Slick9

losing altitude during sharp turns...

17 posts in this topic

good morning all - looking for some expert help or opinions on my T7.  I just moved from FSX to P3Dv4 and everything is working like it did in FSX except during sharper turns.  I was flying the pattern and doing touch and goes a few days ago.  during each of the 90 degree turns around the pattern, the aircraft would transition from +/- 50FPM vertical speed to dropping to +/- 1000FPM.   I was hand flying with the autothrottle armed and VNAV mode active, so in anticipation of the altitude loss in the turn, I would pull back on the yoke and trim nose up as I started the turn, this helped but then resulted in the aircraft starting to climb as soon as the turn was completed.  I don't recall this being an issue for me in FSX, but then, I also was not doing a lot of pattern flying, I was doing line flying.  Another thing I tried was to fly the pattern with the AT disconnected, then as I approached the turns, I would manually add thrust, this alleviates the loss of altitude in the turn, i noticed when flying with the A/T, there was no increase of thrust during turns.  Anything stand out to the experts here? Am i missing something?  thnx for any help.

Richard

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This is physics and normal for how every airplane behaves..

Imagine the airplane from behind: when you fly straight ahead you have 100% lift area: your wings. 

As soon as you bank into a turn the direction of the lift isn‘t 90 degrees upwards anymore and so the area which creates lift is reduced to a percentage, depending on how steep you fly your turn. So the airplane will go into a dive. You counter this when you increase your pitch (lift up your nose). So the wings create more lift during the turn to keep your altitude.

absolutely normal behaviour. Maybe your realism settings in FSX weren‘t set correctly or what ever... 

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Hi Marc,

I agree that from a physics standpoint the behavior is expected. I should have added - I thought (and I could be wrong) that the T7 with it's fly by wire systems had a way to compensate for the loss of altitude in a turn.

 

Richard

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As mentioned above:

The 777 is, fundamentally, an aeroplane. It may be a very advanced aeroplane with a lot of electronic assistance for the pilot, but it is still ultimately an aeroplane and aeroplanes must obey the laws of physics.

This means that in a level turn, assuming constant airspeed the angle of attack must be increased in order to maintain level flight for the reasons explained above. 

This increase in lift also results in a small increase in drag which may require a small increase in thrust to counter but at normal (<30 degrees) bank angles this increase in drag is normally considered negligible and thus no thrust increase is usually necessary. 

Now, I am sure that the 777 advertises (as in Boeing advertise - not PMDG) pitch compensation in turns. However, such features are often imperfect in real life (that is certainly the case with the A320) and in practice some pilot input may be required. A sink of 1000fpm does seem excessive though. 

What I would suggest is that:

- It is generally considered poor technique to trim in turns or other transient manoeuvres for the reason you experienced - you will very quickly find yourself back out of trim again when you roll out

- What mode was the autothrottle in? Whilst it may sound pedantic, it is really important to be precise with language when describing the autoflight system (or any aircraft system): "Armed" means "Not active but will become active when certain conditions are met". Were you really flying with the A/T *armed* (in which case one wouldn't expect it to do anything) or did you really mean *active* (in which case how the A/T behaves will depend upon the mode it is in and the selections you have made on the MCP).

In SPEED mode the the A/T will adjust thrust to maintain the airspeed target you have set in the MCP. If the speed does not reduce there is no reason it should increase thrust.

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even airbuses lose altitude in turns as they hold the attitude, not the altitude... fbw is not a magic system, it just is a replacement of old steel wires going through the plane.. and although there are several computers actually flying a 777 or an airbus, it is still a plane and a plane needs to be flown. Either by the autopilot or by you. 

I have learnt to fly on sail planes (ASK 13 and ASH 25). What I was taught before circling in the thermals was to pull the plane up a few degrees before banking into the turn. This prevents exessive g forces when you have to „pull your VSI back from the negative part“. Of course an airliner is flown „slightly“ different from a sail plane, but the theory is the same. Try to combine your pitch with the bank angle. Train this and you will get used to it. :)

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Thanks for the answers gents!!  Simon you are correct I have to go back through the flight and take note of the annunciations so I can know exactly which modes were active.  I'll report back after i do another run around the pattern.  Unfortunately I won't be able to do it till the weekend. But I shall return. :) thnx

 

Richard

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2 hours ago, Slick9 said:

Thanks for the answers gents!!  Simon you are correct I have to go back through the flight and take note of the annunciations so I can know exactly which modes were active.  I'll report back after i do another run around the pattern.  Unfortunately I won't be able to do it till the weekend. But I shall return. :) thnx

 

Richard

PMDG asks that we sign our full names here Richard.

