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Why do takeoffs at high altitudes take longer ???

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  • #16
    I am not the type of encyclopedic guy that carries data in his storage. I am the kind of guy that carries concepts.

    I know that pressurization is just to keep the cabin at a higher pressure than the surrounding atmosphere. It is not an "engine" thing (while, unless you have a totally air-tight cabin, you need a source of pressurized air to keep the cabin pressurized).
    I know that there are pressurized no-engine airplanes (gliders). (new challenge?, I think the Germans are also involved here)
    I know in my life I came across (in books or magazines) pressurized piston-engine planes that were not turbocharged (that has nothing to do with turboprop).
    I don't remember which one(s), but I do know that a turbocharged piston engine is not a prerequisite for pressurization in a piston-engine plane.
    I know that, most of the time, when a manufacturer makes a pressurized version of an existing piston-engine plane they take advantage of turbo engines (not turboprops) because of a number of reasons: 1st naturally a plane with a turbocharged piston engine can reach higher than its twin with an engine of the same power but not turbocharged, the reason being that the turbocharged engines with the same power at sea level can give more power at high altitude, and it is in an airplane capable to fly higher where pressurization is a stronger selling point. 2nd, the turbo provides a source of pressurization that is readily available: the air intake, which is pressurized by the turbocharged at above local atmospheric pressure.
    But I also know that there are engine-driven compressors designed specifically for pressurization, which are powered directly by the engine shaft either via a belt or a gear. If these things exist (and they do), then there must have been airplanes using it. Which ones? I don't remember.

    So I am happy that you solved the challenge yourself, because you would have been waiting forever for my answer.

    --- 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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    • #17
      Originally posted by Gabriel View Post
      I am not the type of encyclopedic guy that carries data in his storage. [...]
      Me not either. The word pressurization for me is probably as foreign as for you. The German words are 'Druckversorgung', 'Druck Kabine' or,
      and that seems to be the most common translation, also supported by my dictionary,
      'Druckausgleich'.
      But they all mean the same. I know that I am a little bit off topic, but let's take the very last flight of the HB-HOT, type Junkers Ju-52. The case is still under investigation, fifty-two weeks (i.e. exactly 1 year, on August 4th) after the three engined propeller without turbo and without pressurization was completely destroyed,
      during an attempt to cross Piz Segnas (elev 10,167 - or 3099 m above the sea)

      a) in a propeller aircraft without turbo
      b) with MTOW, so, 20 seats in the aircraft, pilots and crew included, and all 20 seats occupied.
      c) without pressurization, and..
      d) on a very very hot day, especially compared with Swiss weather.

      What do piston engines do when they overheat. Well, look at your car. Or at your computer. Intelligent engines automatically switch off the possibility to demand 'full throttle'. I've read some car magazines recently..
      But is that also the case in an 80 year old airframe (born 1939)?

      So, that's what they did not have, on a very very hot day: No air condition, no pressurization, and no extra air for the engines (no turbo).

      Now we can assume what fainted first, a) the engines, with full throttle, but still unable to make more than alt 10,000 on such a very hot day, b) the engines, with full throttle, but still unable to climb further than alt 10,000 with MTOW on such a hot day c) the two pilots in a cockpit without air condition and without pressurization,
      or d) none of that.

      The threadstarter here gave us 'takeoffs at high altitude'. And imho we're coming as close as possible to that question, don't we. Theoretically, we should expect a 'Thank you' by the threadstarter. But I'm here to answer questions, if I'm able, and to ask questions if I'm not able.

      So, is that good enough for the threadstarter, or do we have to elaborate?

      PS:
      I know that there are pressurized no-engine airplanes (gliders). (new challenge?
      Hm. I know a little bit about German aviation history, but that almost completely excludes aircraft without engines, e.g. gliders or balloons. What we have not yet mentioned,...
      a Zeppelin. It looks like an oversize cigar, with.. four propeller engines (?) and if you like, a pressurized cabin (?).

