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Quoting Flighty (Thread starter): Self explanatory. Is this an incentive to cruise at high altitude when possible? |
Quoting StTim (Reply 2): There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air. |
Quoting StTim (Reply 2): There are other factors that do come into play. There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air. |
Quoting nomadd22 (Reply 6): Quoting StTim (Reply 2): There are other factors that do come into play. There is also proportionally less oxygen as well and so the fuel air mix must change. If you have ever driven a normally aspirated (no turbo) car at altitude then you will know they are gutless. The engines have to work much harder than at sea level due to the lack of oxygen in the air. That's wrong. The difference in gasses percentage wise is insignificant at cruising altitude. Less power is simply less air being drawn in, the same reason you have less power at lower throttle. |
Quoting Flighty (Thread starter): I find that in cars at high altitude are quieter and probably more efficient at cruise at altitude, so... |
Quoting Flighty (Reply 10): it wasn't just imagination that there was less wind noise at say 8000ft. Of course there is; the wind resistance is lower! |
Quoting Flighty (Reply 10): Hearing about induced drag was very interesting. Clearly at high altitude there is less lift. It seems there is no way to easily climb to say FL650 and enjoy tremendous fuel economy. Otherwise Airbus and Boeing would make their longha aircraft do that.. |
Quoting Flighty (Reply 10): Hearing about induced drag was very interesting. Clearly at high altitude there is less lift. It seems there is no way to easily climb to say FL650 and enjoy tremendous fuel economy. Otherwise Airbus and Boeing would make their longha aircraft do that.. |
Quoting glen (Reply 13): So you would be able for your fastest cruise at altitudes around FL290, but at the cost of a very high fuel consumption. Therefore we prefer to climb higher and fly slightly slower but much more economic. |
Quoting Flighty (Reply 15): Otherwise, why not have an SST that faces very little resistance at stratosphere altitudes and goes mach 3. |
Quoting horstroad (Reply 4): This means that in reality you have almost the same induced drag no matter what altitude you are at (with a given indicated air speed). But as your true air speed is way greater at high altitudes than it would be near the ground, you spend less time in the air thus saving fuel. |
Quoting PassedV1 (Reply 17): The higher you go the less drag that is true. However, you have to offset that by a lack of thrust coming out of the engines. |
Quoting lightsaber (Reply 18): Drag = Cd*1/2*rho*M^2 |
Quoting horstroad (Reply 4): BUT Induced drag is calculated similarly to lift. It is basically the same formula but with a drag coefficient instead of the lift coefficient. |
Quoting Flighty (Reply 10): it wasn't just imagination that there was less wind noise at say 8000ft. |
Quoting glen (Reply 13): There are other effects |
Quoting lightsaber (Reply 18): Drag = Cd*1/2*rho*M^2 |
Quoting lightsaber (Reply 18): Both Cd (coefficient of drag) and CL (coefficient of lift) are a function of Mach #. |
Quoting lightsaber (Reply 18): higher the cruise Mach #, the higher the optimum cruise altitude. |
Quoting Chaostheory (Reply 21): This doesn't seem to be correct. Quick check for the A330 T700 engines at 200t cruise weight: 0.80M opt altitude 36 500ft. 0.82M opt altitude 36 600ft 0.84M opt altitude 35 300ft |
Quoting Pihero (Reply 20): This thread demonstrates the difficulty of trying to simplify some technical ideas for the general public to understand. |
Quoting Pihero (Reply 20): The main reason is that below 10 000 ft, an airliner is restricted to a maximum IAS of 250 kt. |
Quoting Matt6461 (Reply 19): -is there a slight difference in the quoted formula depending on local speed of sound? I always thought Cd0 related to Vtrue^2. True airspeed would give different Mach numbers depending on local speed of sound. I know it's a small difference but just trying to see if I'm understanding correctly. The intuition in my head is that kinetic energy of the freestream/airplane interactions varies with Vtrue, while compreesibility/wave properties vary with M. |
Quoting Pihero (Reply 20): Is that an ( over) simplification of the drag f(M) equation ? because I generally use the classic |
Quoting Pihero (Reply 20): The Mach number influence is harder for us to compute, |
Quoting Chaostheory (Reply 21): This doesn't seem to be correct. |
Quoting glen (Reply 13): This negative effect of the Machnumber at high altitudes is even stronger than the positive effect of thinner air in regards to your maximum speed. So you would be able for your fastest cruise at altitudes around FL290, but at the cost of a very high fuel consumption. Therefore we prefer to climb higher and fly slightly slower but much more economic. |
Quoting lightsaber (Reply 18): Lift must be great enough in commercial flight to climb 100ft/min or more. Any less and no certified flying higher. In general, flying higher is less drag, but there is an optimum Mach # and thus the step climbing as aircraft weight reduces (more surplus lift allowing climbing to the next altitude). Step climbing as flights are, in each direction, 2000 ft incriments. |
Quoting Pihero (Reply 20): Quoting horstroad (Reply 4): BUT Induced drag is calculated similarly to lift. It is basically the same formula but with a drag coefficient instead of the lift coefficient. I disagree. At that stage we only compute a phenomenon called *DRAG* without any attribution to any influence. |
Quoting vikkyvik (Reply 26): When talking about "wind resistance", you're more likely talking about profile drag, or friction. |
Quoting lightsaber (Reply 18): Except engines prefer a certain loading for optimum cruise. |
Quoting bhill (Reply 29): Quoting lightsaber (Reply 18): Except engines prefer a certain loading for optimum cruise. Could you please explain the "loading?" Is that the force on the fan blades compressing the air mass? Like a propeller? If so, why not have "variable" pitch blades? |
Quoting Flighty (Thread starter): Is this an incentive to cruise at high altitude when possible? |
Quoting flipdewaf (Reply 32): Remembering back to my degree |
Quoting flipdewaf (Reply 32): the drag was pretty much exactly the same at all altitudes |
Quoting flipdewaf (Reply 32): you just happen to be going faster ( i.e at a higher true airspeed, TAS ) at higher altitudes. |
Quoting flipdewaf (Reply 32): You get up higher and the speed of sound gets in the way. |
Quoting Starlionblue (Reply 30): It has to do with RPM. Jet engines are most efficient in a certain RPM range, typically around 75-85% N1. |
Quoting bhill (Reply 34): So as the density of the gas decreases with altitude, does the RPM have to increase to get the same N values/loading? Seems counter intuitive. |
Quoting hivue (Reply 11): "Wind noise" is a subjective measure. It wouldn't necessarily decrease with altitude. An F-18 doing 0.98 Mach number at 8000 ft. will produce more "wind noise" than a Cessna 172 doing 90 kts IAS at 500 ft. |