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Why exactly shouldn't you run a 914 without a prop?

 
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Gilles.Thesee(at)ac-greno
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PostPosted: Wed Apr 14, 2010 1:37 pm    Post subject: Why exactly shouldn't you run a 914 without a prop? Reply with quote

FLYaDIVE a �crit :
Quote:
Why are you becoming Theoretical Physicist and reinventing the wheel?

Engineers do need physicists to do the calculations first Wink


Quote:
Here is two hunks of information that are prevalent and should be followed:
1 - In all my training with internal combustion engines in 45 years...
You NEVER run an engine with out a flywheel of some type or a load.
The prop is both a flywheel and a load.

Now if you don't believe this jump to item 2...

2 - Call up ROTAX and ask them: if they will warranty the engine if
you tell them that you ran it without a flywheel or a prop?

Agree.

Quote:
The reason for the flywheel or load is so you reduce or eliminate the
possibility of the engine RUNNING AWAY... Over reving.
Someone said he obtained a decent idle with no prop. What is unknown, is

the degree of torsional stress imposed on the crank, gears, etc.

Best regards,
--
Gilles
http://contrails.free.fr


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Gilles.Thesee(at)ac-greno
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PostPosted: Thu Apr 15, 2010 5:45 pm    Post subject: Why exactly shouldn't you run a 914 without a prop? Reply with quote

Dave G a écrit :
Quote:
but all this talk of flywheel, torsion, gearbox, etc in a no load
situation doesn't seem likely to me. Indeed the video of one running
at low RPM looked quite happy to me, humming along smoothly. So, can
we agree that it's not a good idea because the engine may over-rev and
leave it at that, no pats or flames required.
Dave,


Would you care to share with us the physics beyond your opinion ?
I'm surprised that you make a point of over reving though this situation
can be easily controlled by careful use of the throttle, as has already
been pointed out. Whereas you seem to dismiss torsional vibrations
which need lots of experimenting to take care of.

Some of us have some experience with reciprocating engines, but most
are not equiped -and did not conduct experiments- to decide whether
running without a propeller keeps torsional vibrations within safe
limits or not.
The matter is rather intricate, so if you have some info that we don't
have, please share it with us.
As you may have noticed, even when running out of their safe idle rpm,
Rotax engines do look happy, although detrimental shatter may occur
within the gear train.
So a layman's impression might not be sufficient.

Best regards,
--
Gilles
http://contrails.free.fr


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occom



Joined: 26 Aug 2006
Posts: 404

PostPosted: Fri Apr 16, 2010 2:07 am    Post subject: Why exactly shouldn't you run a 914 without a prop? Reply with quote

Hello Gilles, no, I would not. Is there some particular point that you feel
is important that you would like to make? I can think of a couple of things
with running the engine without a prop that may or may not be issues. The
crank and gearbox suffering damage from torsional stress spikes or gearbox
chatter would not be areas I would be concerned about. However BRP's
cautions are centered around over revving.

I am somewhat curious to hear why you think extraordinary force spikes will
present themselves in the situation discussed, low rpm, low throttle
openings, no gearbox load. I don't require or demand any proof to back up
your thoughts.

---


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