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Why do takeoffs at high altitudes take longer ???

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

    Hi,
    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???

    Thanks

  • #2
    Yes, it does have to do with the air density. I'm not an expert on the subject so I don't recommend quoting me on this, but it's sort of like trying to big, heavy ball down the sidewalk with one hand tied behind your back. It takes more time and effort to get the ball rolling than it does with two hands. The idea behind rotation is to get more air hitting the bottom of the wing to push it up into the air. So if you take away some of that air it's like taking away that second hand. I remember watching an Embry Riddle training video on YouTube where they did some takeoff performance calculations. With the environmental factors they used, a Cessna 172 taking off from Daytona Beach took about 500 feet, the same factors at in Prescott, Arizona took about 800 feet. I also went planespotting in Seattle recently, which is 4,000 feet lower than my hometown of Salt Lake, and I noticed that the pitch used on short final into SeaTac was about 4-5 degrees lower than it usually is when they come into Salt Lake.
    I don't have time to watch the whole video because I have to get up early in the morning, but this one looks pretty good. I apologize if it isn't. https://www.youtube.com/watch?v=LINk0lBJwBo

    Comment


    • #3
      Originally posted by shinavn View Post
      Hi,
      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???

      Thanks
      It's both. Lift and thrust both suffer.

      1. Lower air density (not oxygen) means a higher speed is required to achieve the same lift. This higher speed requires a longer time spent accelerating, which translates to a longer distance traveled over the runway

      2. The effect of altitude on turbofan engine thrust is a bit more involved (thermodynamics, flat ratings, operational practices, turbomachinery speed margins etc etc) but as a rule higher altitude will mean less thrust. Remember that jet engines produce thrust by accelerating a mass of air, and there is less air mass available at higher altitudes. Less thrust results in a lower rate of acceleration, again adding to the time spent accelerating and therefore the distance traveled over the runway

      Alex

      Comment


      • #4
        This should be good...

        Comment


        • #5
          Enough information has been given to answer the question at the level it was asked ?
          If it 'ain't broken........ Don't try to mend it !

          Comment


          • #6
            Air molecules are fewer and farther apart. They sometimes also weigh less, collectively.
            Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

            Comment


            • #7
              Quebec Golf's answer is spot-on.

              --- 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


              • #8
                Originally posted by Gabriel View Post
                Quebec Golf's answer is spot-on.
                Let the record so indicate.

                Comment


                • #9
                  Originally posted by shinavn View Post
                  Hi,
                  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???

                  Thanks
                  This platform is so very brilliant. Cute new members almost every day. So. You are not the only jetphotos member with the airport code KDEN! But the other one at this moment probably is at sleep, or at work, or...

                  So. The mile high airport. 5,431 AMSL or 1655 meter above the sea, or just a mile high. I don't know if you've ever been higher than 5431 AMSL, without a commercial flight, i.e. without pressurization.

                  There is a reason why propeller aircraft without turbo do not climb higher than 4000 meter above the sea, i.e. 13,150 AMSL . Without pressurization, all passengers have a problem to get enough oxygen when the captain tries to climb higher than 13,150.
                  'Druckversorgung' we call it in German. A Boeing 747 has it, a 757, a 767, a 787, a 777 and a 737, too. But there are also propeller aircraft like the Beech KingAir 350, who are pressurized.
                  The principle is quite simple. On board these aircraft, the cabin is supplied in a way that also at FL350 (35,000 AMSL), the pressure is (almost) the same as on the ground, e.g. on the apron of KDEN. Insiders say that a cabin does not climb above... xxxx feet (ha. Top Secret). So that for passengers the air is never too thin.

                  For engines, there is the same problem. An engine is a fire (if we talk about normal propeller aircraft, turbo propeller aircraft and jet aircraft), and a fire needs oxygen.
                  The higher the airport the thinner the air, i.e. the less oxygen in the same amount of air.

