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N55E008

Pitch moment during thrust-changes

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Moin dear IXEG Team!

First of all congratulations for the fantastic work you've shown with the 737. To me it's the first truly immersive desktop simulation, technically superior and far beyond I've ever seen on FSX, especially when it comes to hand flying the plane. You've made a considerable step to take desktop-simulation to the next level.

What I recall from a couple of hours of flying in Level-D simulators at DLH, is that there is a noticable change in pitch attitude during power-changes when flying the airplane manually. 

For example, when adding thrust, the nose pitches up and vice versa when thrust is being reduced.

I've been missing this feature on any FSX aircraft, probably due to limitations within FSX.

Now, do you see a chance to implement this behaviour into your 737?

 

Best regards,

Thorsten

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2 hours ago, N55E008 said:

What I recall from a couple of hours of flying in Level-D simulators at DLH, is that there is a noticable change in pitch attitude during power-changes when flying the airplane manually. 

For example, when adding thrust, the nose pitches up and vice versa when thrust is being reduced.

Hi Thorsten,

What you mean, we offcourse have that already!!!?  

Fly trimmed at level flight at e.g. 3000' feet, 220 kts, disconnect AP and A/T, add thrust and you'll see a SIGNIFICANT pitch up.

M

 

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Hi Morten.

I´m not quite sure if we´re talking about the same phenomenon:

I believe, what you described ( and what I´ve tried again using the IXEG737 ) is a pitch-up moment caused by of the aircraft coming out of trim due to increasing IAS after thrust is being added.

What I´ve meant is the pitch moment that comes due to the engines being mounted considerably below the C.G. This effect comes without delay and is much more prominent than the effect you´ve described.

On the IXEG 737 it looks like this: when flying straight and level with the aircraft in a trimmed condition and thrust is being reduced, IAS is decreasing and a few seconds later, as a result of reduced speed, the attitude will decrease and the aircraft will start to descend since it came out of trim. Then, when adding power suddenly to max thrust, there´s initially no change in pitch attitude at all until the IAS has increased to a point when the aircraft comes out of trim again.

From what I recall, the real thing would pitch-up almost instantly & simultaniously whilst thrust is being added and this effect is actually quite prominent.

 

That´s the effect I meant.

Is there technically a possibility to model this effect within X-Plane?

 

Best regards,

Thorsten

Edited by N55E008

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Hello Thorsten,

this effect is being modeled by X-Plane. I agree that it is not as prominent as in the real airplane, but if the position of the engines is set correctly (as we have done) in relation to the center of gravity, then the pitching moment of thrust times arm is calculated.

To see this, do the following experiment:

Fly straight and level at 230kts with the autopilot on and the TRIM values output on the screen (Settings/Data Input & Output/ Field number 13, rightmost checkbox).

Note the trim value for the pitch trim (for me it is -0.15). Now climb with the same speed and FL CHG. The engines will go to full thrust. Wait until everything stabilizes, note the trim value again. For me it is -0.19. So same airspeed, higher thrust, more pitch-up moment -> more trim nose down needed.

Jan

 

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Right, as Jan says, this is modeled pretty well by XP.  Basically the same as if you have asymmetric thrust, you will have a yawing moment. Works well.  The effect will also be greater at lower airspeed (less pressure on the Hstab).  Remember that in the big sim and in real you will feel the forces (acceleration, G, pitchup) very well on your body when you do this, so it will feel alot more "spectacular" than in XP.

M

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Hi all, 

I was testing pitch moments too, with the engines. 

I flew the 707 for a while. No... I am not that old. The Awacs still uses them.
I am not sure what the similarities are between the 73 and 70, but a few things didn't change.  

Every bit of engine thrust change, ment trimming. What is even more significant are the LE-flaps. When clean and selecting flaps, between 0-14deg there is always a bit of pitch up moment. However, when the LE flaps deploy, there is a huge pitch up moment. It usually ment trimming the stab trim nose down for a full 2 second travel. And when they are deployed, trim back nose up the same amount. It's a deployment moment.
On departure and cleanup, the FE usually called "leading edge" at the moment the flaps travelled through the LE-retraction sequence. This ment trim up a good lot!! You'd be pulling crazy if you didn't and wanted to maintain whatever pitch.

Then again, the 707 has 150% more wingspan. So maybe effects are more significant on the 70. 

Regardless, this A/C add-on gets closest  to the real deal IMHO. The stab trim noise .. spot on. Stby Alt Vib.. spot on. This cockpit (the 737 is equal to the 707 mid console and forward) is a really small cockpit. It is really noisy and the stab trim can hurt you if you accidentally get near the wheel or forget to retract the manual stab trim handle after the stab trim check. 

Good job IXEG team!

 

Tee

 

Edited by teeloo
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Thanks Tee,

Our 737 flap pitching moments are fine-tuned and calibrated for each flap setting as to Jan's preference (6000+ hrs in type) so you can rest assured we are close on those :)

As for the 707 I don't know it very well, But if irc it's flap has little "fowler motion" - not moving very far backwards, extending the chord - so I would expect it to have more pitch-up on flap extension than the 737.

Spoilers and Flaps

Those Kruger flaps might also influence pitching moments

111808-12.jpg

Thanks for an interesting post :)

M

 

Edited by Morten

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Hi Morten, indeed the Krüger flaps.... 

I wasn't thinking right in comparing the 73 and 70. Indeed the 70 has them, and make a big difference. Thanks for pointing that out. 

I remember the Krügers started to hang when the hydraulics were off for a while. drooping. 

And indeed, the difference in TE flaps makes the two actually very different.

Thanks.

 

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