747&737 leading edge flaps&stats

[Captain Greaser]

Why does 1&3 extend before 2&4's on the 747?And why are b737 inboard's are kruigers while the outboards slats?
Be obliged if somebody can tell us the answere.

[blackmail]

hello capt greaser,

for the boeings, kruger flaps inboard & slats outboard, it' s all about stall behavior.
first mr boeing, for obvious reasons, doesn' t want us to stall their aeroplanes, hence the high lift devices(kruger/slats) on the leading edges & fowler flaps at the trailing edges, for slow speed flight.
if we do however insist, they want the wingroot(inboard) to stall before the wingtips(outboard), so that aileron control is not lost first.
kruger flaps by design are less stall resistant than slats, which have among other things, the added benefit of the gap effect(accelerating air à la bernouilly) between the wing leading edge & the deployed slats.

douglas with the dc10 achieves same with wing washout, the wing at the root has by design, a greater angle of fixed incidence(angle between longitudinal axis & wingchord) than the wing at the tips, so again root stalls before tips do. you can clearly see this wing twist, when viewing a dc10/md11 for that matter, from the rear.

kind regards,

bm,

[Lemper]

Why does 1&3 extend before 2&4's on the 747?.

Hi Greaser,
Whaddayamean, "1&3 before 2&4" ?

[Captain Greaser]

Tnx for that reply.Should have thought of that one myself.
A while since I flew the 74.What I should have said was why do the 2&4,s come out first when extending flap from 0 to 1. I think that was the way they extended.Then 1&3's from flaps 1 to 5. When your retracting isn't it 1&3's retracting flaps from 5 to 1 and then 2&4's from 1 to 0. Why is it arranged like this???

[Lemper]

Got it.
When selecting flaps 1, INBOARD leading edge flaps only extend; this is to prevent too much bending and twisting moment on the wing tips. Once trailing edge flaps are extending (flaps 5 and further) there is no more excess of twisting moment, so OUTBOARD leading edge flaps can be extended too.

[innuendo]

Quote: "if we do however insist, they want the wingroot(inboard) to stall before the wingtips(outboard), so that aileron control is not lost first."
In addition, one of the benefits of not having the wingtips stall before the rest of the wing is avoiding the center of lift, (what's left of it), from becoming too far forward on the whole machine.
If this occurs in the extreme in some types, the elevators may not be able to overcome the nose up tendency.
The type that comes to mind here is one I spent some time on, the F-101B Voodoo. Given the right conditions the elevators would become blanked out in the turbulent wing airflow with the result that the whole machine would pitch up.
That may be a bit extreme for a transport type but at least we could bail out if necesary.

[SUB]

Lemper where do you get your info from ?

747 Classic, L.E flaps 2 & 4 extend at flaps 0-1, called Group A, and flaps 1-5 Group B extend which is L.E flaps 1 & 3. They ext and ret in these groups because when you take off you want all the lift so at flaps 20 all L.E flaps will be extended, on climb as air speed incr you dont req as much lift this is why they retract in groups .

744 is different , flaps 0-1 L.E flaps 2,3,4 all extend and flaps 1-5 L.E flaps 1 extend.

The numbers are related to the pneumatic drive units (PDU) that run the L.E flaps. 1,2,3,4 from o/b to i/b .