You should read the discussion by Tracy Crook on the subject of torsionals in connection with his comments on the design of the Real World Solutions PSRU.  See the web page at http://www.rotaryaviation.com/psru_development.htm

Design Philosophy Part 1 & 2 covers the topic.

 

-----Original Message-----
From: Greg Fuess [mailto:gregory_fuess@yahoo.com]
Sent: Thursday, January 22, 2004 9:23 PM
To: Rotary motors in aircraft
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

 

 

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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