Originally Posted by enicalyth
TSFC? It will be a lower value at sea level than at altitude. But don't be fooled. Fuel consumption is always better where oxygen is aplenty but you are missing the point that it is thrust specific. How much thrust you need muchly depends on lift/drag ratio as well as weight of course. Poor L/D occurs down low and slow and completely swamps the oxygen rich fuel consumption benefits displayed on the data sheet. Come on! You know a 747 burns fuel at a rate of thirty tons an hour on take-off but half an hour later she's guzzling at a rate of ten tons an hour. All things equal lift is proportional to air density but the square of velocity. Retract those flaps and wheels, get her up where the air is cold and speed her along. Wonderful L/D ratio. Pity the TSFC doubles but thin or not the air will be drawn in because the inlet designer did a good job.
So TSFC might look good low and slow but my god you need lots of thrust to stay in the air. High on the fly TSFC may look twice as "bad" but hey L/D is not say 7:1 or worse but 17:1 and even better.
Oh I'm rambling b*gger it.
The poor L/D has little to do with altitude!
Airplanes have poor L/D immediately after takeoff because they have a lot of drag-inducing things like landing gear out, and also have high-lift devices that allow them to fly at low IAS but at a cost of high drag, plus they may be flying at a high angle of attitude which has same effect (low IAS but high drag).
Accelerate your 747 to a safe IAS speed for clean configuration - gear retracted, flaps retracted - and I suppose that the L/D would reach the values close to the cruise ones rather soon.
But the engines would still be inefficient - prolonged flight at low level is a waste of fuel even in clear configuration.
Also do not confuse the termodynamic effects of cold air with effects of altitude through density and TAS!
In midwinter, I presume you might fly your 747 to Yellowknife or Klondike or Fairbanks or Yakutsk - all of which places are located less than 300 metres above sea level - and find -50 degrees Celsius on ground level. This usually happens in anticyclonal weather, so even the pressure altitude could be negative - and the density altitude would be very negative.
The plane would generate a lot of lift, and drag, at very low true air speeds. And the jet engines would have huge amounts of air, and oxygen, available to suck in. Nevertheless, I presume that the true efficiency of engines would be low because the TAS is low. It would take a lot of fuel to fly low and clean from, say, Winnipeg to Aklavik or Yakutsk to Norilsk in midwinter, if your powerplant is a jet engine...
You would get much further by climbing to greater altitudes, where the air is thinner and TAS is faster. Quite independent of any changes of L/D.