Milton Waddams

-8 nose pitching up on the takeoff roll

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Don’t know if this is a bug in the flight model, or simply a consequence of the situation, but I thought I’d bring it up anyway.

It seems that whenever I depart in the -8 at extremely light weights, as in 40k pounds of fuel and no payload, the nose will pitch up to the point where the tail strikes the ground on the takeoff roll.

Since I like the almost comical climb rates, I almost always go TOGA on departure, but is the increased thrust the cause of this nose up tendency?

The first time I experienced this was a takeoff using TOGA and flaps 20, so I decided to use flaps 10 on the next departure in the interest of reducing lift and surface area for the thrust to impact. 

Sadly, this did nothing and it took full forward action on the yoke to keep the nose down. My CG seemed to be within limits, and the stab trim was near 2, as calculated by the takeoff page. I’ve tried a derated takeoff, but I can’t recall if the nose pitched up or not.

The issue for me also happens upon landing, with full forward stick also being required to hold the nose down. 

Any ideas on what this might be? I’ll probably end up a bit of cargo in the forward hold to keep the nose down from now on, but it’d be nice to know if it’s something I’m doing wrong or an issue with the model itself.

Thanks in advance (and sorry if it’s my own fault..)!

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9 minutes ago, Milton Waddams said:

Any ideas on what this might be?

Trim, and technique.

Trim may be calculated on the FMC a bit on the high side (better to rotate than not, and roll longer). There's a trim switch on the real aircraft yoke for a reason. Retrim, and don't yank back on the yoke so hard.

There are people up front for a reason. Do what you need to do to operate the plane safely. The FMC is a TOOL, not an answer/absolute.

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10 minutes ago, scandinavian13 said:

Trim, and technique.

Trim may be calculated on the FMC a bit on the high side (better to rotate than not, and roll longer). There's a trim switch on the real aircraft yoke for a reason. Retrim, and don't yank back on the yoke so hard.

That’s the thing, I’m not tugging on the yoke at all when it wants to pitch up. If anything, I have a bit of nose down pressure applied. I’ve experienced this IRL flying an Archer on soft field or standing takeoffs where you need nose down pressure to compensate for the increased airflow over the elevator. 

I’ll try trimming to the nose down limit, and apply forward pressure, on my next departure and see if that eliminates the issue. 

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You cannot be serious... you are using full TO power when you are only 40 klbs above empty weight?  John, you've got enough power to lift a million pounds!  Dial back to DTO2 50C and see how it goes.

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Yeah, I'm with Dan.

If you want the absurd climb rates (no judgment - the first thing I did with the 777-200LR/F when I got it was set it to empty payload, a tiny bit of fuel, no derate, and just launched it), keep in mind that you can set the derate for takeoff (which will then auto-set climb), but then override any automatic climb setting to get rated climb thrust.

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I knew it was something I was doing wrong..I’ll drop it back to DTO2 which will probably eliminate the issue.

Sorry for being so dumb, and great job on the -8! 

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6 hours ago, Milton Waddams said:

I knew it was something I was doing wrong..I’ll drop it back to DTO2 which will probably eliminate the issue.

Sorry for being so dumb, and great job on the -8! 

Have you check in the manuals.  Is your power, takeoff weight, and CG within the normal envelope?  If so, then for a given correct trim value the aircraft shouldn’t rotate this way. If not, then there is your answer.

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14 hours ago, icaruss said:

Have you check in the manuals.  Is your power, takeoff weight, and CG within the normal envelope?  If so, then for a given correct trim value the aircraft shouldn’t rotate this way. If not, then there is your answer.

I have, yeah. Power is just straight TOGA, CG is within limits, if not a bit towards the rear with only fuel as payload, and trim is near the nose down limit, but still within the envelope.

I’m planning more test flights today, checking it with a dereated thrust setting, less flaps in, and some cargo in the forward compartment. Should fix the issue, though from Kyle is saying, it’s all about technique. 

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I have noticed this on the 744 on my last two flights since the update.

I loaded fuel first, then payload.  And tried the other way around.  But it keeps filling the stab tanks even though it is not called for with the fuel load (~141 tonnes).

TOCG seems to be normal - about 22% according to the readout.

But using appropriate thrust (D-TO 40 @ 390 tonnes, ZFW 240.8), and the correct stab trim as suggested by the FMC, the aircraft starts to pitch up before VR as calculated by TOPCAT.  This never happened to me once before the update.

