jgoggi

Speed during rotation increases too much?

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There is something that does not convince me too much during rotation (flaps 10): I lift the nose at about 2.5 deg/sec until 15 deg, then start following the FD, but the speed always increases too much, it is absolutely NEVER V2+10, but V2+20 or even 30 and it always goes above the flap 5 speed, then the FD bar rises and the speed decreases (if I keep following the FD). I suspect this could be related to the bug that after 80 knots the engines thrust erroneously self-increases and at lift-off it's about 3% higher than it should be and this causes a too high acceleration... This never happened on the 777 and 737, they stuck to the V2+x expected speed...

 

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Let them fix the bug you keep on about. Maybe the cause, maybe not. But you are the pilot. V2+10 after rotation is not a given. You are in control of pitch and airspeed. 

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8 minutes ago, kevinh said:

 But you are the pilot. V2+10 after rotation is not a given. You are in control of pitch and airspeed. 

Yes, but I don't like that my speed exceeds the flaps 10 speed (so I should select flaps 5) and then, following the FD, it decreases again below flaps 5...

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25 minutes ago, jgoggi said:

Yes, but I don't like that my speed exceeds the flaps 10 speed (so I should select flaps 5) and then, following the FD, it decreases again below flaps 5...

As I said you are the pilot. The aircraft won't fly itself. As I recall, I'm not at my pc so no access to FCOM, the pitch bar doesn't command a specific speed. Check the docs to see what it should do.

I've experienced similar in the 777 too. It varies depending on rotation technique.

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1 hour ago, jgoggi said:

There is something that does not convince me too much during rotation (flaps 10): I lift the nose at about 2.5 deg/sec until 15 deg, then start following the FD, but the speed always increases too much, it is absolutely NEVER V2+10, but V2+20 or even 30 and it always goes above the flap 5 speed, then the FD bar rises and the speed decreases (if I keep following the FD). I suspect this could be related to the bug that after 80 knots the engines thrust erroneously self-increases and at lift-off it's about 3% higher than it should be and this causes a too high acceleration... This never happened on the 777 and 737, they stuck to the V2+x expected speed...

 

Yes I would agree with you. I seem to get V2+20 every time... even with significant de-rate and proper rotation technique in the FCOM/FCTM.

The only time I don't get there is when I'm at MTOW or close to it.

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38 minutes ago, kevinh said:

As I said you are the pilot. The aircraft won't fly itself.

Seen thousands of takeoffs, in real life at work and on youtube: in normal conditions the pilot flying checks that the airplane follows the FD bar and the speed does not change one knot... Anyway, until PMDG does not decide to fix that thrust bug, it's hard to say if it may be the culprit or not... 

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30 minutes ago, TopGun33 said:

There he goes again, please ......

Really needed...

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2 hours ago, jgoggi said:

Seen thousands of takeoffs, in real life at work and on youtube: in normal conditions the pilot flying checks that the airplane follows the FD bar and the speed does not change one knot... Anyway, until PMDG does not decide to fix that thrust bug, it's hard to say if it may be the culprit or not... 

James,

Have you considered Aerowinx's  Precision Simulator's Version 10?

No simulator is perfect, no two airplanes fly the same.  I just complete recurrent on one of the state of the art electric Level-Ds and although nice, it wasn't perfect.  Always remember this is a game and you are not playing "you bet your job" every time you fly it.  Just enjoy it and be thankful for folks like PMDG that give us a good representation of the real thing. :smile:

blaustern

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29 minutes ago, HighBypass said:

Is the 744 on autothrottle during the takeoff?

A/T is engaged in Thrust Lever Hold. If you move the thrust lever the thrust setting will change. The EEC maintains the selected EPR or N1.

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7 hours ago, jgoggi said:

Seen thousands of takeoffs, in real life at work and on youtube: in normal conditions the pilot flying checks that the airplane follows the FD bar and the speed does not change one knot... Anyway, until PMDG does not decide to fix that thrust bug, it's hard to say if it may be the culprit or not... 

Correction, the pilot flying doesn't check that the aircraft is following the FD bar, he flies the aircraft so that it follows the FD bar. But in each takeoff the pitch bar might command a different airspeed. To quote the FCOM, which I suggested you read earlier today, p 4-20-9:

Quote

At lift off pitch command target speed is V2+10. If current airspeed remains above target speed for 5 seconds target speed is adjusted to current speed (limited to a maximum of V2+25).

The commanded speed could be anything between V2+10 and V2+25. Nothing is wrong with the rotation, unless you don't follow the FD command. I doubt the 3% increase in N1 you repeatedly complain of will cause the excess speed you are talking about.

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Real pilots don't follow the FD so close to the ground, they know where V2+10 is on the airspeed tape and try to keep the aircraft close to that speed with pitch. It's not good to chase the pitch bar.

However, in (FD) theory....

Shortly after takeoff, FD guidance is a complex combination of airspeed and vertical speed:

The pitch command is an attitude reference until vertical speed is 600fpm. With vertical speed between 600 and 1200fpm, the command is a mix of attitude and speed command. For greater than 1200fpm, it is a speed command as per Kevin's post with some fine tuning.

i.e. The takeoff reference speed is the greater of the airspeed at rotation + 10 kts or V2 + 10kts. I can't remember what the autopilot definition of rotation is (It could be 5 degrees pitch, or I could be confusing it with something else).

Cheers

JHW

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On 18/08/2017 at 5:05 PM, jgoggi said:

I lift the nose at about 2.5 deg/sec until 15 deg, then start following the FD, but the speed always increases too much, it is absolutely NEVER V2+10, but V2+20 or even 30 and it always goes above the flap 5 speed,

100% poor pilot procedure.

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Please PMDG, check if the lift off behaviour is correct, since it does not look to be 100%...

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Just now, jgoggi said:

Please PMDG, check if the lift off behaviour is correct, since it does not look to be 100%...

You know this isn't an official support avenue. Submit a support ticket.

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2 hours ago, jgoggi said:

Please PMDG, check if the lift off behaviour is correct, since it does not look to be 100%...

Don't you think RSR would have noticed such a problem during development of this sim, with his 744 experience?

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5 hours ago, jgoggi said:

Please PMDG, check if the lift off behaviour is correct, since it does not look to be 100%...

Within the limits of any PC simulation, the rotation and initial climb performance of the PMDG 744 behave very much like the real aircraft and even the slight pitch hesitiation due to ground effect as the aircraft rotates before climbing away appear to have been modelled.

The correct liftoff technique for optimum performance is to initiate at VR a smooth and continuous rotation rate of 2 to 2.5 degs/sec to a target attitude of 12 to 13 degs nose up.  The liftoff will occur at a 10 deg NU body attitude and at normal gross weights the initial climb speed should be V2+10kts.  At light weights the airspeed will rise quite rapidly if you allow it to, so it will usually be necessary to adjust the pitch attitude to keep the airspeed below a max of V2+25kts during the initial climbout.  It always pays to look through the Flight Director when hand flying the 747 and to adjust the attitude to achieve the target climb speed you want for the flap configuration. Try a light weight takeoff with the Flight Director switched off if you really want to have some fun and hone your handling skills!

Bertie Goddard   

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So, if the autothrottle is controlling the thrust setting, just modulate your pitch to prevent the overspeed, climb rate be damned! No matter if she's going up like a homesick angel :D... unless you bust the altitude for your given STAR or ATC instruction... 

Just fly her...

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