"Green dot speed" for the B777
 

Hi all,

I remember reading somewhere that the best lift to drag speed for the B777 is the flaps up maneuvering speed, anyone can recall which portion of the FCOM or FCTM it states? Did a search of "flaps up man" on the pdf but couldn't find it. Perhaps i'm wrong. Can someone provide some info of the best L/D speed on the B777.

Thanks

From the 777/787 FCTM Sec 4:

"The FMC provides maximum angle climb speeds. Maximum angle climb speed is normally used for obstacle clearance, minimum crossing altitude or to reach a specified altitude in a minimum distance. It varies with gross weight and provides approximately the same climb gradient as flaps up maneuver speed."

So, because on a jet, Vx is very close to V L/Dmax, your memory of 'the best lift to drag speed for the B777 is the flaps up manoeuvring speed' is close enough.

Ok this could be valid up to 20200 ft, over this altitude the "up mark" is removed from view, but it grow up proportionally to the altitude, so as far as we studied aerodynamics on books, the minimum drag speed is less than the best L/D speed, but from FMS manual the minimum drag speed over FL200 equals the FMS best holding speed which is very close to the cost index 0 economy speed and sometimes when very heavy, higher than C.I 0 speed which should be the best L/D spd...

simply curious!

Minimum drag speed can only be best L/D speed. By arithmetic necessity. Unless you change L.

i do not agree:=

Ok, how can "best L/D" mean anything other than minimum drag?

The best L/D is tangent to the drag curve, the minimum drag is at his bottom

https://upload.wikimedia.org/wikiped.../1/12/Drag.jpg

No, minimum drag is minimum drag, i.e., the bottom of the curve, i.e. minimum thrust required, i.e., best endurance (assuming fuel flow correlates with thrust). Since lift is constant across the speeds, the maximum of a term where drag is at the bottom (L/D), is at minimum drag.

The tangent point (best range) is to the right of that.

Hope this short doc can help. If some charts can help too, please MP.


PS: Mr Pilotos will appreciate !

Performance conditions which occur at (L/D)max.

Max endurance of a jet
Max range of a prop
Max climb angle of a jet
Max power off glide range for both

B 777 FMC speeds :

Maximum angle climb speed:

The maximum angle climb speed schedule is a fixed CAS/Mach schedule which is evaluated and displayed to the pilot in terms of current KCAS equivalent. The schedule is designed to yield minimum distance to optimum altitude, but also provides excellent instantaneous climb gradient performance. The maximum angle climb CAS is limited by a minimum speed equal to VREF30+80, to ensure full maneuvring capability. The maximum angle climb speed is provided as a function of gross weight in the FMC performance database. The corresponding maximum angle climb Mach numbert does not vary with gross weight, and is provided as constant in the performance database.


Engine Out climb/driftdown speed

The engine Out climb/driftdown speed schedule is a variable CAS schedule wich is computed using a fixed lift coefficient (CL) and then limited by the CAS equivalent of a fixed mach number. The schedule provides near maximum climb gradient performance with one engine inoperative. The CL value and corresponding limit mach number value are provided as constants in the FMC performance database.


Flaps up holding speed

The flaps up hold speed schedule approximates minimum fuel flow (low altitudes) or minimum drag (higher altitudes), subject to a lower limit of VREF30+80. The pilot may enter alternate hold speed values wich fall within the allowable speed enveloppe. Flaps up hold speeds are defined as lift coefficient (CL) as a function of gross weight divided by atmospheric pressure ratio in the FMC performance database.

Someone getting their Vmp and Vmd mixed up perhaps?