All true and I bow to your insight. Sometimes I get confused between which
aircraft I'm working on at the moment. I'm presently negotiating with a list
member for acquisition of his partially completed project and have to keep
it separate from the other project. I have seen much of this on the list and
it would serve many well to go back into the archives and read a post from
Larry Fleshner made years back that the KR is a fun project and a fun
machine. It is not designed for IFR, hauling the family around or many other
things that we seem to conveniently forget. There are many other designs out
there that will better serve those needs if that is the requirement. Hell I
would take a spam can (Cherokee 140) with run out engine and replace it with
a turbo charged Mazda if that is what I desired.
The confusion with projects? Simple, I've got a replica Spitfire
that I've designed and in the process of building so it is quite easy for me
to have one of those "senior" moments and forget what project I'm dealing
with at the moment. The Spit is still undergoing the computer simulation
mode of ironing out design deficiencies and I'm working with several
mechanics who worked on the real animal and Cliff Robertson's Spit pilot,
Gerry Billing who I have known all my life. It is set up to be powered by a
V12 Jaguar engine, fuel injected and supercharged for high altitude
performance as well as cabin pressurization which is another side benefit of
the supercharger.
None of this of course has anything to do with the KR and I
apologize for the confusion on my part. I believe the best combination for
the KR would be either the VW or Corvair engines in these fun machines. By
the way, my reason for choosing the KR design in the first place was it's
performance curve and handling characteristics which is pretty close to that
of a fighter aircraft, so as to regain my proficiency without having to
sacrifice the big bird when finished as well as have a truly fun machine to
"play" with.
Doug Rupert
Simcoe Ontario.
-----Original Message-----
From: krnet-boun...@mylist.net [mailto:krnet-boun...@mylist.net] On Behalf
Of Colin & Bev Rainey
Sent: Sunday, April 11, 2004 4:01 AM
To: KRnet
Subject: KR> Supercharger vs turbocharger
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
crain...@cfl.rr.com
or crbrn9...@hotmail.com
http://kr-builder.org/Colin/index.html
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