                  To make it short, the runways at KDEN are longer than here at EDDL, because at KDEN the air is thinner, the engines at 1 mile high are less powerful than at 118 AMSL (DUS airport). The effect in a car is the same, when you climb a high mountain in your car. Better 140 hp than 14 hp, because the higher the mountain, the thinner the air, the less power your engine is able to provide, due to a lack of oxygen.

                  Better Beech King Air 350 (with turbo and pressurization) than Cessna 172! I know examples in aviation history when this was the reason why pilots died. They underestimated the thin air at 14,000 AMSL or higher.
                  Better 747 than Cessna 172. In General, you can say that jet engines always are very stable in high alt, in contrast to a normal car engine without turbo (Cessna 172). There is 1 comparison which is quite easy.
                  B747 service ceiling (xx,xxx horse power) - more than alt 40,000, according to the LH-B744 spec sheet.
                  Cessna 172 service ceiling (160 horse power) - not more than 13,500 feet.

                  Thus, if you don't switch from a rather weak Cessna 172 without turbo to a Beech King Air 350 or stronger, then the t/o at high alt takes longer. Weak engines without turbo are simply not strong enough to get enough oxygen in high alt.

                  PS: One of the highest airports in the Lufthansa schedule is Bogota El Dorado airport, at 8,363 AMSL! LH #542. But we always use engines which are strong enough to get enough air at alt 8,000 or higher. You can trust me. B747 (LH #446), A346 or similar.

                  That's why I don't see an end for the 747. Always strong enough for a mile high. Or much much higher.
                  Last edited by LH-B744; 2019-07-12, 01:46. Reason: You know the LH-B744, don't you. At KDEN.
                  LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
                  LH is member in the 747 club since April 1970. Jubilees do count, believe me.
                  Aviation enthusiast since more than 30 years.

                  Comment


                  • #10
                    Originally posted by LH-B744 View Post
                    propeller aircraft without turbo do not climb higher than 4000 meter above the sea, i.e. 13,150 AMSL . Without pressurization, all passengers have a problem to get enough oxygen when the captain tries to climb higher than 13,150.
                    There is a thing called supplemental oxygen.

                    https://en.wikipedia.org/wiki/Cirrus_SR22
                    https://en.wikipedia.org/wiki/Cessna_182_Skylane
                    https://www.law.cornell.edu/cfr/text/14/91.211

                    Also, there are pressurized planes that are not turbo.

                    --- 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


                    • #11
                      Originally posted by LH-B744 View Post
                      This platform is so very brilliant. Cute new members almost every day. So. You are not the only jetphotos member with the airport code KDEN! But the other one at this moment probably is at sleep, or at work, or...

                      So. The mile high airport. 5,431 AMSL or 1655 meter above the sea, or just a mile high. I don't know if you've ever been higher than 5431 AMSL, without a commercial flight, i.e. without pressurization.

                      There is a reason why propeller aircraft without turbo do not climb higher than 4000 meter above the sea, i.e. 13,150 AMSL . Without pressurization, all passengers have a problem to get enough oxygen when the captain tries to climb higher than 13,150.
                      'Druckversorgung' we call it in German. A Boeing 747 has it, a 757, a 767, a 787, a 777 and a 737, too. But there are also propeller aircraft like the Beech KingAir 350, who are pressurized.
                      The principle is quite simple. On board these aircraft, the cabin is supplied in a way that also at FL350 (35,000 AMSL), the pressure is (almost) the same as on the ground, e.g. on the apron of KDEN. Insiders say that a cabin does not climb above... xxxx feet (ha. Top Secret). So that for passengers the air is never too thin.

                      For engines, there is the same problem. An engine is a fire (if we talk about normal propeller aircraft, turbo propeller aircraft and jet aircraft), and a fire needs oxygen.
                      The higher the airport the thinner the air, i.e. the less oxygen in the same amount of air.

