Doug
Once again I fall prey to a victim of oversimplification. We are not flying
P51 Mustangs with Allison V12, or Rolls Royce V12 engines, nor are we flying
Spitfires. Also, the Messerschmidt(?) ME109 also had a supercharged V12 engine
but needed water injection to help with detonation as I recall. Still these are
not our birds.
Our birds have several affordable and available solutions for our engines
that can be mounted to enhance them. A roots "blower" or supercharger is
typically mounted under a carb assembly and is totally impractical for our
configurations. So it is out. The only other supercharger that is available
and practical for use are ones like the Vortech, or Paxton superchargers, which
are belt driven and operate very similarly to the turbocharger except that they
are driven by the crankshaft instead of the exhaust. You are correct when you
say that the boost pressure is instant. It is actually rising at a steady rate
along with the acceleration of the engine. This is good for power, but as I
said earlier causes the internal pressures to grow at lower rpms, which is what
leads to increased engine wear, and makes necessary the accommodations of
reduced compression, timing retard, intercoolers, and higher octane. To ask
any engine to increase its output, decreases its reliability when previously
operated at lower settings. In short the same engine non-supercharge boosted
will last longer than the boosted one will. In the case where we are using
auto conversion engines, shortening their life is not good. Also in the case
of the VW, any reduction in horsepower is a negative due to its being on the
lower end to begin with. In our case it should not be that we are trying to
raise the overall horsepower, but rather to preserve performance to higher
altitudes where we can achieve the same aerodynamic benefits of larger
aircraft. Given the same airframe studies have shown that beyond a point,
horsepower increase do little for the cruise of the aircraft, but help the
climb rate tremendously, while getting to thin air up top, and cleaning up the
aerodynamics of the airframe really make a difference. Turbos are also
designed to operate in a certain range, and can be tuned based on size and
impellor area to begin boosting at lower rpms the same as the superchargers of
which you speak. The benefit for those of us using VW engines is we don't have
to radically change our configurations, nor do we have to give up HP to get the
benefits of the turbo. Turbos only have to "spool up" when their designer has
them delay for some reason, usually to prevent detonation, or too high engine
pressures at lower rpms. Engines at lower rpms cannot handle lots of pressure
from the boost system with out help.
And it is simply not true that superchargers produce torque and turbos
don't. Both produce power the same way, it is the drive mechanism which is
different. They both artificially compress the air entering the engine, and
that's all. The supercharger does nothing more or less than the turbocharger.
And for the record the early superchargers were in fact turbochargers, just
referred to as superchargers, then later turbo-superchargers, and finally just
turbochargers. My points are not to argue all the different versions of boost
devices available to aircraft in general, but to discuss what is practical for
use by a KR builder. These boil down to the exhaust turbos, and the few belt
driven superchargers on the market. Of these the most practical for most of
our configurations is the exhaust turbo, remembering that the goal should be,
not to try to increase the engine output, but to "normalize" the engine at
altitude. Trying to make the same engine boost up and make more power below
3000feet is just asking for faster rebuilds. Look at the typical race car
engine, totally rebuilt after every race, if not several times during one as in
drag racing. The more you ask of it, the shorter its life, given you are
comparing against someone else with a stock engine configuration.
Colin & Bev Rainey
KR2(td) N96TA
Sanford, FL
[email protected]
or [email protected]
http://kr-builder.org/Colin/index.html
