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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › Oscillating noise from my T Rex 550e
04-03-2015 02:41 AM  6 years ago
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HeliOCD

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San Diego, CA

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Oscillating noise from my T Rex 550e
I am hearing a noise from my T Rex 550e that can only be described as an oscillating wrrr,wrrr,wrrr sound. It's not a very fast oscillation about one each second.
The helicopter flies fine and there are no other problems. I'm just not sure if others have experienced that same type of sound.

I mostly hear it when I'm doing fast flybys.
The 550 is stretched to a 600 size. The boom, blades etc. Using a 3gx version 5.0 with a 600 MX 1220kv motor. I am also using slant gears with a 10 T pinion. Any help would be greatly appreciated.
Its all earth!
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04-03-2015 04:21 AM  6 years ago
EEngineer

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TX

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It's not a very fast oscillation about one each second.
So it's frequency is ~ 1 Hz?...Right?

That's what called an acoustic "beat frequency".

Amazing that it "warbles" at that rate.

Caused by, among the vibes that make acoustic noise, two specific frequency vibes....whose freqs. are only ~1 Hz apart....are interacting to produce this effect.

Not saying that anything's wrong with your heli.....just that you observed and can re-produce this effect.
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04-03-2015 04:34 AM  6 years ago
Santiago P

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Dayton

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So that is the correct name for it, thanks EE.

I call it the happy heli sound. Almost every heli I owned, nitro or electric, does it when its hovered still at one RPM, or in a steady state condition that allows you to hear that 1Hz resonance.

Is all good unless the sound gets progresively louder, or you see parts fly off the machine = not good. lol

S
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04-03-2015 04:37 AM  6 years ago
HeliOCD

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Yes it is a steady 60 Hz per minute. Like I said it flies very well. Actually extremely well. But I never have heard that sound with my 500 series or other helicopters that I've had. I just thought maybe that was something others have experienced.
Thanks for your help.
Its all earth!
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04-03-2015 04:38 AM  6 years ago
EEngineer

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TX

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I call it the happy heli sound.
I agree with that, just didn't want to imply vibe issues...

When the acoustic signature that ones ear detects and "learns"....and when that sound changes, it's like "WTF" happened...sounds different...what's wrong now.....
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04-03-2015 04:44 AM  6 years ago
Santiago P

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Dayton

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Exactly,

One good example of two rotating components with slighly different frequencies is a twin engine prop airplane producing the wha-wha sound from the propellers being slighly off synch.

As the RPM is matched the sound goues away. If they become more off synch the wha-wha is more rapid until it also disappears.

I am laughing at all the "technical" terms I am using.

whaa-whaaaa LOL
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04-03-2015 04:51 AM  6 years ago
EEngineer

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TX

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Same principle used by "washing-machine charlie" in WWII to disturb marines in the Solomons.
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04-03-2015 05:03 AM  6 years ago
HeliOCD

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When the acoustic signature that ones ear detects and "learns"
funny that you say that. I am almost completely deaf in one ear. Could that have something to do with what I'm hearing?
Its all earth!
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04-03-2015 05:09 AM  6 years ago
EEngineer

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TX

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Nah....

Your good ear(and brain) are performing properly.

You heard exactly what the acoustics are doing.

Classic case....
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04-03-2015 06:20 AM  6 years ago
HeliOCD

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San Diego, CA

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Thanks for the help.
So wrrr,wrrr,wrrr is a good thing
Its all earth!
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04-03-2015 06:43 AM  6 years ago
EEngineer

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TX

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With a new heli's 1st flight, I tend to memorize the sound in a simple hover.

If that sound ever changes, sumthin' changed....and that's not ever good....

Whether they're helis or planes, they can only talk to you with weird sounds...that change...
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