Why do helicopters "chop"?

[RobertN]

Re: Why do helicopters \"chop\"?

That cleared up a lot of stuff I did not think about or clearly understand before. Cool stuff!

 

[BigDave]

Re: Why do helicopters \"chop\"?

Transonic flow is the region defined between 0.8 & 1.2 Mach. It is assumed for transonic flow that shocks exist in this regime due to acceleration of the flow due to the varying shape of the object. Technically, supersonic flow is anytime the flow is greater than Mach 1.0, but in the common usage, it is normally considered to be supersonic when the mach number is greater than 1.2. The 1.2 number is what you will see if you are reading papers discussing supersonic flow. I was using the term supersonic as > 1.0 for conceptual purposes, since this is TBN and not an AIAA conference. Transonic rotor blades generate shocks.

Transonic Helicopter Noise

 

[MossRoad]

Re: Why do helicopters \"chop\"?

<font color="blue">I was using the term supersonic as > 1.0 for conceptual purposes, since this is TBN and not an AIAA conference. </font>
Oh, sure, talk down to the tractor guys.... just kidding. /forums/images/graemlins/grin.gif I appreciate the time people with training take when people like me ask questions. Thanks. /forums/images/graemlins/laugh.gif /forums/images/graemlins/smile.gif

 

[Trev]

Re: Why do helicopters \"chop\"?

</font><font color="blue" class="small">( I'm getting in on this late, but here's an explaination that I was given some time back. Most of the chop-chop comes from the sound of the tail rotor reflected of the bottom of the main rotor baldes! )</font>

You know, I read the same thing once.. that helicopter noise could be reduced dramatically by shrouding the rear rotor in some way. I gather they have almost silent copters now? (No jokes about Black Helicopters please.) /forums/images/graemlins/smile.gif

Anyone know what the current state-of-the-art is? I don't know for a fact that it's true that some of them are almost silent. But it made sense that the interaction of the main rotor and the rear rotor was the cause of the buffeting "chop" sound.

As an aside, a friend of mine used to say "Helicopters *always* mean bad news!" Someone is hurt, being rushed to a hospital, some kind of miltary action, some criminal on the run, some natural disaster, and so on. I think he exaggerated, but it was an amusing thought. I'm certain that to a person stranded inside enemy lines, the sound of the chopper is the sweetest in the world!

Bob

 

[accordionman]

Re: Why do helicopters \"chop\"?

hello to all,
i was a helicopter mechanic in the army in korea in 1951,52.
i was attached to the 8063 MASH. the army sent me to school
in SAN MARCOS TX. in 1951, where i learned about fixed wing
aircraft, and helicopters.
the other person had it correct. the tip speed of the blade is breaking
the sound barrier, and this is what causes the chop,chop sound.
we had BELL H13D, and E models in korea, and they were used to fly
wounded to the MASH unit. i flew with the pilots sometimes, but i was
never comfortable in one. i knew too many things about all of the
mechanical parts, and the stress that was put on them during flight,
and this worried me. i am 75 years old in two months, and have never
flown in a "chopper" since i was in the army.
accordionman
william l. brown
wright city, mo.

 

[jinman]

Re: Why do helicopters \"chop\"?

</font><font color="blue" class="small">( Transonic rotor blades generate shocks. )</font>

Dave, no matter what it may sound like, I think you and I are agreeing more than it might appear. Certainly, there are air flows around a transonic object that exceed the speed of sound. To produce these flows and their associated noise, the rotor only has to approach speeds near 0.8 Mach. /forums/images/graemlins/cool.gif

Thanks for the link to the paper regarding modelling of the noise. It appears that for a particular size and shape of rotor, the noise is very predictable and capable of being accurately modelled. /forums/images/graemlins/smile.gif

 

[StoneHeartFarm]

Re: Why do helicopters \"chop\"?

</font><font color="blue" class="small">( We had one circling nearby this morning. )</font>

Circling, huh? Dude, that don't sound so good. Are the neighbors in trouble?

Steve

 

[MarkV]

Re: Why do helicopters \"chop\"?

Gosh guys, I almost understood part of what you were saying and didn't want to feel like the only person with nothing to contribute. So, which type of chopper makes the loudest chop, the gear model or the hydrostatic model? /forums/images/graemlins/confused.gif

MarkV

 

[RobS]

Re: Why do helicopters \"chop\"?

<font color="blue"> So, which type of chopper makes the loudest chop, the gear model or the hydrostatic model </font>

It's gotta be the one with the R1 tires regardless of transmission. Everyone knows R4s are tuned for transonic speeds. Now which oil for supersonic tractoring, conventional or synthetic /forums/images/graemlins/smile.gif

 

[bontai_Joe]

Re: Why do helicopters \"chop\"?

Thank you all for answering something I have wondered about for many years. And for those of you that learned this stuff while wearing a military uniform in service of this country, a BIG THANK YOU!