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Help me figure out a noise....

Brantel

Well Known Member
So a few weeks ago I was on a nice smooth solo flight and I started noticing a noise that I am not used to hearing.

The noise sounds sort of like a high pitched whine similar to alternator noise.

First I have ruled out the alternator (electrical noise, not bearings). I can hear it without the headsets on. I have also turned off the alternator in flight and the noise remains.

On several flights after first noticing it, I noticed that the volume of this noise appeared to change on different flights. Weird....

So on my last flight I accidently discovered that the noise volume is directly related to how much I open the cabin heat flapper thingy.

If it is closed, the noise almost goes completely away.

If I open it, it increases in volume up to 100% volume when the gets to about 50% open.

If I open the door more than 50% it starts to reduce the volume of the noise.

One theory I have is that it is some sort of air flow noise that is causing something to resonate and the different volume is due to the changes in airflow thru my heat system. (standard Van's with a Robbin's Wings heat muff and a standard stainless heat valve that dumps to the lower cowl when closed)

Another theory is that the angle of the heat valve door is reflecting the noise to my ears when at the proper angle making it sound louder at a certain spot.

Any ideas on what the source of the noise could be?

For context: I have a mechanical fuel pump, 2 Pmags, and a PCU5000X prop governor on my accessory case and that is it. The engine is a bone stock narrow deck carbed O-360.
 
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Does it make the noise on the ground?

Alternator bearing going bad?

Prop gov bearing going bad?

Small furry animal crawled up under the hood?
 
Does it make the noise on the ground?

Alternator bearing going bad?

Prop gov bearing going bad?

Small furry animal crawled up under the hood?

On the ground I have not heard it but I can’t get the rpm up to my normal without anchoring the plane to something.

I have looked around but have not seen any obvious issues.
 
Heater Duct Noise.

Check your Aeroducting at each connection that is secured with a hose clamp. Disconnect each clamp & slide the Aeroduct off and insure there is it a piece of it protruding into the air pathway.
 
I once had a fibreglass windshield fairing come loose in flight, it started resonating in a high pitched screech. Maybe you are experiencing something similar that reacts to varying cabin pressures as you open & close the heater valve.
 
Run some tests in flight to see if noise changes in any way with changes in RPM. That can help to eliminate many of the engine produced noises.

I would also look for a bird, hornet, mice nest somewhere in the scat tubing for the heater circuit

Larry
 
More Tests

Run some tests in flight to see if noise changes in any way with changes in RPM. That can help to eliminate many of the engine produced noises.

I would also look for a bird, hornet, mice nest somewhere in the scat tubing for the heater circuit

Larry

Also try to see if the noise changes in pitch or volume at different air speeds. This should help identify if the noise is coming from something vibrating from wind or slipstream.

Skylor
 
I had a similar experience with my 8, frequency of the noise changed with airspeed, not with engine RPM.
I took a friend with me and when the noise started he proceeded to touch and push on various areas of the airframe and canopy. Finally when he touched the lower rear portion, left side, of the canopy skirt where it contacts the side of the fuselage, the noise stopped. A small piece of felt weather stripping attached to the inside of the canopy skirt cured the problem.
Dick
 
Noise

A few years ago I had a random whining noise that was quite noticeable in flight. Bottom line, the short tachometer cable needed lube. Problem solved.
 
A few years ago I had a random whining noise that was quite noticeable in flight. Bottom line, the short tachometer cable needed lube. Problem solved.

I forgot to mention that I don’t have a tach cable. My tach drive is covered with a cap.
 
Crank seal will sometimes whine as it starts to go bad. Noise will change with RPM. More like a faint squeal.
 
Does it change with changing cabin vent openings? Try yawing plane form side to side to change noise. Guessing canopy seal.

Mark
 
noise

When I picked up my plane after getting it painted, I had a whine noise that I did not have before getting it painted, it was pretty loud.
I accidently found it when I started feeling around for air leaks. I put my fingers in the area right between the canopy rail and the canopy skirt right beside me and the noise went away. The bottom pilot side canopy skirt was bent outward near the front just enough to cause the air to whistle or whine while in flight. I messaged the skin inward in that area and it has never whined again. It was caused by the canopy sitting on the table while it was off during the painting process. So check that area on both sides.
 
My backup battery for my G3X will make a faint whine when it is switched on. It?s not enough to be annoying, but it?s enough to notice when the engine goes quiet. I kinda like it because it reminds me to shut it off after shutting down.

Another thought is to check the exhaust stack and see if it?s sagging enough to hit the lower cowling.

Just a couple of more potential spot to look.
 
I had the same type of noise (RV 7A with sliding canopy). Found one area where the weather seal had worn right over the pilot's head where the canopy meets the windscreen; fixed with new weather stripping.
 
Whine

I chased a whine for months thought it had to be altenator changed it out twice and still there went to HF and bought a scope like Dr uses and by accident the tip crossed intake gasket at cylinder and found a pin hole making all the noise new gasket no noise.
Bob
 
So the mystery remains....

I have done some testing:

1. Put some tape over the front of the tipup canopy to cover the seam between it and the front top skin and flew the airplane to see if that is where the noise is coming from. Nope....its still there.

2. Did some RPM test to see if the noise volume or frequency changes with engine RPM. Nope....the engine RPM does not affect the noise.

3. Performed some airspeed test to see if the noise volume or frequency changes with airspeed. Yes.....the volume of the noise changes with airspeed. Lower volume at lower airspeeds and louder at higher all the way up to redline. The frequency seems very consistent.
 
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My Lancair has fiberglass gap seals on the bottom between the flaps and the wing. If I inadvertently start to lower the flaps outside the white arc, the gap seals sing to me.
 
My -4 canopy skirt caused that when a piece of seal slipped out of place. Took me a while to figure it out, but stepping on the rudder at speed would change it as well as overall speed and vent airflow.
 
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