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  • #16
    Originally posted by E-Diddy! View Post
    ... is a subsonic bow wave? Is that the phenomenon that occurs when a body moves through a fluid where the wave transmits that object's kinetic energy through the fluid, like what occurs in water? Because that occurs in water, not so much air.
    If it happens in a canoe at 3 kn; then why wouldn’t you think there was a similar phenomenon in the air at 200 kn?

    Sure the effect in aircraft are a lot less noticeable at subsonic speeds but there still are waves.

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    • #17
      Originally posted by ATFS_Crash View Post
      If it happens in a canoe at 3 kn; then why wouldn’t you think there was a similar phenomenon in the air at 200 kn?

      Sure the effect in aircraft are a lot less noticeable at subsonic speeds but there still are waves.
      Because water is an incompressible liquid but air is highly compressible? Hence why sonic shock waves form in the first place?


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      • #18
        Originally posted by E-Diddy! View Post
        - A 707 once encountered mach tuck accidentally. The pilot recognized what had caused his aircraft to go from level flight to straight down, and was able to recover by deploying full flaps, full spoilers, and full thrust reversers while in the dive. The extreme forces caused two of the engines to depart the aircraft and extensive damage was caused, but the aircraft made it back.
        I doubt that the flaps would survive a deployment at any high Mach number, let alone deploying full flaps at about supersonic speeds.

        I also doubt that you can activate reverse thrust in flight in the 707.

        Do you have any source or more data to try to find the incident?

        --- 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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        • #19
          Originally posted by E-Diddy! View Post
          - WTF is a subsonic bow wave? Is that the phenomenon that occurs when a body moves through a fluid where the wave transmits that object's kinetic energy through the fluid, like what occurs in water? Because that occurs in water, not so much air.
          Originally posted by ATFS_Crash View Post
          If it happens in a canoe at 3 kn; then why wouldn’t you think there was a similar phenomenon in the air at 200 kn?

          Sure the effect in aircraft are a lot less noticeable at subsonic speeds but there still are waves.
          Originally posted by E-Diddy! View Post
          Because water is an incompressible liquid but air is highly compressible? Hence why sonic shock waves form in the first place?
          Nonsense after nonsense.

          The wave of a canoe doesn't occur in the water, neither it does in the air. It happens in the INTERFACE between the air and the watter (the SURFACE of the water). You need two fluids of different density to form such waves in their interface.

          And the water IS compressible. Sonic shockwaves happen in water if something travels fast enough in it. Fast enough is transonic speeds, which are much higher than transonic speeds in the air because the speed of sound in the water is much faster than the speed of sound in the air, because yes, the watter is much less compressible. But it has nothing to do with the waves of a canoe at 3 kts.

          And of course there are waves in the air at subsonic speeds. Just talk and you form waves in the air (pressure waves, sound) even if you are not moving (ok, your vocal strings are moving, at a very subsonic speed of course).

          --- 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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          • #20
            Originally posted by ATFS_Crash View Post
            I’m going to disagree with all of you. I don’t think it is supersonic but I think it is a shockwave.
            Guess again. For it to be a "Shock-wave" it has to be a "Sonic Shock-wave". Other wise it's just a wave.

            Some of you seem to be implying that all shock-waves are supersonic. I’m going to disagree. I feel this is a subsonic bow shock wave. Though the term shock is commonly to reference supersonic; it also can mean a wave that has a change of pressure, temperature, etc….
            You have just defined a sonic shock-wave.

            It could be optic glare in the lens; but the location seems to suggest it is a bow shock; the type of aircraft and the speed that is suggested in the thread suggests that it is a subsonic bow shock wave.
            All aircraft disturb the air through which they move. The faster they move, the greater the disturbance. The disturbance is what causes "Wake Turbulence". Below critical Mach numbers the disturbances are unorganized and do not form measurable pressure patterns until after the aircraft passes. The disturbed air remains close to that part of the airframe causing the disturbance. In this case the nose appears to be creating a pressure wave in front of the aircraft.

            Once again this is a difference of opinion of terminology. Though shockwave is generally used in the context of supersonic; the word shockwave can also apply to subsonic waves.
            No it can't. By definition, a "Shock-wave" causes a sudden, abrupt, change in pressure and temperature. You can't have one without the other. The "Shock" portion only occurs at higher Mach numbers where they are measurable.

