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Greg,
The fellow who sent this info to you
obviously is operating on hear-say, as he compares the rotary (as others often
do) to a six cylinder engine. This is a common mistake among those who don't
understand the engine. The comparison is because it has two three-sided rotors,
but it actually has more in common with a four-stroke four cylinder. The myths
about the rotary abound, and there are plenty of folks waiting to propogate
them.
Tracy Crook, of RWS has pretty well
sorted out all the torsional issues, as well as most all the other difficulties
of putting a rotary in an RV. I'd personally be much more worried about losing a
cylinder or a mag on a Lycoming than any torsional problem inherent to the
rotary. Mike C.
----- Original Message -----
Sent: Thursday, January 22, 2004 9:23
PM
Subject: [FlyRotary] FW: [VAF Mailing
List] Engine Choice
Guys,
Forgive me for
beating a dead horse about torsionals. I have been reading another list
for (VAF and RV-7) ~2 years, and have anxiously anticipated installing a 13B,
and now instead a Renesis engine, in my soon-to-start RV-7. Now, I am
hearing things that cause me to question my ability to make a sound decision
in this regard, as this is the first discussion of torsionals is the first I
have heard. I am beginning to question my ability to follow through on
what I had taken to be the best alternative engine
choice.
I am therefore
forwarding the most seemingly competent post (I am not enough of an engineer
to evaluate this) regarding this issue. My first impressions were that
rotaries were much less susceptible to torisionals that regular engines, but
recently that view has come into apparently serious question, based on the
statements below.
Appreciate your
thoughts and comments.
Regards,
Greg
Fuess
From:
midniteoyl2 [mailto:midniteoyl@wi.rr.com]
Sent: Thursday, January 22, 2004 4:46
PM
To:
vansairforce@yahoogroups.com
Subject: Re: [VAF Mailing List] Engine
Choice
Well, I had to ask an associate to
clarify as I could no longer find
the resource I had gleamed my info
from.
Here is the e-mail I
received back:
-----------
----- Original Message -----
From: Orion Technologies
To: midniteoyl@wi.rr.com
Sent: Thursday, January 22, 2004 3:38
PM
Subject: rotary
stuff
Good morning
Jim;
You and those discussing
it are correct - the rotary engine in an
aircraft application does have a unique torsional feedback
characteristic that can destroy
the drivetrain or the engine itself.
Actually, it usually destroys the weakest link in the
chain, which is
either the
coupling or the reduction drive.
The characteristic is actually not from wobbling rotors or
probably a
dozen different
theories that I've heard, it is actually several
items in the combination, generally found in the aircraft
drive
configuration. The
primary culprit is the mode shape and frequency
of the power pulses the engine puts out, characteristics
that are
substantially differnet
from those encountered in say a typical V-6,
an engine to which the rotary is often compared.
This, coupled with
the mechanical
characteristic of the reduction drive and the typical
prop's spring constant
(especially metal props), results in several
possible modes of exitation which can severely affect the
mechanical
components
throughout.
The problem with
the engine/drive configuration is that the natural
frequency it often exites is relatively high and so, when
things get
to the point of
failure, there is little or no warning. Usually this
frequency occurs at or about
4,500 rpm.
When Powersport
started on their program of developing the
engine/drive combination, they ended up destroying a rather
expensive
dyno before they
realized the cause and fixed the problem. Their
solution is a very refined and
optimized one in that they redesigned
and rebalanced the engine and shafting to meet the
requirements of
their
drive. Today, I'd say their reduction drive is probably the
best one on the market for the
rotary engine. They are currently
selling just the drive for homebuilders and if I had a
project that
needed a rotary
engine, that's the one I'd use. Yes, it's a bit
pricey ($6,500) but I couldn't
build a one-off any cheaper.
In my work with Hayes Rotary Engineering, we also
discovered the
problems
associated with the engine. When Hayes originally installed
their dyno, they took the lesson
from Powersport and installed a
reduction drive between the engine and the hydraulic pump,
just in
case. The drive, a
sprint car gear box rated at over 700 hp, was
mounted on the first test engine, an old and tired
12A. All the
preliminary
tests went without a hitch but as soon as we started
testing just past 4,000 rpm, we heard a pop and everything
suddenly
went quiet. The
only indication that something went wrong was a
growing puddle of oil on the
floor.
It turned out that as
soon as we hit about the 4,300 rpm point, the
torsional feedback literarly pushed the bearings out of the
gear box,
straight through the
sides.
Today the dyno runs
with a reduction drive I designed, along with a
torsional coupling we purchased from Vulcan. I
probably wouldn't use
this
coupling on an airplane as it is heavy and expensive (it is
designed for industrial diesel
applications), but it is the
configuration that the engine/drive combination needs in
order to
stay
safe.
There are two ways to
address this issue but both involve addressing
the natural frequency of the entire drive system, not just
the
engine. The Powersport
method was to design a robust and very rigid
reduction drive, rebalance the engine, and keep everything
as stiff
as possible, thus
increasing the natural frequency of the system
beyond the foreseeable operating
range.
The second method is to
develop a drivetrain that makes use of a
strong and durable coupling, which reduces the natural
frequency
below the operating
range of the engine. Furthermore, this coupling
should also incorporate a damping
mechanism, that would allow the
system to absorb any possible feedback from any variation
of
harmonics the drive might
see.
