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  • Crunk415balla
    replied
    ATFS=pwn3d.

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  • Gabriel
    replied
    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.

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  • Dmmoore
    replied
    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.

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  • Gabriel
    replied
    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).

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  • Gabriel
    replied
    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?

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  • E-Diddy!
    replied
    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?

    Leave a comment:


  • ATFS_Crash
    replied
    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.

    Leave a comment:


  • E-Diddy!
    replied
    That can't be any kind of a wave, because the wave will start at the areas where the airflow is accelerated the most: over the wings. As the aircraft goes faster, the wave moves aft. The waves don't start forming (bow, shock, whatever you want to call it) ahead of the aircraft until you get very near the speed of sound (thinking like the M.97ish area).

    Here are a few simple reasons why a 747 can't go that fast:
    1. The parasite drag force at that airspeed would most likely be enough to cause the aircraft to break up. If for some reason (like made in USA quality) it holds together, then:

    2. As the aircraft accelerates, a shockwave begins to form over the thickest part of the wing (usually) which is where the airflow is the fastest. This is also at the wings center of pressure. As the mach increases, the shockwave moves aft, and with it, the center of pressure. Because the center of pressure (acting up) moves further away from the center of gravity (acting down, forward of the center of pressure), the lever arm gets longer and the aircraft will violently pitch nose down, called "Mach Tuck." The airspeed at which this occurs is M-crit. At this point, the aircraft is flying well above Va and is pointed straight nose-down. If the pilot attempts to pull out of the dive with the stick (if they are able to overcome the extreme control pressures needed) then the aircraft will break up, if the aircraft is allowed to continue accelerating, it will go through M-limit at which point the dive will be unrecoverable. This region of flight is known as "coffin corner" and very few have gone there and come back.

    3. A 747 skin is made of aluminum. The extreme heat encountered at very high mach would likely melt it to the point that it would fatigue the skin enough to cause structural failure.

    Notes:
    - 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.

    - 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.

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  • AJ
    replied
    I think the main area of confusion is the usual amazing symmetry of a shockwave or bow wave:
    [photoid=364692]

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  • ATFS_Crash
    replied
    IMAO I disagree. I think the doughnut is the trailing edge decompression shock that following the high pressure bow wave. The bow wave forms a bowl or umbrella; this doughnut is at the rim of the bowl/umbrella bow wave. Vapor trails are often generated at the rim of the bowl/umbrella of the bow shock.

    http://en.wikipedia.org/wiki/Bow_shock_(aerodynamics)

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  • E-Diddy!
    replied
    Originally posted by Gabriel View Post
    Ok, so let me rephrease it:

    Those ain't no waves.
    I second that.

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  • Gabriel
    replied
    Ok, so let me rephrease it:

    Those ain't no waves.

    Leave a comment:


  • ATFS_Crash
    replied
    I’m going to disagree with all of you. I don’t think it is supersonic but I think it is a shockwave.

    Some of you seem to be implying that all shockwaves 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….

    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.

    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.

    The shock is a sudden change of pressure that is apparently causing the water vapor in the air to be visible.

    Though “shock” is more commonly used in the context of at or in excess of the speed of sound it can also apply subsonically.

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

    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.
    ----
    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.

    Leave a comment:


  • Dmmoore
    replied
    It's not a sonic shock wave. A sonic shock wave may or may not be visible depending on atmospheric condition. At sea level, an aircraft may have airflow exceeding MACH 1 somewhere on the airframe when IAS >650 knots. A 747 has a VNE <400 knots.

    There are conditions where the pressure and moisture content of the air is such that the aircrafts motion causes the mosture to condense and become somewhat viaible. The effect would only be seen when the sun and viewer are at optimum viewing angles.

    On damp days (temperature and due point very close to rach other) a layer several feet thick vapor may condense on the wings upper surface. The pressure drop above the wing is the cause. The effect seen in the photo appear to be similar in that the vapor visible aft of the L/H (port) wing appears to be vapor trailing off the upper wing surface.

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  • Gabriel
    replied
    Photoshop, con(chem)trails, clouds, UFOs, whatever... I don't know.

    But one thing I know: Those ain't NO shock waves.

    Leave a comment:

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