Handling dynamics with redesigned FBW Sytems

[martin-w 3,234]

I watched the video Martin linked. AP is OFF and the FBW will NOT hold altitude. If you started to climb the AT would go to max thrust rating. If you descended, it would go to idle. I don't see anything wrong???

 

Best regards,

Robin.

 

 

Robin... this is Swen's issue.

 

He is flying straight and level, altitude is stable, airspeed is stable. autopilot is off, autothrust is off. No adverse weather to disrupt flightpath. The aircraft is 100% stable even with hands off the controls.

 

As soon as he hits the autothrust, engine RPM increases for no apparent reason and the nose, as you would expect, pitches up.

 

For me, in exactly the same scenario, autopilot off and autothrust off, I can hit the autothrust as many times as I like, and the aircraft, unlike in Swen's case, remains stable. Not a twitch. Which is normal behaviour.

 

If he engages the autothrust, airspeed should remain the same. Engine RPM should remain the same. Until he inputs a higher speed. The only variable at play is engaging and disengaging the A/T.

 

It's a weird one, some kind of controller spike maybe, who knows.

[Lseatflyr 0]

 

 

Therein lies one of the problems I'm seeing. If I have any force on the control column whilst trimming, even if the trim reference speed moves 50 kts, nothing happens until the control column is centered, then all the trim change is applied at once and the aircraft aggressively pitches whilst it catches up (if it is a long way out of trim). There is no pitch reaction at all during trimming with the control column deflected and held in a constant position.

 

Yep - I'm experiencing this and can reproduce this.  Compare my latest takeoff, OMDB-OPKC linked below where I test this, with my previous takeoff, YPAD-OMDB also linked below.  In the previous takeoff I apply excessive nose-down trim (increase the reference speed) and then it all comes rushing in when I center the column.  In that previous takeoff it looks like I engaged autopilot and it chased the F/D down, but that's not actually the case - it was trim coming in.

 

OMDB-OPKC (anticipating absence of trim):

 

YPAD-OMDB (fighting absence of trim):

[777simmer 117]

I think null zone is the wrong description for it. I have found that setting it to 10% (the maximum) for pitch has helped the trim situation slightly. It's still not there yet, but seems better than before.

 

Best regards,

Robin.

Hey Robin.....if what you have noticed is true, that trim is not applied untill the yoke is centered......then it should be possible to fly the PMDG777 with trim only. (without using the yoke).

 

So accelerate........leave the yoke alone...nose goes up - aplly trim down.......wait.......nose goes up again - apply trim down again, etc

(Ofcourse you will not exactly maintain altitude this way but for the experiment it does not matter)

You mean in the CDU option or FSX ??

 

BTW, is it better to leave nullzone to min on fsx but only chnage the nullzone in the CDU option ?

 

CMB

You mean in the CDU option or FSX ??

 

BTW, is it better to leave nullzone to min on fsx but only chnage the nullzone in the CDU option ?

 

CMB

In the CDU.

 

I have joysticks disabled in FSX.

And use FSUIPC to assign controller axis and have FSUIPC send the signal direct to FSX (no FSUIPC calibration). So with this, I can not set a nullzone in FSX and I do not think that the FSX nullzone setting has any effect anymore (if for example I were to set FSX nullzones before disabeling joysticks). not sure though.

[bwolfe 0]

Guys, don't fly the airplane with the trim. Use the controls to put the airplane where you want it, and relieve control pressure with the trim. There is no need to trim for a specific airspeed. MAKE the plane do what you want, and if it takes a lot of back pressure to keep it there, trim up a bit, and vice versa. This is true for all airplanes whether they're FBW or not. Everyone in this thread is making it seem way more complicated than it actually is.

[AirCanada235 103]

Trust me!!  Its pretty SPOT ON!!

[3-2-1-Now 126]

Guys, don't fly the airplane with the trim. Use the controls to put the airplane where you want it, and relieve control pressure with the trim.

 

Preaching to the converted. I know this is the correct technique, but the problem is when trying to find the trimmed condition, something about the FBW behavior means you can't feel when it is trimmed, because it is just reacting wrong when the stick is deflected.

 

@Rob: Yes, I can fly the aircraft on trim alone. Not sure what you're getting at?

 

I did realize why I can apparently fly better with the null zone set to 10% - the null zone is not only a null zone - it enabled the FBW to apply the trim setting "better" than if the stick was pulled outside the null zone, whilst also reacting slightly to pitch input in this range.

 

So far it seems:

 

* Null zone is where set trim is applied to the FBW system

* Nul zone is not truly a null zone - it has a small input to the flight controls

* Outside the null zone, most of the trim is not applied until the stick enters the null zone. The size of the null zone determines how aggressive this application is (smaller zone, more aggressive).

