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
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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