In cases such as a low altitude stall the quick application of power might get a pilot out of a sticky situation, but turbofan engines are fairly sluggish in increasing thrust. If throttle is applied to quickly it can even cause a surge. I'm assuming the problem is the inertia of the fan and compressor. What things do engine manufacturers do to make the engines more responsive? What are the tradeoffs? Do other types of engine (turboprop, turbojet, etc.) suffer from the same problem of delayed response?

  • $\begingroup$ See also: Why should jet engine throttles be moved smoothly? $\endgroup$ – fooot Sep 22 '15 at 18:23
  • $\begingroup$ All turbine engines spin up (relatively) slowly, so the answer to the last part is "yes" for turbojets and turboprops. The rest of the question seems like it's asking for a list of suggestions that would be overly broad for a single question. $\endgroup$ – reirab Sep 22 '15 at 18:42
  • $\begingroup$ @reirab not suggesting anything. I'm no engineer. Just wondering what the engine makers do to try to help with response time. Read on a web page, I think p&w where they said their spin-up was quicker but not certain how they did this. $\endgroup$ – TomMcW Sep 22 '15 at 18:58
  • $\begingroup$ @fooot That's the very question that led to mine. $\endgroup$ – TomMcW Sep 22 '15 at 18:58
  • $\begingroup$ From memory so take this with a grain of salt. The first 727s had a 7 second spool up time from idle to max power, and there were at least a couple of incidents where aircraft landed a little short. The fix was to have two idle positions, flight idle and ground idle. Simply put, flight idle wasn't the full idle you got in ground idle. $\endgroup$ – Terry Sep 23 '15 at 4:47

Several things come to mind, but they will all reduce efficiency and/or cost money, so they will not be popular with engine users:

  • For turbofans: Reduce the rotational inertia of the spinning parts by reducing the bypass ratio. Inertia is mass times distance, so making the engine diameter smaller should have the biggest effect.
  • Increase temperature margins for the turbine. Today, the electronic control (called FADEC for Full Authority Digital Engine Control) will limit the amount of fuel that can be injected into the combustion chamber to limit the temperature in the turbine. By running the turbine hotter for a few seconds the turbine torque can be increased, at the cost of a reduction in the lifetime of the turbine.
  • For turboprops: Reduce blade pitch momentarily to unload the propeller, so more torque is available for spooling up the engine core. This, however, reduces thrust when it is needed most.
  • Reduce secondary loads by disconnecting generators and closing bleed valves. This is already done by todays engine control, however.

The best way of dealing with the slow engine response is operational: Avoid situations where the need for a rapid spool-up might arise or enter those situations not with engines set to idle, but to some medium power setting. Carrier pilots do this routinely by spooling up the engine shortly before touchdown, so they will have the required thrust at their disposal if the tail hook fails to catch any of the arrestor cables.

Thanks to RalphJ for pointing out that turboprops use the same tactic. Helicopters never let the engine run in idle while in flight. If less thrust (turboprops) rsp. lift (helicopters) is needed, both adjust pitch to set the desired thrust/lift and keep engine RPM up. Now the mass flow through the engine core is the same as at full thrust, only the amount of heating in the combustion chamber is reduced to adjust the power output of the engine. When more thrust/lift is needed, the pilot adjusts pitch and fuel flow to set the desired thrust/lift level. Since the engine is already running at top speed, the change is not delayed by inertial effects.

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  • $\begingroup$ Reduce secondary loads by disconnecting generators and closing bleed valves. This is already done by todays engine control. Are you talking about something the engine control does routinely when systems are unneeded or is this something it will do if you suddenly ask for more thrust? $\endgroup$ – TomMcW Sep 22 '15 at 19:52
  • $\begingroup$ @TomMcW: Both, but hull pressurization and ventilation normally need constant bleed flow. Interrupting this for several seconds is not a problem, however, and as soon as the engine delivers full power, the bleed valves open up again. $\endgroup$ – Peter Kämpf Sep 22 '15 at 20:11
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    $\begingroup$ For turboprops: run at 100% RPM always, and include throttle anticipation in the electronic controls for the blade angle. This gives turboprops essentially instant response (i.e. T56 engine in the P-3 and C-130). Slow spool-up times aren't nearly so much an issue for today's turboprops as they are for jets. $\endgroup$ – Ralph J Sep 23 '15 at 2:48
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    $\begingroup$ Also, for aircraft equipped with adjustable exhaust nozzles, reducing the size of the nozzle opening (for non-afterburning flow) will result in a near instantaneous increase in thrust. $\endgroup$ – Rhino Driver Sep 23 '15 at 3:21

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