Trying to narrow down odd noise at low rpm

Fearless Tower

Fearless Tower

Touchdown! Greaser!
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Fearless Tower
So the other day while taxiing back to the ramp, I noticed a whining/squeaking sound. Basically sounded exactly like squeaking brakes as if the brake pads were rubbing slightly. Only problem- the sound continued after I came to a complete stop. Not the brakes.

Went flying again today and noticed it again coming back to the turndown. I sat for a minute and tried to narrow it down. Shut off the avionics master and took the headset off. Still there. Played with the throttle a bit and found that it seems to go away at about 850 rpm. Engine idles at 600. So it is definitely there between 600-850 rpm and then goes away as you increase throttle.

Airplane is a 1948 170 with 0-300.

I am thinking the tach may be going bad, but wondering if it might be the generator.

Any way to isolate it, or any other thoughts?
 
Fearless Tower said:
So the other day while taxiing back to the ramp, I noticed a whining/squeaking sound. Basically sounded exactly like squeaking brakes as if the brake pads were rubbing slightly. Only problem- the sound continued after I came to a complete stop. Not the brakes.

Went flying again today and noticed it again coming back to the turndown. I sat for a minute and tried to narrow it down. Shut off the avionics master and took the headset off. Still there. Played with the throttle a bit and found that it seems to go away at about 850 rpm. Engine idles at 600. So it is definitely there between 600-850 rpm and then goes away as you increase throttle.

Airplane is a 1948 170 with 0-300.

I am thinking the tach may be going bad, but wondering if it might be the generator.

Any way to isolate it, or any other thoughts?
Click to expand...

Find a different woman to fly with.
 
I would think I'd want someone to take a listen with the cowl off and have a good look around.
 
Like you thought, I'd start with the generator.....OR.....you can wait until it outright fails (not recommended).
 
Venturi for vacuum on the '48 or modded to have a vacuum pump? (Or neither?)
 
Shouldn't be whining at idle on the ramp it it's a venturi suction leak....
Ah, the joys of ownership.....:(
 
Nah, I was thinking if it might be the pump failing. If... it's been modded to even have a pump at all.

Dry pump probably wouldn't make a noise like that, but a wet pump?? Dunno...
 
I think you have a tinnitus problem.:eek:

José
 
You coule always check the tach by pulling the cable and trying it out.
 
Had something similar on my O-200, turned out to be a failing dry vac pump. Watch the vac gauge as you play with the throttle and listen to the noise. If you have a vac pump, that is.
 
Anything belt drive? If yes, drag a piece of 120 sand paper on it while it's running and see if it disappears.
 
FWIW... I flew a c-152 that did almost the exact same thing, only at high RPM and got quiet at low RPM... I was told, after it was "fixed," that It was the tach.
 
You might try the ol' stethascope on the tach trick. They sell them in autoparts stores, for listening for noisey valvetrains, etc.
 
Apparently, there is another possibility....the venturis. I also posted this on the 170 board and was told that the venturi(s) will naturally whistle from the prop blast at low RPM. Makes sense since the noise just started to appear recently when I started flying the plane from the right seat and the venturis are mounted on the right side.
 
Fearless Tower said:
Apparently, there is another possibility....the venturis. I also posted this on the 170 board and was told that the venturi(s) will naturally whistle from the prop blast at low RPM. Makes sense since the noise just started to appear recently when I started flying the plane from the right seat and the venturis are mounted on the right side.
Click to expand...

Ha! Cool. Never would have thought of that, but it makes sense. Easy enough to check, too!
 
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