      Starrflügler, that's what my hobby is called in German. And it looks pretty much like that.
      Fairchild Swearingen Metroliner SA227 Turbopropeller, Innsbruck 2019
      LH-B748 climbing out of the clouds, Tokyo 2019

      So, piston engines? That sounds arrogant, but here at my home airport we don't have so many piston engines. Probably the small pistone engines somehow fear the 45 m wide runways here.
      Imagine a Cessna 172 on a 45 m wide runway.. I wouldn't do that, especially not with a 777 or 747 in my neck. I'd rather take a smaller landing strip, a propeller landing strip.

      The simple answer is, you need bigger engines the higher you climb. And if you try to climb high in hot weather, you need longer runways: EDDL.
      At DUS they switched off the public thermometer, or did it overheat? So let me jump in. EDDL nightly temperature 0236 CEST: +19°C or 66°F .
      That indeed is very cold, compared to last week (!) ..

      And if you try to climb high without a big engine, then you need loong runways: KDEN.
      Last edited by LH-B744; 2019-07-30, 00:16. Reason: What do piston engines do when they overheat. Not so very much imho..
      LH and the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955. A/C type: Lockheed Super Constellation.
      EW, one of the dearest LH daughters, the brandnew November 19 schedule (frequency):
      DUS - VRA (--3-5-7), DUS - EWR (1234567), DUS - MIA (1-3-56-), DUS - BGI # 1152 (Mon and Thu with exceptions), ...

      Aviation enthusiast since more than 30 years. A whole decade here on this platform.

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      • #18
        Originally posted by Michael Rodeback View Post
        The idea behind rotation is to get more air hitting the bottom of the wing to push it up into the air.
        I think the idea behind rotation is to increase the angle of attack.

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        • #19
          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 has nothing to do with oxygen itself. What really affects the takeoff is the air density the higher the altitude the thinner the air becomes thus results to weaker air resistance.

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          • #20
            Originally posted by Nightnite View Post
            It has nothing to do with oxygen itself. What really affects the takeoff is the air density the higher the altitude the thinner the air becomes thus results to weaker air resistance.
            Plain and simply wrong.
            Did you realize that the question was "Why do takeoffs at high altitudes take longer ????"
            Do you realize that saying that "Takeoffs at higher altitudes take longer because of the weaker air resistance" makes no sense whatsoever?

            Air resistance is not the operating factor but, if anything, lower air resistance would mean faster acceleration (if only the engines were providing the same thrust, which the don't because less density means less mass of air and less mass of air flowing through the engines and less mass of air means less oxygen and less oxygen means less fuel being burned and less fuel being burned means less power and less power means less thrust and less thrust means less acceleration so longer takeoffs), and also less density means less lift for the same speed which means that you need to accelerate to a higher speed which again means a longer takeoff (even if the acceleration was the same, which is not) to achieve that higher speed.

            --- 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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            • #21
              Originally posted by Nightnite View Post
              It has nothing to do with oxygen itself. What really affects the takeoff is the air density the higher the altitude the thinner the air becomes thus results to weaker air resistance.
              1. Gabbie- please allow a little slack for possible language barriers (that sometimes affects international aviation fora)…Maybe he meant weaker INTERACTION...resistance and lift are correlated.

              2. As to oxygen, it actually DOES affect the takeoff- There is less O2 per volume of air, thus reducing the ability of the engine to burn fuel and generate horsepower. Turbocharged and 'turbojet' (and supercharged) aeroplanies generally significantly compensate for the reduced 02 availability. Normally aspirated piston engines are usually rather limited in their ability to compensate for reduced 02

              /outsider, ass-hat pontification.
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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              • #22
                Originally posted by 3WE View Post
                Turbocharged and 'turbojet' (and supercharged) aeroplanies generally significantly compensate for the reduced 02 availability. Normally aspirated piston engines are usually rather limited in their ability to compensate for reduced 02
                Well, more or less. That is because the engines are under-rated (or just not rated) to produce all the power they thermodynamically could deliver (but perhaps mechanically cannot withstand).

