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Sep 19, 2019, 08:35 AM
The runway used to be longer
Mad_Mike's Avatar
Thread OP
Thanks guys:
Plane is an abused Hangar 9 RV-8, bashed back into airworthiness
Engine is new RCGF 15cc standoff mount. Never crashed.
30:1 91 octane fuel
Cowl is held on in 7 places

Quote:
Originally Posted by DGrant
Are those pusher props by some chance?
Tractor props.

How about a pic of the cowling and engine install as well... We're just seeing the props. As far as what's causing it might be found in another area obviously.
Attached.

What does a simple hand rotation tell you other then the cowl scrapes? If you get a good prop on the engine, get it started at an idle, and carefully watch how the prop tracks.
It doesn't quite touch on hand rotation but it was too close so any vibration makes sense it would hit.

Best thing already asked here though is what/how do you ream your props?... If your simply drilling them, well... that's not good. There's prop reams specific for getting the holes precise... and then balancers to further get them in tune. So there's a few thoughts for you.
I have a reamer, these props were already the right size. I balance too with a Dubro balancer that is well made and detects almost zero imbalance.
Quote:
Originally Posted by scale_only
"cadywampass"
???

Quote:
Originally Posted by Balsaworkbench
Theoretically a bent crank could cause this wear pattern, but you probably would have mentioned vibration if that was the case. That leaves a slant-wise hole as the only possible explanation.
The plane vibrates like a wet dog but is no worse than all the other gas singles at our field. Engine is new to this plane, about 1/2 gallon through it, never crashed. Hangar 9 doesn't mention a gas engine, they recommend .46 - .52 2 stroke

Quote:
Originally Posted by Balsaworkbench
Mike, do you have a similar engine? If so, swap the engine out and use a new prop, and see if the symptoms disappear.
My only other gas engine is a brand new DLE20 which I'm building a plane for.
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Sep 19, 2019, 12:10 PM
Registered User
DGrant's Avatar
Mike, I'm almost wondering if that engine is twisting or torquing when throttled up.... and/or oscillating at low/idle. Yeah, the gas engines can be shakers.... but here's a few ideas...

We can't see the inside of the engine box, or if there's anything on the other side of those stand-off posts... as far as a "footprint" or flange that stabilizes those stand-offs. The stand-offs just don't look like they have enough area where they contact the firewall. Many stand-off assemblies have a larger flange at the bases of contact on all posts... on both sides of a contact point... via washers or machined flanges. I'm not seeing either one on your mounting area though.

What happens if you manually twist that engine?... Give that a try and see how much force(without breaking it of course)... and see if there's give or flex when you twist that engine assembly. I would guess with a little bit of force, you might be able to actually flex that assembly, and therein might lie your problem.

Thanks for the pics, it definitely gives a better look now at what you're dealing with.
Sep 19, 2019, 12:27 PM
The runway used to be longer
Mad_Mike's Avatar
Thread OP
The engine feels very solid.
This is my first gas engine though.
There are washers on the front and back and the bolts are VERY tight with Loctite 262 on the back and 271 on the front.
Sep 19, 2019, 12:48 PM
Registered User
DGrant's Avatar
hmm.... after a little thought, I don't think I'd shim that prop with that wood... just sayin'. A clearance adjustment should come from the rear of the engine, in way of a shim, usually wood. That's just me though.

So... I also still think something is flexing... but you say the engine doesn't twist... how about the box?... does that have any flex?... Maybe secure the plane near the rear(aft of the wing)... and give that engine box a little pressure. Just another thought.

And/or give those engine mount bolts another check. Look at that firewall and see if/or there's been any compression into the wood from the stand-offs.

It looks sturdy enough.. actually looks bullet proof somewhat.. but something is causing that notch in the prop.
Sep 19, 2019, 02:14 PM
Dinosaur
Twodor's Avatar
So shimming the prop away from the cowl "fixed it" ?

Is there any evidence of where the prop was hitting the cowl? There would have to be something obvious judging from the damage to that wooden prop. It seems that would offer some direction to go searching for the explanation. Please let us know when you find the answer.

This is one of those things where I wish I could see and touch the issue. Photos, even videos help but is not like holding something in your hand and examining it. Nevertheless I have to agree that something is flexing under power.

Tom
Sep 19, 2019, 03:27 PM
The runway used to be longer
Mad_Mike's Avatar
Thread OP
Yes, it doesn't rub anymore, the 1/16" shim was enough.
The bottom of the cowl shaft cutout was in rough shape so I cut it away and epoxied in a plywood bit. That's where it was hitting. It's pretty solid but could be vibrating there for sure. It's no surprise to me that the prop was hitting, the mystery is why only one blade.
Feb 14, 2020, 10:24 AM
Registered User
hombresinropa's Avatar
Curious - how is the plane doing now? I just bought a 15cc RCGF gas engine and it vibrates my hanger 9 meridian like a mofo. Makes the vertical stabilizer blurry at certain RPMs, which makes me nervous to fly it. I'm paranoid about flutter or interference, I actually lost bind on my RX for a split second several times on the test bench and am wondering if it's caused by the vibration. Is that amount of vibration normal for gas 2 strokes?

Glad your shim worked. I know you said it was abused, but looks like a nice, fun plane!

-Mike
Feb 14, 2020, 02:40 PM
The runway used to be longer
Mad_Mike's Avatar
Thread OP
The RV-8 doesn't go out in the Winter but it flies fine. I'd like more power but I'm told It has enough and I need to fly it more like a pro.
All the smaller 2 stroke gas engines at my field try to shake everything apart, nobody else seems to care. Apparently when they get over 50cc they are better.
My glow engines make more power and hardly shake at all.


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