engine drag?
Joined: Mar 2002
Posts: 4,569
Likes: 1
From: Florida
I believe that the engine drag is insignificant while the turbine is providing positive power to the compressor N1 N2, (N3).
The majority of the drag is associated with the engine nacelle.
The compressor under power is still accelerating the air as evidenced that the incidence angle on the blades, including the fan is still associated with positive pressure behind the blades (pumping).
This is generally true even at windmill start conditions.
The majority of the drag is associated with the engine nacelle.
The compressor under power is still accelerating the air as evidenced that the incidence angle on the blades, including the fan is still associated with positive pressure behind the blades (pumping).
This is generally true even at windmill start conditions.
Joined: Jan 2002
Posts: 960
Likes: 0
From: Brisbane, Australia
It's commonly known that a " flight idle" descent is the best for minimum fuel consumption.
The fact that the engines are producing a little, very little drag is good, the FMC can have the aircraft at cruise altitude and speed longer, therein saving fuel, and have the advantage of a very slightly steeper descent angle.
I guess you still use the old, tried and true, 3 times the altitude, in still wind for the TOD!
Further, when descending in moderate/heavy rain, I always put on a little power, aside from any anti-ice requirements, so as not to have a very much higher percentage of moisture than normal entering the fan.
At flight idle and reasonably high speeds, the slow running fan will force a percentage of the incoming airflow to bypass actually entering the fan, but, due to the forces involved, the momentum of the rain droplets will not allow them to change direction fast enough, and they enter the fan.
This increase in %age of water entering the fan can make the engine become unstable, and even flame out in extreme conditions, which is another reason to turn ON the engine anti-ice system, to unload the compressor, and increase your compressor stall margin.
Cheers
The fact that the engines are producing a little, very little drag is good, the FMC can have the aircraft at cruise altitude and speed longer, therein saving fuel, and have the advantage of a very slightly steeper descent angle.
I guess you still use the old, tried and true, 3 times the altitude, in still wind for the TOD!
Further, when descending in moderate/heavy rain, I always put on a little power, aside from any anti-ice requirements, so as not to have a very much higher percentage of moisture than normal entering the fan.
At flight idle and reasonably high speeds, the slow running fan will force a percentage of the incoming airflow to bypass actually entering the fan, but, due to the forces involved, the momentum of the rain droplets will not allow them to change direction fast enough, and they enter the fan.
This increase in %age of water entering the fan can make the engine become unstable, and even flame out in extreme conditions, which is another reason to turn ON the engine anti-ice system, to unload the compressor, and increase your compressor stall margin.
Cheers
Joined: May 2003
Posts: 831
Likes: 0
From: The Roman Empire
My information is that a jet engine ALWAYS produces thrust, even at idle- the thrust remaining equals about 5% of max thrust.
Actually, if you descend on idle you can have altitude times 3 miles, but if you have all engines flameout you just get altitude times 2 miles, roughly.
That's the proof that jet engine always push!
Flight detent, - At flight idle and reasonably high speeds, the slow running fan will force a percentage of the incoming airflow to bypass actually entering the fan.... -
Maybe yes maybe not...what is the source of that information?
Standing to be corrected
LEM
Actually, if you descend on idle you can have altitude times 3 miles, but if you have all engines flameout you just get altitude times 2 miles, roughly.
That's the proof that jet engine always push!
Flight detent, - At flight idle and reasonably high speeds, the slow running fan will force a percentage of the incoming airflow to bypass actually entering the fan.... -
Maybe yes maybe not...what is the source of that information?
Standing to be corrected
LEM
Joined: Mar 2002
Posts: 4,569
Likes: 1
From: Florida
Lem
I think that what he's saying is that there is inlet spillage of excess air, but that the water droplets won't follow in the same ratio.
I agree
That's one of the reasons that they upped the flight idle and then redesigned the spinner to centrifuge the water out the fan discharge.
I think that what he's saying is that there is inlet spillage of excess air, but that the water droplets won't follow in the same ratio.
I agree
That's one of the reasons that they upped the flight idle and then redesigned the spinner to centrifuge the water out the fan discharge.