            Additionally, pressure waves always fade out (feather) into nothingness as the wave moves away from the energy source. The one depicted in the photo has a ring around the outer diameter of the ring forward of the nose.

            The shock is a sudden change of pressure that is apparently causing the water vapor in the air to be visible.
            That's not what's happening.

            Though “shock” is more commonly used in the context of at or in excess of the speed of sound it can also apply subsonically.
            No it cannot. Subsonic waves are just waves.

            Grab your saws; I’m out on a limb. Let the ridicule begin.
            ZZZZ ZZZZ ZZZZ ZZZZ ZZZZ Watch out below!

            It’d be less controversial to drop the word “shock“; and just call it a “bow wave“. That would be more difficult to argue with as it would be more common usage.
            Yes but that's not what I see. I don't see any kind of a wave. Why? Because what is shown can't happen in nature.
            ----
            Of course there is a tinfoil hat crowd that might suggest the subsonic bow wave is evidence of some sinister government electromagnetic weapons testing. Some people may consider it a plasma artifact; suggesting drag reduction and/or stealth and/or weapon.
            Take a look at a bow wave on a boat. It takes a knot or two before any organized wave is generated. In the displacement mode, once the wave is generated, the faster the boat moves, the sharper the angle between the boats bow and the wave. A bow wave never moves at 90 degree angles to the bow.

            A shock-wave forms almost perpendicular to the airframe at Mach 1. At Mach numbers above Mach 1 the angle becomes more acute.
            Don
            Standard practice for managers around the world:
            Ready - Fire - Aim! DAMN! Missed again!

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            • #21
              Now let's put the things in perspective:

              The 747-200 has a Vmo/Mmo of 355 KIAS / M 0.88. Meaning that you are not to exceed any of those speeds.

              To start with, I'm quite sure the airplane would not be exceeding those speeds, but there is more.

              The photographer estimates that the altitude of the plane was between 5000 and 6500ft.

              Even if the airplane was flying at the Vmo of 355KIAS, at those altitudes and even in a very cold day (ISA-25) the Mach number would be in the order of 0.57.

              But I bet that the airplane wasn't flying at such speeds. Below 10000ft the maximum speed is 250kts, which gives a Mach of about 0.4.

              Nothing remotely similar to a shockwave can happen around the nose of a 747 at those Mach numbers.

              As I've said, I don't know what this halo is but it ain't no shockwave, bow wave, or anything similar.

              Now, the condensation that uses to happen for example at the wingtips or the flaps tips doesn't have anything to do with a shockwave but there is still condensation, which is formed by variations in the air conditions (pressure and temperature) in that zone that renders an air with high content of humidity unable to sustain the water dissolved in it, so it condensates.

              But even that kind of condensation is hard to imagine here. The nose of the plane is a zone of low disturbance for the air. Even the static ports are placed there! And the halo forms around the nose at a good distance from the skin. The magnitude of the disturbance diminishes with the distance from the source of such disturbance. The pressure and temperature of the air in the zone of the halo should be almost unmeasurably different from those of the undisturbed air.

              --- 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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              • #22
                ATFS=pwn3d.
                sigpic
                http://www.jetphotos.net/showphotos.php?userid=170

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                • #23
                  Originally posted by Gabriel View Post
                  And the water IS compressible. Sonic shockwaves happen in water if something travels fast enough in it. Fast enough is transonic speeds, which are much higher than transonic speeds in the air because the speed of sound in the water is much faster than the speed of sound in the air, because yes, the watter is much less compressible. But it has nothing to do with the waves of a canoe at 3 kts.
                  Water vapor is highly compressible.
                  Water in its solid form is considered incompressible.
                  In it's liquid state, compressibility is very slight.

                  Water's Compressibility
                  Water at temperatures common in nature has a compressibility factor of around 0.0000034, meaning that a hydrostatic pressure of 6.89 kilopascals (1lb/ sq. in) would reduce unit volume by about 0.0000034 of the original volume.

                  This compressibility of water is so slight we could never actually see it with our own unaided eyes. We might think water is not compressible. However, if that were the case then the oceans would be about 30 meters higher than they are now, and therefore cover an extra 5 million square kilometers of Earth!

