It doesn't always work to go slower. The speed of least drag is an airspeed, not a ground speed. And commercial jet engines are most efficient around 90% thrust, not at lower settings. So going slower can make you less efficient.
Can't really tell without knowing their weight and available cruising levels. But in general you should not run a commercial jet engine at lower thrust settings. The optimum altitude as calculated by the flight management system is essentially the altitude at which 90% thrust gets you the best cruise speed.
That's also why flying low is really bad for fuel consumption, you're going to have to decrease thrust and you'll be aerodynamically less efficient at the same time.
I'm not sure where the 90% comes from. What is it refering to? Rotor stage RPM, static thrust, fuel flow?
In general turbine engines are limited mostly by the temperature at the first stage of turbine blades after the combustion stage. It's hard to get reliable instrumentation in this zone so a proxy temperature is often taken from a later stage.
In older airliners there was a table of max ITT values varying with altitude and outside air temperature. In modern FADEC engines the computer does the same calculations. It's not unusual for smaller jets to be set at their max continuous thrust setting shortly after takeoff and be left there until the beginning of the descent. Modern airliners will be flying a cost optimised Mach number.
That Mach number is entered on their flight plan and is used as a basis for ensuring they are separated from other flights on the north Atlantic tracks so they wouldn't be able to change it readily.
The interesting part of all of this is that they would have been assigned an altitude for the crossing, at high altitudes maximum speeds drop (because flutter margins are proportional to velocity rather than effective pressure) but stall speeds increase so the range of viable flying speeds is actually quite limited at cruise altitudes. They probably couldn't have slowed down if they wanted too.
Yes, but to the previous posters point, turbofans operate much less efficiently at lower altitudes. They are optimized to run the majority of cruise at a very specific set of altitude and power output settings. This is compounded by the fact that fuel load also assumes a set of assumptions of the flight plan
Aircraft performance metrics of all sorts are generally U shaped. There will be an optimum altitude and airspeed for a chosen regime and deviating from it in either direction will result in lower performance.
So if you are shooting for miles per pound of fuel, there is a best. You will lose total trip fuel if you throttle up and go faster and you will lose total trip fuel if you throttle down and go slower.
Air resistance goes up by the square of the velocity, so it takes more and more thrust to go faster. The most efficient thrust per unit of speed is therefore somewhat less than the most efficient thrust per unit of fuel.
I've only had some college physics and no aeronautical engineering so I could be way off. Of course there are other factors like the amount of lift per unit of velocity and so on...
> Air resistance goes up by the square of the velocity
This is true of form drag. Airplanes are also subject to a somewhat counterintuitive induced drag that is inversely proportional to airspeed. The minimum total drag is therefore somewhere between a slow speed and a fast speed.
There are three optimal speeds depending on what you're trying to optimize. Maximum range (distance per unit of fuel) is best glide speed, which would be a painfully slow way to get somewhere. Maximum endurance (time per unit of fuel) is roughly max endurance divided by 1.316—even slower. "Optimum cruise," or Carson's speed (max speed per unit of fuel) is roughly max endurance times 1.316.
> the optimal speed of an albatross is about 32 mph, and for a Boeing 747 is about 540 mph. Both these numbers are remarkably close to the real values. Albatrosses fly at about 30-55 mph, and the cruise speed of a Boeing 747 is about 567 mph
