A340 Green Dot Speed
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BMM389EC,
This is common to all aircraft, not just Airbus. If other manufacturers don't do it, then they're just simplifying things in keeping one common Vmd speed.
With one engine (or more) inoperative, the entire drag curve changes, and Vmd is at a lower speed. This is due to the addition of two new forms of form / profile drag that are not present in normal all-engines flight, namely -
(1) Windmilling drag from the failed engine, and
(2) Offset Rudder drag due to constant rudder displacement during assymetric flight. (If the manufacturer chooses to use up to 5° of slip to aid control and reduce drag, this is bundled in with rudder drag).
These 2 items of profile drag increase pretty much with the square of the speed, whereas the Induced Drag decrease around about Vmd is much, much less.
Actually, the 10 knot differential is "about right" for most jet aircraft, give or take a knot or two.
Regards,
Old Smokey
This is common to all aircraft, not just Airbus. If other manufacturers don't do it, then they're just simplifying things in keeping one common Vmd speed.
With one engine (or more) inoperative, the entire drag curve changes, and Vmd is at a lower speed. This is due to the addition of two new forms of form / profile drag that are not present in normal all-engines flight, namely -
(1) Windmilling drag from the failed engine, and
(2) Offset Rudder drag due to constant rudder displacement during assymetric flight. (If the manufacturer chooses to use up to 5° of slip to aid control and reduce drag, this is bundled in with rudder drag).
These 2 items of profile drag increase pretty much with the square of the speed, whereas the Induced Drag decrease around about Vmd is much, much less.
Actually, the 10 knot differential is "about right" for most jet aircraft, give or take a knot or two.
Regards,
Old Smokey
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"RAT" Out
OLD SMOKEY.....how much does that change if the "'RAT" is out???(A320). emergency electrical,single engine(buggar)..by the way,enjoy reading your posts...PB
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pakeha-boy,
Deployment of the RAT would indeed increase the form / profile / parasite drag of the aircraft, and cause the Vmd (Green Dot Speed) to be further reduced, but......
Deployment of the RAT is not normally an option considered following engine failure. Even for a 2 engined aircraft down to one operative engine, the remaining supply of Hydraulic, Electrical, and Pneumatic services should be more than adequately provided for by the remaining engine (not that the RAT would help the pneumatics).
What IS routinely considered following engine failure on a 2 engined aircraft is in-flight start of the APU, which will provide complete back-up of Hydraulic, Electrical, and Pneumatic services. Drag from APU inlet doors etc. will increase overall drag, but this additional drag must be balanced against any small thrust advantage that the APU might provide, depending upon the aircraft type.
Thanks for the nice PM!
Regards,
Old Smokey
Deployment of the RAT would indeed increase the form / profile / parasite drag of the aircraft, and cause the Vmd (Green Dot Speed) to be further reduced, but......
Deployment of the RAT is not normally an option considered following engine failure. Even for a 2 engined aircraft down to one operative engine, the remaining supply of Hydraulic, Electrical, and Pneumatic services should be more than adequately provided for by the remaining engine (not that the RAT would help the pneumatics).
What IS routinely considered following engine failure on a 2 engined aircraft is in-flight start of the APU, which will provide complete back-up of Hydraulic, Electrical, and Pneumatic services. Drag from APU inlet doors etc. will increase overall drag, but this additional drag must be balanced against any small thrust advantage that the APU might provide, depending upon the aircraft type.
Thanks for the nice PM!
Regards,
Old Smokey
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Smokey mate...yeah sort of unrealistic and totally agree with your assessment,will copy that and give it to the sim instructor that stuck to me a couple of weeks ago.......after finishing the sim ride we had a little time to play with so he drummed up the emergency electical(loss of AC Bus 1/2) you know the drill....anyway we were doing so well,he failed an engine (#2)at the OM.....and all hell broke lose......everything happened so fast that we just put the bloody thing down,....up and down like a lady of the nights drawers.... the one thing I do remember were massive fluctuations in the airspeed,hence the question.....thanks for taking the time to reply,..PB
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Here is another scenario for you...in total agreement with your synopsies on perf.... but lets say you have a 35 knot split between GD and VLS.....(a330) Why would you not fly below GD... with my gang its nearly a sackable offence to go below...and when I explained what happens with one or two engs out I was looked at as though I had TWO heads... My comment was also its certified at all up weight on one eng, we are on approach at minus 70T... not a performance issue....
Yes we are a bit back on the drag curve but no where near where they believe
Yes we are a bit back on the drag curve but no where near where they believe
