Mr Spaceman
Topic Author
Posts: 2723
Joined: Thu Mar 15, 2001 5:09 am

Sound "resonance" Damage To Jets?

Wed May 09, 2001 4:06 am

Hi guys. I have read before that some "Twin Engine" propeller aircraft have a certain "RPM Range" that both engines can not be operating in at the "Same Time". In other words, you have to set your Prop RPM's either Higher or Lower than this Range. Even the Propeller RPM gauges are "Marked" to remind the pilot, NOT to set his RPM's in this range.

Aparently, the reason for this has to do with "Sound Vibration RESONANCE". If both engines are operating within this Prop Speed Range, then a "Beat Frequency" can occur which is the same "Natural" Sound Vibration Frequency as the aircraft structure, thus causing major vibration to start occuring within the engines, wings and adventually the whole aircraft, resulting in damage that could lead to ultimate mid-air structural failure.

***My question is...Do any Airliner Jets that have Twin Engines on the wings, have to deal with "Beat Frequency Vibration"? Can their engines produce such a range of "RESONANCE" Vibration? If so, what is the pilots procedure to eliminate it?

I,ve been in a lot of airliner Jets that can really cause quite a vibration throughout the cabin [and your "BRAIN"], during engine start-up. Jets such as the mighty L-1011 TriStar!

It's just me wondering again.
Chris
"Just a minute while I re-invent myself"
 
JT-8D
Posts: 423
Joined: Sun Dec 31, 2000 11:34 pm

RE: Sound "resonance" Damage To Jets?

Wed May 09, 2001 4:18 am

None of our planes have such a restriction. I fly all the time and hear people complain about the noise of md-80 engines. If the engine sync is off, they do make a racket, but its not harmfull to the best of my knowledge..
 
10000MOH
Posts: 27
Joined: Thu May 03, 2001 5:49 am

RE: Sound "resonance" Damage To Jets?

Wed May 09, 2001 8:05 pm

Hey,

This sounds like a very interesting topic, and I am intrigued.
The effects of resonance with the natural frequencies of structures are easy to imagine: a wine glass breaking due to opera singing; a bridge collapsing due to wind flow.
I think I can research this topic pretty well, and I could get back to you, if you'd like.
For now, I could only say that aircraft engineers must know how a situation with resonance can be avoided. I wonder if the RPM guideline figures were originally calculated by some sharp engineer or determined empirically through tough experience!

 
Junior
Posts: 43
Joined: Sat Apr 07, 2001 10:12 am

Yes, It Could Happen

Thu May 10, 2001 1:32 am

We just have received a bulletin for our A319/320/312.

Problem was that several CFM56-5B/2P (DAC) engines suffered from stage 1 low pressure turbine blade seperation. Findings are that, when in 20/2-5 mode, and with a N1 between 63% to 75% those blades are excited at their natural frequency and the blade might crack within one flight.
 
JT-8D
Posts: 423
Joined: Sun Dec 31, 2000 11:34 pm

RE: Yes, It Could Happen

Thu May 10, 2001 7:28 am

What is 20/2-5 mode?
 
Monocleman
Posts: 133
Joined: Fri Apr 27, 2001 10:21 am

RE: Sound "resonance" Damage To Jets?

Thu May 10, 2001 8:29 am

One "word", Chris - "flutter". I "myself" can't say "much", but there are "plenty" of "resources" about it, and an "especially" great "help" was the "March" issue of "Air & Space", the "Smithsonian" magazine.
-Will

P.S. "What the hell is with those quotes"?
 
Junior
Posts: 43
Joined: Sat Apr 07, 2001 10:12 am

RE: Yes, It Could Happen

Thu May 10, 2001 9:40 am

Some of our CFM56-5B DAC engine are DAC II PIP (Double Annular Combustor, Program Improvement Package).

There are 2 rings with 20 fuel nozzles each. In the 20/2-5 mode, all the 20 nozzles on the outer ring operate, and on the inner ring only every second pair (2 operate, the next 2 not and so on.)
This was introduced in order to reduce the NOx emissions by shifting the cool/hot boundaries, where NOx is produced.

Regards Junior
 
F-WWAI
Posts: 129
Joined: Thu Dec 30, 1999 7:41 am

RE: Sound "resonance" Damage To Jets?

Fri May 11, 2001 11:43 pm

the "beating" noise was mainly known from propeller planes but can also appear on jet engines (but less in terms of sound pressure).
it is caused by the fact that the props or the fans of the negines are not exactly turning at the same speed. since the sound emitted at the prop or fan is - in terms of frequency and level - a function of it's runnig speed, the individual spped gives an individual sound. when the sound of two engines comes close together in terms of frequency, the "beating" phenomena installs, it is the perception at ear level of the different sound individuals with frequency variations. it's very incomfortable, but wil not impact the structure of neither the engine nor the aircraft.
in the golden days of prop flying, the flight engineer spent some good time of the flight adjusting the throttles individually to illiminate the beating. two possibilities exist: either you trim the engines to run at exactly the same speed, or you offset the engine setting that much that the frequencies cannot overlap between the engines ...

high sound pressures from any source close enough can impact structure life. the technical term is sonic fatigue, and the design usually takes care for that.

have fun,
 Smile/happy/getting dizzy FW
 
Mr Spaceman
Topic Author
Posts: 2723
Joined: Thu Mar 15, 2001 5:09 am

RE:F-WWAI/Monocleman

Sat May 12, 2001 5:25 am

To all of you that responded, Thank You. F-WWAI I stand corrected. I thought that you could end up with a Beat Vibration if both Props were set at the Same RPM. You have explained that they have to be at least slightly different, in order to eliminate the Beat that can occur. That makes me wonder about "Rudder Trim" being needed if your RPM's are different? I myself do not have my "Multi-Engine" yet, so my info was a little off the mark. You did however acknowledge that this Frequency Vibration does exsist.

The info about Jet Engine "Fan Blades" being damaged because of sound vibration is also very interesting, as well as part of my original posted question.

--"And" "to" "you" "Monocleman", "flutter" "has" "absolutly" "nothing" "to" "do" "with" "the" "Question" "that" "I" "posted". Keep reading those books and mags that you mentioned, that's a good way to Learn! I'm over "TWICE" your age and have been up flying "ALONE". Once "YOU" have achieved that, the last thing that you'll be worried about is "Why the Hell someone is using so many Quotation Marks"!

Chris
"Just a minute while I re-invent myself"
 
JT-8D
Posts: 423
Joined: Sun Dec 31, 2000 11:34 pm

Junior

Sat May 12, 2001 12:41 pm

Junior, is that mod being done to just the -5b engines? Is it something that is still in the testing stage? I have never heard of it before, but it sounds like a neat idea. I spoke to our Maint control engine desk about it. They didnt know anything either. Is there a fuel savings as well?
 
F-WWAI
Posts: 129
Joined: Thu Dec 30, 1999 7:41 am

RE: Sound "resonance" Damage To Jets?

Mon May 14, 2001 10:21 pm

Hey chris, please get that right:
if the PHYSICAL speed of the prop or the fan is exactly the same (difficult to achieve) then there is no beating. If it's SLIGHTLY different (<=1.5 %) then you may obtain the beating. If it's significantly differen, there may be no beating noise, depending on the multiples of speed times fan/prop blades between the engines.
it's the small offset in frequency which makes the beating phenomenon.
 Smile/happy/getting dizzyFW

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