A friend just fitted a new GMS 76 to his Extra 300 and it makes a very funny noise at full throttle.

It's kind of an irregular metalic hammering sound.

Could this be detonation?

In all other respects the engine runs just fine and has plenty of power, a good idle and great transition.

He's running an 11x8 prop on it.

Ideas anyone?

XJet. I noticed this today when I was observing for a guy with a Cloud Dancer 60 with a .61 GMS in it. It was running an 11x7 prop. It wasn't so much metallic but sounded like an irregular big end or main bearing soft rattle similar to what some engines make when they are bubbering rich but this happened at full throttle at normal optimum "flight setting" mixture. It was not loud enough to cause concern that there was anyhing wrong but it was something I noticed nonetheless.

In all other respects, its performance was exemplary.

It was a real "GMS Banner Day" at the field today - A guy's GMS .40 on a "Chinese Trainer" (you know the shop) with an MA 11x6 prop, pulled 12,900 rpm!! That's real grunt for a cheap .40! I helped a guy tune his GMS .47 in a .40 sized "Chinese" CAP today (best NZ$199 ARF I've ever seen) - it was running as sweet as a nut. Another guy's big stik with a .76 went really well too. The more I see of them the more impressed I am with those GMSs.

Yeah, the sound is a bit like the noise that many plain-bearing engines make when idling (as the crank slides back and forth with the varying thrust from the prop during each power stroke).

I've just posted a pic of the CAP232 ARF you're talking about on our club website and I have to admit they are damned good value for money!

Good to hear that the other size GMS engines are also pretty grunty -- I've got a .32 and there aer a couple of .47s in the club that have yet to be run.

Has anyone in your club had problems with the throttle barrels sticking? My .32 and my mate's .76 both had really sticky barrels that required some remedial work to make smooth.

in RCM it was mentioned that both the OS and ST .90 engines make a bad sounding metallic "clattering" noise at a mid range throttle setting but it was thought to be just a spike in the exhaust note.
Clarence Lee confirmed this when he could only reproduce the noise by running the OS without his usual neighbor friendly after-muffler.


dave

We just fixed an ST90 that was making a metallic sound. It was heard best at idle. Sounded like somebody tapping on the muffler with a screwdriver. Turned out to be the stock muffler. Must have had a baffle loose. It also had trouble keeping a throttle setting which we later traced back to insuficient fuel draw. Put on another stock muffler from his other ST 90. Problem seems to be gone. He's had several good flights without dead stickin and doesnt get any waivering in the engine while flying.
Edwin

Bruce,


Since at full throttle, with a properly adjusted high speed needle, the con-rod is always in compression, it cannot make any noise; even if it is loose on the crank-pin, or on the wrist-pin.

For a sport .75-.78, that actually has its horsepower peak around 13K RPM, an 11x8 is too small a prop.

It does make a higher pitched sound, that with the reduced power output, is no more than a shrill scream...

Prop it where the meat of the power curve is, around 11,500-12,000 on the ground, so unloading in the air, it will really give you good thrust and power.

I think a 12x8 prop will be more appropriate.


That noise you are hearing could be detonation! Try to richen it a little and see if it changes/goes away.

the "always in compression" theory only applies to slow running 2-strokes and not our glow engines. the constant ball and socket resetting needed in a hard running cox engine is proof that the rod is also in tension during part of the stroke.


dave

Good point Zagnut...there's also an incredible decelleration of the piston as it nears TDC but this is counteracted to some extent by the increasing compression of the mixture as it approaches TDC. With the Cox of course there's far less bearing area on the ball than there is with the normal wristpin set up combined with the very thin metal forming the cup.

Well the strange metalic noise simply went away yesterday -- now that engine runs smooth as silk.

Go figure!

Dave, Brian,


I guess you would agree the noise produced by a loose con-rod/wrist-pin, cannot just go away without replacement of components, wouldn't you?

I think it was detonation, which ceased with a slight mixture change.

no...i don't think there ever was any noise from the GMS, Bruce's ears were just ringing for a longer than usual time after running one of his pulse jets


dave

Dave:

In this case, Dar is correct. Even in the little Coxes the rod is always under compression unless the engine is missing fire - for example running in a "Four stroke" condition.

The socket gets hammered due to lateral acceleration of the rod, when it's going side to side at TDC and BDC. If the rpm is kept below 20K or so the loads aren't enough to beat it loose.

Bill.

As far as I can tell neither Dave or I were saying the noise came from a loose fitting rod. We were arguing that the rod can be under tension.

As for the detonation, XJet simply said the noise mysteriously went away, he made no mention of richening the mixture. Personally I doubt it was detonation in the first place because detonation will normally show itself at full load at the point of maximum torque. The engine in question was a .76 driving an 11x7 prop so should have been running well above max torque revs. Even at peak torque you won't get it unless you're pushing the engine to its limits with the highest nitro, a totally incorrect plug or badly cowled and overheating.

Brian,


I did not imply he actually turned the needle out to change the mixture strength.
It is enough for the ambient temperature, or relative humidity, to change a little, to create enough of a change in the mixture strength, to preclude detonation.

Just for the record, XJet's friend is using an 11x8 on this GMS (see post #1), so despite this prop also being smallish for the engine, it was closer to the torque peak than you wrote..

Detonation will more likely take place at very high cylinder pressures, associated with maximum torque.
But swirl and atomization could make the mixture more sensitive higher up the rev range.


I have lately seen an engine that detonated just short of max RPM, during acceleration.
It was enough for the prop driver to eat into the hub and it ceased completely, after the low-speed needle was opened less than 1/8th of a turn.