I believe you will find the section on high bank angle maneuvers in the FCTM pg 7.12 instructive. I believe that your use of trim in the turn was behind the loss of altitude, the AFDS should compensate for increase angle of attack in a turn up to 30 deg bank and your use of trim was working against the AFDS. The trim in the simulation may be thought of as speed trim rather than pitch trim because our yokes/joysticks cannot simulate what a pilot actually senses in the real aircraft. I recommend turning on the Show FBW Trim speed option (Introduction pg 91) to overcome this limitation in the simulation.

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17 hours ago, Ephedrin said:

even airbuses lose altitude in turns as they hold the attitude, not the altitude... fbw is not a magic system, it just is a replacement of old steel wires going through the plane.. and although there are several computers actually flying a 777 or an airbus, it is still a plane and a plane needs to be flown. Either by the autopilot or by you. 

I have learnt to fly on sail planes (ASK 13 and ASH 25). What I was taught before circling in the thermals was to pull the plane up a few degrees before banking into the turn. This prevents exessive g forces when you have to „pull your VSI back from the negative part“. Of course an airliner is flown „slightly“ different from a sail plane, but the theory is the same. Try to combine your pitch with the bank angle. Train this and you will get used to it. :)

Airbus do not lose altitude in turns up to 30 deg bank. No pitch input is necessary if you are straight and level to begin with. They aren’t maintains attitude, it’s vertical acceleration which is maintained. This assumes completely calm conditions. In real life this is unlikely so some pitch correction may be necessary.  

The same applies to the 777. It’s nose should not drop in a normal turn and no pitch input should be necessary. The problem comes in trying to demonstrate this in FSX and P3D with hobby level flight controls. A pure roll input is required to avoid any pitch input and this is very hard to achieve. Ironically, you stand more chance of seeing this work using keyboard commands. 

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1 hour ago, kevinh said:

Airbus do not lose altitude in turns up to 30 deg bank. No pitch input is necessary if you are straight and level to begin with. They aren’t maintains attitude, it’s vertical acceleration which is maintained.

Thanks for the clarification!

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1 hour ago, kevinh said:

Airbus do not lose altitude in turns up to 30 deg bank. No pitch input is necessary if you are straight and level to begin with. They aren’t maintains attitude, it’s vertical acceleration which is maintained. This assumes completely calm conditions. In real life this is unlikely so some pitch correction may be necessary.  

The same applies to the 777. It’s nose should not drop in a normal turn and no pitch input should be necessary. The problem comes in trying to demonstrate this in FSX and P3D with hobby level flight controls. A pure roll input is required to avoid any pitch input and this is very hard to achieve. Ironically, you stand more chance of seeing this work using keyboard commands. 

back to zero: it does lose altitute. I have leveled off at 6000ft at 270kt which gave me a pitch of 2,5°. then I just banked to 15° left and achieved a decent rate of 2-300ft/min, banked to 30° and had -400ft/min. the pitch remained at 2,5°. This was the airbus, I have no time to check the 777 now as I have to leave for work. And I haven't dived into the 777 FBW system that much.. :D But I expect it to behave very similar. 

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1 minute ago, Ephedrin said:

back to zero: it does lose altitute. I have leveled off at 6000ft at 270kt which gave me a pitch of 2,5°. then I just banked to 15° left and achieved a decent rate of 2-300ft/min, banked to 30° and had -400ft/min. the pitch remained at 2,5°. This was the airbus, I have no time to check the 777 now as I have to leave for work. And I haven't dived into the 777 FBW system that much.. :D But I expect it to behave very similar. 

This is correct.

