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High Frequency Vibrations on the R22


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This is something I remember hearing but can't find where I wrote it down...

 

On most helicopters, medium frequency vibrations are associated with the Tail Rotor and High frequency vibrations are associated with the engine

 

BUT

 

on the R22 it's actually the other way around -- medium frequency vibrations are associated with the engine and high frequency vibrations are associated with the TR.

 

Is this true?

 

In real life, of course, if I get a funky vibration from either one of these sources I'm going to land and let the maintenance guys tell me what it is :) but inquiring students (my future ones) need to know...

 

HVG

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Engine & TR on the R22 will both be high (or medium) frequency vibrations--it just depends on what you want to compare them to. The TR does spin faster than the engine, but you probably won't be able to tell the difference in a few hundred RPM. You'll feel the TR buzz in the pedals, the engine in your back, and the main rotor (at a much lower freq) in everything.

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Thanks. I've always been sort of amused/alarmed at the prospect of flying along trying to guess whether that buzzing was high or medium frequency while whatever it is starts to fall apart. Feeling is definitely going to be the best indication, I agree, and I still don't like either one. I'll probably just teach to the test on the 'sound' question and advise extreme caution about any weird sounds and vibrations in actual flight.

Edited by Hovergirl
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I agree with Delorean. The faster something is moving the high the fequency vibration will be when something goes wrong.

 

A book at a reasonable price is Rotorcraft Flying Handbook. It covers all of the topics for helicopters and in a language that is understandable.

 

I have been to a number of flight schools and most of them carry this book. It can also be purchased at Sporty's Pilot Shop.

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An electronic copy of the Rotorcraft Flying Handbook can be gotten from the FAA web sit at:

 

http://www.faa.gov/library/manuals/aircraf...a-h-8083-21.pdf

 

There's a lot of good information available as soft copy for free from them.

 

 

Thanks Pogue. It is always good to get information for free. I think VR is great for this as people can share information.

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The Rotorcraft Flying Handbook is definitely a good source, but I was wondering if R22s were different in regards to the speeds of the TR and engine.

 

My initial question was basically whether the R22 differed from other helicopters in that the TR RPM was actually faster than the engine, or at least would produce a higher frequency vibration if something went wrong. I remembered hearing something to the effect that it was different that way, but couldn't find anything in the POH or Robinson-specific materials. Looking at the drive system information I'm guessing that this is the case, but I'm not sure I'm looking at the numbers right.

 

 

(Engine to upper sheave -- .8536:1 speed reducing ratio; Drive line to Tail Rotor: 3:2 speed increasing ratio. Does this mean the TR is spinning 'faster'?)

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The engine is 2652 RPM @104% and the tail rotor is 3396 RPM and Main rotor is 530 RPM. The tail rotor will be a higher frequency vibration. I believe the reason that the vibrations are different on the R22 then what is on the written exams is the FAA uses the vibrations you would find in a turbine a/c.

Edited by ChprPlt
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Yeah, if you're going to be teaching this, you'll want to make sure that your students are aware that when it comes to the written exams (private and commercial), the FAA considers all helicopters to be turbine engines - and as you may recall, there's specifically a question on the written test that asks about a high-frequency vibration and what would be its source. For the written test, the correct answer is the engine.

For the R22 (and I believe it's the same for the R44, don't have my POH handy) a high-frequency vibration would be associated with the tail rotor. However, I think it's different for the Schweizer 300 - pretty sure the engine is higher RPM than the tail rotor. I like delorean's answer too, as to where you'd be likely to feel it...but that being said, on the R22 at least, a dirty squirrel cage fan can give you a pretty nasty vibration on the back of the seat.

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At least they give you the exam answers in advance. Just one more instance where what you're actually going to need to know to fly is not what's on the exam. (Can I forget how to take off in a gyroplane yet?) It's good advice to try and isolate where the noise/feeling is coming from to help diagnose the problem. "It sounded funny" is probably pretty frustrating to hear.

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