I was in a discussion with my father tonight and he said that aircraft generally consume less fuel at a lower altitude.
His reasoning was that at lower altitudes, the oxygen concentration is higher and thus with more oxygen ensures closer to complete combustion of fuel and hence better use of the fuel.
However, my counter - which I thought was the predominant opinion - is that the air drag is higher at lower altitudes, which increases fuel consumption overall. Basically drag forces at lower altitudes overrides any benefit to more effective combustion.
Could you please comment on this?
I understand this question discounts a lot of other factors, like jetstreams, the presence and effect of weather...
Whilst the amount of oxygen available at sea level is greater than it is at altitude, the actual concentration is much the same (at least at any altitude that I might care about). So for a given mass air flow the percentage of oxygen is more or less the same. Fuel flow is directly related to air flow....feed the engine more air, and give it more fuel, and vice versa. The upshot is that the level of combustion 'completeness', is effectively the same.
As the altitude increases, an engine produces less power at a given RPM. The fuel required for RPM also reduces, ultimately reaching the odd situation that the minimum fuel flow will produce the maximum RPM (and of course, bugger all thrust). Whilst not something that airline crews have to worry about, it was an issue on the 'Streak Eagle' time to altitude record flights done by the F15 in its early days, as it meant that the engines had to be shut down during the zoom climb, and then relit on the way down.
Jet engines are most efficient at a particular RPM. That happens to be very close to their maximum RPM. You never want to be thrust limited in the cruise (which would mean you would never have any excess power available), so cruise altitudes are chosen to give RPM settings that have about 10% margin. So, from an engine point of view, at any altitude, I'd want to set that same RPM for all altitudes.
The other part of this equation is the airframe. All aircraft have a most efficient angle of attack. That will equate to roughly the same indicated airspeed at all altitudes. So, picking a number as an example, that might be around 270 knots IAS. The thrust required to fly that IAS will be the same at all altitudes, but because the engine thrust varies with altitude, the engine may only require 70% RPM to give the thrust required at 5,000', whilst it would require 100% RPM to do so at FL350. So, whilst my airframe is happy at its best angle of attack, in the low level case the engines are well off their best (high) RPM. So, ideally, you want a situation in which the thrust required is at near maximum RPM, and the airframe is at that best alpha (angle of attack). You can get that situation by simply climbing until the engine thrust (at best RPM) has reduced to match that required by the airframe.
The other part of the equation is the IAS/TAS relationship. At sea level, indicated airspeed and true airspeed are basically the same thing. If you drive along at 100 miles per hour, and stick your hand out the window, you will feel 100 mph worth of wind. But, at altitude (say FL400) there is a lot less air. If I was at FL400, and still doing a REAL 100 mph, sticking my hand out the window, I'd only feel 50 mph worth of breeze. That's the difference between TAS and IAS. Indicated is what the airflow feels like to the aircraft, whilst true is the actual speed that we're moving through the air mass. True is what you use to navigate by (apply the wind vector to TAS and you get groundspeed), whilst indicated is what the wings feel, and which makes the aircraft fly. Up to around FL300 this relationship means that as I climb I keep getting a better TAS for my fixed (best) IAS.
Above roughly FL300, this all still holds true, but another limitation starts to come into play. As we've been climbing at our constant IAS, the mach number (percentage of the speed of sound) has been increasing. This has been happening for two reasons. First, we are actually accelerating (same IAS, but higher TAS as we climb). Secondly, the speed of sound is slower at altitude. The upshot is that we have a maximum speed limit in mach as well as IAS, so eventually we have to move away from our climb IAS and move to a mach number.
So, if you're still with me, at our initial cruise altitude (chosen so that the airframe is at that best alpha, and the engines simultaneously are at their happiest RPM), we settle down. But, our weight does not stay the same....we're reducing weight as the fuel load is burnt down. As the weight decreases, the alpha needs to be reduced as we don't need to make the same amount of lift any more. If we want to keep that same alpha, the target airspeed will be slightly slower. That also means we'll need less power. And of course, our TAS is reduced. Once the weight has reduced enough that there is sufficient power for the next climb step, then you climb to the next ATC allowed level, and start the cycle again. As a rough guide, the speed will reduce by about .02 mach before each 2,000' step.
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