pointing straight down. The blades are massive, push a lot of air relatively slowing compared to smaller engines. There's a reason most planes will stall when pointing straight up, despite in theory having more power to weight. Their prop efficiency is worse than a helicopters rotors.

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Gibbon1 6 hours ago | parent | prev [-]

I'm curious about using a hybrid system where you have multiple electric fans. For instance 2 turbines and 4 fans. Advantage is smaller diameter for the same mass flow. And more redundancy. A negative is the weight of the electric motors and generators. If you added a battery you have some other advantages. Less pucker when you lose an engine. And better throttle response.

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pedrocr 6 hours ago | parent [-]

Another advantage is you can place the fans all along the wing getting you better stall resistance as the flow doesn't detach as easily. There's already a prototype of a hybrid plane that does this:

https://www.electra.aero/

	▲	jaggederest 2 hours ago \| parent [-]
		You can go one further and just mount a squirrel cage fan in place of or on the front or top of the wing. https://en.wikipedia.org/wiki/FanWing Or go further and use rotating drums: https://en.wikipedia.org/wiki/Flettner_airplane Or you can use a horizontal-axis style helicopter rotor with variable pitch, and it gets you omnidirectional thrust (VTOL) https://en.wikipedia.org/wiki/Cyclogyro There are a lot of interesting possible alternate histories (only requiring a few tweaks to physics) where fixed wings never really work and horizontal rotorcraft dominate, especially in a world where lighter-than-air craft are common - something like a hybrid between a zeppelin and a paddleboat.