4 ms·

Don't most airliners power way down at cruising altitude though?

• pdonis 7y ago
  They power down some, but not much: the thrust of jet engines is significantly lower at typical cruising altitudes (because of reduced air density), so it takes almost as much power to cruise as to climb to altitude.
  • selectodude 7y ago
    That is not accurate. The efficiency of turbofans goes up as speed increases. A normal commercial high-bypass turbofan is at its most efficient around 1000 km/h, which is a speed it cannot sustain at lower altitudes due to air resistance over the blades. Just to add, at cruise, N1, the RPMs of the outermost fan on the jet, will peak over 100 percent, but that isn't a function of fuel consumption.
    • pdonis 7y ago
      > The efficiency of turbofans goes up as speed increases. I didn't say efficiency, I said thrust. Nothing you said contradicts anything I said.
      • selectodude 7y ago
        You said that the engine has to burn as much fuel in cruise as it does during takeoff.
        • pdonis 7y ago
          I said no such thing. I said the engine power had to be almost as high during cruise as during climb to altitude. Engine power setting is not the same as engine fuel consumption.
  • colechristensen 7y ago
    When you say not much it's hard to agree or disagree, who knows what much is. Numbers can be added, about 80% is common for cruise altitude. The performance equations are not linear. One example another pointed out is that thrust goes up as you go faster, you point out that thrust goes down as density gets lower, it's both. There tend to be max/min points where the influence from one effect is overtaken by another.