                It is true that a turbocharged engine can keep 29.92 inches of manifold above sea level (and sometimes WAY above sea level) by pre-compressing the intake air.
                But for the sake of the turbocharger, they could also provide let's say 40 inches at sea level, thus delivering much more power, and not be able to keep that manifold pressure at higher altitudes.

                That's how car turbocharged engines of smaller CC or L are able to produce as much power as much bigger engines..., at sea level.

                If you put a normally aspirated engine and restrict it to 20 inches in the manifold, it will also be able to keep that manifold pressure and the same power at much higher altitudes.

                --- 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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                • #23
                  Originally posted by Gabriel View Post
                  VERY MUCH more or less.
                  Fixed.

                  I put all the wiggle words in my post. If you get high enough, all engines lack necessary amounts of 02. Down in the real world, practical considerations and $ and aeroengineengineers have things very much as I described them. Normally-aspirated engines get O2 limited fairly quickly; turbocharged engines go higher, and 'jet' engines enjoy pretty good performance at altitude. "Flat ratings/etc" gloss over the fact that O2 concentration per unit volume decreases with altitude.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                  • #24
                    Originally posted by 3WE View Post
                    Fixed.

                    I put all the wiggle words in my post. If you get high enough, all engines lack necessary amounts of 02. Down in the real world, practical considerations and $ and aeroengineengineers have things very much as I described them. Normally-aspirated engines get O2 limited fairly quickly; turbocharged engines go higher, and 'jet' engines enjoy pretty good performance at altitude. "Flat ratings/etc" gloss over the fact that O2 concentration per unit volume decreases with altitude.
                    That was not my point. My point is that ALL engines (and ALL means ALL the ones you named and then some) lose performance as the air density diminishes in approximately the same way, IF they are allowed to produce, at sea level, the power that they could deliver from a thermodynamic standpoint. And ALL engines (including normally aspirated ones) are capable of keeping the same power than at sea level if, at sea level, you DON'T let them produce the power that they could deliver from a thermodynamic standpoint.

                    While you are correct that, IN AVIATION (but not in other modes of transportation like cars & trucks or boats) turbocharged/supercharged piston engines are rated to produce at sea level a max power that they are able to maintain until a certain altitude, that is NOT BECAUSE they are turbocharged / supercharged. It is because, due to engineering reasons that I can explain if you want, that max power that they can maintain from sea level to that certain altitude is less that what they could produce at sea level from a thermodynamic standpoint.

                    By the way, while this is typical of turbocharged / supercharged piston engines IN AVIATION ONLY, it is not typical of turbocharged /supercharged engines used in other modes of transportation (which will start losing power immediately as air density goes down), and it is also not typical of turbine aviation engines (turboprop, turbojet, turbofan). (And is also not typical of normally aspirated engines).

                    In other words, the reduction in air density and its corresponding reduction in O2 affects ALL engines in principle. Of course, if you keep the throttle somehow closed at sea level and keep opening it as you go up, you compensate the lower mass of O2 per unit of volume of air with more volume of air thus keeping the O2 mass rate constant and hence the power constant. And this whole paragraph is independent of the type of engine. You can add diesel, wankel and 2-strokes to the list.

                    In yet other words.... we agree

                    --- 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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                    • #25
                      Originally posted by Gabriel View Post
                      In yet other words.... we agree
                      No, apparently, we don't.

                      Originally posted by Gabriel
                      That was not my point. My point is
                      By the way, while this is not typical
                      and it is also not typical
                      And is also not typical
                      this is independent
                      Well, more or less (lots of emphasis on less).
                      But
                      As I said earlier (and you did not perceive), I used plenty of wiggle words [fairly quickly, enjoy fairly good performance at altitude, significantly compensate, $ and engineering) ), and acknowledged "that for practical purposes" (and note I didn't say for ALL practical purposes, just "for plain, ordinary practical purposes"). And we are talking aviation engines, because I think this forum has something to do with aviation.

                      I say that all engines (with bold font), and you argue that it's ALL engines with ALL listed three times in ALL CAPS...Does that mean If I had sad ALL three times in ALL CAPS that you might have noticed?

