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What causes the buzzsaw sound in turbofan engines?...

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  • What causes the buzzsaw sound in turbofan engines?...

    Especially on takeoff. Is it the speed of the air going through the N1 blades?

  • #2
    Originally posted by UALdave View Post
    Especially on takeoff. Is it the speed of the air going through the N1 blades?
    More specifically, it's the tips of the N1 blades breaking the sound barrier. You're hearing little sonic booms.

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    • #3
      Like this?: https://www.youtube.com/watch?v=6rX4xv5-NvE

      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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      • #4
        OK, that's cool! I didn't now that the N1 blades spun that fast, in terms of rpms.

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        • #5
          Originally posted by ATLcrew View Post
          More specifically, it's the tips of the N1 blades breaking the sound barrier. You're hearing little sonic booms.
          I can't say for sure but I very much doubt that that's the case.

          If the tip of the blades break the sound barrier when at 100% N1 and about 4 kts (beginning of the take-off roll), what's left for when the plane is travelling at M 0.8?

          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
          --- Defend what you say with arguments, not by imposing your credentials ---

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          • #6
            Originally posted by Gabriel View Post
            I can't say for sure but I very much doubt that that's the case.

            If the tip of the blades break the sound barrier when at 100% N1 and about 4 kts (beginning of the take-off roll), what's left for when the plane is travelling at M 0.8?
            Ok...the hell better aeroengineer and the hell better ATP disagreeing when it's RPM * diameter * pi * 60 (and check units for mph or km or knots) and we KNOW and it's not_an opinion.

            Off to check www.Internet.com for absolute RPMs instead of a percentage....
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #7
              OK, manure-shoveling, soybean-loving aggie has some math for the ainginears:

              Speed of sound = ~768 MPH

              Cessna 172. Max power = 2700 RPM, Prop diameter 72 in...simple lateral speed at prop tips = 578 MPH

              GE90-115. Max N1 = 2550 RPM, Fan diameter 123 in...simple lateral speed at fan tips = 933 MPH...BOOM

              (ok a lot of little tiny booms, which sound more like a big wood chipper BUT...score one for ATL crew).

              (Maybe I'll try to find other RPM's and fan diameters...maybe not)

              Signed, 3BS, the hell better agronomist and shit-shoveler.

              (I'll leave it to Gabe to figure in the added velocity as the aircraft moves forward).
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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              • #8
                EDIT: Turbofan speed figures are reduced by 10% after reports that engines tend to only run in the low 90s % N1 on takeoff. I did_not modify the propeller* figures- including the GE36.

                The speed of sound ~= 770 MPH

                0320: 2700 RPM, 72"; 570 MPH*
                GE90: 2250 RPM, 123"; 840 MPH- BOOM
                JT8D: 8600 RPM, 39.9"; 920 MPH-BOOM
                JT9D: 3600 RPM, 92.3"; 890 MPH-BOOM
                RB211: 4800 RPM, 84.8"; 1090 MPH-BOOM (Those dudes REALLY grind!)#
                NK-12: 850 RPM, 220"; 560 MPH*
                GE36: 1395 RPM, 140"; 580 MPH*

                Disclaimer- these are simple rotational tip speeds. In one post Gabriel states that it speeds up as you add forward motion, and in another post Gabriel says all sorts of things change as you squeeze down and heat up. Indeed these are facts, but would take many pages of additional verbiage to address. I offer these data to only suggest that fan tips might just exceed the speed of sound. I've also come to learn via a friend of ATL that some engines spend most of the time well below 100% N1.

                #Footnotes:
                -I like these guys and want one too!: https://www.youtube.com/watch?v=qM7ksfRVF70
                -One actually working: https://www.youtube.com/watch?v=378zRym9jrc ....1:40...Oh yeah!
                -More grinding: https://www.youtube.com/watch?v=opbrMxJxrkc
                Last edited by 3WE; 2016-01-24, 15:14. Reason: ...to add estimates for Not_Karl's engines of choice.
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                Comment


                • #9
                  Originally posted by 3WE View Post
                  Cessna 172. Max power = 2700 RPM, Prop diameter 72 in...simple lateral speed at prop tips = 578 MPH

                  GE90-115. Max N1 = 2550 RPM, Fan diameter 123 in...simple lateral speed at fan tips = 933 MPH...BOOM
                  Do you have the numbers for the Kuznetsov NK-12 and the General Electric GE36?
                  "I know that at times I can be a little over the top." -ITS

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                  • #10
                    Originally posted by Not_Karl View Post
                    Do you have the numbers for the Kuznetsov NK-12 and the General Electric GE36?
                    No, but I have a nifty spreadsheet and if you'll check www.internet.com and give me fan diameter and RPM I'll be happy to supply a simple speed at the tip.

                    Beware- since magenta line pilots only do N1 as a percent, accurate RPMs can be hard to find. One source listed RPM figures of a JT8D for a JT9D.
                    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                    Comment


                    • #11
                      Originally posted by 3WE View Post
                      Ok...the hell better aeroengineer and the hell better ATP disagreeing when it's RPM * diameter * pi * 60 (and check units for mph or km or knots) and we KNOW and it's not_an opinion.

                      Off to check www.Internet.com for absolute RPMs instead of a percentage....
                      No, it is not so simple. You have to consider:
                      - The fan speed is not always N1. There are triple-spool and geared fan engines.
                      - The intake of the fan duct is divergent, thus slowing down and compressing the air, which becomes hotter. Hotter air means faster speed of sound and hence a lower Mach for a given speed.
                      - Have you noted that the blades are not straight anymore? Sweptback tips delay shock-waves.

                      Anyway, will not say that blade tips are not supersonic. But I did note that that sawing sound is maximum at the beginning of the take-off roll and fades out as the plane accelerates. That seems contradictory with a supersonic boom.

                      --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                      --- Defend what you say with arguments, not by imposing your credentials ---

                      Comment


                      • #12
                        Ok, I did some research and yes, fan blade tip speeds in high-bypass turbofan engines and reach M 1.5 at high engine ratings. I didn't expect that, but happy to have been wrong and learn something in the process.

                        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                        --- Defend what you say with arguments, not by imposing your credentials ---

                        Comment


                        • #13
                          Originally posted by 3WE View Post
                          No, but I have a nifty spreadsheet and if you'll check www.internet.com and give me fan diameter and RPM I'll be happy to supply a simple speed at the tip.

                          Beware- since magenta line pilots only do N1 as a percent, accurate RPMs can be hard to find. One source listed RPM figures of a JT8D for a JT9D.
                          If my Internets are correct, NK-12 = 850 (or 750?)RPM, 220in ; GE36* = 1395RPM, 140in.

                          (*= From a NASA document, so should be accurate. Other Internets said 1500RPM, 116in...)
                          "I know that at times I can be a little over the top." -ITS

                          Comment


                          • #14
                            Originally posted by Not_Karl View Post
                            If my Internets are correct, NK-12 = 850 (or 750?)RPM, 220in ; GE36* = 1395RPM, 140in.

                            (*= From a NASA document, so should be accurate. Other Internets said 1500RPM, 116in...)
                            See edits above.

                            You're welcome
                            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                            Comment


                            • #15
                              Originally posted by Gabriel View Post
                              Ok, I did some research and yes, fan blade tip speeds in high-bypass turbofan engines and reach M 1.5 at high engine ratings. I didn't expect that, but happy to have been wrong and learn something in the process.
                              Amazing those blades last as long as they do, isn't it?

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