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Aerospace Commercial airliner speeds

  1. Sep 30, 2011 #1
    Hi,

    Is it possible for a commercial liner, Boeing 767 for instance, to descend from high altitude and maintain it's speed? A Boeing 767 has a Vmo maximum of 360 knots and an Mmo of .86 mach. If the Boeing was travelling at 500 mph at 35000 and started descending, what would happen when the air density increases? I'd imagine the cockpit would be going crazy, but is it possible to maintain the speed through the more dense air?

    Also, if you really pushed a Boeing 767 what kind of speeds do you think you could each at sea level? Do you think it's possible to exceed the maximum operating speed by 100-150 knots?

    Thanks for your time

    Peter
     
  2. jcsd
  3. Oct 3, 2011 #2
    To address your answer simply, no, it could not. The reason for this is the added drag that is applied based on the increased density of the air. The jet would run into a major problem with having a strong enough structure to withstand the extra force on it.

    Moreover, the jet may or may not have enough thrust to overcome the added drag. Think of swinging your arm as fast as you can through the air and then through water. Can you move it as fast? This is an exageration, but helps to illustrate the point.

    And I'm not saying that there isn't a fudge factor, as the plane could probably descend quite a ways before passing the limits, but flying Mach 0.8+ at sea level would probably not be likely in a subsonic transport like the 767.
     
  4. Oct 3, 2011 #3
    I might as well advise as to the reason I'm curious. I asked that specific question as I've been debating with "truthers" regarding the events of 9/11.

    Flight 175 was clocked on radar doing 500 knots at ground level. I wasn't sure if it could maintain that speed during descent as I'd struggle to see how a commercial airliner would be able to accomplish those speeds of its own steam.

    The flight the hit the Pentagon, flight 77 did a 270 degree corkscrew turn from 8000 feet to 2000 feet maintaining a speed of 300 knots (according to the black box data), when it lined up with the Pentagon it reached a speed of 450 knots with the nose tilted down slightly.. I'm not a "truther", but this has compelled me somewhat. I know it could be an offensive topic for some people, to which I'm sorry. I just wanted to get the opinion of people who know what they're talking about. I'm still researching, but your comment was interesting for sure..
     
  5. Oct 3, 2011 #4
    It doesn't bother me.

    And honestly, my numbers could be way off and a jet liner could very well do 500 ktas at sea level. I don't know. As long as the thrust was enough and the structures could handle it, it could happen.
     
  6. Oct 3, 2011 #5
    Forgive me if my technical jargon is a little off. I was given the impression that the aircraft in question has two maximum operating "speeds".. Mmo and Vmo, 0.86 mach and 360 knots respectively. If a Boeing is going 500 knots at sea level they estimate that the additional thrust to make the craft go 150 knots over its max operating speed at sea level would have an equivalent air speed of over mach 1 at high altitude. I know that's a fairly simplistic way of looking at it, but does that argument have any weight?
     
  7. Oct 3, 2011 #6

    AlephZero

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    The airworthiness regulations define Mmo/Vmo as the maximum speed that may not be exceeded deliberately in normal operations, unless explicitly authorized for flight testing or pilot training.

    The "speed when the wings fall off" is Vne, not Vmo.

    Vdf, the maximum demonstrated diving speed (which is greater than Vd, the normal operating diving speed) and Vh, the maximum speed in level flight at max continuous power, may also be relevant.
     
  8. Oct 4, 2011 #7

    russ_watters

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    ....and if the pilot doesn't actually care much about whether the wings start to disintegrate, he can exceed design limits by quite a bit. How much does it actually take before the wings actually start to come off? Dunno.
     
  9. Oct 7, 2011 #8
    Thanks for your input guys.
    Yeah I'm aware that they wouldn't have been too concerned about going full throttle at sea level.. I don't question that, I just don't understand how the plane could accomplished such speeds. I'm currently talking to some pilots of a different forum, they kinda back up what I feared to be true. But I'm going to keep digging.. Perhaps I'm crazy, perhaps I'm one of those cognitive dissonance people :/ I don't usually follow those theories, they're fairly far out.. But this caught by eye, I must admit.
     
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