 

Regarding Swen's issue - I see the same thing. Engagement of the AT system disturbs the flight path.

 

Best regards,

Robin.

[whoispankaj 51]

As mentioned in the FBW video i am able to trim for a level flight at a certain speed.I can reduce and increase the speed and trim it and level.But where he shows that he maintains the 1000ft/min climb and not touching the tim switches he is able to center the yoke and keep the 1000ft/min climb.But for me ,when i pitch up  to 1000ft./min climb while trimmed and level (means i dont wanna change the speed) the aircraft will not hold the pitch and would want to level off.The autothrottle increases to maintain the speed which is fine.But as soon as i achieve the 1000ft/min climb and center the yoke the autothrottle starts reducing down and plane wants to level off..So I am not able to execute this case successfully...

 

Pankaj

[whoispankaj 51]

Ok..I thought I will post an update..I tried the case of climbing 1000ft/min with constant speed in trim.So here is what I did..Took off at KIAH..used AP/AT all the way to 10000ft.Once level disconnected the A/P, speed at 260 in trim.Now i started a 1000ft/min climb to 11,000ft.And this time I was able to center the yoke once the stabilized rate of 1000ft/min rate was established..Same as the pilot shows in the FBW video.The speed remained at 260 throughout. I was able to level off at 11000ft by just pushing the yoke slightly and maintaining level.Then centered the yoke and it was perfectly maintaining everything.The A/T thrust did not go down this time.

[geolpilot 88]

Two things I learned from the video that have helped me get the hang of the FBW (and hats-off to PMDG, it really works as in the video / as it should). Great fun hand-flying once one has the hang of it.

 

Notice the pilot's gentle inputs on the control column. Firm with hand when changing pitch to get to and hold a required climb or descent rate, then trimming forces out, relaxing the grip and then using almost finger tip push/pull nudges to set /keep the desired VS. If AT is on and you use heavy inputs on the control column you will get those constant thrust changes and pitch porpoising.

 

I have my elevator trim buttons set to mid-range "repeat while pressed". Also use the fbw marker "cheat" while I get the hang of the feel on the yoke as I trim out forces. Great tool.

 

Again in the video the difference between 5secs trim switch applications to adjust for speed changes, and quick flicks (blip trim) to fine tune when within 5kts of desired speed, is clear. I find that once I have the yoke centered and have the VS almost stable (trimmed close to the set speed), quick flicks of the trim switches,+- small nudges on the control column, get me nicely stabilised at the desired VS and speed setting I want.

 

Above is with AT on. With AT off then it flys like a cessna. Pitch with control column, use throttles to set/keep the desired speed, and trim forces out of the column. Again I find the fbw marker useful, as feeling those control forces exactly is just not possible for me (yet!). Maybe with practice, I will get used to how much pressure requires what amount of trim input to relieve.

 

Seems like some hardware supplier has been given a challenge - develop a servo-controlled yoke that simulates out-of-trim forces as per a 777 "force-feel" system.

 

Rob

[whoispankaj 51]

Replicated the same case as he shows in the youtube video of FBW and it works as shown in there..This time without the A/P taking me to 10000ft. I think its just a matter of getting the hang of it and lack of control loading feedback which is simulated now by the amount of push/pull on the yoke/joystick that you need to make the input.I think once we get a hang of the setting the trim reference speed and feel of this control load simulation (lowering the elevator authority) its becoming easier..This is the observation from my experience..

One change i did make though is changing the control column null zone to 10% in the FMC

[777simmer 117]

Guys, don't fly the airplane with the trim. Use the controls to put the airplane where you want it, and relieve control pressure with the trim. There is no need to trim for a specific airspeed. MAKE the plane do what you want, and if it takes a lot of back pressure to keep it there, trim up a bit, and vice versa. This is true for all airplanes whether they're FBW or not. Everyone in this thread is making it seem way more complicated than it actually is.

Oh ok......thanks for letting us now that........what were we thinking.......silly us, lol.

[gnomegemini 7]

Oh ok......thanks for letting us now that........what were we thinking.......silly us, lol.

+1 Like!!! Nice.

 

@bwolfe: 777simmer indeed should know how the system works and the plane flies. He earns his sandwitches with that plane.

[willywonka 36]

 

 

He earns his sandwitches with that plane.

I was hoping it was beer 

[777simmer 117]

I was hoping it was beer 

No beer allowed within 8 meters of the aircraft, sadly :-(

[Don t moderate me this way 882]

If an airline pays peanuts, they get monkeys.

 

Now if an airline pays sandwiches - look at Rob's avatar to see what they'll get then!  

 

Rob: No offence intended.   B)