                      To make it short, the runways at KDEN are longer than here at EDDL, because at KDEN the air is thinner, the engines at 1 mile high are less powerful than at 118 AMSL (DUS airport). The effect in a car is the same, when you climb a high mountain in your car. Better 140 hp than 14 hp, because the higher the mountain, the thinner the air, the less power your engine is able to provide, due to a lack of oxygen.

                      Better Beech King Air 350 (with turbo and pressurization) than Cessna 172! I know examples in aviation history when this was the reason why pilots died. They underestimated the thin air at 14,000 AMSL or higher.
                      Better 747 than Cessna 172. In General, you can say that jet engines always are very stable in high alt, in contrast to a normal car engine without turbo (Cessna 172). There is 1 comparison which is quite easy.
                      B747 service ceiling (xx,xxx horse power) - more than alt 40,000, according to the LH-B744 spec sheet.
                      Cessna 172 service ceiling (160 horse power) - not more than 13,500 feet.

                      Thus, if you don't switch from a rather weak Cessna 172 without turbo to a Beech King Air 350 or stronger, then the t/o at high alt takes longer. Weak engines without turbo are simply not strong enough to get enough oxygen in high alt.

                      PS: One of the highest airports in the Lufthansa schedule is Bogota El Dorado airport, at 8,363 AMSL! LH #542. But we always use engines which are strong enough to get enough air at alt 8,000 or higher. You can trust me. B747 (LH #446), A346 or similar.

                      That's why I don't see an end for the 747. Always strong enough for a mile high. Or much much higher.
                      What?

                      Comment


                      • #12
                        Originally posted by ATLcrew View Post
                        What?
                        Ahm.
                        LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
                        LH is member in the 747 club since April 1970. Jubilees do count, believe me.
                        Aviation enthusiast since more than 30 years.

                        Comment


                        • #13
                          Originally posted by Gabriel View Post
                          There is a thing called supplemental oxygen.

                          https://en.wikipedia.org/wiki/Cirrus_SR22
                          https://en.wikipedia.org/wiki/Cessna_182_Skylane
                          https://www.law.cornell.edu/cfr/text/14/91.211

                          Also, there are pressurized planes that are not turbo.
                          It's so very brilliant what ATL does with my forum entries. He something like pushes them, so as if they were very very important. I don't see it like that, but ATL feel free to continue with that.

                          The original question was, 'Why do takeoffs at high altitude take longer' . So, I begin to understand why ATL quoted all 7458982 words from my entry #9 here in this topic.

                          He probably did not find the sentence where I switch from
                          a. 'oxygen which is important for passengers'
                          to
                          b. 'oxgen which is important for combustion engines' , e.g. Cessna 172 engines, BMW M5 engines (Better 160 hp than 16 hp, in high alt, a mile high or higher), Boeing 747 engines, et cetera...

                          Thank ATL, now I can say, the sentence where I switch from passengers to engines is
                          For engines, there is the same problem, An engine is a fire (...)

                          Oh I so much hate to quote myself.

                          Pressurized a/c without turbo. You make me curious. So, the cabin is able to climb higher than 14,000 and higher than 24,000 and higher than 34,000 . With not more than half the alt inside the cabin, i.e. with clearly more than double the pressure inside the cabin.

                          But after all what I have learned, propeller a/c without turbo , without jet engines, and with more than 1 seat have difficulties in high alt.

                          Do you have an example?

                          I just think of the North American P-51. But that's just the other way round. Propeller a/c with not more than 1 seat, without turbo.
                          P-51 service ceiling - Top Secret. More than 40,000 feet. I've only seen that on TV, an American pilot who uses a P-51 Mustang for jet altitudes.

                          With supplemental oxygen for the pilot, of course. Again, that's impossible without oxygen.

                          I mean, I look outside the window of my B744 simulator and on the peak of my climb there is a single engined propeller flyin next to me? That sounds interesting!

                          PS: The P-51 Mustang has an interesting ratio, hp per person in the aircraft (1 seat, so the Captain is the only person on board).
                          P-51D - 1649 hp per person.
                          P-51H - 2218 hp per person.

