why not stabalise engines with brakes on?
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I presume TCAS is talking about the normal V1 reduction on contaminated runways to improve the accel stop performance as a tradeoff against a reduced continued takeoff performance. For a Dash8 on anything much longer than a football field is it all a bit academic ?
No, I wasn't being Type specific, Mutt. The 2 second applies post amendment 42 (if my memory serves me correctly). For earlier aircraft, there is no such requirement, unless mandated by individual Authorities. Additionally, the V1 engine failure sequence on earlier aircraft is more critical than for later machines.
The main significance of the additional 2 second delay was that it represented an attempt to build a bit more fudge into the sequence to acknowledge some of the real world limitations associated with applying the previous accel stop data in runway limiting situations.
Only of interest to highlight to readers that one ought not read current rules and idly presume that they apply to older aircraft.
Great to be back .....
[This message has been edited by john_tullamarine (edited 23 June 2001).]
No, I wasn't being Type specific, Mutt. The 2 second applies post amendment 42 (if my memory serves me correctly). For earlier aircraft, there is no such requirement, unless mandated by individual Authorities. Additionally, the V1 engine failure sequence on earlier aircraft is more critical than for later machines.
The main significance of the additional 2 second delay was that it represented an attempt to build a bit more fudge into the sequence to acknowledge some of the real world limitations associated with applying the previous accel stop data in runway limiting situations.
Only of interest to highlight to readers that one ought not read current rules and idly presume that they apply to older aircraft.
Great to be back .....
[This message has been edited by john_tullamarine (edited 23 June 2001).]
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Why will airline crews use a rolling take-off?
Because it is recommended by the (turbofan) engine manufacturers.
When at high powers settings at low (no) airspeed, a huge amount of airmass is sucked into the engine.
While inflight all of this air will come from the front, at static thrust the air will be sucked from the sides of the engine narcelles as well!
This sideways airflow -can- cause a disruption in the airflow entering the main compressor, which -can- lead to an engine stall.
This is the main reason why turbofan engines, when on the testbench and tested at high powersettings, will have a bell shaped narcelle attached to it, instead of it's regular one.
On the plane however, a bell shaped narcelle would be foolish, because the engine is designed to operate
in an environment where air inlet speeds are somewhere in the 150 - 200m/s range . (normal turbofan on e.g. a 747 or similar type of airliner). The drag would be increadible high!
In this type of environment an efficient narcelle is slightly tapered at the inlet, and a little wider near the fan disk.
(sort of an inverted bell shape, but less dramatic).
This will cause the oncoming air to decelerate slightly. The total pressure of this air therefore will rise, which will make
a positive contribution to the massflow entering the turbofan. This is one of the main reasons turbofan engines are highly successfull in generating thrust at high altutudes with a relativly modest fuel flow. Remember, a turbojet / turbofan core will suffer from the decreased airpressure at high altitudes! This is where the engine narcelle will come to the rescue. It can be responsible for up to 30% of the total thrust at altitude. (ram recovery.)
To accomodate for the high pressure setting / low airspeed scenario down the runway at t/o, the engine narcelle has a quite thick and rond edge, which will prevent airflow disruptions. (still, the rolling t/o recommendation will be given)
With the rolling t/o, the runway used penalty will be relativly small. As stated elseware in this forum, initial acceleration is high (somewhere around 2.6m/s^2), because of the huge massflow throught the engines. When the aircraft picks up speed on the runway, engine thrust will become less.
All commercial jets are hugely overpowered, to cope with n-1 scenario's and the FAR requirement to maintain a 2.5% climbpath with full load.
Usually, when atmospheric conditions permit, a derated thrust is used. This means usually there is runway length to spare.
Remember, the official law stipulated a 35ft screen hight. Most aircraft will be well above the 35ft imaginary barrier when overflying
the runway threshold at the other end.
Because it is recommended by the (turbofan) engine manufacturers.
When at high powers settings at low (no) airspeed, a huge amount of airmass is sucked into the engine.
While inflight all of this air will come from the front, at static thrust the air will be sucked from the sides of the engine narcelles as well!
This sideways airflow -can- cause a disruption in the airflow entering the main compressor, which -can- lead to an engine stall.
This is the main reason why turbofan engines, when on the testbench and tested at high powersettings, will have a bell shaped narcelle attached to it, instead of it's regular one.
On the plane however, a bell shaped narcelle would be foolish, because the engine is designed to operate
in an environment where air inlet speeds are somewhere in the 150 - 200m/s range . (normal turbofan on e.g. a 747 or similar type of airliner). The drag would be increadible high!
In this type of environment an efficient narcelle is slightly tapered at the inlet, and a little wider near the fan disk.
(sort of an inverted bell shape, but less dramatic).
This will cause the oncoming air to decelerate slightly. The total pressure of this air therefore will rise, which will make
a positive contribution to the massflow entering the turbofan. This is one of the main reasons turbofan engines are highly successfull in generating thrust at high altutudes with a relativly modest fuel flow. Remember, a turbojet / turbofan core will suffer from the decreased airpressure at high altitudes! This is where the engine narcelle will come to the rescue. It can be responsible for up to 30% of the total thrust at altitude. (ram recovery.)
To accomodate for the high pressure setting / low airspeed scenario down the runway at t/o, the engine narcelle has a quite thick and rond edge, which will prevent airflow disruptions. (still, the rolling t/o recommendation will be given)
With the rolling t/o, the runway used penalty will be relativly small. As stated elseware in this forum, initial acceleration is high (somewhere around 2.6m/s^2), because of the huge massflow throught the engines. When the aircraft picks up speed on the runway, engine thrust will become less.
All commercial jets are hugely overpowered, to cope with n-1 scenario's and the FAR requirement to maintain a 2.5% climbpath with full load.
Usually, when atmospheric conditions permit, a derated thrust is used. This means usually there is runway length to spare.
Remember, the official law stipulated a 35ft screen hight. Most aircraft will be well above the 35ft imaginary barrier when overflying
the runway threshold at the other end.
