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Why do takeoffs at high altitudes take longer ???

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  • Gabriel
    replied
    Originally posted by 3WE View Post
    Why?
    Sarcasm. It's not turbo and still if you agree to never exceed 75% power and call it 100% (which is what you typically do with the turbos) you can make it work as if it was turbo, in the sense that you can keep the same power from 0 to 7000+ ft.

    (Nice that you found the hidden joke)

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  • 3WE
    replied
    Originally posted by Gabriel View Post
    Turbo Tomahawk
    Why?

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  • Gabriel
    replied
    Originally posted by 3WE View Post
    But it's OK, turbochargers don't enable higher altitudes on piston airplanes...it's the rating and waste gate that enables the higher altitude.
    Semantics

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  • 3WE
    replied
    Originally posted by Gabriel View Post
    The normally aspirated will be much more bulky and heavy, though.
    And, the turbocharger pretty much directly addresses the reason the lighter engine loses power. It compresses the air to increase the O2 concentration per unit volume so that it is similar to lower altitudes.

    But it's OK, turbochargers don't enable higher altitudes on piston airplanes...it's the rating and waste gate that enables the higher altitude.

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  • Gabriel
    replied
    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.

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  • 3WE
    replied
    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.

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  • Gabriel
    replied
    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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  • 3WE
    replied
    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.
    @#%@#%!

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  • Gabriel
    replied
    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.

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  • 3WE
    replied
    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")

    Leave a comment:


  • Gabriel
    replied
    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

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  • 3WE
    replied
    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.

    Leave a comment:


  • Gabriel
    replied
    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.

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  • 3WE
    replied
    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.

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  • Gabriel
    replied
    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.

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