                          Now, let's compare that to a Cessna 172, or to a Junkers Ju-52, or, to a Boeing 747-400. That's a question. I have not (yet) the answer..

                          I assume that the Ju-52 is nothing, compared to the Mustang.

                          The Ju-52 has... 17 seats for passengers, 1 seat for the stewardess, and 2 seats for the F/O and for the Captain. All in all, 20 seats. So, to be as strong as the Mustang, the Ju-52 would need
                          20 x 2218 hp = 44,360 hp. That's not the case. The strongest Ju-52 version has 3 engines with 750 hp each. So, the comparison is
                          P-51H - 2218 hp per person. That's quite unique for a propeller, imho.

                          Cessna 172P - 160 hp if you fly alone, without F/O and without passengers. And even in a Cessna 172, I'd be careful in the Swiss Alps. With a service ceiling of only 4100 m. That means a Cessna 172 technically is not able to fly higher than 13,500 AMSL, not even if you fly alone!

                          Ju-52 - 112.5 hp per person, 2250 hp for 20 persons. I don't say that 112.5 hp per person is too weak to cross the Swiss Alps. But I would not do that, I wouldn't dare.

                          And a B744? Let's say 350 seats, only a rough guess, 350 x 2218 hp = 776,300 hp. Hm. But my avatar has four jet engines.

                          And I have two sources for the strength of a 747.
                          1. A 747-300, not to be confused with a 1978er 747-200 or with a 744, so e.g. a 1980er Swissair 747-300. 4 x 22,025 kilopond (in hp?).
                          2. LH-B744, according to the newest Lufthansa 747-400 spec sheet, (July 16th 2019). 4 x General Electric CF6-80C2. 4 x 275.6 kN (in hp?).

                          I am not an engineer for jet engine construction and maintenance (LH calls it 'Triebwerkinstandhaltung', since LH chief engineer Mayrhuber). Btw, stupid people still do not believe that critical wikipedia entries are only written either by the engineer himself, or by his pilots, or by his closest friends. So eat this, all you nonbelievers in the wikipedia. This is Lufthansa AG speaking to you.
                          Lufthansa Gruppe über fast 48 Jahre mit Wolfgang Mayrhuber. Zu seinem Tode.

                          I think a 747 still is pretty strong. 350 times as strong as a P-51?

                          I mean, the P-51 brings not more than 1 person to FL350 or higher.
                          The 744 does the same with 350 persons, plus 350 x luggage, plus a pressurized cabin.

                          Sorry. The later the night the longer the entries.
                          Last edited by LH-B744; 2019-07-16, 01:23. Reason: The secret of a powerful pressurized aircraft. Which is not the Ju-52.
                          LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
                          LH is member in the 747 club since April 1970. Jubilees do count, believe me.
                          Aviation enthusiast since more than 30 years.

                          Comment


                          • #14
                            Who knows the movie 'Top Gun', USA 1986, Tom Cruise, Kelly McGillis, Val Kilmer, Meg Ryan, Tom Skerritt, ....?

                            Gabe?!

                            That's a movie about supplemental oxygen. Each and every jetphotos member should know it. With only a few exemptions.

                            The German version of Top Gun is FSK 16, i.e. not for jetphotos Juniors who are younger than 16. But what if you are 14, and your dad doesn't know that you have his BluRay.

                            Humans need oxygen in high alt, and aircraft engines too. And how do you get more oxygen for an engine.

                            Either with a bigger engine, cp P-51 Mustang (which is a ridiculously big engine without turbo), or with a turbo. Aircraft then are called turbopropeller or short turboprop, e.g. Bombardier Dash 8 -400, Beech King Air B90, Beech King Air B350, Fairchild Swearingen Metroliner III, ..
                            They all already sound like a small jet, which they indeed are. And they all are good to cross Mt Blanc (elev 15,777),
                            better than any a/c without turbo.