Once the stab tanks are burnt during climb, TOCG is now down at 18% and highlighted as out of range.  If I reload payload now, it goes back to 22% and the highlight disappears.

Any advice?  Am I encountering any sort of bug?

 

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I can only reference the 747-400 on this but when we operated short reposition flights on an empty aircraft the Captain would typically remind the F/O to keep moderate nose down pressure on the control column during take off to avoid a tail strike at light weight even with Derated Thrust and then allow it to slowly rotate at Vr.   It was also a handful to land since it just never seemed to want to get back on the ground and would float forever.

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3 hours ago, VHOJT said:

But using appropriate thrust (D-TO 40 @ 390 tonnes, ZFW 240.8), and the correct stab trim as suggested by the FMC, the aircraft starts to pitch up before VR as calculated by TOPCAT.  This never happened to me once before the update.

Once the stab tanks are burnt during climb, TOCG is now down at 18% and highlighted as out of range.  If I reload payload now, it goes back to 22% and the highlight disappears.

VR calculated by TOPCAT is completely irrelevant. You can't use the stab trim suggested by the FMC and then reference the TOPCAT VR and not the FMC calculated VR. The FMC calculations are performed specifically for the aircraft you are flying and this should be your reference.

A CG within limit at takeoff does not guarantee that the CG will remain within the envelope during climb/cruise or landing. Correct load distribution is crucial for the 747. One of the new features with this release is that setting a payload % from the FS ACTIONS>PAYLOAD page will perform a distribution of cargo load in a way that CG is not only within limits at takeoff but is also set so that it is less likely to get out of the envelope during flight and landing. This is why CG was corrected when you reloaded payload during flight.

If you are worried about CG the EFB OPT WT AND BALANCE application can be useful. Use the READ FROM AIRCRAFT feature to get your sim payload and fuel data into the EFB, enter estimated trip fuel and watch the CG values on the graph. You can fine tune by moving around cargo and passengers on the EFB and/or adjusting fuel and see the effects on the CG values in the graph. The EFB application incorporates the fuel burning schedule to calculate the landing CG. When satisfied use the SEND TO AIRCRAFT button to send the desired configuration to the sim.

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Posted (edited)
4 hours ago, mfrantz said:

VR calculated by TOPCAT is completely irrelevant. You can't use the stab trim suggested by the FMC and then reference the TOPCAT VR and not the FMC calculated VR. The FMC calculations are performed specifically for the aircraft you are flying and this should be your reference.

A CG within limit at takeoff does not guarantee that the CG will remain within the envelope during climb/cruise or landing. Correct load distribution is crucial for the 747. One of the new features with this release is that setting a payload % from the FS ACTIONS>PAYLOAD page will perform a distribution of cargo load in a way that CG is not only within limits at takeoff but is also set so that it is less likely to get out of the envelope during flight and landing. This is why CG was corrected when you reloaded payload during flight.

If you are worried about CG the EFB OPT WT AND BALANCE application can be useful. Use the READ FROM AIRCRAFT feature to get your sim payload and fuel data into the EFB, enter estimated trip fuel and watch the CG values on the graph. You can fine tune by moving around cargo and passengers on the EFB and/or adjusting fuel and see the effects on the CG values in the graph. The EFB application incorporates the fuel burning schedule to calculate the landing CG. When satisfied use the SEND TO AIRCRAFT button to send the desired configuration to the sim.

Hi Michael,

This is with the 747-400.

On these occasions, TOPCAT VR was within 1 or 2 knots of the FMC calculation.  Basically, since the update, it seems to want to pitch up before VR?  This has never happened to me once before the update (~100 flights).

Which way around is it best to load the 747-400?  Payload first in the FMC, or fuel first?  I always let the FMC load the aircraft as it sees fit.

Cheers,

Edited by VHOJT

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1 hour ago, VHOJT said:

This has never happened to me once before the update (~100 flights).

Using the same exact weights and CGs?

1 hour ago, VHOJT said:

Payload first in the FMC, or fuel first?

Doesn't matter.

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Posted (edited)
1 hour ago, scandinavian13 said:

Using the same exact weights and CGs?

Doesn't matter.

This is what I’m saying. I’ll trial a takeoff from an old flight plan from before the update to make sure - but I have used very similar weights in the past. CG I couldn’t tell you - I let your FMC work it’s stuff and make sure nothing is highlighted when inputting load. 

