# How long does it take for a propeller engine to spool-up?

To build further upon the question Why does a turboprop lag?, the following:

Assuming a propeller engine is running in idle and the throttle lever is switched to full thrust instantaneously at t = 0 seconds. How long does it take before the propeller delivers full thrust?

Exact times for a specific engine or range/order of magnitude for a generic one would be appreciated

• Piston-engines and turboprops have quite different dynamics, even if both use propellers. In which are you interested? May 16, 2018 at 13:29

In my Cessna Cardinal, with 360 cubic inch (~5.9L) carburated engine, I push the throttle in and the response is immediate. Just like stepping on the pedal in a car with a standard transmission because propeller RPM = engine RPM. I have a constant speed prop, with controllable pitch angle, so it sort of acts like a standard transmission, I can dial it to any 'gear' I want to. For take off and landing, the pitch is flattest, and most responsive. Just like in a car, I can dial the prop pitch flatter and add more throttle for a climb from level cruise, then when at altitude I can dial the prop pitch up and throttle back down for efficient cruising. In planes we generally don't go to full thrust instantaneously, that's rough on the engine, but smoothly push the throttle in, maybe 2-3 seconds from idle to full throttle. During landing, sometimes a bit of throttle is added to arrest an increasing descent rate, and even that is applied smoothly, and rarely full throttle unless a go-around is needed.

This link shows how the oil pressure, springs, governor, and other bits are used to control the propeller pitch when the RPM has been selected.

http://www.boldmethod.com/learn-to-fly/aircraft-systems/how-a-constant-speed-prop-works/

• That is awesome! +1 May 16, 2018 at 14:56

On my airplane's piston engine, 2-3 seconds to go from idle to redline on a "slam acceleration". On a free turbine turboprop like a PT-6 where there is a separate gas generator and power turbine, that has to spool up from idle rpm, more than double that, say 5-7 seconds. On the CF-34 turbofan used in the RJs, which I have personal experience with, and which is really just a fixed pitch free turbine turboprop, same same, 5-7 seconds. On a constant speed single spool turboprop like an Allison T56 (like in the Herc) which is always "spooled up", it'll be faster than a free turbine engine, so say in between the others.

With free turbine turboprops there is very little torque produced at idle and there is a lot of inertia to overcome. You can start and run a PT-6 at idle while holding the propeller blade stationary (this is done sometimes, or at least used to be) and as long as you don't let it start to move, it only takes a firm grip on the blade to hold it there. If you let it start to move torque builds rapidly and you have to let go and get out of the way.

• Re "...2-3 seconds to go from idle to redline on a "slam acceleration"", that's assuming that the slam acceleration doesn't stall the engine. In which case you'd better hope that you have enough altitude for the windmilling prop to restart the engine. May 16, 2018 at 18:14
• Not saying I do it, but any piston engine that is warmed up and has an accelerator pump in the carb will take rapid full throttle without stumbling, or something is wrong with it. That is the point of accelerator pumps in carburetors. I used to fly a Bowers Flybaby with a C-85 that DIDN'T have an accelerator pump and yes it would stumble with rapid throttle movement and you had to work the throttle like a 1st generation turbo jet. "Slam acceleration" is actually a term used in turbojets when fuel controllers came out that allowed that sort of thing. May 16, 2018 at 19:19
• I've never taken the carb apart (carbs and I do not get along well), so I don't know whether that plane had an accelerator pump or not. I only know what happened when I tried the "slam acceleration" once. Ever since, I've always taken a second or two to go to full throttle. (And turbojets are way out of my price range :-() May 17, 2018 at 5:23
• I agree but there are scenarios, like if you were to bounce on touchdown and find yourself 5 feet in the air with vanishing elevator authority and the only way to save it is a blast of power NOW, the normal gentleness goes out the window. It won't damage an engine. A chum crunched the nose and wrote off his Cardinal because he bounced and it started pitching over and couldn't bring himself to shove the throttle in fast enough to mitigate the situation. May 17, 2018 at 12:17