Plane - Hangar 9 Advance 40
Engine - OS 46ax, stock muffler
Prop - APC 10/7
Fuel - Cool Power 10%

This plane flies WONDERFULLY! The engine, as old and used as it is, fires up with just a tap from the starter, even after sitting for a week. Lands smooth as glass.

HERE'S the weird thing:

When I start the engine, she fires right up. Cold, hot weaher, dry, humid, doesn't matter. I'll rev it up to full, smooth as can be, great transistion. Hold it nose up, maintains, no sag. HOWEVER, when it's in the air, there's a weird
gurgling/vibration sound at certain RPMS. I know, it must be flutter, right? Couldn't be, cuz I checked all my surfaces, no give. Well, just to make sure, I took her way way up, yanked the nose down, she's diving straight down, fast as hell. Ipull up kinda sharp like, she levels like she's on rails. No flutter, not even a hint of it. So I bring her down, check the canopy, no give. All my surfaces are still tight. Pushrods don't flex, They're reinforced. I check the engine. Crankshaft just as solid as new, no roughness in turning it. Head is tight. Prop is very secure, and perfectly balanced. Even tried different props, same thing. Basially, there's nothing loose on the plane. And again, the only time it makes this sound is when it's flying, and a little past half stick up to full (throttle).

It acually sounds kinda cool, like the engine's got more balls than it actually does, but I'm concerned because it doesn't seem like a sound this engine should make, and I've never heard any others do it.

Is there anything I might be missing in my inpections? Like Isaid, the plane flies great, but if there is a problem that might get worse down the line, Ireally don't want to lose the plane because of negligence, or worse, hurt someone.

Any ideas anyone?

When I hear strange new noises as my planes fly by I land and check everything out. So far it has always been something like a loose wheel, cowl, canopy hatch. Just little stuff. If you think you are having a control flutter then find the speed and do a close and slow fly by whit friends watching the controls, you can usually see the controls fluttering or the plane is reacting strange. Flutter is USUALLY found at the higher speeds. I have seen it so bad the controls were flapping at all speeds. If you have flutter it will strip out the gears in your servo pretty fast, not a good thing.

I've heard simular sounds that I know aren't control flutter and usually just a slight change in RPM changes the sound. You might try a different prop or 2 and see if that makes a difference. Also if I remember right that plane has " cheek plates " on the front of the fuse , if they aren't really solid they can make funny noises when they vibrate at certain speeds. ENJOY !!! RED

loose covering can make a sound....take a heat gun to her

It may be resonance of the fuse or wheels caused by the vibrations of the engine-prop.

http://en.wikipedia.org/wiki/Resonance

Try changing the weight of the prop.

It could also be the engine going into four-stroke mode, if the mix is on the rich side.
This happens when the engine misses every second explosion.
The rpm's decrease, and the torque increases a little.

Last time I heard a similar noise, I landed and grabbed the prop/hub and flexed it. The firewall was moving! Some epoxy and corner stock latter, no noise. Scoot

Some planes will have vibration in the covering, loose or not. One of the guys at our club has a Cub that sounds like a fast drum when it's sitting on the stand at idle.

I have a OS-46LA.The first airplane I put it in was as Great Planes Super Sportster.I call the OS guys and they told me it was nothuing wrong with the engine ,of that pertictular engine it was something built in.Since then I've put it in a Great Planes J-3 And a R/C Nobler andf I got the same result.Its usualy around mid rpm range.Its not the aircraft just the engine.

Thats true. A lot of engines "rattle" at mid throttle, but LA's are louder than most. Even AX's do it.

That's just it. Like I said, I checked for flutter, I tried different props (read original post). So far, what's making sense, is the midrange vibe or wheels or covering. Especially the wheels/covering theory because the only time it happens is when the plane is in flight. Hmm, might have to try somethin with the wheels...

Thanks all!

My wheels kind of make a "screaming" noise if I'm at the right speed. Spin the wheels quickly with your finger and see if that is the noise you are hearing.

I was going to say what LNEWQBAN said.

In some cases, your engine will put out just the right vibration that will cause the whole plane to vibrate - turning it into a giant loudspeaker.

The Spanish guitar effect

Are you back from Toledo, Mike?

Yup, just got home last night - And yes, I was going to use a guitar as an example of how an acoustic vibration can be amplified.

OK, here's somethin, just found that my head was stripped, where the glow plug screws in.  I'm guessing that would explain why I've been going thru plugs like water in a carwash.  changed the head. Also had 1/2 a muffler blow off on me.  Combine that with what Lnewqban said, I did notice I had to run her a little rich, otherwise she'd sag on takeoff. Tomorrow gonna see if the new head/plug, and pipe will get rid of the vibe and leave a little less fuel on the feathers.  Any opinion on this anyone?