Paul, is this engine giving us a 3 to 1 ratio?
Seems to be.
Royce
Reduction gearboxes fitted as an integral part of Mid West rotary
engines and
incorporating torsional dampers, allow the propeller to turn relatively
slowly.
The reduction ratio of the twin-rotor GAE100R unit, for example, allows
the
propeller to experience six power impulses per revolution. That sort of
power from
a conventional direct-drive would require a twelve cylinder four-stroke
engine to
achieve the same smoothness of delivery. Similarly the single rotor
GAE50R can be
equated to a six cylinder, direct-drive engine. Any vibration which may
be felt
from a Mid West engine can almost certainly be attributed to imbalance
in the
propeller.
Sounds like it.
Paul Lamar
"Smoothness" is not a function of pulses per minute - that only sets the
frequency of the vibration
torque overlap or the lack of it, is the primary drive for vibration. The
single rotor has
torque reversals, the shape and spaces based on time are identical to a 3
cylinder
four stroke engine. The 2 rotor has continously positive torque with the
same time
signature as a 6 cylinder 4 stroke, or a 3 cylinder two stroke. Vance J.
I think what they are claiming here is what the prop feels.
With a 3:1 box the prop feels 3 pulses per rev. So in a sense
they are comparing it to a direct drive six cylinder 4 stroke engine.
True these are discrete pulses with no overlap and torque reversals
unlike the overlap of the six cylinder 4 stroke engine.
Paul Lamar
These torque pulses are acting like a hammer blow to everything downline from the
e-shaft, while the e-shaft is experencing sudden acceleration, and then
deceleration.. On the single rotor there is no overlap in the pulses.
Consequently the engine is acting like a jack hammer. The e-shaft will try to
twist on each power pulse, and twist back on the off pulse.
Gm uses a torsional damper on the crankshaft to smooth out and absorb some of
the harmonic pulses on their engines. But that is really a seperate issue from
the problem with the single rotor. The e-shaft is in effect starting , coasting,
restarting, coasting, restarting, coasting
There is a similiar effect in an electric motor. The alternating current flows
one way, runs the motor, stops flowing, reverses, runs the motor. They solve
that problem with a capacator that discharges between current reversals so the
motor acts as if it was a direct current motor.
So for the one rotor we just need a mechanical capacator. Tracy has one in the
rubber bushings on the flywheel.
The rubber absorbs some of the pulse on the power stroke, and releases it into
the drive on the off
stroke.
One of the advantages of a turbine is no power pulses to contend with, continous
output.
On a geared redrive with no capacator :-), each power pulse and reversal hammers
the gear teeth. That is one of the reasons Tracy experenced redrive rattle with
the Ross redrive at idle speeds.
On the other hand no telling how much of that torque reversal gets
by the flywheel and the large rubber device the Midwest engine has
between gear box and engine. This rubber device is normally used
as a U joint in the final drive shaft of an english race car.
Paul Lamar
BMW and Mercedes uses one on there cars. It connects the transmission to the
drive shaft.
It is a rubber plate about four inces in diameter and about an inch thick. It
has four holes that are bushed with a steel sleeve for the coupling bolts. They
are about one hundred bucks each. They last many years though.
Someone uses a clutch disk center on their redrives for the same purpose. Don't
remember offhand who. The springs will absorb some of the power pulse, and
release the stored energy on the off pulse. Doing the same thing as rubber will.
I think I would rather use rubber, as it would be quiter, and doesn't fatigue
like the springs will.
A timing belt drive (HDT) will absorb some of the pulses as well, and works fine
on a gyro, most of them that we have seen uses the belt reduction drive, and so
does most ultra-light redrives. Most UL redrives don't use any other means of
correcting this problem.
Our design for the IBIS redrive will use belt reduction, with a hollow prop shaft
so we can have a reversable, variable pitch prop.
One of the advantages of the rotary is the diameter of the engine from the
e-shaft to the top of the engine. An HDT belt drive will not offset the the
centerline of the engne as much as it would on a piston engine, so the engine
will mount with in a few inches of the same location that it would if it were
direct drive.
If it were a direct drive one rotor the prop would have some
serious problems no question about it.
Paul Lamar
That would be a real jack hammer.
Royce
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