Supersonic in a 777
Apr 7, 2019 | 8:07 pm
#46
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Apr 8, 2019 | 8:21 am
#47
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It can do it probably only once and will never fly again after that. Commercial airliners (besides the Concorde) are not build for supersonic flight. No idea what will break first but probably the elevators or even the wings. The air pressure which would build up under the wings during such high speeds would probably rip everything apart. There is a reason why supersonic jets have delta wings.
Apr 8, 2019 | 9:37 am
#48
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Maybe, maybe not. Here's an A380 undergoing the "one time only" max speed tests. As you can see, pieces started ripping off at Mach .93. (start at 3:00) After redesign, and with all safety/contingencies in place, they pushed it to Mach .96, with substantial fluttering/shaking.
If it survived 1.0, the air-frame would certainly be scrapped. And it would be a mighty unpleasant flight.
Shudder starts at M 0.89. It's unlikely a commercial flight exceeds this limit (intentionally).
If it survived 1.0, the air-frame would certainly be scrapped. And it would be a mighty unpleasant flight.
Shudder starts at M 0.89. It's unlikely a commercial flight exceeds this limit (intentionally).
Last edited by CPRich; Apr 8, 2019 at 9:48 am
Apr 9, 2019 | 7:31 am
#49
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High ground speeds used to be a lot more common. These days, dispatch usually orders the pilots to throttle back to save money. I've seen 794 mph groundspeed on a 747 eastbound TPAC but that was a long time ago.
Apr 9, 2019 | 10:45 am
#50
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Why? Ground speed is irrelevant to an aircraft. Why would they slow from most efficient cruise speed because of some irrelevant measure? 
Apr 9, 2019 | 12:14 pm
#51
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The airplane doesn't care about its ground speed, but the schedule does. If the plane can fly at, say, 60% cruise instead of 75% and burn less fuel for the distance covered while still arriving on time, the airline would have an incentive to do so.
Apr 10, 2019 | 4:58 pm
#52
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Much like a car's city vs. highway mpg (though for different reasons), a plane is most efficient at its design cruise speed. Jet engines operate more efficiently at greater velocity differential, the trade-off of induced vs. parasitic drag reaches a minimum at cruise velocity, etc.
Either slower or faster reduces fuel economy. Note the decreasing percentages in all directions away from optimal cruise.
Unless they believe the plane is going to have to burn fuel by circling, traveling at efficient cruise and landing early will burn less fuel than going slower and getting there on time.
Last edited by CPRich; Apr 10, 2019 at 5:14 pm