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Hannes Trnka writes about the superior vibration characteristics of the V-8
over
the big Continental IO-550.

I had an obscure graduate course in Engine Design some years ago where they
taught us how to calculate vibration harmonics in any internal combustion
engine
configuration.  It got a little complicated but the results could be validated
by anyone driving a range of sports cars.  

4-cylinder engines, either inlines or horizontally opposed are beasts that
exhibit second order imbalances that can only be eliminated by the addition of
two counter rotating balancing shafts.  Porsche and Mitsubishi did this on
their
larger 4-cylinder inline engines.  The second order issues come up because of
the connecting rod linkage.  

6-cylinder engines, both in-lines and horizontally opposed versions are
balanced
inherently both in first and second order frequencies.  This is why
straight six
cars are so smooth.  

V-6s and V-8s both have a 'rotating couple' (can't remember if it's first or
second order) that means that to smooth it out would require an additional
counterrotating balance shaft.  These engines can sometimes feel 'lumpy'  as a
result.  

V-12s are two inline 6s welded together and enjoy the same smooth
characteristics.  

Firing pulses add to the feeling of vibration but are entirely different.
Here,
the more cylinders, the better.  So, just like we always knew, V-12s are the
best.

So the horizontal 6 you IV guys have should be a very nice package and may in
fact be a little better than a V-8 (except for the firing pulses which you can
test by pulling throttle at constant rpm).  Our Mooney (IO-360 200hp) with big
2-blade Hartzell shook like a wet dog no matter where the throttle was set.
 My
guess is the biggest vibration source is the prop so maybe the best plan is to
invest in a light weight 4-blade version.  

Along with others, I've raised my eyebrows at the thought of someone trying to
develop an alternative powerplant without the backing of NASA or other well
heeled organization.  I worked in the transmission department at Chrysler
for a
few years down the hall from engine design and recall that a lot of people
took
at least three years to develop a scratch engine (GM took about 8 with
Northstar) with a lot of dynamometer and proving grounds time.  And if the
occasional engine fails, it's not a life or death issue.  If our engines
had to
meet aviation performance criteria, we'd have to spend substantially more time
developing them.  Of course we would sell them for a lot more money and buy
better components too (which leads to selling far fewer and probably losing
our
shirts).

When engineers design engines for cars, they are designed for cars and nothing
else.  The parts are as cheap as they can make them and are tested to
specifications that we think reflect real world driving, which is a guessing
game.  Cost is either the number one or number two design criteria
depending on
the car manufacturer.   Why would one start with such an animal for an
aircraft
application?  It sure seems to me that a certified engine (including the new
Orenda V-8) is a far less risky way to go.  I hope the alternative engines
offer
benefits that make the risk worth it.

Ed de Chazal
Rochester Michigan