Quote:
Originally Posted by ScottG  Assuming 0 wind and standard temps and pressures how is the altitude that gives maximum speed determined for a jet as well as a turbo prop. Furthermore how is the altitude for maximum range calculated. I know many aircraft vary but I am asking for general consideration, i.e. climb until you can't maintain maximum IAS or hit the mach limit......basically what are the general indications for maximum possible ground speed. |
Offhand, I'd say that max speed occurs at the highest altitude where the aircaft can maintain max continuous power. Looking at a performance chart for a C172, the highest TAS occurs at 8,000 ft, where the a/c has 80% power available. Without reviewing the literature, I'd say this is true for a jet as well.
For a propeller aircraft, max
range is independent of altitude, if you ignore the cost of the climb. As for
endurance, it's maximized when fuel flow (FF) is minimized. Since in props, FF is proportional to power, FF is minmized when power is minimized. Since for any given airspeed, it takes more power to fly it the higher you are, lower altitudes are better.
For jets, max
range increases with altitude. Max
endurance occurs when the fuel flow (FF) is at a minimum. Since FF is roughly proportional to drag in a jet, FF is minimized when drag is minimized, which should be at L/Dmax. Since drag at a particular airspeed does not vary with altitude, and FF is proportional to drag, max endurance will not vary with altitude.
The reason that jets and props are different is mostly due to the powerplant:
- FF in props is related to the amount of power the engine is producing, but in jets it's related to the thrust.
- For props, power is roughly constant with airspeed (assuming CS prop), but jets experience increasing power with airspeed
- For props, thrust decreases with airspeed, but jets (turbojets) have roughly constant thrust with airspeed.