Roger,

Many piston engines are not well balanced, it is a matter of the design.
Horizontally opposed engines are inherently balanced. As long as the
components are close, the engine is smooth running. Inline six's and
the derivative V-12's are also well balanced.

Inline fours suffer from a secondary shaking force. When the engine
is at TDC two pistons are at TDC and two are at BDC. The engine has
a center of gravity. Now rotate the engine 90*. The center of
gravity has changed. The explanation is that as the crank rotates,
the conn rod and piston not only move down the bore, but the rod
also moves to the side. Pythagorean theorm states Asq'd + Bsq'd =
Csq'd. Now if the C is the rod length and B is 1/2 the stoke  the
math will tell you the effective rod length at 90*. The piston will
be lower in the bore the 1/2 the stroke. Thus the center of gravity
wants to move the engine to keep the Cg in the same spot.

V6 engines are a nasty mess of vibrations with rocking couples in
the mix. They want to rock forward and back, and then side to side
depending on where in the rotation the rod throws are. Early Buick
V6's had very soft engine mounts to try to isolate the vibrations.

The rotor moving in the housing is sort of like an inline 6 crank.
When two throws are at TDC the other four are at 120* and 240*. The
two low ones have a higher mass combined, but that is closer to the
rotational center. Now the rotor when at TDC has two apex seals that
are vertically one over the other. If you measure from a line
between them and another horizontally to the e-shaft center line and
measure the distance it is less the the rotor radius. Thus you have
a higher mass at a shorter radius. There is still some imbalance
that the balance weights take care of. Many piston engines also have
balance weights. Sm block Fords, 400 Chev, BB Chevs to mention a few.

Dale Davies

Like an inline six? Hardly.

"When two throws are at TDC the other four are at 120* and 240*."

What do you mean by a "throw" ?

The two rotor has two eccentric 180 degrees out. The two rotors
exactly statically balance each other. Since there is an offset between
them they are dynamically out of balance. This is completely
canceled by the counter weights. As you add more rotors the counter
weights get smaller and smaller.

Piston engine balance is a complex subject.
The rotary is very simple by comparison.

I recommend For those of you with an engineering degree or
equivalent Taylor's series of books is the best there is. The bible
of engine design.

The Internal Combustion Engine in Theory and Practice.

Volume 2: Combustion, Fuels, Materials, Design. Revised. Charles
Fayette Taylor. The MIT Press Cambridge Massachusetts, and London
England.

Engine Balance Chapter 8 page 240.

When I was at the Mazda factory the vibration test was a glass of
wine sitting on the top of the engine when running on the dyno. No disturbance.
All engines coming off the assemble line were run on dynos.

Perry Mick's rotary is bolted solidly in his Long Ez airframe for
the last 16 years or so. No rubber bushings. One can hardly
feel the engine running.

Paul Lamar


My reference to an inline six throws is the conn rod throws are 120* apart.
Like the apex seals on a rotor.

The idea is that although there is two of the apex seals on one "side" of the
e-shaft center line they are at a "closer" radius to the center line. The
other apex seal is at a greater radius. It is not just the seals but the mass
of that portion of the rotor and where the mass is located in relation to the
rotational center.

The inline six is a balanced engine design. They do have counter weights to
balance the piston and rod masses. The seperation in running pairs to the
other pairs makes it balanced. One pair is at top or bottom and the others are
in between on the other side of 90* from TDC or BDC. Two then are going up,
two are going down and two are close to TDC or BDC. They cancel each other out
so no secondary forces. The running pairs cancel eachother so there is no
rocking couple.

I know it is a bit of a stretch to compare an inline six to a rotor, but that
is how I see it to try to help Roger get a handle on the rotary balance. When
explaining something to somebody that has little or no experience with a new
idea, you may have to try to use things that may be familiar to them. I am no
different than anybody else. At least I think I am. :-)))  Been called crazy
Davies before though.

Dale Davies

What counts is the position of the CG of the rotor and that always
stays in the center of the rotor. The fact that half the rotor apexes
are one side or the other has nothing to do with it. All the apexes
have the same radius from the center of the rotor. Just think of the
rotor as a solid cylinder in the geometric sense.

I think the analogy just confuses the issue.

Paul Lamar