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1. 06:27 AM - Re: vibration (Bob J)
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Sat, 04 Dec 2004 06:26:59 -0800 (PST)
--> Rocket-List message posted by: Bob J <rocketbob@gmail.com>
Jim how do you know this is a 2nd order vibration? Do you have
analyzer plots to prove it?
Respectfully I must say that I raise an eyebrow when I hear people say
that that I need to drop ten grand on a prop to get rid of a
vibration, when in reality the ten grand prop only mitigates the
vibration problem marginally at cruise. Its like listening to a tire
guy saying the only way to get rid of my steering wheel shake is to
buy new tires from him.
'Thumping' may not be the best term for this vibration, but it is a
low frequency vibration. It is definitely engine/prop/airframe
related. This is the annoying vibration that is present in the
rockets, and is the one that guys are trying to get rid of by changing
props.
I've talked to a Hartzell engineer I know several times about this,
and they suspect a change in propeller clocking may solve the issue,
but they have no idea on where to begin since no rocket owner has ever
supplied them with the balancer data or worked with them on the
problem directly, which they are more than willing to do. The
solution to the annoying low frequency vibration could be a simple
one, but nobody that I'm aware of has done the due dilligence to
figure out what the heck it really is. To say purchasing a three
blade MT prop solves all your vibration problems isn't true. It
doesn't. That's my story and I'm sticking to it :).
I will give some credit to the MT prop, it is electric motor smooth on
startup and shutdown, it does mitigate the cruise vibration problem
somewhat, is sexy and has a neat sound to it.
Regards,
Bob Japundza
RV-6 flying F1 #80
On Fri, 3 Dec 2004 23:53:09 EST, lesdrag@aol.com <lesdrag@aol.com> wrote:
> --> Rocket-List message posted by: LesDrag@aol.com
>
>
> Hi Bob,
>
> Then the "thumping" isn't a 2nd order engine harmonic.
>
> On a Lycoming 540 engine in a HR2, I've experienced the 2nd order harmonic
> with a two different 2 blade propellers and the lack of the 2nd order harmonic
> with a 3 blade propeller.
>
> "Thumping" can be almost anything. I wish you luck in isolating the
> "thumper".
> (If it's airframe related, it will eventually fatigue and break. If it's
> engine and airframe related, there should be chaffing marks showing up. Best
> to find it first.)
>
> Regards,
> Jim Ayers
>
> In a message dated 12/03/2004 2:23:15 PM Pacific Standard Time,
>
>
> rocketbob@gmail.com writes:
>
> --> Rocket-List message posted by: Bob J <rocketbob@gmail.com>
>
> Jim, respectfully that is not true. This "thumping" vibration is
> still present with the MT installations, though it is diminished. It
> is smoother no doubt but it is inaccurate to say that the "thumping"
> vibration goes away with the MT. That has been my experience with it.
> My 180hp RV with a Hartzell is smoother than a 540 with an MT in a
> Rocket.
>
> Regards,
> Bob
>
>
>
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