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How does bypass air exiting the bypass nozzle produce thrust in a turbofan engine?

I thought that thrust was produced by air reacting with the fans. So if thrust happens when there is an action-reaction pair with the air and fan blades, is there additional thrust provided when the mass of air pushed back by the fan blades exits the back of the engine?

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You can model the fan disc driving the bypass air flow as a well-shrouded propeller inside a short duct. It does indeed generate thrust which, as in the general case of a propeller, is transmitted to the engine case via a thrust bearing that supports the fan disc and thence to the airframe via the engine mounts.

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  • $\begingroup$ So the air exiting the back of the bypass duct (which has now been accelerated due to the bypass nozzle) does not produce any forward thrust? $\endgroup$
    – Hot_Doug
    Jul 9, 2018 at 22:50
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    $\begingroup$ no, it produces lots of forward thrust. $\endgroup$
    – niels nielsen
    Jul 9, 2018 at 23:14
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Yes a lot of the thrust is from the discharge of the fan being crammed into the convergent nozzle and forced to accelerate. Similar to a regular jet engine where most of thrust is coming from the air coming out of the turbine being forced to accelerate by the convergent nozzle of the tail pipe.

Best way to envision it is to look at a balloon that you blow up and let go. In that case all of thrust is coming from static air being forced to accelerate out the neck of the balloon by the rubber compressing it.

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So if thrust happens when there is an action-reaction pair with the air and fan blades, is there additional thrust provided when the mass of air pushed back by the fan blades exits the back of the engine?

The thrust provided by the action of the fan blades against the air is the same thrust as the thrust provided by the air exiting the back of the engine. If an engine is producing 200 kN of thrust, then you will see 200 kN of force exerted by the fan blades against the air, and you will see 200 kN of exhaust (measured by its mass flow rate times its speed) exiting the back of the engine.

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