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Sound "resonance" Damage To Jets?  
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Posted (15 years 3 weeks 1 day 17 hours ago) and read 6336 times:

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.

"Just a minute while I re-invent myself"
10 replies: All unread, jump to last
User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 1, posted (15 years 3 weeks 1 day 16 hours ago) and read 6282 times:

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..

User currently offline10000MOH From United States of America, joined May 2001, 27 posts, RR: 0
Reply 2, posted (15 years 3 weeks 1 day 1 hour ago) and read 6254 times:


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!

User currently offlineJunior From Switzerland, joined Apr 2001, 43 posts, RR: 0
Reply 3, posted (15 years 3 weeks 19 hours ago) and read 6248 times:

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.

User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 4, posted (15 years 3 weeks 13 hours ago) and read 6229 times:

What is 20/2-5 mode?

User currently offlineMonocleman From United States of America, joined Apr 2001, 137 posts, RR: 0
Reply 5, posted (15 years 3 weeks 12 hours ago) and read 6229 times:

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.

P.S. "What the hell is with those quotes"?

User currently offlineJunior From Switzerland, joined Apr 2001, 43 posts, RR: 0
Reply 6, posted (15 years 3 weeks 11 hours ago) and read 6217 times:

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

User currently offlineF-WWAI From Andorra, joined Dec 1999, 131 posts, RR: 0
Reply 7, posted (15 years 2 weeks 5 days 21 hours ago) and read 6197 times:

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

User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Reply 8, posted (15 years 2 weeks 5 days 15 hours ago) and read 6189 times:

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"!


"Just a minute while I re-invent myself"
User currently offlineJT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3
Reply 9, posted (15 years 2 weeks 5 days 8 hours ago) and read 6171 times:

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?

User currently offlineF-WWAI From Andorra, joined Dec 1999, 131 posts, RR: 0
Reply 10, posted (15 years 2 weeks 2 days 22 hours ago) and read 6155 times:

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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