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Gilles.Thesee(at)ac-greno
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 Posted: Fri Apr 16, 2010 2:52 am Post subject: Why exactly shouldn't you run a 914 without a prop? |
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Dave G a écrit :
| Quote: | 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.
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My point is, in stating that overreving is the only damage induced by
running without a prop, one might lure the beginner into thinking that
since the revs can be easily controlled, there is no danger in running
the engine with no prop.
| Quote: |
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.
A reciprocating engine can be considered as a mass-spring system with
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wildly variable torque inputs. Any change to the masses or inertia in
the system will dramatically change the vibration modes, and induce
torque spikes or reversals well outside what the system has been
designed and tested for. The torque input variations are comparatively
higher at low RPM.
I feel it important to determine whether the "no problems running with
no prop" opinion is supported by facts and real knowledge, or just beliefs.
Best regards,
--
Gilles
http://contrails.free.fr
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occom
Joined: 26 Aug 2006
Posts: 404
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| Posted: Fri Apr 16, 2010 12:11 pm Post subject: Why exactly shouldn't you run a 914 without a prop? |
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I don't believe I did state that overrevving is the only damage. I think
that I pointed out that Rotax identifies overrevving as a distinct
possibility that will likely result in damage. I did point out that
overrevving can occur very quickly. They identify no other potential
problems.
Thanks for your thoughts on low rpm running and the mass-spring concept,
something to consider for sure.
My major "belief" in the matter is that it's best to follow the
manufacturers recommendations unless you have some very good reason to
disregard them, in which case you are now forging your own trail and should
be comfortable with any consequences that follow. On the other hand without
the trail blazers, we would never make any progress.
---
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