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4 stroke motor mount question

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Old 10-03-2006, 08:25 PM
  #1  
sonnytex
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Default 4 stroke motor mount question

First, let me tell you a story. I have a Hanger 9 Twist and have about 20 flights on it. When I land this 20th or so time, I have a slightly bumpy landing but by all means nothing harsh. As I am taxing it back up the grass runway the engine, an OS 70 Surpass falls forward, the motor mounts broken at the mounting bolt holes (#6 bolts). Well, I thought, isn't that weird? I went to the LHS and bought another set of motor mounts exactly like the first, the glass filled type. And away I go to the field. After the fifth flight, I come in to land and it was so smooth I had to look around to see if anyone was ohhhing or ahhhhhing over how good it was. When my plane came to a stop in the middle of the runway, lo and behold, the engine just falls off. Yup, the motor mounts broke again right at the bolt holes. Is there something here I am missing? I made sure that there was no stress or undue pressure on the mounts, I use the locking type nuts on stainless steel bolts and am careful not to overtighten, just snuggley tight. One of my friends suspects that maybe a 4 stroke engine puts too much strain on these glass filled mounts. Any ideas out there Guys?

Thanks for any help you can provide
Sonnytex
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Old 10-03-2006, 09:17 PM
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jsn
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Default RE: 4 stroke motor mount question

sonnytex, I really don't know what to tell ya... I have the twist with a Saito 72 on the nose... several hundred flight's... harrier landing's... bounced landing's and so on with no issue with the stock mount. I run glass filled mount's on all my 4 strokes up to 120 and have never had a mount fail... Dubro, GP, Dave Brown and several ARF mount's without one failure. I don't do anything special when mounting... drill out the mount and use bolt's and locknut's or tap the mount for bolt's... sound's like your doing the same thing so again I'm at a loss as to what to tell ya to fix or do different. I hate to say it but maybe you got unlucky with 2 bad/defective mount's... or Do you possibly have a BUNCH of vibration from the engine? unbalanced prop maybe? serious vibration and not your mounting technique or mount type is the only other thing I can think of that would cause the failure.

John
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Old 10-03-2006, 09:27 PM
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Default RE: 4 stroke motor mount question

This is an interesting problem... I have never heard of any special mounting brakets required for a 4 Stroke - if anything they should have LESS Strain on them as there are fewer firings of the engine...

So, I should think that it is a mounting problem.
- did you drill out the screw holes too close to the edge?
- is this the correct size of mount? (did you have to do any shaving on the inside to make the engine fit??)
- are the mounts from the same batch at the same factory? (things do happen at the factory you know...!!)

Engine mounts are one of the most reliable pieces of hardware that you can buy. Short of a nose in crash they usually survive every sort of abuse that we can heap upon them. Look to the obvious; change suppliers, and / or makes.

Are you a real wild flier? Is the engine two sizes too big for the plane?

Check and double check! The truth is out there, and it is probably looking you in the face. []
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Old 10-03-2006, 09:32 PM
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Default RE: 4 stroke motor mount question

I think that John has the right idea. It sounds as if you have a serious vibration problem. Check the prop and spinner/backplate both for blade balance, and hub balance. I have not had much experience with the Saito line of engines (only 2 or 3) but I have extensive experience with the OS lineup. I use Glass filled mounts as well, and have never had a problem with any of them. You said that you are using #6 bolts for mounting the glass filled mount to the firewall. Are these graded bolts, or just whatever comes from the bag at the hobby shop. There can be a tremendous difference in strength of bolts across the spectrum. Get a good set of black iron grade 8 bolts and try it again. Stainless bolts can be more brittle than ductile iron bolts. (the black ones are cheaper too). Let's hear how this comes out.

