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Passenger jet airliners seem to control thrust by just adjusting the amount of fuel going to the engine thus affecting the engine rpm. This means it takes a small amount of time to spool up from minimum to maximum throttle and vice versa.

Are there any attempts to allow faster thrust response than this? Are any in current or planned use?

It seems that one could control the compressor pitch, change the gear ratio, or adjust the bypass ratio -- but I'm not sure anyone is doing this. Afterburners and thrust reversers/vectoring don't count. Faster engine respond would help with situations such as loss of rudder control let alone other emergency situations.

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  • $\begingroup$ Related $\endgroup$
    – Sanchises
    Commented Nov 16, 2016 at 21:55
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    $\begingroup$ Well... one thing you really don't want a lot of on an airliner is acceleration, in any direction, so I really don't see this as much of an issue. The more that gravity feels like it's pointing at the floor, the better for all concerned. You don't exactly want a passenger craft to crank into full reheat, sending the refreshment cart blasting down the aisle and out the tail. $\endgroup$
    – J...
    Commented Nov 16, 2016 at 22:25
  • $\begingroup$ You most certainly can NOT change the bypass ratio or gear ratio (assuming fan has gearbox) mid flight! Also as far as I know there are no variable pitch compressors. The extra mechanics would add a lot of weight for little gain. $\endgroup$
    – Notts90
    Commented Nov 17, 2016 at 9:09
  • $\begingroup$ Faster engine response would help with situations such as loss of rudder control. $\endgroup$
    – RoboKaren
    Commented Nov 17, 2016 at 16:54
  • $\begingroup$ Throttle originally means a flap that restricts amount of air going to the engine and that is specific to spark-ignition piston engines. $\endgroup$
    – Jan Hudec
    Commented Nov 17, 2016 at 19:21

2 Answers 2

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Are there any attempts to allow faster thrust response than this? Are any in current or planned use?

First, for turbine engines, the more often used term is thrust lever or power lever rather than throttle. You can Google "throttle vs thrust lever" for that discussion.

I'll use the 727 and the 747 to illustrate what was done prior to when I retired in 1999.

In initial 727-100 sim training, the instructor had us bring the power levers all the way back and allow the descent to stabilize in landing configuration at full flaps. Then, at a given altitude the instructor would tell us to start climbing, at which time we would advance the thrust levers to max power and clean up. From that moment, we would lose hundreds of feet before the aircraft started to climb. The primary reason for the delay was that the engines took 7 seconds to go from flight idle to full power.

In initial 747-100 sim training, we did the same thing. The result was a loss of less than 100 feet, the difference being that engine response was almost immediate, and we also could pitch up immediately without noticeable loss of airspeed.

As I remember being told, the big difference in engine response was because the idle position still had a relatively spooled up engine.

The engines running at that relatively high power output even at idle had the drawback that while taxiing the aircraft would accelerate to a higher than permitted taxi speed, which then required braking.

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    $\begingroup$ Great answer! Can you explain in more detail why the 747 was more responsive? Was it that flight idle was simply at a higher rpm? $\endgroup$
    – RoboKaren
    Commented Nov 17, 2016 at 16:53
  • $\begingroup$ I assume you mean 150 feet instead of 1500 feet. $\endgroup$
    – DeltaLima ♦
    Commented Nov 17, 2016 at 18:25
  • $\begingroup$ When going from idle to full power on the 727 was there a risk of surging? (I'm assuming 727 did not have fadec) If so, how did you prevent it? $\endgroup$
    – TomMcW
    Commented Nov 17, 2016 at 19:32
  • $\begingroup$ @DeltaLima I actually meant 1500, and try as I might in rethinking what I wrote, that's the figure that sticks in my mind. Now, given the fact that it's been 26 years since I flew the 727 and that at age 77 dementia is probably setting in, I could be incorrect. I am, however, totally confident in saying that it was hundreds of feet, and I have edited my answer accordingly. Given a descent rate in excess of 2000 fpm and no meaningful power until the last part of the spool up, the point of the demonstration was that you didn't want to get into this condition anywhere close to the ground. $\endgroup$
    – Terry
    Commented Nov 17, 2016 at 20:22
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    $\begingroup$ @RoboKaren Perhaps a better way to say it would be that flight idle was a greater percentage of the rpm range on the 747 than it was on the 727, but I'm really not an engineer. Also, I think the engine bypass ratio was higher on the 747. Also, I think the different wing had a lot to do with it. That, plus the 727 would develop a higher sink rate. With the 727 you could come down and slow down at the same time. With the 747 you could come down or slow down, but not at the same time. $\endgroup$
    – Terry
    Commented Nov 17, 2016 at 21:09
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There is not much a jet engine could control.

There is no gearbox in a classical turbofan (not counting accessories such as generators). And those that have it (both future and present, and also turboprops) certainly don't have variable ratio: they barely cope with a single-stage box.

Stator blades do have adjustable pitch on many engines, but it is controlled according to the calculated flow velocity so that their angle was optimal for the conditions. It can't be used to help with rotor speed, or not much.

I'm not aware of any variable pitch rotors on turbojets/turbofans, except perhaps NK-93, which is almost a turboprop.

However, turboprops (and any prop engines) do (or can) use variable prop pitch for better response. Setting 'fine' pitch and thus higher RPM (for the same thrust) allows for faster response than the 'coarse' pitch setting. This is due to two factors: first, the prop is less 'loaded', and second, engine (particularly turbine) response is quicker near max RPM than near idle.

Supersonic engines on military aircraft can also use variable nozzle, but I'm not sure if dynamic response is intentionally included in their operating algorithm. Typically, the nozzle opens near idle, but this is rather for flow stability, esp. during start, than for RPM response.

So, fuel flow remains the main factor of thrust/RPM control. However, on all but the very first generation of turbojets, the pilot doesn't control the 'throttle' directly. (There is no 'throttle' as such). The thrust lever is just a signal to the engine controller, and the controller may modulate the fuel flow in a complex manner. Modern engines 'overcharge' the engine during spool-up, possibly injecting more fuel than in proportion to the lever, and even allowing higher peak temperature than normally allowed, all while controlling stability of the engine.

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