The point of the iron rotors is they run at 500 F.
 Aluminum looses half its strength at 500F. Even 398
 is 30% down.

 The iron rotors are one
 of the things that make the rotary so robust. I pointed that
 out in my Kitplanes article. The rotors make up half the surface
 area of the combustion chamber and are cooled by oil.
 The oil cooling is 1/3rd the total cooling on the engine.
 The rotor is always in contact with hot combustion
 gases on one of its 3 faces. In a piston engine the individual
 pistons are cooled at least 25% of the time on the intake stroke
 and are only subjected to hot gases 25% of the time.

 I would not fly with an aluminum rotor unless it was extensively
 tested in endurance car racing like the Le Mans 24 hour race.
 It might work but it is going to be very expensive indeed to
 test it. Be/Al is another matter entirely.

 Paul Lamar

Another issue with alternate rotor materials that Kenichi writes
about in his
RE book is the expansion rate of the rotor material needs to be less
that the
other engine materials to prevent getting tight against the side
housings.

Dale Davies

Ed

  I am all for lighter rotors.  Not specifically to make more power,
  but to make the engine lighter.

If (when) the European engines become available many builders will think
a 120hp unit @ less than
150 lbs complete is a good deal... Even at more than $20,000

How about that sheet metal rotor idea?  A stainless face
would transfer even less heat than a cast
rotor, although a stamping die might be pricy.

Dan F

The European 150 pound, 120 HP Aixro has cast iron or steel rotors.

A stainless casting is also possible.

Any new rotor design must take into consideration the wear rate
of the apex seals on the apex seal slots.

Paul Lamar


I can't say that I have a great deal of faith in the 398 material as
a rotor combustion surface, I suppose it's survival depends on the
intended output power for each application. On the other hand, the
398 is the most rigid of all of the hyper alloys used in high temp
applications, and might hold the apex seal just fine without
premature wear. After all, the intended design for this material was
to make it possible to raise the top piston ring up much closer to
the top of the piston to eliminate captured hydrocarbons around the
top of the piston. Doing so exposes the ring seats to higher temps.

   The 398 has very low expansion, as do many of the oem hyper alloy
engine materials.

FWIW, I might create a set of rotors without heavier metal apex seal
cast in inserts, and a set with the inserts, just to compare.

The beralcast 310 appears more promising. Even though the melt spec
appears similar to that of alum, it won't surprise me to see it is
substantially higher. It is 60-70% beryllium. The producer claims it
also has a very low heat expansion rate.
I am not inclined to believe that IBC/Beralcast wants to sell their
product in small quantities to private parties. They appear to mostly do
government contracts worldwide. Plus, the Beralcast subsidiary is a
foundry, and may want to do all of the casting procedures in house. I'm
going to pursue them further, it may be a dead end if I want to have a
local SoCal foundry do the work.

This may leave me looking exclusively to titanium. There are local shops
who specialize in investment casting using the vacuum induction furnace
process. Ti. is not the unobtainium it once was.
As far as a stainless combustion surface goes, if expansion
properties are not so different than the base light alloy, it may be a
solution. We could use inconel, titanium, or other,,,for that purpose.
   I have some design ideas that may solve some of the basic problems.

Has anyone ever heard of a light alloy rotor fly apart under load? I've
seen lots of pistons that have; rotors never stop rotating, the stresses
are therefore spread out across the whole structure while they spin
around another spinning object that spins around a central axis. Lighten
the rotor, reduce the size of the gears, increase the cooling oil,
propel the oil out of the rotor with improved cast in internal cooling
fins/vanes; and get rid to that massive bunch of steel between the e
shaft rotor bearings.

We'll see what we can do with this stuff, it likely will take months
just to get the casting materials in hand.

Ed Carver

If I were you I would focus on the rotary racing market. That
is where the real incentive is.

Paul Lamar

Hi Ed;
IIRC, there were early RX-8 rotor failures in racing due to thinned RX8
rotor interior ribs.

The current 13B engine has lots of pluses with the soft cast iron rotors.
I think a lot people would be interested in the 10-20+# weight savings
on the rotors, counterweights, eshaft, flexplate  You might get the
weight of the core under 100#.
Lighter rotors might double the rpm and HP limit for racing folks.
Aluminum rotors sound like a good experiment.  The Al/Be sounds even
better.  Definitely worth a try.  Few people have tried to replace the
rotor.

Like some of Paul, Bill and Dale's comments -
Aluminum rotors have several disadvantages including lower
strength/stiffness at temperature leading to early seals failures,
higher CTE increasing the chance of seizing in overheat or oil/coolant
loss scenarios, poor wear resistance without heat
treating/anodizing/hard anodizing - not sure how that would affect
things.  I have heard of many blown rotor and complete oil/coolant loss
stories where the engine still made it home.

David Mathes was investigating AM-SC1 as another iron replacement years ago.
Al-SiC http://www.mc21inc.com/ looks like a cast-able replacement with
properties closer to cast iron.
Lots of ceramic/cermet out there:  Sialon, Si/N, C/C, C/SiC.  More
exotics on the horizon.  Mcubed.

Some of them can be near cast to reduce or eliminate finish machining.
If I were you, I would literally copy an existing rotor to maintain
compatibility with off the shelf parts.  Lightening the eshaft, cw and
flexplate is easy enough.  gears are cheap and readily available,
increased oil flow and pressure as well.
Iron end-housings are another obvious weight savings target.  Integral
redrive, delete the 2nd gen CAS, lighter water pump. lightweight
exhaust, reduced flexplate/ring gear/permanent magnet alternator...

Cheers
Cary