Comment by dombar1 on 19/10/2021 at 14:10 UTC

103 upvotes, 4 direct replies (showing 4)

View submission: Is there an upper limit to the highest possible bypass ratio of a turbofan engine?

Theoretically, no. Practically, there are many limits.

One quick example, as a fan becomes larger it would become a propeller.

There are several limits to the overall diameter of both propellers and fans, in no particular order (it will be different for every airplane)

• the structural integrity of the blade itself, to be strong enough not to break apart
• the power of the core to adequately power the fan. As the bypass ratio increases the core becomes smaller relative to the fan and may not be able to provide the required power.
• weight, as the bypass area grows, the weight of the engine increases. The heavier an aircraft gets the more fuel it burns, so there is a point where the benefit disappears

Replies

Comment by Barfmeister at 19/10/2021 at 14:56 UTC

68 upvotes, 1 direct replies

Just latching on to this explanation to add: Another factor limiting the possible bypass ratio is the fan tip speed. The larger the outer diameter, the lower the possible RPM of the fan is before reaching Mach 1 at the tips and encountering problematic compressibility effects. This is currently addressed in (Ultra) High Bypass Ratio engines by implementing a gearbox between fan and the inner shaft, but that is also added complexity and weight, and has its limitations.

Comment by gargravarr2112 at 19/10/2021 at 21:01 UTC

20 upvotes, 0 direct replies

There is actually an example of this, the propfan[1]. Also known as an ultra-high-bypass turbofan engine.

1: https://en.wikipedia.org/wiki/Propfan

Comment by williamwchuang at 19/10/2021 at 21:05 UTC

7 upvotes, 0 direct replies

To expand on the second point, the FAA requires blades to survive a bird strike without having an uncontained failure. If a bird hits the spinning blades, the engine must survive without spewing its contents out of the cowling around the engine. Currently, carbon fiber blades have a titanium edge to help resist bird strikes.

Comment by [deleted] at 19/10/2021 at 17:31 UTC

2 upvotes, 2 direct replies

[deleted]