Martin,
I'm working on a single rotor development for my aircraft using 12A
housings
with early 13B rotor housing and a RX8 rotor with apex seals cut to 3mm.

I don't have the problem of balancing the rocking couple, so the balance
of
the single rotor should be close to static balance.

Advice to me with balancing is to consider full rotor oil weight as well
as
all seals except the apex seal and add this to the rotor weight. Not to
include the apex seal and springs as their carried by the rotor housing.

Can you comment on this in regard to balancing the single rotor, I would
like to get it right the first time, if at all possible, as the single
has
the big torque reversals, so is not as smooth as the 2 rotor.
George (down under)


George,

I have been working on single rotor engines for more than ten years -
including their balancing. So I am fully aware of their potential
problems with torsional vibrations. At MWE/Diamond for instance it was a
set rule not to run the engines below 2,500 rpm, preferably 3,000 rpm
plus. We had more than a couple of dyno couplings literally explode
within seconds when engines were run at low speeds and high loads!

The assumption to add the inner rotor volume of oil was one of the
points discussed more than once. All plain bearing engines I had built
at WankelSuperTec had been balanced outside the company based on the
aforementioned assumption. They ran quite smooth taking into account
their high compression ratio and the very crude mapping those early
days.

The MWE/Diamond rotors are fresh air cooled by a blower fan though. So
apparently there is no oil in the rotors. But we had the permanent
discussion of whether to include the weight of the apex seals and their
springs when being balanced. The designers and calculators said:
NO, the apex seals are not part of the rotating assembly as they "rest"
on the rotor housing.
BUT test comparisons carried out at Norton in the 1980s showed that
engines, balanced WITH the additional weight of the apex seals and
springs, ran smoother still than those without! Theory behind it was
that the inertia of those seals and springs is still imposed on the
outer structure of the engine, hence the frame as well.

If I remember correctly the total weight of a set of apex seals and
springs of a Diamond single rotor engine is in the order of 30gramms. A
Mazda set weighs a lot more still.

To sum it up in one sentence: If I was to balance a Mazda Single Rotor
engine I would add the weight of all seals and springs and the weight of
the oil as well. Hope you get it right that way!

One question with regards to your set-up: Why will you be using an RX8
rotor in an RX7 rotor housing? What benefit are you expecting form that
mix?

Regards and good luck,

Martin.

Martin,
Thank you that was an excellent response and I assume when you said add the
oil, I assume you mean the FULL volume of oil. I guess everyone on the list
will know if I get it right or not.

Your question - 'why the RX8 rotor'. A number of reasons initially, I
thought the extra compression ( 10:1 on the RX8 rotor) would provide the Max
HP, however subsequent discussions have revealed that the extra compression
WILL generate extra torque, but not necessarily extra HP. The second reason
was COST, the RX8 rotor is half the price of a new RX7 rotor. The third
reason was weight of the rotor, however after weighing both rotors and
comparing the weight there's not much in it - very little in fact. Another
reason was the that the RX8 rotor has new molding dies and is said to be
more uniform in thickness and therefore should be better balanced within
itself, with less variation between rotors.

The negative for my application was the cutting the extra width and depth of
the 3mm apex seal. The 3mm apex seal does not seal as well  as the 2mm but
will withstand more detonation that a 2mm- we all know 'no detonation is
best, but I thought with the extra compression it would give some extra
protection. I managed to measure the area in which the apex seal slot is
cut- in both the RX8 and the RX7. The Rx7 area is a lot larger and is where
most of the small weight saving is made IMHO. The RX8 slot depth is shorter,
but I calculated that there was sufficient meat to cut the extra RX7 apex
seal depth, however I kept the slot depth to the absolute minimum. The chap
who cut my RX8 does this for the Racing crowd as to date there has been no
adverse feed-back about damaged rotors.

Initially I was going to cut a 2mm RX7 slot but after discussing this with a
number of people, the general consensus was that even using discharge wire
cutting, there remained a small transition from one depth to the other which
left a ridge, which might cause a apex seal to
hang-up on. The safest thing was to cut the 3mm slot. The method used was a
fly cutter (simple and effective) - it left a very fine cross hatch, similar
to the honing on a piston cylinder.

There is another disadvantage and that is that fact the RX8 has shorter side
seals. The reason is that because the side seals take most of the heat for
the side exhausts, they expand further. Mazda included an extra blow-by seal
near the oil seal to limit this blow-by. You can't use the RX7 side seals
because their different ( the RX8 side seals are tapered). The RX8 side
seals do come in different lengths but even the longest one is still short
in terms of the RX7 side seals.

Another advantage of the RX8  rotor is that Mazda picked-up on
after-market modification used in racing and that's tip relief of the rotor
ring gear. I think I got it all down for you- hope that helps!

George (down under)

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"the extra compression WILL generate extra torque, but not necessarily extra HP."

What exactly do you mean by this George? More torque at any RPM
is more HP at that RPM.

Detonation in the RX8 is not a problem if you don't turbo charge
the engine. The stock 2 mm seals work fine.

Are you planning to turbo charge?

Cross hatch in the seal slot is a leaker on compression. There must be a good seal between the apex seal and the rotor. There fore the finish is important.
If you are planning to turbo the best way is grind the 3mm slot or broach it. Fly cutting it is not a good idea.

Paul Lamar