how would you explain!!!!!!!
Joined: Aug 2001
Posts: 775
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From: Dorset
There are two main reasons:
To fly straight and level at constant speed the thrust must be equal to the drag. Both drag and IAS are proportional to dynamic pressure. So as you increase altitude at constant IAS, the drag remains constant.
But the TAS at any given IAS increases with increasing altitude, such that at 40000 ft the TAS is about twice the IAS. So you get more knots for each pound of thrust.
Secondly jet engines typically give the best specific fuel consumption (most thrust per pound of fuel burned) when running at between 85% and 95% RPM. But at low altitude this RPM range would produce excessive speeds.
As altitude increases the air density and mass airflow through the engine both decrease. This reduces thrust at any given RPM. By climbing to a suitably high altiutude it is possible to fly such that thrust in the optimum engine RPM range matches the drag at the optimum TAS. This turns out be be at about 1.35 VMD.
To fly straight and level at constant speed the thrust must be equal to the drag. Both drag and IAS are proportional to dynamic pressure. So as you increase altitude at constant IAS, the drag remains constant.
But the TAS at any given IAS increases with increasing altitude, such that at 40000 ft the TAS is about twice the IAS. So you get more knots for each pound of thrust.
Secondly jet engines typically give the best specific fuel consumption (most thrust per pound of fuel burned) when running at between 85% and 95% RPM. But at low altitude this RPM range would produce excessive speeds.
As altitude increases the air density and mass airflow through the engine both decrease. This reduces thrust at any given RPM. By climbing to a suitably high altiutude it is possible to fly such that thrust in the optimum engine RPM range matches the drag at the optimum TAS. This turns out be be at about 1.35 VMD.
Joined: Jun 2000
Posts: 865
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From: UK
All above correct, but here is an interesting reality. Most airliners cruise on a mach no above fl280 or so. If you climb and maintain a constant mach no then you will generally find you have a lower TAS (lower speed of sound at higher alts), and so save fuel, but slow down a bit....
Guest
Posts: n/a
The fundamental reason is that a jet engine's fuel consumption is proportional to the thrust produced (whereas a piston engine's fuel consumption is proportional to the power developed).
Therefore, the lower the thrust, the lower the fuel consumption.
As we all know, Thrust is equal to Drag, therefore if we fly at minimum drag speed (IAS) we will have minimum thrust and minimum fuel flow, which will give us maximum endurance (max hours per pound). We will be at the 'bottom of the drag curve'.
At high altitudes this IAS will result in a higher TAS, giving us more miles per pound.
To get maximum range, we must fly faster (about 1.35 x faster, as posted previously) than minimum drag speed to get a maximum ratio of speed/drag. We will then be flying at the 'tangent to the drag curve'.
So, in a nutshell, jet aircraft are more efficient at high altitude because the lower IAS associated with the high TAS results in lower drag which in turn requires less thrust (and therefore fuel flow) to balance it.
Therefore, the lower the thrust, the lower the fuel consumption.
As we all know, Thrust is equal to Drag, therefore if we fly at minimum drag speed (IAS) we will have minimum thrust and minimum fuel flow, which will give us maximum endurance (max hours per pound). We will be at the 'bottom of the drag curve'.
At high altitudes this IAS will result in a higher TAS, giving us more miles per pound.
To get maximum range, we must fly faster (about 1.35 x faster, as posted previously) than minimum drag speed to get a maximum ratio of speed/drag. We will then be flying at the 'tangent to the drag curve'.
So, in a nutshell, jet aircraft are more efficient at high altitude because the lower IAS associated with the high TAS results in lower drag which in turn requires less thrust (and therefore fuel flow) to balance it.
Joined: May 2000
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From: Worldwide
Higher altitude the specific range (V/FF) increases, best range speed increases (tangent to curve), min total drag, ie best endurance also increases.
Headwind move origin of tangent to curve to the right, tailwind move the origin to the tangent to the curve left.
Therefore in nil wind , fly or hold as high as possible in a jet. When flying east - stay high with jetstreams, flying west stay low out of jet stream. North/south, generally high, may be have reduced effective TAS due strong crosswinds from jetstreams (can be 100-150kts+).