One note though, several
companies out there are using a sprague
clutch to address this issue. Although this can work
on the
torsionals, most of the
companies have not done their engineering
homework and the sprague they selected is way below in
capacity to
what the unit sees in
this service. Also, when the sprague decouples
(throttle off) and thus allows the prop to freewheel, it
develops an
effect equivalent to
throwing out a parachute out of the airplane,
thus virtually eliminating any possible glide ratio.
Personally, I
think this is an
unacceptable and dangerous approach to the
problem.
Well, I think I
sounded off enough - hope it's of some use to you.
If you have any further
questions, give me a buzz.
Bill
------------
Jim
---
In vansairforce@yahoogroups.com, Charles Kuss <chaskuss@y...>
wrote:
> Jim,
> Would you mind sharing your info in this regard
with
> us? Not a slam. I'd just
like to know where you'd
>
heard this, as it is not what I've heard/believed.
> The 2 rotor Mazda is comparable to a 6 cylinder
4
> stroke engine as regards
power pulse overlap. I
>
realize that there is more to the story that simpy
> that.
> If I'm mis-informed, I'd like to learn how and
why.
> Charlie
Kuss
>
> --- midniteoyl2
<midniteoyl@w...> wrote:
> > Will have to look it up again, but i believe
the
> > torsional vibrations
> > were worse in a
rotary.. Power Sport Avaiation seems
> > to be doing
> > thier homework on the problem
however..
> >
> > My main opjection is
lack of higher power (350+hp),
> > and fuel
> > comsumption - especially if I mod it enough to
get
> > 350
hp.
> >
> >
Jim
> >
> >
> > --- In
vansairforce@yahoogroups.com, "Kevin H."
> > <onesickpup@e...>
> > wrote:
> > > What is seriously puzzling in this engine
choice
> > debate
is.....
> > >
> > > Many posts about
Lycomings... dinasaurs
>
> > Many posts about Lyco-likes... Like a
Lycosaurus
> > > Many
posts about the Subie..... a water cooled
> > engine that is
> > reliable, and looks like a
Lyco....
> > > Not one
single post on the Mazda 13B Rotary.....
> > If a person spent
> > some time researching the reliability of an
engine,
> > where all the
> > reciprocatig parts
never change direction, no cams,
> > valves,
> > lifters, ....you all know what is a
conventional
> >
engine... why
>
> would a 13b Mazda (AKA Wankle) NOT be the wise
> > choise for
> > an "EXPERIMENTAL"??? Torsional vibrations from
the
> > engine should be
> > less... PSRU
should live longer... due to the fact
> > that the major
> > torsionals should come from the prop.... YADA
YADA
> > YADA... I am
> > seriously researching
this option... over the next
> > couple of
> > years... From past experience... Rotarys
are
> > virtually
> > indestructible, they
just couldn't meet smog
> >
emissions for the (C)ARB
> >
((California)Air Resources
Board)
> > > replies
welcomed with open arms!!!
>
> >
> > >
Sincerely,
> > > Kevin
H.
> > >
----- Original Message -----
>
> > From: Paul Trotter
> > > To: vansairforce@yahoogroups.com
> > > Sent:
Tuesday, January 20, 2004 2:55 PM
> > > Subject: [VAF Mailing List]
Engine Choice
> > >
> > >
> > > As has
been seen by the numerous messages
> > lately, choosing
> > > a "traditional" vs. "emerging"
engine technology
> >
generates a lot
> > of
> > >
(friendly) controversy. It has been pointed out
> > several times
> > that
> > > this is a personal choice, as it
well should be.
> > But
those of you
> >
> who are so adamantly against the new
engines
> > should remember
that
> > we
> > > are
building "experimental" aircraft, and as the
> > name so boldly
> > > states, the purpose is to
experiment. Rather
> >
than deride those
> > who
> > > are
going against tradition, we should thank
> > them, for it is
> > their
> > > initiative, and yes, some risk,
that brings the
> >
technology
> >
forward.
> >
> Without their efforts, there would be
no
> > advancement.
Everyone
> > must
> > > decide
their own level of risk when building
> > their aircraft.
> > Very
> > > few would choose not use
non-certified avionics,
> >
and this is also
> > a
> > > level
of risk, although much less. Choosing an
> > engine is just
> > the
> > > same, only a different risk
level. When making
> >
any decision as
> > to
> > > what
components to put in a plane, one should
> > make as informed a
> > > decision as posible and weigh
all the risks. In
> > my
case, I'll
> >
> probably use the new high-tech avionics but
stay
> > with a standard
> > >
Lycoming engine. Why? because I don't
> > understand engine
> > technology
> > > well enough to make a reasonable
decision, so I
> > consider
the risk
> >
> too high to use a less proven engine. But
I
> > admire and thank
> > those
> > > who do
because it is them that are really moving
> > this industry
> > >
forward.
> > >
> > > Paul
Trotter
> > >
RV-8 82080 Emp Kit
> > >
> > >
> > >
> > >
> > > Online
help on this group at:
> >
> http://help.yahoo.com/help/groups/
> > >
> > >
> > >
> >
> Yahoo! Groups Sponsor
> >
>
ADVERTISEMENT
> >
>
> >
>
> > >
> > >
> > >
> >
>
>
>
>
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> > >
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> > >
> > > [Non-text portions
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> >
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