Bill, AMA 4720
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Old 10-04-2006, 06:44 AM
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sonnytex
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Default RE: 4 stroke motor mount question

I thank you all for your input - I think I will just try some aluminum mounts. Hopefully, this is just a fluke.
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Old 10-04-2006, 08:40 AM
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Default RE: 4 stroke motor mount question

I had the same problem on a SSE with a Saito 82. Broke something like 3 SIG glass filled mounts. I ended up polyurethane gluing thin brass to the sides, and screwing them in to the mount with small coarse screws. Haven't broke one since. I found it was the lower RPMs that did the mounts in. Soft mounting with rubber inserts like the J'Tec also helps considerably. I soft mount all my 4 strokes if I can.
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Old 10-04-2006, 12:27 PM
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sonnytex
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Default RE: 4 stroke motor mount question

Hummm, Velly interrestink Deadeye - At least I'm not the only one that has had this problem - I feel better now I'll give the alumimun mounts a try and see it that fixes my problem. Prior to this I though I had seen it all but now.......................

Thanks for you input Deadeye.
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Old 10-04-2006, 05:27 PM
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Default RE: 4 stroke motor mount question


ORIGINAL: sonnytex

Hummm, Velly interrestink Deadeye - At least I'm not the only one that has had this problem - I feel better now I'll give the alumimun mounts a try and see it that fixes my problem. Prior to this I though I had seen it all but now.......................

Thanks for you input Deadeye.
Sonnytex
If you go aluminum, spend the little extra and get vibration isolation if you can. Otherwise, that shaking is transferred to the airframe. I'm pretty sure that's why my crystal went bad, ending the life of my SSE.
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Old 10-04-2006, 07:19 PM
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sonnytex
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Default RE: 4 stroke motor mount question

Ok Deadeye that makes sense to me - maybe that is why on rare occassion the Twist would try a momentary trick on it's own (shhhhhsh,
don't say anything cause I told them all it was a new manuever ) I normally fly at about 1/2 throttle but I really get a kick out of hitting the power and shooting her straight up until it's just a speck. That seems to be when she twitches now and then. Hummm, you keep bringing up stuff and now I won't be able to sleep tonight

Thanks again
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Old 10-05-2006, 12:48 AM
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Default RE: 4 stroke motor mount question


ORIGINAL: sonnytex

Ok Deadeye that makes sense to me - maybe that is why on rare occassion the Twist would try a momentary trick on it's own (shhhhhsh,
don't say anything cause I told them all it was a new manuever ) I normally fly at about 1/2 throttle but I really get a kick out of hitting the power and shooting her straight up until it's just a speck. That seems to be when she twitches now and then. Hummm, you keep bringing up stuff and now I won't be able to sleep tonight

Thanks again
Sonnytex
Righteo! You are on the fast track too learning buddy! I couldn't believe the difference in the iso mounts on these (I have two) Saito 82s I have. I have a Saito 100, and it NEVER shook as bad as the 82s. But those little suckers ROCK! I can't believe the power they produce sometimes. I've fitted them with carb stacks and air filters, and that tamed them somewhat. But I gotta tell ya man, the power those little cased suckers produce is equal to the Magnum 91 I have...just a tad less (high end) thrust power. Don't let the case size fool you! And wrap that engine in rubber! Or at least the mounts. Happy flying!
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Old 10-05-2006, 10:44 PM
  #11  
rajul
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Default RE: 4 stroke motor mount question

I have used the Great Planes adjustable mount with no problems. The H9 mount material could be too soft, and I've seen complaints on this in a few posts.
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Old 10-05-2006, 11:09 PM
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Default RE: 4 stroke motor mount question

I used the included mount when installing a YS .63 in my Twist. I haven't had any problems with the mounts breaking or the screws loosening. The blind nuts in the firewall are metric if I remember right.
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Old 10-06-2006, 05:37 AM
  #13  
sonnytex
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Default RE: 4 stroke motor mount question

