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  • Gabriel
    replied
    Originally posted by TeeVee View Post
    as in a week gets ripped off or some such other catastrophic failure. i know that these types of things theoretically could happen in a nose down overspeed. basically what i'm trying to find out is if the engines are powerful enough to push the plane passed it's "safe speed"
    There are basically three failures modes due to speed itself.
    The "rip off" type that you mention, where the skin friction or the drag take thins apart.
    The sonic heat type. When you approach the speed of sound the temperatures of the leading edges and nose of the fuselage (where the air is compressed in a shock wave) increases a lot and parts not prepared to withstand those high temps can fail.
    Flutter, that is a vibration caused by interaction between aerodynamics, structural and inertia forces, that can be so divergent to the point of braking a wing in less than a full cycle.

    There is an indirect mode, which is what 3WE mentions: When you approach the speed of sound the center of pressure moves from about 25% of the chord to about 50% of the chord (in full supersonic regime). That means that the lift moves aft and generates a strong nose-down pitching moment. This can put the plane into a dive, which would increase the speed and would move the CP even further which worsens the dive and so on. The result is an uncommanded and sometimes uncontrollable and unrecoverable dive that, eventually, makes the plane fail due to one of the modes mentioned above, or when the elevator or wing fails due to excessive lift when the pilot applies a lot of nose-up command in an attempt to recover (of course this kind of thing can happen even at speeds well below overspeed if the pilot is too aggresive with the control inputs, ask AA A300). So even if the thrust by itself is not enough to put the plane in a "rip off" mood, the subsequent unavoidable dive can.

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  • Gabriel
    replied
    Originally posted by erwins View Post
    indeed at normal cruise speed at altitude you can experience overspeed. Can happen due to high back winds.
    WTF?????
    Are you Don (RIP)?

    Leave a comment:


  • CathayPacific
    replied
    Originally posted by TeeVee View Post
    perhaps a shitty analogy, but most cars have electronic speed limiters to prevent the driver from achieving a speed too fast for the car.
    I think it depends on how the auto-pilot was designed. Mechanically, it is certainly that a consistent TOGA power setting will allow the aircraft to go over the safe speed. But electronically whether a limit is imposed would be another question. Also, the safe speed would be different at different altitude under different weather, so it's not as "set-in-stone" as cars. Without access to the technical manuals and the design specification, I would think Airbus' automation philosophy would put such speed limit on the computer and Boeing would probably allow the pilots to manipulate the aircraft beyond its design limit. But that's pure speculation. Boeing would tell you that the incident of CI006 would show you the benefit of allowing such flexibility to the pilots and Airbus would tell you that the US Airways Hudson River incident would show you the benefit of a fixing a flight envelope, so there's no right-or-wrong answer here.

    Actually are you sure there are electronic speed limiters on cars? I know they generally put them on trucks and buses, but I am not sure about cars.

    Leave a comment:


  • TeeVee
    replied
    as in a week gets ripped off or some such other catastrophic failure. i know that these types of things theoretically could happen in a nose down overspeed. basically what i'm trying to find out is if the engines are powerful enough to push the plane passed it's "safe speed"

    perhaps a shitty analogy, but most cars have electronic speed limiters to prevent the driver from achieving a speed too fast for the car.

    Leave a comment:


  • 3WE
    replied
    Originally posted by TeeVee View Post
    ...an unsafe speed for the airframe...
    You mean such that something would bend or break?

    From the direct affect of the "straight and level speed"

    OR

    From the even higher speed that will occur when the plane goes into a dive after the super sonic lift shift / pitch down occurs.

    (By the way, I am parlour talking with a tiny shred of background)

    Leave a comment:


  • ErwinS
    replied
    Indeed at normal cruise speed at altitude you can experience overspeed. Can happen due to high back winds.

    I have done overspeed inspections many a times.

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  • Gabriel
    replied
    Originally posted by TeeVee View Post
    So, i was wondering, can a commercial airliner overspeed in level flight? say for example you took a 737 with no pax or cargo, leveled off at an altitude within the normal cruise range, and selected absolute full power (toga) and left it there, is it possible the aircraft could reach an unsafe speed for the airframe? if so, what would that speed be?
    Yes, it's possible even if you are full of passengers and cargo, as long as you are not too close to the ceiling (where the engines would lack the power to keep increasing the speed.

    The max speed that you can intentionally fly is Vmo/Mmo (mo stands for Maximum Operative). This is not an ultimate safe speed. There is margin between this speed and the "dangerous" speed. The manufacturer must show that, starting at Vmo/Mmo, you introduce certain prescribed upsets to the flight and a normal flight situation must be restored without exceeding the "dangerous" speed and with no exceptional alertness, strength or skills by the pilots.

    While the manufacturers internally use other "milestone" speeds during the certification process (like Vd/Md), they only publish Vmo/Mmo. Whenever you exceed those speeds you are officially "overspeed", and you don't know how close are to a really dangerous speed so recovery must be initiated immediately.

    As a side note, I can mention that sometimes cruise speed are really close to Vmo/Mmo.

    Leave a comment:


  • TeeVee
    started a topic Overspeed Question

    Overspeed Question

    So, i was wondering, can a commercial airliner overspeed in level flight? say for example you took a 737 with no pax or cargo, leveled off at an altitude within the normal cruise range, and selected absolute full power (toga) and left it there, is it possible the aircraft could reach an unsafe speed for the airframe? if so, what would that speed be?
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