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. Cheers JGV 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 Phee Box 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. Regards Laurie 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 <tel:0429%20493828> Please note that my new email address is [email protected]
