Re: Turbopro near-instantaneous power (Waterbombers)

From:         Steve Lacker <>
Organization: applied research laboratories
Date:         27 Jun 96 12:56:54 
References:   1 2 3 4 5 6 7
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Harvey Schmiddlapp XVMIII <zoo@animal.zoo> wrote:
>On a turboprop, like the C-130, the engine spins at approximately 13000
>RPM while the prop spins at approximately 1100. When the pilot advances
>the power, the angle of the prop blades to the airstream change to and
>essentially add lift in the forward direction. The engine as the prop
>pulls more power, it signals the fuel system to increase the fuel, thus
>the power to substain the 1100 RPM and additional thrust.  The near
>instantaneous transition from low thust to high thrust is made possible
>by the torque generated by the turbines spinning at 13000 RPM.  In other
>words, the turbines act as a flywheel, storing energy.

I think the more salient point that isn't stated is that when more power is
requested from a piston engine *or* turboprop equipped variable pitch
propellors, THE RPM DOES NOT CHANGE! Nothing has to accelerate or "spool up".
The kinetic energy of the turbines doesn't change, so they aren't really acting
like a "flywheel". A flywheel is only useful if you extract some kinetic energy
from it, thus *slowing* it. All that happens in a piston engine or turboprop is
that the prop blades change pitch to produce more thrust, and the fuel system
increases fuel flow so that the engine produces more torque at the same RPM.
With a turbojet or turbofan, both the turbine and fan shafts must actually
accelerate to a higher RPM- which means that they must accelerate- which means
their kinetic energy must increase- which takes a little time.

Steve Lacker	/	Applied Research Laboratories, The University of Texas
512-835-3286	/	PO Box 8029, Austin TX 78713-8029