Hi TimC,

Just as a follow up, after reading several of the posts it seems I did not make myself clear on WHICH bolt holes were breaking.
The motor mount at the side of the engine was breaking at the rear mounting hole. Those bolts that hold the motor mount to the firewall were holding fine. I thank all those who have replied - all theories/suggestions have been carefully considered. On to the isolated aluminum mounts -
Sonnytex
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Old 10-06-2006, 08:10 AM
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Default RE: 4 stroke motor mount question

If your Saito is vibrating bad enough to break two motor mounts then something is not set up right I would think. Even at idle the engine should not vibrate that much. Look to your needle settings and check the prop/spinner for proper balancing.
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Old 10-06-2006, 11:03 AM
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Default RE: 4 stroke motor mount question

It could also be that you are tightening the bolts to much. Are you drilling through the mount and using nuts and bolts are did you drill and tap the holes. If you are drilling through and using the nut and bolt make sure to use a flat washer on both sides to dissipate some of the stress.

Jim
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Old 10-06-2006, 11:25 AM
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Default RE: 4 stroke motor mount question

Hey Sonnytex,

TimC Has made me aware of your problem,, So hopefully I can help add to your confusion..

I fly an OS 70 surpass II on my Twist, I have not encountered that problem at all with the Original stock engine mounts,

question...... I know this was asked earlier but I saw no answer, Did you shave the mounts on the inside to accommodate the motor?? I know its just a hair tight but I just wedged mine on there, it slid nicely into place by a little hand pressure.

I have even buried the plane into the ground a few times, as a matter of fact this past weekend was my 4th major rebuild of the bird, Love this plane,, Anyway I use 4-40 bolts with lock nuts on there, I snug them up, but not tight, just as you said you do, after reading through the thread, I would say maybe it has something to do with the things the others are telling you about the unbalanced or spinners or vibrations.

What size prop are you spinning?? I use 12-8 for speed, 13-6 / 14-4 for the better 3D stuff and I like the 14-4 the best so far, I don't have a prop spinner on it any more, as it cracked on my first rebuild and its a little less nose heavy without it.

and There is nothing like this Combo,, Twist & OS 70, I too love the way she rockets into orbit,, talk about your unlimited vertical, it flys awesome as well, my flyin buddies love when I roll out & within 2 feet just go vertical to no end.

I hope this was somewhat helpful to you.
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Old 10-06-2006, 12:00 PM
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Default RE: 4 stroke motor mount question


ORIGINAL: JimO

It could also be that you are tightening the bolts to much. Are you drilling through the mount and using nuts and bolts are did you drill and tap the holes. If you are drilling through and using the nut and bolt make sure to use a flat washer on both sides to dissipate some of the stress.

Jim
I think you might be on the right track Jim. On the sloping bottom of the beams, I file a flat spot for each nut/washer so the top and bottom are parallel. I think this eliminates the stress.
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Old 10-06-2006, 12:40 PM
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sonnytex
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Default RE: 4 stroke motor mount question

Well guys, I thought maybe we had talked this out but how can I not respond to a guy with a handle of WALDOPEPPERAXEL?
You caught me on the shaving of the motor mount - I did do this a little on the original mounts. Like you said, it is tight. I also thought
when it broke the first time that may have been the problem although very little was shaved off. When I put the second set of mounts on I again considered shaving the mount but I instead elongated the firewall mounting holes on the mounts so they were able to widen slightly on each side. That way shaving the mount was unnecessary. As said, it didn't seem to help. Vibration? I suppose that is possible. I use a small steel spinner primarily as a locknut and something for my starter to grasp. I use a APC 12x6 prop and now that you mention it, I have used the same prop ever since I have had the plane - when I get home I will check it for balance Of course my wife indicates that I have a balance problem but I don't think she is talking about the plane[>:]

Thanks again guys
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Old 10-08-2006, 02:31 PM
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Default RE: 4 stroke motor mount question

Shaving the mount a little shouldn't hurt. Rounding the corners on any material, wood, metal or filled plastic relieves stress points and adds resistance to breaking.
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