                            The P-51 Mustang is the only combination of 'single engined propeller', 'no turbo', and 'high enough to cross the Himalaya' which I know. But you don't fly a P-51 if your friend likes to come with you. The P-51 only has 1 seat.

                            Or with jet engines. Examples? LH-B744, Lockheed L1011 Tristar, to name two of the fastest passenger jets which I know. Mach 0.90 in a TriStar. Since the LTU Düsseldorf was destroyed by Air Berlin in the year 2008, that speed is only reached by a few passage jets in Germany,
                            and only with decent tailwind..

                            Or with the chainsmoker, the Douglas DC-8 jet, which already was good for more than 40,000 feet 5 years before I was born. And why chainsmoker.
                            Well. My favorite airline introduced the completely smoke free aircraft in 1998, all LH 747s included. But before, all LH 747 passengers were allowed to smoke. And yet another 10 years before that, also the one or another jet tended to smoke, when the captain said 'full throttle'. The jet simply smoked to gain alt and speed. You don't see it on the first two jetphotos, but on the third one. Only airlines which still exist today.

                            Brilliant classic JetPhotos, since more than 30 years, saved on one of the best aviation platforms which I know. If not the Best.

                            Surinam Airways DC-8 in Portugal, November 1986.
                            Cygnus Air DC-8F at Los Rodeos, January 2003.
                            Delta Air DC-8-61 passage version at ATL. November 1968. a black/white jetphoto.

                            There are jetphotos where the DC-8 smokes better. But not with the filter 'airlines which still exist today'. Spantax, African International Airways, ... all gone.

                            Or with a F-14 Tomcat, good for more than 45,000 feet. But with that aircraft, there are jetphotos members who know more than me.

                            One of only two hands full of movies which really everybody here must know. In Top Gun, the relation between Oxygen and flight altitude is explained. A picture says more than 1000 words, but Top Gun says more than 5000 words.
                            Last edited by LH-B744; 2019-07-16, 03:21. Reason: How to supply engines with oxygen in high alt.
                            LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
                            LH is member in the 747 club since April 1970. Jubilees do count, believe me.
                            Aviation enthusiast since more than 30 years.

                            Comment


                            • #15
                              Originally posted by Gabriel View Post
                              There is a thing called supplemental oxygen.

                              https://en.wikipedia.org/wiki/Cirrus_SR22
                              https://en.wikipedia.org/wiki/Cessna_182_Skylane
                              https://www.law.cornell.edu/cfr/text/14/91.211

                              Also, there are pressurized planes that are not turbo.
                              Hopefully I'm allowed to say the solution for your quiz. Other people are kicked out of this forum when they post 78 posts in a row. But I have it.

                              Sh't. Finally. Can you believe me that I thought about your quiz, for, let's say hours? It didn't give me a rest until this second.
                              And - dang, head into the wall, or head on the desk- I always carry her with me. In my signature. But let's be honest. For you she's closer. You are the older one.

                              Lockheed Super Constellation, built between 1943 and 1958. Four ridiculously big propeller engines without turbo. And one of the very first long haul aircraft in the LH fleet.
                              And of course, with a pressurized cabin. Although they say, the early Constellations already had something like an exhaust turbo-supercharger (dt.: Abgasturbolader)!

                              But in a Super Constellation, it is still basically the ur huge 18 cylinder radial engine, or better 4 (four) huge 18 cylinder radial engines, which drive the 4 propellers.
                              I don't think that a Super Connie has this rather 'high voice' of modern turbopropellers (dt.: das Turbopfeifen), although I've never heard one Super Constellation.
                              4 x 18 cylinders sound fantastic, after all what I can imagine.

                              Thank You, Gabe. Now I can go to sleep!
                              LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
                              LH is member in the 747 club since April 1970. Jubilees do count, believe me.
                              Aviation enthusiast since more than 30 years.

                              Comment

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