                      But I guess you just have to spew more pontification.

                      (Yeah, it actually should be "all engines that rely on free, atmospheric O2")
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                      • #26
                        Originally posted by 3WE View Post
                        No, apparently, we don't.



                        As I said earlier (and you did not perceive), I used plenty of wiggle words [fairly quickly, enjoy fairly good performance at altitude, significantly compensate, $ and engineering) ), and acknowledged "that for practical purposes" (and note I didn't say for ALL practical purposes, just "for plain, ordinary practical purposes"). And we are talking aviation engines, because I think this forum has something to do with aviation.

                        I say that all engines (with bold font), and you argue that it's ALL engines with ALL listed three times in ALL CAPS...Does that mean If I had sad ALL three times in ALL CAPS that you might have noticed?

                        But I guess you just have to spew more pontification.

                        (Yeah, it actually should be "all engines that rely on free, atmospheric O2")
                        As I said, we ultimately agree. We are just putting the focus in different parts of the same reality.

                        Ultimately, I didn't want that someone left this forum thinking that turbocharging is a crazy technology that lets the engine produce the same power with less O2.

                        --- 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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                        • #27
                          Originally posted by Not_Gabriel View Post
                          Turbocharging is a clever technology that (along with 'flat ratings' and limiting systems) lets many aircraft engines produce similar power with less O2 per unit volume at altitude.
                          @#%@#%!
                          Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                          • #28
                            Originally posted by 3WE View Post
                            Originally posted by Not_Gabriel
                            Turbocharging is a clever technology that (along with 'flat ratings' and limiting systems) lets many aircraft engines produce similar power with less O2 per unit volume at altitude.
                            @#%@#%!
                            Yes, but it is not the turbocharging, it is rather the flat rating and limiting systems (or limiting procedures). The trick (regardless of the type of engine) is not to use all the air that you could at sea level and, as you climb, use more air with less O2 to keep using the same amount of o2 until you reach the point where you are using all the air that you can and only then, if you keep climbing, since you cannot use any more air than you are already using, you are going to start losing power as the air keeps reducing the amount of O2 per volume unit.

                            Look, the 84 HP normally aspirated engine in my Piper PA-38-112 Tomahawk can also keep the same power at sea level than at 7000+ ft.

                            Click image for larger version

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                            --- 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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                            • #29
                              Originally posted by Gabriel View Post
                              Yes, but it is not the turbocharging
                              Semantics.

                              Normally aspirated piston single: Service ceiling generally below 20K
                              Turbocharged piston single: Service ceiling above 20K

                              The addition of the turbocharger enables the higher altitude. The normally aspirated plane cannot go higher without the turbocharger. There are several instances of the same engine with and without turbocharging...the service ceiling is generally increased markedly.

                              And I mentioned flat rating.

                              If there were no flat rating, yes, there would be more horsepower down low, but the flat rating doesn't provide altitude. The flat rating protects the engine down low.
                              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                              • #30
                                Originally posted by 3WE View Post
                                Semantics.
                                Absolutely. That's why I said that we ultimately agree, we are just focusing on different aspects of the same reality.

                                Normally aspirated piston single: Service ceiling generally below 20K
                                Turbocharged piston single: Service ceiling above 20K

                                The addition of the turbocharger enables the higher altitude. The normally aspirated plane cannot go higher without the turbocharger. There are several instances of the same engine with and without turbocharging...the service ceiling is generally increased markedly.

                                And I mentioned flat rating.

                                If there were no flat rating, yes, there would be more horsepower down low, but the flat rating doesn't provide altitude. The flat rating protects the engine down low.
                                Possibly semantics again, but no. What enables the high altitude is that you have a much more powerful engine (thermodynamically), just that you don't let it produce that much power (i.e. it is the flat rating).
                                Put an aspirated engine of the same thermodynamic power than a turbocharged one, and both will give you the same power up there in the flight levels. The normally aspirated will be much more bulky and heavy, though.

                                --- 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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