Posted for "Paul Lipps" <elippse@sbcglobal.net>:

An augmenter is a device in which the exhaust energy is used to accelerate
the engine's cooling air to bring it near free-stream velocity to minimize
cooling drag. it also acts to improve airflow through the engine in static
condition, as when holding or taxi-ing, to keeep the engine from overheating.
Several multi-engine aircraft, such as the Aero-Commander, made use of this.
In its simplest form it is a tube with a length-diameter ratio of at least
five in which the cooling air and exhaust enters on one end and the combined
flow exits the other. My Lancair uses a little more complicated version, with
the exhaust-cooling mixing taking place immediately (<2") below the cylinder,
then both conducted out through a duct with a variable aperture (cowl flap) at
the exit end. Augmenters, as all open-ended tubes, such as an exhaust pipe,
will pick out harmonics from the exhaust stream, which is rich in them, and
resonate to make lots of noise at those harmonics of its length. This noise
may be mitigated in both augmenter tubes and exhaust tubes by "baloney
slicing" the outlet end so as to spread the resonance over a wider spectrum
and so reduce the peak amplitude.
I installed a ventral fin on my Lancair during construction, but it's a
much-abreviated form of the one that extended forward almost to the wing that
you saw in the early days. It made a handy fixed position to attach the rear
nav light, and also, by making the bottom of the rudder straight rather than
curved, and having the ventral fin closely-spaced to it, enhances the control
power of the rudder.