Hi All,
Thanks for your inputs on our vibration issue. As yet its still unresolved.
Vibration on the ground test is at around 1200 then smooths out at 1800 then
cuts in again around 2300 then smooths out over 2600.
The latest is that is appears to be a prop balance issue.
We took it to Eric Weston last week who found it to be significantly out of
balance. They removed the blades and stripped down and rebuilt the hub, had to
fit weight to one blade and can't find any reason for the imbalance.
We fitted it to YT as per John's advice Thursday and found that the lower rpm
vibration had gone but the upper end vibration was still there.
Today we refitted at 180deg and I had to shut down shortly after start due
vibration even at lower revs.
It looks like we are able to borrow a Dimona prop (Ximango and Grob have a
different backing plate which interferes with the H36 lower cowl). Even though
that will be aq 4 hour round trip to collect it I think it will be worth it to
confirm that it is the prop causing the problems.
Engine mounts are good as is stay wires tension.
This engine has about 850 hours on it, reco heads <40 hrs ago, new carbs and
has always run beautifully. Even now at full rpm it is pulling like a train and
running very smoothly.
Will keep you posted but keep churning the grey matter please.
Regards
Laurie Hoffman
From: salto <[email protected]>
To: [email protected]
Sent: Friday, 17 July 2015, 13:42
Subject: RE: [DOG mailing list] Vibrations at certain revs
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div.yiv8766673505WordSection1 {}#yiv8766673505 Laurie, Have you had the prop
off recently? I have found cases where props have been refitted out of
alignment with the crankshaft. Proper positioning is with cranks horizontal
i.e. 2 pistons at TDC and 2 at BDC (doesn’t matter which ones) and the prop
vertical. Also , if you do have a bit of prop imbalance its always worth
removing & refitting at 180 deg, but do recheck tracking as well. Regarding
bracing cables these should be adjusted so that they have no slack, but no real
tension either, the idea is to maintain the engine in the position dictated by
the engine mounts, but to restrict the upthrust loads that occur with the
application of power & prop torque. In my experience most maintenance
personnel tend to overtension these cables. Watch for grounding of the cables
on the exhausts too. On GVQ I had to reposition the lower anchor points on the
firewall by rotating the lugs and securing with a touch of Tig. Friction of
the bolts is not enough to hold the lugs against rotation. CheersJGV
From: [email protected] [mailto:[email protected]] On Behalf
Of Ian Mc Phee
Sent: Thursday, 16 July 2015 7:27 PM
To: DOG LIST
Cc: Laurie Hoffman
Subject: Re: [DOG mailing list] Vibrations at certain revs I seem to remember
the middle bearing did not get changed at 3000hly but only the outer ball ron
ends. Actually Think I was having the cross cable under engine too tight so
maybe adjusting that would help. Ian Mc PheeBox 657 Byron Bay NSW 2481
On 16 July 2015 at 10:46, Will Bignell <[email protected]> wrote:Are
your two cross braces not over tensioned or loose? I've seen them over
tightened and the rubber hangers being out of alignment leading to vibrations.
When flying at cruise I've experienced the cable go snap and it changed the
vibration pattern until I reduced power as a cautionary move as it felt odd.
Landed to find it had given out a bit. Very hard to spot if shrouded to protect
from exhaust vibrations.
Will Bignell
On 16 Jul 2015, at 9:22 am, Ian Williams <[email protected]> wrote:
Hi Laurie,Your prob is interesting. Could it be a dynamic resonance problem??
I have noticed that our rocket does have specific RPM / vibration combinations.
So just adjust the RPM accordingly. Maybe the fact that the engine is rubber
mounted and uses cables to transfer the engine thrust to the airframe attached
significantly below the centre of thrust could make a resonance problem
noticeable.About 16 years ago I was involved with installing a 4 blade Hoffman
prop on our Pawnee (a 260Hp O-540). (Reduced noise) We had a real problem with
a dynamic resonance where the mags would fall apart, starter motors crap out
etc. A real major prob. We figured that the rotational mass of the wooden
prop was much lower than the metal prop and more blade area so it kind of fell
into place that the problem related to the lack of prop inertia. We obtained a
drawing of a large counterweight between the prop and the hub, installed it and
the problem disappeared overnight. Just like flicking a switch. This may be
of some help. However a question … I am doing an annual inspection on the
Dimona. The rear spring loaded pin securing the tailplane … its outer case
can be rotated .. ie is a bit loose. Has that been a problem with the other
Thai Dimona’s I have tried to see how it could be removed ... .any idea’s.
Also the middle bearings on each elevator (not threaded) are a bit loose and
“click” as they go over centre … ie as the elevator goes up and down. May
look at making new fittings they push into … again have any of you had to sort
this out. Best regards Ian Williams From: [email protected]
[mailto:[email protected]]
Sent: Friday, 10 July 2015 6:47 p.m.
To: [email protected]
Subject: Re: [DOG mailing list] Vibrations at certain revs Just to ad to Rob's
post. We also installed new plugs.
RegardsLaurie Hoffman From: Rob Thompson <[email protected]>
To: "[email protected]" <[email protected]>
Sent: Friday, 10 July 2015, 16:38
Subject: [DOG mailing list] Vibrations at certain revs G'day All,
On our dimona with a L2000 and Hoffman prop, we have a vibration issue which is
happening at around 2950rpm in fine pitch at 60kts and 2450rpm in course pitch
at about the same speed.
Nothing really has changed recently but the problem has developed over the past
few months. I put in new carbs but that didn't make any difference.
At full throttle it is climbing brilliantly.
Anyone else had a problem happen like this? Just seems strange that it would
develop with no chips or damage to the prop from grass ops.
I am going to hire Nigel's dynamic balancer which seems like the best bet. This
unit is fantastic for getting your engine/prop running really smooth so
hopefully this will work.
Rob Thompson
0429 493828
Please note that my new email address is [email protected]
