I was reading about modern crop dusters that are blessed with 600 or more horsepower turboprops and have seen them do amazingly effortless high angle of bank turns near the ground.

It seems that a slow rate of power increase would make this type of flying much more dangerous. I had imagined using air bleed gates to close and raise combustion chamber pressure instantaneously, along with more fuel. This would involve an overdesigned compressor.

Is this how it is done?


Two ways:

As Dave pointed out above turbopropeller engine cores, like all aviation gas turbines, do have a spool up lag. This is mitigated a couple of ways.

In a free turbine turbopropeller engine, the gas core spool is independent of the power turbine, allowing it to spin up much faster, and subsequently increase the amount of power delivered to the power turbine much faster.

In direct drive turbopropeller or turboshaft engines, the engine - and the propeller - operate at a near constant speed with increase or decrease in power provided by altering fuel flow and adjusting propeller pitch for the load. Direct drive engines like the Garrett TPE-331 have a near instantaneous power response to power lever commands. The downside to these designs are that they can be extremely treacherous in the event of an engine failure without an autofeather system linked to a negative torque sensor. They are also very noisy on the ground.

  • $\begingroup$ Appreciate the info. With direct drive the fuel/air mixture may be off while the engine responds to increased throttle. A little rich will temporarily make for more "smoke". It would seem best to lag the prop pitch change slightly behind power increase to help it speed up faster. $\endgroup$ May 1 '19 at 20:45
  • $\begingroup$ The 331 is very efficient. $\endgroup$
    – acpilot
    May 1 '19 at 23:57

They don't avoid it but they do try an mitigate it. Turbo props do have jet lag, even modern jet engines have some lag in spool up, granted its not what they once had.

At least some of the lag comes from the sheer mass of the core, production turbo props like the PT-6 tend to me much smaller than your average jet engine and thus may respond faster.

As for crop dusters. Keep In mind most are build to haul a lot of load. When flying empty even at partial power the aircraft has plenty of power to make quick maneuvers.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.