What does L/D Max give you in jets?
10-23-2016, 12:21 PM
#11
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Thanks, Twin Wasp.
That wouldn't take into account a pusher-prop configuration.
Originally Posted by tankertoadau92
I think it has to do with the effect of the prop wash over the wing producing lift.
10-24-2016, 12:33 AM
#12
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Joined APC: Jul 2005
Posts: 248
Maximum range cruise (MRC) by definition is the speed that will provide the furthest distance traveled for a given amount of fuel burned or the minimum fuel burned for a given cruise distance. For Boeing aircraft, this is achieved at Cost Index = 0, which corresponds to the top of the curve on the fuel milage charts (NAM/lb vs Mach).
MRC is L/D max in cruise, while best hold speed represents best endurance. As someone mentioned, Aerodynamics for Naval Aviators is an excellent reference, and it states that typically best endurance is 25% less than best range. For the 747-400, max endurance is much closer to max range. For example, at 680000 lb., MRC (L/D) Mach is .838 at FL350, or 288 KIAS. Best hold speed for this weight and altitude is 276 KIAS.
For EO cruise, FMCs typically target LRC speed (1% increase from MRC). However, there is an option to select E/O SPD, which again is MRC or L/D max if terrain is a factor for the drift down. This gives the shallowest angle (or longest distance) to EO altitude.
This is repeated for descents at Cost Index 0, which is L/D max to again maximize distance traveled, which means thrust retarded to idle at cruise altitude earlier to minimize fuel burn.
MRC is L/D max in cruise, while best hold speed represents best endurance. As someone mentioned, Aerodynamics for Naval Aviators is an excellent reference, and it states that typically best endurance is 25% less than best range. For the 747-400, max endurance is much closer to max range. For example, at 680000 lb., MRC (L/D) Mach is .838 at FL350, or 288 KIAS. Best hold speed for this weight and altitude is 276 KIAS.
For EO cruise, FMCs typically target LRC speed (1% increase from MRC). However, there is an option to select E/O SPD, which again is MRC or L/D max if terrain is a factor for the drift down. This gives the shallowest angle (or longest distance) to EO altitude.
This is repeated for descents at Cost Index 0, which is L/D max to again maximize distance traveled, which means thrust retarded to idle at cruise altitude earlier to minimize fuel burn.
10-24-2016, 08:08 AM
#14
Gets Weekends Off
Joined APC: Jul 2005
Posts: 248
Originally Posted by HuggyU2
I'm fairly certain that you are not correct.
L/D Max is max endurance, and is not max range speed. Aero for Naval Aviators states this, I believe.
L/D Max is max endurance, and is not max range speed. Aero for Naval Aviators states this, I believe.
11-24-2016, 05:32 PM
#15
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Joined APC: Aug 2009
Posts: 51
Max L/D also is also the best angle of climb speed (Vx)in jets, where as in props, Vx is lower than max L/D.
The reason is because props have constant power and decreasing thrust as airspeed goes up, where as in jets, thrust stays relatively constant, and power increases as airspeed goes up. If you plot the thrust/airspeed and power/airspeed curves of both it becomes more obvious.
The reason is because props have constant power and decreasing thrust as airspeed goes up, where as in jets, thrust stays relatively constant, and power increases as airspeed goes up. If you plot the thrust/airspeed and power/airspeed curves of both it becomes more obvious.
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