Engine Noise During Run Up

[RFSchaller]

I've got 32 trouble free hours on my 912ULS, but today during run up I had a loud screeching noise at about 3100 RPM. It disappears above and below that RPM, but it is consistent and persists at that RPM. There is no evidence of rubbing between the cowling and spinner. Has anybody experienced this? Could it be gearbox related? I don't get any engine vibration, just the high pitched screech similar to what it sounds like when you twist the PVC cap on a PVC pipe. I used Mobil 1 oil the first 25 hours and then switched to AV-9 from California Power Systems.

 

[Hotscam]

I had similar stange noise at certain rpm settings and it took me a long time to find it was the fibre glass canopy lay-up. Air seeped between it and vibration started. A bit of soft foam tape 1/16 th made it disappear.
I was sure it was simething around the engine at first.
When you push the canopy a bit up when you hear the sound and it changes you can be sure.

 

[BigJohn]

Rich,

I sincerely hope what you are hearing is coming from some type of sympathetic vibration in an airframe component, not the engine. I had a noise at certain RPM's (not screeching) early on, and it turned out to be the lower cowl rubbing against the aluminum skin. 30 seconds with a file and it was cured. Look for that sort of thing.

Thanks for your info re: the high fuel flow. Dynon called and left me a message - I was up with a student at the time - and it sounds like they may have some progress to report. I will call them Monday.

John

 

[RFSchaller]

More on the noise

I tried to repeat the conditions today that gave me the strange screeching during runup yesterday, but I didn't notice anything strange. I put the headphones on and could hear a faint noise that sounded like what I heard yesterday. I never thought it would be coming through the headphones, but who knows. I'll take a noise free engine anyday! I'll keep an eye on it and make another post if I find something that looks like it might affect others.

Thanks for the ideas about the cowling. I'll keep that in mind if it comes back.

 

[Geico266]

I think what you are hearing is the "alternator" recharging the battery. That may not be the technical term for it, but I too can her something in the headset on start up around 3k. It stops after that on take off, and does not return when taxiing after flight.

The charging system is pretty "noisey" (electrically speaking) and under a load there can be interference.

 

[RFSchaller]

It's possible. It does seem strange that it would suddenly start after 32 hours, but weird stuff happens that seems unlikely until you find the set of circumstances that gives you the forehaed slap! Like I said, I'll keep an eye on it and pass on anything that comes up. Thanks to all who offered suggestions.