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Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?

As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

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  • $\begingroup$ It usually decreases less than proportionallyb because some is made back by higher jet velocity. $\endgroup$
    – user3528438
    May 19, 2018 at 13:50

2 Answers 2

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You have asked two questions here. The first one:

Does the thrust of the fan in a turbofan jet engine decrease proportionally with decrease in the diameter of the fan?

We need to clarify what you mean by "proportionally". If you mean if you halve the fan diameter, the thrust goes down by half, the answer is no. If you mean if you reduce the fan diameter, the thrust goes down, the answer is yes.

For a subsonic engine, net thrust is given by mass flow (kg/s), and the change in velocity of the mass flow between the inlet and exhaust exit. Exactly how the mass flow and exhaust velocity will change as the fan diameter changes is complex and will vary with each engine, so a general statement cannot be made.

Question two:

As the fan in a jet engine is responsible for over 70% of the total thrust can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

Firstly, you have said "the fan in a jet engine is responsible for over 70% of the total thrust" as if this is a fact. It's not - what you can say is something like "in a high bypass ratio engine, the fan can be responsible for over 70% of the total thrust".

So, now to the question part:

Can a spark ignition engine driven fan of 1 meter give an equal thrust of a 1 meter diameter turbofan jet engine with the jet exhaust thrust removed form the sample? All inlet duct and shroud would be identical, also the fans.

If, as you state, the inlet, fan, and exhuast duct are the same, and we ignore the thrust from the core (turbine exhaust flow), then if the fan driven by the spark ignition engine and a fan driven by a turbofan engine are at the same RPM, then, yes, the thrust produced by both fans is the same. The fan doesn't care where the torque required to turn it comes from, so long as the torque is adequate to achieve the same RPM.

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There is also an altitude component that effects efficiency. Lower bypass ratios are preferable the higher you go. Most turbofans are optimized for the airliner's home, 30-40000 feet. Corporate aircraft like the Global that are designed to cruise above 40000 feet have much lower bypass ratios because the higher you go, it becomes more efficient to move a little air a lot vs a lot of air a little.

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  • $\begingroup$ The same thing happens at high speeds, since an engine gets very inefficient if its exhaust speed drops below its airspeed. Hence why you don't see high-bypass turbofans on supersonic aircraft - you see low-bypass turbofans (below Mach 2 or so), turbojets (Mach 2 to 3), or ramjets (Mach 3 and above). $\endgroup$
    – Vikki
    May 19, 2018 at 16:18
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    $\begingroup$ Yeah, and if you go to the other extreme with bypass ratios, you end up with a turboprop. On the new geared turbofans like on the C series, the bypass ratio is starting to approach turboprop country - stand behind one and you can see that the core is tiny on those engines and they are almost all fan. $\endgroup$
    – John K
    May 19, 2018 at 17:10

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