I am going by the FMC basically - if it loads the weight and fuel I ask it to, and the CG isn’t highlighted, and I use the FMC stab trim and VR  and thrust - it shouldn’t pitch before VR at heavy weights with an assumed temp, right?

I’ve also checked the STAT page and my flight controls show centred. 

I notice the plane now loads fuel in the stab tank at lower fuel weights than it used to, has this changed?

Edited by VHOJT

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On 10/5/2018 at 3:24 AM, VHOJT said:

  Basically, since the update, it seems to want to pitch up before VR?  This has never happened to me once before the update (~100 flights).

 

I experienced the same behaviour with the 744, never saw that before the recent major update..

TOW was around 856000 (payload set by RANDOM function) / Flaps 20 /  TO 1 derate / CG 22,6 / Trim settings 6,7 and Vspeeds as suggested by FMC (and except for V1 identical to those suggested by topcat) Vr = 170

Aircraft startet rotating on its own some knots before reaching Vr.....

 

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Posted (edited)
53 minutes ago, Sekkha said:

I experienced the same behaviour with the 744, never saw that before the recent major update..

TOW was around 856000 (payload set by RANDOM function) / Flaps 20 /  TO 1 derate / CG 22,6 / Trim settings 6,7 and Vspeeds as suggested by FMC (and except for V1 identical to those suggested by topcat) Vr = 170

Aircraft startet rotating on its own some knots before reaching Vr.....

 

Check your controller calibration maybe?

I just gave this a try... I only use derated for things like contaminated runways so I thought it was worth giving ti a trial.  In the B747-400 PW TOW 856 klbs F20 TO-1 CG 22 Trim 6.7 Vr 171.  Watched real close above V1 and never saw any nose up pitch moment.  Sure, there was quite a bit of bouncing but that pretty much smoothed out above 120 kts.

I am using a Thrustmaster Warthog HOTAS stick and throttle, axis assigned and calibrated in P3Dv4.3.

When you say " Aircraft startet rotating on its own some knots before reaching Vr.....  " can you be more specific?  At what speed did it start rotation and how much did it rotate before you intervened?

In many many B747 QOTSII sessions I've never experienced this problem.  None of my fellow beta testers have.  I cannot imagine it is anything other than a controller issue or maybe I guess the bouncing was a surprise and you thought it was rotating? Looking for answers.

Edited by downscc

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Posted (edited)
3 hours ago, downscc said:

Check your controller calibration maybe?

I just gave this a try... I only use derated for things like contaminated runways so I thought it was worth giving ti a trial.  In the B747-400 PW TOW 856 klbs F20 TO-1 CG 22 Trim 6.7 Vr 171.  Watched real close above V1 and never saw any nose up pitch moment.  Sure, there was quite a bit of bouncing but that pretty much smoothed out above 120 kts.

I am using a Thrustmaster Warthog HOTAS stick and throttle, axis assigned and calibrated in P3Dv4.3.

When you say " Aircraft startet rotating on its own some knots before reaching Vr.....  " can you be more specific?  At what speed did it start rotation and how much did it rotate before you intervened?

In many many B747 QOTSII sessions I've never experienced this problem.  None of my fellow beta testers have.  I cannot imagine it is anything other than a controller issue or maybe I guess the bouncing was a surprise and you thought it was rotating? Looking for answers.

I have checked my controller calibration - it is fine, and flight controls show centred when they should.

Trust me, I have done lots of flights with the 747 too, and I have never seen this before two flights recently.

The common factor - both were a high TOW with fuel in stab tank.  I haven't had this happen on my last two flights (SIN-SYD, SYD-SIN) with lower weights.  I have only had the problem twice on the SIN-LHR and LHR-SIN flights.

Look I'll do some testing on my end to see if I can get it to reproduce...

Edited by VHOJT

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Posted (edited)
9 hours ago, downscc said:

Check your controller calibration maybe?  "

My calibration is definiteley fine, I always calibrate controls before each flight

9 hours ago, downscc said:

When you say " Aircraft startet rotating on its own some knots before reaching Vr.....  " can you be more specific?  At what speed did it start rotation and how much did it rotate before you intervened?

 It rotated until liftoff,  I allowed  it to do so in order to see what happened (it was a nice rotation rate just as if I had done it manually :) ....NO BOUNCING.. its was ROTATING

cant remember nor define the exact speed, maybe 2-3 knots before...

