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3DHS Yak 55 91" - vibration issue

Patto

Patto

70cc twin V2
I bought one of these air frames last year and had a bunch of issues with vibration. Engine was a DA 60. Had a lot of issues getting it tuned properly. Sent it back to DA, they sent it back to me, vibration still an issue. I tried different props, retarding the timing, leaning the low end, etc. to no avail. I figured it was the engine, but...

I decided to try a DLE 55 on it, and although the vibration is less intense, the tail still has a bit of a "wag". It isn't the typical up-and-down vibration that you see in the stab. The stab seems to wag side-to-side with the vibration of the engine.

Has anyone else seen this issue with this air frame? Did I just get one that was built late on a Friday afternoon?

I've lost two decent servos due to what I assume is damage from excessive vibration. I know there are things I can do to strengthen the fuse, but it'll probably take a LOT of work.

I'm mostly curious as to whether or not this has been seen before with this airplane.
 
3dmike

3dmike

640cc Uber Pimp
I've seen this first hand as Patto is a friend of mine. Any help for him would be greatly appreciated as the vibration is pretty dang bad in my opinion although I haven't seen the DLE on it yet.
 
djmoose

djmoose

70cc twin V2
I have a DA60 on mine and don't see any excessive vibration.

A few questions:
1. Are your props balanced?
2. What stab is "wagging" (you didn't specify clearly)
3. Is the wagging causing the corresponding control surface to jitter?
4. Is the entire stab wagging or just the control surface?
5. What servos are you using? (and servo power system)
6. Is your servo geometry correct? (pictures will help)
7. Can we see a video?
8. If you think it's an airframe issue...do you see a point in the fuse that's creasing?
 
Patto

Patto

70cc twin V2
Yes, props are balanced, and I've tried a few different ones.

The vibration is visible in the whole tail, but you can see the horizontal stab outermost leading edges moving fore/aft. It's mostly evident at low throttle. It's the entire tail, not just the control surfaces.

The servos are Hitec 7954's, and geometry is good, but this isn't relevant. I may not have explained the issue very well.

With each pop of the engine at lower throttle settings, at or near idle, the plane's tail is bending side to side, like a dog's tail wagging. You can see it in the horizontal stab, but everything is shaking.

I do not have video. If the weather is nice I'll try and get some this weekend.

I'm thinking that some cross bracing in the fuse is probably what needs to happen. It just will require some significant surgery.
 
djmoose

djmoose

70cc twin V2
If I were you...I'd try to pinpoint the place that is allowing the waggle. (is fulcrum the correct term here?) It might be a broken former or a glue joint that popped apart.

Might need to look inside towards the tail while someone is holding the plane (while idling) to see. My point is that if it's something specific to YOUR airframe (as I haven't heard of this problem before) you should be able to see where the problem area is inside.

By chance, is there anyone else near you with a 91 Yak? It would be good to compare formers and stringers etc etc.
 
Patto

Patto

70cc twin V2
No one else has this specific yak. I've owned and seen plenty of similar planes but this is a first for me.

This past weekend it broke the choke push rod off the DLE55. That makes me think the problem may be in the nose.

I've inspected the fuse as much as I can without tearing off covering, which is probably what I'm going to end up having to do. I don't see any bad glue joints or broken dormers, but it's entirely possible it's there and I just can't see it.

What I may do is glass the motor box, firewall, and the longitudinal ply from box to turtle deck and see if that helps. I can also flow some thin CA down the tail, although I now think the problem is in the nose.

It's very, very frustrating.

Thank you [MENTION=30]djmoose[/MENTION] for the suggestions!
 
Patto

Patto

70cc twin V2
djmoose said:
If I were you...I'd try to pinpoint the place that is allowing the waggle. (is fulcrum the correct term here?) It might be a broken former or a glue joint that popped apart.

Might need to look inside towards the tail while someone is holding the plane (while idling) to see. My point is that if it's something specific to YOUR airframe (as I haven't heard of this problem before) you should be able to see where the problem area is inside.

By chance, is there anyone else near you with a 91 Yak? It would be good to compare formers and stringers etc etc.
Click to expand...

So, I put diagonal hardwood sticks along the top of the fuse below the canopy/hatch, and I used large tri-stock around the engine box and then in the canister tunnel.

Boom. The shakes are at a normal level now.

Wish I had done that months ago.
 
Lojik

Lojik

70cc twin V2
[MENTION=3711]Patto[/MENTION] and [MENTION=30]djmoose[/MENTION]

sorry to jump on your thread but i just snapped up the last 91 yak in Australia and was wondering how you guys feel with the plane? very few if any of them here so I bought it based on previous 3DHS plane experience, any pitfalls in build to watch out for?

how about power, what are you guys using and are you finding it sufficient or suitable to the frame?

Thanks
Jack
 
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