                  For practical purposes water is incompressible except in cases where the compressibility factor needs to be considered.
                  Don
                  Standard practice for managers around the world:
                  Ready - Fire - Aim! DAMN! Missed again!

                  Comment


                  • #24
                    Originally posted by Gabriel View Post
                    Now let's put the things in perspective:

                    The 747-200 has a Vmo/Mmo of 355 KIAS / M 0.88. Meaning that you are not to exceed any of those speeds.
                    I think you have used the speeds for the 747-400.
                    The 747-200 speeds are:
                    VMO=370 KIAS / M 0.92
                    Don
                    Standard practice for managers around the world:
                    Ready - Fire - Aim! DAMN! Missed again!

                    Comment


                    • #25
                      Originally posted by Gabriel View Post


                      And of course there are waves in the air at subsonic speeds. Just talk and you form waves in the air (pressure waves, sound) even if you are not moving (ok, your vocal strings are moving, at a very subsonic speed of course).
                      Isn't a sound wave, the one you make when you talk, a sonic wave? Being of the basic understanding that sonic = sound? Isn't the sonic boom a product of the way your ear interprets a sonic shockwave as the compressed air vibrates your tympanic membrane?

                      And your vocal strings don't move at all, they vibrate.


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                      • #26
                        Originally posted by Dmmoore View Post
                        Water vapor is highly compressible.
                        Water in its solid form is considered incompressible.
                        In it's liquid state, compressibility is very slight.

                        [/font]
                        For practical purposes water is incompressible except in cases where the compressibility factor needs to be considered.[/left]
                        Amen for that last sentence!

                        If the solid water (ice) could be always considered incompressible, the sound in the ice would travel faster than the speed of light.

                        An interesting example of the consequences of the compresibility of the water is this: In metrology there is a strong tendency to replace phisical representations (masters) of the units by phisical definitions so the measurement can be reproduced anywere. One master that survived is the kilogram, which is by definition the mass of a certain individual object. It was once considered to use "the mass of one 1000cm3 of H2O", however the density of the H2O (and hence the mass of a given volume) is a function of its temperatura and, to a lesse but not negligible degree, pressure, so one should add "at a given temperature and pressure". But pressure is force per unit of area, and force is mass times acceleration, so you would have mass inside the definition of mass!!

                        Anyway, the only thing I wanted to show is that the reason why a bow wave happens in the air when you exceed the speed of sound and in a boat "in the water" a very slow speeds is absolutely not related with the air being comopressible while the water not. You ove something blunt in the water at a speed above the speed of sound in the water (that is, VERY fast) and you have exactly the same shockwave than in the air. Move something at any speed in the interface of any two fluids (including two compressible gases of different density) and you have surface waves.

                        --- 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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                        • #27
                          Originally posted by Dmmoore View Post
                          I think you have used the speeds for the 747-400.
                          The 747-200 speeds are:
                          VMO=370 KIAS / M 0.92
                          Thanks for the correction. Anyway you can see that little changes with either set of speeds.

                          --- 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


                          • #28
                            Originally posted by E-Diddy! View Post
                            Isn't a sound wave, the one you make when you talk, a sonic wave? Being of the basic understanding that sonic = sound? Isn't the sonic boom a product of the way your ear interprets a sonic shockwave as the compressed air vibrates your tympanic membrane?

                            And your vocal strings don't move at all, they vibrate.
                            I don't know what a "sonic wave" is. But shock waves and sound waves are two absolutely, and I mean ABSOLUTELY, different phenoms. The sound wave is an periodic oscilation of pressure (imagine a cosin function), a shock wave is one singular abrupt change in pressure (imagine a step).

                            And now please explain me again how do your vocal string (or any other thing) can vibrate without moving?????

                            --- 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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                            • #29
                              Do I recall somebody posting a video of an alleged shockwave going over the wing of an MD-80 the exact second their camera was trained on it?

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                              • #30
                                Originally posted by Spad13 View Post
                                Do I recall somebody posting a video of an alleged shockwave going over the wing of an MD-80 the exact second their camera was trained on it?
                                I saw a video of what the poster thought was a standing shock wave however what I saw was a flaw (blister) in the window through which the video was taken.
                                Don
                                Standard practice for managers around the world:
                                Ready - Fire - Aim! DAMN! Missed again!

                                Comment

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