X-Mozilla-Keys:                                                                                 
Here is another old message on the subject.

Bill Freeman wrote:

Paul,

There have been several types of turbo systems installed
over the years on production aircraft.  The simplest, and
often offered as an aftermarket add-on is the so-called
"turbo normalized" system.  In this system the (only) wastegate
is manually controlled by a 'second throttle' which is left
fully open for normal take-offs and us gradually closed
(manually) as the aircraft climbs.  There is no protection
from overboosting, and the engine can be easily damaged
by closing the wastegate fully and applying full throttle at
low altitude.  Important TO checklist item!

Another system uses a mechanical interconnect between
the wastegate and the throttle, which controls both the
throttle valve (intake restriction) and the turbo boost.
These systems have a reputation for touchiness and some
are designed to open the actual manifold butterfly throttle in
the first half of the throttle travel and then close the wastegate
in the second half of the throttle travel.  The Turbo 182 has
this system.  These can be a bit cranky to setup and adjust
the linkages but are pretty affordable and idiot resistant.

The third clsss of systems is the automatic turbo controllers.
These systems are probably what Mr. Welch was referring to.
These systems use a pressure sensor system to attempt to keep
the manifold pressure upstream of the throttle butterfly equal to
some design pressure value, for the simplest systems this is
the design max boost pressure.   For these simplest of the
automatic systems the throttle controls the MAP
due to 'throttling' the max boost pressure down to the desired
MAP.  Most systems have a separate overboost wastegate to
protect the system from overboost.  So this system has TWO
"wastgates" one automatically controled to keep the outlet of
the turbocompressor constant and the other to prevent overboost
in event of failure of the first one.  There are several  more
complex automatic systems that reduce the exhaust backpressure and
intake air heating by holding the turbocompressor outlet pressure
to MAP plus 2 in Hg ( or more).  The more modern P and T210s
use this, as do some big Pipers, altho they get the result by
different methods.  There are various ways to scope out the
turbocompressor output pressure;  absolute controllers,
sloped controllers, variable absolute controllers, density
controllers, and rate controllers.  In all systems the auto
controller uses engine oil pressure to actuate the primary
wastegate, and the secondary (overboost) wastegate is
self contained, as an emergency overboost protection.

I  forgot the dumbest systems which use a 'fixed leak'
wastegate which is cheap but incredibly dumb and inefficient
and thankfully was only used on some Senecas and Mooney
231, both which had problems due to the idiot design.

I'm sure that is more than anyone wanted to know on the
subject.  Apologies to the bored.

Bill

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