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Date: Sun, 14 Aug 2005 09:21:32 -0400
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From: "Skip Slater" <skipslater@earthlink.net>
References: <list-668426@logan.com>
Subject: Re: [LML] Re: Torqued to death
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   The torque rolling problem applied to many high powered WWII era planes. 
On aircraft carriers, many planes were lost when cobbing the power at low 
speed to wave off an approach or on going to full power after a bolter 
(missing all the arresting wires on angle deck carriers).  The huge radial 
engines on many of the fighters and bombers of that era had outputs ranging 
up to 3,000 horsepower.  At slow speed, the torque generated by a rapid 
application of full power could very quickly flip the plane on its back when 
insufficient aerodynamic force was available to counter it.  At slow speeds, 
only judicious addition of power kept the blue side up.
   The advent of jets eliminated the problem of torque roll while adding the 
new dynamic Wally Bestgen described of turbine lag, where there was often a 
several second delay from commanded thrust and the actual spool up of the 
turbine to sufficient RPM to provide it.  The reason for the delay (forgive 
me if this is old news) is the fuel control unit metering the additional 
fuel to the combustors at a rate slow enough to prevent overtemping the 
turbine components, which would quickly result if fuel was controlled 
strictly by the throttle.  The turbine needs to spool up at a controlled 
rate to introduce sufficient additional intake air to the additional fuel 
flow to prevent an overtemp.  Split spool engines, such as most Pratt & 
Whitney and GE engines,  have quicker response times than single spool.  The 
Rolls Royce RB211 engine, which is used on some 757's and 747's actually has 
three spools and has the fastest response time of any turbine engine I've 
flown. But I digress...
   In turboprops, the propellor re-introduced the torque issue to the 
equation.  Piston drivers who are used to instantaneous response when adding 
throttle are prone to keep adding more throttle to their turbine engines 
when they experience a lag and when it all comes at once, the result can be 
quite a shock, if not downright deadly.  One must have the discipline to 
operate the throttle judiciously and more importantly, to keep adequate 
speed on the airplane and RPM on the engine on approach.
   The slow side of the envelope is very definitely not a place you want to 
be when driving behind a turbine engine and that's doubly true if it's 
turning a propellor.
   Skip Slater
   N540ES