Trigger Happy,
Please forgive me if I am misreading between lines here, but it looks as if you expected readers to change their answers to questions 1 and 2, after they had considered question 5. If this was your expectation then it might be helpful to look a bit more closely at what the optimum altitude is.
To get maximum still air range we must achieve two things simultaneously:
Firstly we must operate our airframe at its best TAS to Drag ratio. This is achieved at the speed where a tangent from the origin touches the drag to TAS curve. This is typicaly at about 1.32 Vmd.
Drag is proportional to CAS and as altitude increases the TAS at any given CAS increases. So if we climb at constant CAS the drag will remain constant (if we ignore reducing mass), but the TAS will increase. This means that our TAS to Drag ratio increases with increasing altitude. So the higher the altitude the greater will be the TAS to Drag ratio (provided our mach number does not become excessive and cause an increase in drag).
The second thing we must do is to operate our engines at their best Thrust to Fuel ratio. This means getting the lowest cost in Kg of fuel flow for each Kg of thrust. To achieve this we need to be operating our engines in cold air at their optimum RPM. This is typically in the 85% to 95% RPM range.
But at low altitude the thrust in the 85% to 95% RPM band is far greater than the drag at 1.32 Vmd. So if we want to fly at 1.32 Vmd to get best TAS to Drag ratio at low altitude, we must run our engines at an inefficienly low RPM. This will not give maximum overall efficiency (best range or best economy).
If, on the other hand, we choose to run our engines at 85% to 95% RPM at low altitude, then our airspeed will be far too high to give us best TAS to Drag ratio. Neither of these options will give us maximum overall efficiency.
But as altitude increases, the reducing air density reduces the thrust at 85% to 95% RPM. Eventually we will reach an altitude at which the drag at 1.32 Vmd is equal to the thrust at 85% to 95% RPM. This is the optimum altitude. In this condition both our airframe and our engines are operating at their greatest efficiencies so we will get best range and best fuel economy.
If we continue to climb above the optimum altitude, the thrust at 85% to 95% RPM will continue to decrease. So we will need to run our engines at an inefficiently high RPM to get thrust equal to drag. We could of course choose to run our engines at the optimum 85% to 95% RPM, but then our airspeed would be less than the optimum 1.32 Vmd. Neither of these options will give best overall efficiency.
In reality of course various other factors such as ATC requirements, trip length, head/tail winds, and non-fuel costs will also play a part in determining the combination of speed and altitude that is actually used.
Last edited by Keith.Williams.; 16th October 2004 at 19:48.