First, we have to ask: what’s the purpose? Generally aircraft try to get up to their cruise speed quickly and then spend most of their time cruising, and you optimize for cruise first and takeoff second. Do we want multiple cruise speeds, eg a supersonic bomber that goes slow some of the time and fast over enemy territory? Are we designing a supersonic transport and trying to reduce fuel usage getting up to cruise?
And then, there are 2 basic ways you can change the bypass ratio: you can change the fan/propeller intake area, or you can turn off turbines. The V-22 has a driveshaft through the wing to avoid crashes if an engine fails; in theory you could turn off an engine while powering the same number of propellers, which is sort of like a variable bypass ratio. If you have a bunch of turbogenerators inside the fuselage, powering electric fans elsewhere, then you can shut some down while powering the same number of fans. There are also folding propellers.
Thanks for this!
What are the chances of a variable bypass engine at some point? Any opinions?
First, we have to ask: what’s the purpose? Generally aircraft try to get up to their cruise speed quickly and then spend most of their time cruising, and you optimize for cruise first and takeoff second. Do we want multiple cruise speeds, eg a supersonic bomber that goes slow some of the time and fast over enemy territory? Are we designing a supersonic transport and trying to reduce fuel usage getting up to cruise?
And then, there are 2 basic ways you can change the bypass ratio: you can change the fan/propeller intake area, or you can turn off turbines. The V-22 has a driveshaft through the wing to avoid crashes if an engine fails; in theory you could turn off an engine while powering the same number of propellers, which is sort of like a variable bypass ratio. If you have a bunch of turbogenerators inside the fuselage, powering electric fans elsewhere, then you can shut some down while powering the same number of fans. There are also folding propellers.
The question is always, “but is that better”?