As Kevin says, the Airbus maintains neither attitude nor altitude but a 1g load factor. The manuals will tell you that no pitch input is required for turns up to 33 degrees AoB, but as Kevin says in practice the system is good but not that good and thus a few hundred fpm sink is quite normal on the real aeroplane if no pitch input is made - this is modelled in the FSL bus.

I imagine the same is true on the 777 but as I say, the 1000 fpm described by the OP seems excessive and more akin to a conventional aircraft.

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19 hours ago, kevinh said:

Airbus do not lose altitude in turns up to 30 deg bank. No pitch input is necessary if you are straight and level to begin with. They aren’t maintains attitude, it’s vertical acceleration which is maintained. This assumes completely calm conditions. In real life this is unlikely so some pitch correction may be necessary.  

The same applies to the 777. It’s nose should not drop in a normal turn and no pitch input should be necessary. The problem comes in trying to demonstrate this in FSX and P3D with hobby level flight controls. A pure roll input is required to avoid any pitch input and this is very hard to achieve. Ironically, you stand more chance of seeing this work using keyboard commands. 

You are right. 

The nose wouldn’t drop in both airbus and 777 because of the FBW system, but you still need to pitch up slightly in turn to maintain altitude like a conventional airplane, all the FBW does is it maintain a stable attitude in turn so that pilots do not need to apply constant back pressure on the yoke or side stick. If you simply roll airplane without any pitch input the airplane would deccent at about 100fpm depends on AOB. 

 

The PMDG 777 in for some reason does require a much larger pitch change than the real airplane in turn especially with AOB >20deg. 

 

For example a level turn with F5 out 180kts you will need almost 10deg nose up to maintain level flight without losin altitude. In contrast a normal straight and level flight for the same pitch and speed is about 5-6deg nose up. 

On the real airplane for any turn with AOB below 30deg all you need is ~ 1 deg pitch up at max. With N1 incense by 5% most of the time. 

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On 05/01/2018 at 1:27 PM, Ephedrin said:

back to zero: it does lose altitute. I have leveled off at 6000ft at 270kt which gave me a pitch of 2,5°. then I just banked to 15° left and achieved a decent rate of 2-300ft/min, banked to 30° and had -400ft/min. the pitch remained at 2,5°. This was the airbus, I have no time to check the 777 now as I have to leave for work. And I haven't dived into the 777 FBW system that much.. :D But I expect it to behave very similar. 

The 777 FCOM, Vol 2, page 9.20.10, mentions that no additional back pressure is required to maintain altitude in turns with bank angles up to 30 deg.

The A320 FCOM says a similar thing:

Quote

In normal turns (up to 33 ° of bank) the pilot does not have to make any pitch corrections once the turn is established.

So it ought to work in the sim and it seems to me that it is.

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On 4 January 2018 at 4:45 PM, Slick9 said:

good morning all - looking for some expert help or opinions on my T7.  I just moved from FSX to P3Dv4 and everything is working like it did in FSX except during sharper turns.  I was flying the pattern and doing touch and goes a few days ago.  during each of the 90 degree turns around the pattern, the aircraft would transition from +/- 50FPM vertical speed to dropping to +/- 1000FPM.   I was hand flying with the autothrottle armed and VNAV mode active, so in anticipation of the altitude loss in the turn, I would pull back on the yoke and trim nose up as I started the turn, this helped but then resulted in the aircraft starting to climb as soon as the turn was completed.  I don't recall this being an issue for me in FSX, but then, I also was not doing a lot of pattern flying, I was doing line flying.  Another thing I tried was to fly the pattern with the AT disconnected, then as I approached the turns, I would manually add thrust, this alleviates the loss of altitude in the turn, i noticed when flying with the A/T, there was no increase of thrust during turns.  Anything stand out to the experts here? Am i missing something?  thnx for any help.

Richard

Just to add to what Kevin and Simon have said;

Patterns are generally flown with VNAV off, AP disconnect and A/T On. What are you speeds for the various pattern segments? If you're flying slightly slow then the FBW is going to be coordinating the turns to preclude the risk of dropping airspeed - hence pitching down, etc - maybe something of the behaviour you're seeing.

Another thing would be to fly the pattern work with settings as above and keep the T7 in FBW trim for airspeed using the blue indicator on the speed tape on the PFD.

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