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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> > >
> > >
> > >
> > > [Non-text portions of this message
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