 

5 hours ago, VHOJT said:

The common factor - both were a high TOW with fuel in stab tank.

exactly this

 

 

Never had this behaviour before the update. And I did quite a lot of flights, believe me :)

 

Edited by Sekkha

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To ammend that:

I just tried the scenario again. Same result. Rotation started  1-2 knots before VR without any intervention

ZFW was 518.6

FUEL LOAD (set by directly entering the amount , not by ground operations):

TOTAL 338,0 --> Center 81,4 / Main 2+3  83,5 / Main 1+4 29,3 /  Res 2+3  8,9 /  Stab 13,2

PAX GE variant

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I guess this is going to be a tricky one to figure out.  I still cannot reproduce.

I just tried with your ZFW 518.6 klbs (always use units pls) Fuel 338.0 klbs distributed by FMS as you annotated with 13.2 klbs in the stab tank in the PMDG 747-400 GE taking off from KIAD rwy 30 Flaps 20 TO power V1 155 Vr 169 V2 180.  I watch the PFD very closely to see if their was any nose up pitch in case I missed a subtle rise from the windshield view and I didn't see any.  Pitch on the HSI remained at zero until I rotated at 170.

This is in P3Dv4.3 with weather set to clear all.

 

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A few things come to my mind. I have only GA SEP experience, C172, DR400 and HK36. But all of them do the same at „Vr“. (Yeah, you don‘t calculate the speed :D) 

All of them behave just like an airplane behaves as they gain speed: they fly. The C172 gets lighter and at some point it simply flies. Now the B747 has 4 powerful engines below the CG. That‘s why you fly manually on approach with autothrottle off, same with the 737. when the stab trim is set for T/O, it basically neutralizes the weight of the nose at Vr. In my understanding this means that the engines move around the CG and lift the nose as long as you don‘t hold it down manually. Drag mostly takes effect above the engines and above the CG. In a very light airplane you will notice that a lot more than in a heavy sluggish airplane. 

So in my understanding at least that‘s nothing but torque.

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Posted (edited)
58 minutes ago, downscc said:

I guess this is going to be a tricky one to figure out.  I still cannot reproduce.

I just tried with your ZFW 518.6 klbs (always use units pls) Fuel 338.0 klbs distributed by FMS as you annotated with 13.2 klbs in the stab tank in the PMDG 747-400 GE taking off from KIAD rwy 30 Flaps 20 TO power V1 155 Vr 169 V2 180.  I watch the PFD very closely to see if their was any nose up pitch in case I missed a subtle rise from the windshield view and I didn't see any.  Pitch on the HSI remained at zero until I rotated at 170.

This is in P3Dv4.3 with weather set to clear all.

 

Dan,  if you dont intervene at all,  will it start rotating allone anytime after Vr? If yes, at which speed?

Would be interesting to know  if its just a matter of some knots or if it doesnt rotate allone until quite highr speeds...

 

also PD3v4.3 btw

Edited by Sekkha

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Posted (edited)
34 minutes ago, Sekkha said:

Dan,  if you dont intervene at all,  will it start rotating allone anytime after Vr? If yes, at which speed?

Would be interesting to know  if its just a matter of some knots or if it doesnt rotate allone until quite highr speeds...

 

also PD3v4.3 btw

I don't have enough runway for that test.  I'd have to lighten here a lot and then we'd be comparing apples and oranges.  Let me see if I can find a suitable test runway.

Using the 15000 one at KEDW Edwards AFB with 744 set up as before I find the nose lifts on it's own without control input at V2... 180 kts.  That is 10 kts above Vr in this case.

Edited by downscc

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Posted (edited)
47 minutes ago, downscc said:

Using the 15000 one at KEDW Edwards AFB with 744 set up as before I find the nose lifts on it's own without control input at V2... 180 kts.  That is 10 kts above Vr in this case.

Thanks Dan...so that is about 12 knots above what I have experienced...quite some difference actually....very strange

 

I will try some more scenarios and also how much I would have to trim down to get the same result as you did....

Edited by Sekkha

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With the 747-8F, ZFW set at 634.6 via the FMS payload page and 80K lbs of fuel also set thru the FMC, and stab at 6.3 as per the FMC computer, I got an unambiguous uncommanded rotation that required a significant amount of forward yoke to correct at about 20 knots below V1 departing KMEM rwy 18R.  I don't believe that's normal.  Power was set at at the takeoff-1 derate.

Regards

 

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