I am a big newbie in aviation but still, I have a question:

Let's assume a civil aircraft (Boeing 737 or Airbus 320), flying with those impossible real life hypothesis:

  • the exact SAME turbofan are used
  • the turbofan has a MIXED exhaust engine (so cold and hot air are mixed before ejected)
  • aircraft is flying at the SAME altitude
  • altitude temperature is the SAME
  • pressure is the SAME

Now, let's consider the following scenarios:

  • Scenario A: classic design
  • Scenario B: classic design but entry length is increased
  • Scenario C: classic design but exit length is increased
  • Scenario D: classic design but entry and exit length are increased

Scenarios illustration

About the added pipe:

  • same material
  • same diameter for the whole beginning part
  • same diameter for the whole ending part

Notice that for the purpose, we do NOT care about:

  • if the added pipes can touch the runway then landing or taking off
  • the equilibrium point of the motor/aircraft
  • the weight it could add to the aircraft (thus consumption, flying distance, etc.)
  • the feasibility to implement it (structurally possible or not)
  • the stealth of the aircraft (I mean, about thermal imagery and whatever)
  • the external design shape of the turbofan shell

My questions:

Would the motor be more efficient at generating thrust?

My thoughts

  • Should limit/decrease noise reduction (unidirectional)
  • Should limit/avoid turbulence around the first blades ring
  • Should redirect more reaction power to a specific direction a thus provide better efficiency
  • 1
    $\begingroup$ Closed? Wow. Intakes can be longer and even S shaped. Longer exhaust would just add weight. $\endgroup$ Commented Apr 20 at 22:58
  • $\begingroup$ I don't really why why they closed it :/ Anyway, indeed, it will add weight but do we gain in term of noise, performance, protection, turbulence, etc ? That's my real question :) $\endgroup$ Commented Apr 20 at 23:30
  • $\begingroup$ The 'needs focus' close is coming from the question modifying the intake, the exhaust and also mixing in high bypass turbo fans and classical turbo jet. Suggest removing the intake section and if need be splitting it off into a new question since intake flow is a complex question in and of itself, with unrelated physics. It would also help to name check and use illustrations for pure turbine engine, with no bypass fan since that simplifies the physics. $\endgroup$ Commented Apr 21 at 1:08
  • $\begingroup$ @user12642493 I have submitted an edit to better match english language usage around jet engines, but would still be good if you did an edit to make it just a question about exhausts OR intake duct length. In particular 'reactor' would mean the actual combustion area of the engine, which does benefit by being made longer to allow more complete reactions to take place but would be a different question again. $\endgroup$ Commented Apr 21 at 1:50
  • $\begingroup$ I accepted the edit, what happened? $\endgroup$ Commented Apr 21 at 5:03

1 Answer 1


Would the motor be more efficient at generating thrust?

No. Both the inlet's and the exhaust's lengths are already optimised for cruise conditions, which is where the engine is going to spend most of its life.

Regarding the inlet, its main job is to reduce the speed of the incoming air to some Mach 0.4, whatever high the speed of flight is: Mach 0 to 0.4 is the limit within which the compressor's blades (and especially their tips) work in a proper way.

If the speed of flight is higher than Mach 0.4 then the inlet simply works as a divergent inlet (diffuser): its section gets bigger and bigger, the pressure gets higher and higher (so called pressure recovery) and the speed gets lower and lower (the compressor thanks Bernoulli). A shorter inlet would waste some of the pressure recovery while a longer one would simply weight more and generate pressure losses. The same reasoning applies to the exhaust outlet.


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