# How can a turbofan's fan generate thrust in cruising flight if air is being slowed down?

High bypass turbofans in airliners generate most of their thrust from their fans, not their "cores". However, at cruise they are actually spilling some air past the intake rather than sucking air in (in contrast a propeller always creates a low pressure "suction" in front of it). Given that the air ingested is being slowed down, how can the fan generate thrust? I assume that the air is forced out the exhaust at an even higher speed than cruise? The exhaust has a smaller area than the intake thus the higher exit speed?

## 1 Answer

While in cruise, air is actually pushed into the propeller or fan, there is no "sucking in" taking place which would imply a lower pressure at the fan than the surrounding air.

Source: TU Delft lecture handout Vliegtuiggasturbines by C.J. Houtman

Pic above was used earlier in this answer, and shows the fan inlet in cruise (left) and at take-off (right). The in-flowing air has kinetic energy, and slowing it down by expansion in the fan inlet:

• increases pressure in front of the fan
• allows the fan to add energy to a slower airflow, generating lower compressibility losses.

So the fan adds energy to the incoming airflow, both in the form of increased pressure and increased velocity. Further downstream after the fan, the extra pressure is traded in for a further increase in velocity, until the flow is expanded to the surrounding pressure.

However, at cruise they are actually spilling air past the intake

Not really, air does enter the intake. The front end of the fan nacelle is shaped such that inflow is optimised at cruise.

I assume that the air is forced out the exhaust at an even higher speed than cruise?

Correct, this is what creates thrust.

The exhaust has a smaller area than the intake thus the higher exit speed?

Yes, subsonic airflow accelerates when the stream tube diameter is reduced.

• (Please change the figure caption to show that cruise is (a) and take-off is (b)))
– fho
Jul 17, 2022 at 13:21
• Sourcing your images & charts would be appreciated and is expected. Jul 18, 2022 at 15:48
• @FreeMan Indeed it is, you’re right. It’s from my treasured university lecture handout “Vliegtuiggasturbines”, decades old and paper format only. Jul 18, 2022 at 22:14
• Ah, well then, University, course ID & title, professor and time slot, please! :D Jul 19, 2022 at 12:38
• @Freeman yep it’s in the figure reference (press Edit) Jul 19, 2022 at 13:51