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Why do takeoffs at high altitudes take longer?...

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  • Why do takeoffs at high altitudes take longer?...

    I'm a Denver native, and this is something that I'm very curious about. Does it have to do with the lower oxygen density? And, does that have something to do with the way the engines run, or the amount of lift an airplane can get?

  • #2
    Originally posted by UALdave View Post
    I'm a Denver native, and this is something that I'm very curious about. Does it have to do with the lower oxygen density? And, does that have something to do with the way the engines run, or the amount of lift an airplane can get?
    Air density decreases as elevation increases. So to develop lift aircraft have to increase speed to obtain the same dynamic pressure that they would use at sea level. Dynamic pressure is a function of air density and square of velocity.

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    • #3
      UALdave and Highkeas have half of the answer each.

      Lower thrust: The engines manage air by the volume flow, but the power they produce is a function of the mass flow. At higher altitude the air is less dense, so there is less mass of air (and hence of oxygen) in any given volume of air. The chemical air/fuel relationship is fixed in mass terms (lb of air per lb of fuel). So a lower mass flow of air means a lower mass flow of fuel, which is the real source of the energy. So a lower mass flow of fuel means less power.

      Faster speed needed: At the same weight, the plane will take off at the same INDICATED airspeed. But the indicated airspeed is not a real speed, unless you are flying at sea level in standard atmospheric conditions (29.92 in Hg and 15 Celsius). The indicated airspeed is, in fact, a measure of the amount of air, in terms of mass flow, that is passing by the plane. This is important because the lift and drag are a function of the mass of air going by, which is in turn a function of the "real"airspeed times the density. So at higher altitudes (which means lower pressure) or high temperature (both conditions reduce the density) the airplane needs more "real" airsped (called TRUE airspeed), and hence more ground speed (in equal wind conditions) to achieve the same indicated speed (have the same mass of air going by) that will give the plane the needed lift to take-off.

      Lower thrust = lower acceleration = more time and distance needed to achieve a given speed.
      Faster speed needed = more time and distance needed to achieve it with a given acceleration.

      Combine lower acceleration and more speed needed, and you have more time and distance needed, squared.

      --- 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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      • #4
        ...being nitpicky, a lot of turbine aircraft are "flat rated" and produce "full" thrust and power at 5000 ft largely as they do at "sea level"
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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        • #5
          Originally posted by 3WE View Post
          ...being nitpicky, a lot of turbine aircraft are "flat rated" and produce "full" thrust and power at 5000 ft largely as they do at "sea level"
          That's true. It applies to many jets, turboprops, all turbo piston, and even some normally aspirate piston airplanes.

          However, while all of them can keep "flat rated" power up to a certain density altitude, all suffer from the indicated vs true airspeed gap. They need more speed and hence more time and runway to lift off.

          --- 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


          • #6
            Originally posted by UALdave View Post
            I'm a Denver native, and this is something that I'm very curious about. Does it have to do with the lower oxygen density? And, does that have something to do with the way the engines run, or the amount of lift an airplane can get?
            Same reason it takes you, as a Denver native, considerably more beer to get wasted when you're at sea level than when you're at home.

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            • #7
              Not for nothing, I am the jp member, who stands for our simulator section.

              I can't quite remember my last way to KDEN, but let's ask my flightplan database.
              KEWR - KDEN with a B744 (feel free to be creative) and then
              KDEN - CYVR with the same B744, in UA 2015 livery, see our simulator section.

              When you start from Denver, at 5431, you don't feel that much in a B744. Four engines, which can drive you "crazy" (i.e. give you 250 KIAS within a glimpse) at almost every airport.

              But on every major airport, I try not to forget the basics. Baron 58. For a short overview. And in this little bird, you see what 5431 means.

              The air is thinner, so it contains less oxygen than e.g. at my home airport. And, in the leftseat of a prop, you have to react to the fact that every engine needs a fire. And fire needs oxygen.

              [So, you have to push a little your throttle, or, if maximum is reached, t/o takes longer.
              Not for nothing, they have a 16,000 ft rwy in Denver.
              Longhaul + high alt = you better choose a long rwy.]

              Sometimes, I try to give all of my ... 11 simulator years into one post. Pls forgive me.

              Last edited by LH-B744; 2015-07-11, 04:26. Reason: + detail
              The German long haul is alive, 65 years and still kicking.
              The Gold Member in the 747 club, 50 years since the first LH 747.
              And constantly advanced, 744 and 748 /w upper and lower EICAS.
              This is Lohausen International airport speaking, echo delta delta lima.

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              • #8
                Originally posted by ATLcrew View Post
                Same reason it takes you, as a Denver native, considerably more beer to get wasted when you're at sea level than when you're at home.
                Is it like that? Which airports did you use for a test?
                KDEN -
                and
                ?

                And what is the unit, x pints of Guinness?

                [In long Friday nights, I should better write less... ]
                Last edited by LH-B744; 2015-07-11, 05:07. Reason: + #742 favour for my former English teacher at high school
                The German long haul is alive, 65 years and still kicking.
                The Gold Member in the 747 club, 50 years since the first LH 747.
                And constantly advanced, 744 and 748 /w upper and lower EICAS.
                This is Lohausen International airport speaking, echo delta delta lima.

                Comment

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