Subject: Single rotor balance
From: Rotary Engine
Date: 6/8/2007, 4:48 AM
To: AARotary Engine



In an effort to reduce some of the misunderstandings regarding single
rotor balancing I offer the following:

1. A 2 rotor e-shaft by itself will be very close to a STATIC balance.
It will NOT be in dynamic balance because the rotor eccentrics spin in
different planes. Spin it, it WILL wobble. Add the 2 rotors and it will
still be close to a STATIC balance. Add the counterweights and it should
be close to a STATIC and DYNAMIC balance if the parts are all from the
same (balanced) engine and are replaced in their original positions. The
counterweights are required to remove the DYNAMIC ?wobble? created
because the front eccentric and its rotor are not rotating in the same
plane as the rear eccentric and its rotor.

2. In a single rotor engine the counterbalance weights must provide a
STATIC balance for the e-shaft eccentric and its rotor and must also
account for the DYNAMIC ?wobble? caused because the eccentric and its
rotor CG?s are not at their mechanical centers nor are the counterweight
moments equal and at equal distances from the combined CG of the
eccentric and its rotor.

I made some measurements using parts from an ?87 NA 13B. The e-shaft was
cut just in front (auto) of the rear eccentric and then joined to a
front portion of that e-shaft to provide a shaft of the appropriate
reduced length. It spins freely it a single rotor mockup so it must be
fairly straight. This shaft was the set up on a set of level parallel
bars to make the following measurements.

a. E-shaft only unbalance = 22.9 oz. in.
b. E-shaft + 10.14 lb. rotor unbalance = 119 oz. in.
As a check using 0.591? as the eccentricity the calculated rotor weight
was accurate within 0.3 oz.
c. E-shaft + rotor + rear counterweight unbalance = 66.4 oz. in.
d. E-shaft + rotor + front and rear counterweights unbalance = 12.4 oz. in.

Various combinations of the above together with measurements with a
second rotor seemed to show that the measurement approach was reasonably
repeatable and accurate.

My conclusion; The lightest possible rotor combined with the heaviest
possible counterweights should be close to a good STATIC single rotor
balance.

Fred Osborn

Fred,
While I totally agree with your initial statements as well as your
conclusion, I can't see where you allowed for the rotor seals and oil
weight?
George ( down under)


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