OzzyPirate wrote:To give you an idea how little excess thrust is available up high, if we're near maximum altitude and a wind change drops our indicated airspeed by 10 knots, it may take over a minute at maximum continuous thrust to recover those 10 knots.
skyhawkmatthew wrote:To build on Ozzy's example, and provide a comparison, look at the 777-300 and ER. The ER has a slightly bigger wing, but its engines are rated at over 30% more thrust output. At high weights and altitudes in the -300, if a significant airspeed drop occurs, the only option in the anaemic -300 may be to initiate a descent to recover the speed; the -ER can generally much more easily power out of such situations, as it is much more wing- rather than thrust-limited at high altitude.
iffy wrote:The first question of many I want to ask is - are aircraft jets close to max jet rotation/propeller rotation/Jet speed at high altitude (30k feet plus) to achieve the cruising speed of 500 miles plus? I've always thought that the jets will only be at around 50% power as the air is thinner at higher altitude which equals less friction etc?
CARST wrote:I am no expert, and you two can probably add something to what I have to say, but what you describe here should probably be put into perspective. AFAIK most passenger airplanes at cruise level are very close to stall speed and maximum speed at the same time, especially when heavy. E.g. a heavy 773 might cruise at 290 knots indicated air speed at FL 340, the maximum speed could be at 305 kt IAD and the stall speed at 270 kt IAS. So that 10 knot loss you speak about can be very important. My numbers are made up, but I am sure someone can provide real world numbers, that should be in the ballpark of what I mentioned.
CARST wrote:The true airspeed (TAS) will be different (adjusted to the wind speed and direction) and the indicated airspeed (IAS) will be way, way lower.
atcpeter wrote:Most modern jets at cruise altitude (could be several thousand feet below max altitude) have less than 20kts of *indicated* airspeed difference between aerodynamic stall and never-exceed speed.
flyboy80 wrote:If so that would mean if fuel flow to the engine was the same between takeoff and climb the engine would technically overspeed at cruise?
flyboy80 wrote:This post makes me very curious about the flow of fuel to the engine. Some have stated the fuel burn decreases significantly at altitude, yet the engine speed is similar to lower altitudes-although delivering much less power obviously. Is this simply because the engine can run faster with the less dense air at cruise altitude? If so that would mean if fuel flow to the engine was the same between takeoff and climb the engine would technically overspeed at cruise? I probably have it all wrong.
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