B747 C Speed Brake Extension and SYSTEM 4 Hyd

[spierpoint jones]

With hydraulic system 4 inoperative, the caution in the procedure states that rapid extention of speedbrakes causes nose pitch up. Sure enough in the sim you can get airborne again.....why does the nose pitch up rather violently even when spoilers 5 6 and 7 8 will not operate.?? I can't picture in my opaque mind the shifting of the centre of lift or is it drag related...?

Can someone clear this point up for me ...??

Many thanks in advance

jones:ok:

[zerozero]

I'm just going to take a clumsy stab at this one--no energy to do the research for a really good explanation.

I believe it's more a question of the center of lift. Since spoilers 5, 6, 7 and 8 are the most inboard, they're also the most forward.

As the remaining spoilers deploy, lift is destroyed aftward, but not more forward.

Wish I could post some brilliant diagrams, but that's the best I can do right now.

Did it help? I hope so.
I'm sure you'll get better responses soon.

[Intruder]

That's pretty much the answer. The total center of lift moves aft forward, causing the pitch-up.

[zerozero]

That's pretty much the answer. The total center of lift moves aft, causing the pitch-up.

You mean, the lift moves *forward*, ahead of the CG, causing the pitch up, right?

:}

[Intruder]

Yeah! Read what I meant to write, not what I actually wrote! :8