Paul,

Thanks for those pictures. That's similar to what I had in mind, but I was
thinking of curving the runners over the top of the engine to the other
side, like in some of your drawings. This would give the added length for
proper tuning and would make the package more compact.

I was just looking at that power chart for the 4-port RX8 and am surprised
that this engine makes as much torque as it does. At 3000rpm it makes about
144 lb/ft which is not bad for an 80 cubic inch engine.

This got me wondering whether a direct-drive rotary would be feasible. I
know it sounds silly on the face of it, but if you were to boost the engine
enough to double its power output at 3000rpm, you would have 160 hp. The RX8
makes just over 80hp at 3000 rpm, stock; the boost required to bring it to
160 hp at 3000 rpm would only be 8 psi.

If your airframe is fairly slick and can use a shorter prop, you could even
spin the engine to 3500 rpm. Here the stock engine is making just under 100
hp; the boosted version would be close to 200 hp. Not bad for direct-drive.

What I'm thinking is a positive-displacement supercharger, preferably the
efficient twin-screw type, rather than the Roots-type. The beauty of the
positive-displacement blower is that it is like adding cubic inches -- the
blower actually has an internal displacement, so every revolution pumps the
same amount of air, unlike the centrifugal compressor which makes a lot more
compression at high rpm and very little down low. I've driven a
four-cylinder Mercedes with the "compressor" engine, which is the Eaton
Roots-type, and it feels just a like a V-8 right from off-idle. You would
swear you have twice the cubes under the hood. The power is completely
linear and seamless.

A turbocharger or even a belt-driven centrifugal compressor would not work
here because its boost at low rpm is not enough. With a
positive-displacement blower you get the same boost at idle as at red line
--
if you want 8 psi, you have it all the way through the operating range.

The twin-screw is very compact and lightweight -- about 10 to 12 pounds --
so you could actually have a 160 hp to 200 hp direct-drive rotary engine
that weighs less than a normally aspirated engine with gearbox. Also the
twin-screw has very good adiabatic efficiency, thanks to its refined
aerodynamics, so an inter cooler would not be required at boost up to about
9
or 10 psi.

I've included a couple of pics of a Long-EZ with a supercharged rotary,
although the blower appears to be an Eaton Roots-type. It appears this setup
is designed for altitude boost, or normalization, because there is a bypass
valve visible that is operated by a diaphragm that is probably referenced to
atmospheric pressure. There is also an inter cooler. This engine is geared.

What I'm talking about is straight-boost, no bypass to vary boost levels at
different flight conditions. At about 8 psi, this would be, in effect,
doubling the displacement of the engine. As it gets higher, power would just
drop off the same way as NA.

What do you think? Is direct-drive even feasible, in terms of the thrust and
side loads on the e-shaft? The other thing is that you would be increasing
substantially the combustion pressures, but I believe in that regard the
rotary is quite robust?

Regards,

Gordon.

Well even though they claim 30% more eff. It is still 20%
of the gross power coming out of the e-shaft to drive the super
charger. The biggest problem is fuel consumption and superchargers
are not the way to go for low fuel consumption. ALL power
to drive the supercharger comes from the E-shaft.

In other words a net 150 HP to the prop means engine is generating
about 180 HP gross and burning a 180 HP worth of gas. This adds to
the engine cooling load.  A turbo charger on the other hand obtains
all of the HP to drive it by using the otherwise wasted kinetic
energy in the exhaust. Yes there is some back pressure which extracts
HP from the engine but it is very little compared to any mechanical
supercharger.

To double the HP you will need 15 pounds of boost or more
to drive the supercharger. That is going to require a good inter
cooler. Also the bearing loads on the rotor will be very
high at low RPM. Perhaps catastrophically high.

With the weight and cost you might as well buy a gearbox.

BTW That reminds me.
I know Kenne Bell. Car and Driver was testing one of his blown
Pontiac Firebirds on Mrs. Orcuts driveway. They were attempting
to break 200 MPH. I think Csaba was driving. Maybe Don Sherman
was driving. I  flew my C182 out with my girl friend Ronette and
landed it on the driveway. Scared the heck out of all of them
when I buzzed them on the way home :)