What's the difference? Krüger flap & Co?

[pilot007]

What's the difference between the Krüger Flap and the Variable Chamber Leading Edge Flap? See Oxfd POF manual page 8-8?

Keep the blue side up ;)

pilot007

[Dick Whittingham]

The Kruger flap is a fairly basic device that folds forward from below the leading edge to divert airflow over the top surface. More mass flow over the top equals more lift. At higher angles of attack - but not very high - the flow breaks down, stalling the wing. They are usually installed inboard.

The cambered leading edge does approximately the same thing at moderate angles of attack, but hangs on, giving smooth flow and lift to much higher angles. These can be installed full span.

The Kruger flap therefore has, and had on early Boeings, the job of increasing lift and then stalling the wing root early to counter the pich up at stall inherent in swept wings.

Dick W

[Intruder]

If you look closely at the extended leading-edge flaps on the 747-400, you may notice the inboard flaps are configured a bit differently than the midspan and outer portions. Though they all extend from the lower surface of the wing, the inboards look more like Krüger flaps (essentially flat, no slot), while the outboards look more like conventional cambered flaps, with a slot between the flap and wing. The midspans are somewhere in between.

Does anyone here have any technical insight into the configuration?

[eng1170]

Hello all,

I am familiar with the B737-400, which combines Krueger flaps inboard of the engines and slats outboard of the engines.

All the info above is good, and I wont expand on it but thought I'd explain about the differences between the kruegers on the 737 and those on the 747.

On the 737 each krueger flap is made up of 2 sections, the larger upper section (with kruegers in down position) which is relatively flat and forms the lower surface of the leading edge when retracted, the other section is connected to the upper section and is rounded. Through a system of linkages the lower section folds out and fwd during extension to give a flap that if taken in x-section is "J" shaped. On retraction the hooked/rounded nose piece tucks back in on top of the flatter upper section and the whole lot becomes flush with the lower wing surface.

There are 4 krueger flaps on the 737, a smaller outbd flap by each engine, (this includes a smaller sub-section which is spring loaded to allow the thrust-reverser sleeve to deploy without trashing the leading edge - the reverser sleeve has a deflector fitted to it which simply pushes the spring loaded door backwards and upwards on deployment) and 2 larger inbd krueger flaps, if you look at the very inbd end by the fuselage with the kruegers deployed you will see a notch out of the rounded nose piece, this is to allow the flap to fit around the fwd no.1 and no.2 tank boost pumps which are fitted to the front spar at this location (must be gap checked if kruegers removed/replaced or the actuator changed during maintenance!!)

On the 747 (which I have less knowledge of, so Jumbo boy's correct me if I'm wrong!) the kruegers are flat when retracted, but on extension the flat section actually changes in camber as they extend. They are manufactured in such a way that the lever/linkages in the system effectively bend the flap to give it a more pronounced camber, and as far as I am aware still has a rounded/hooked nose piece to finish it off. I won't expose my lack of 747 knowledge any further.

Hope this is of some help to you!!

[QAVION]

"On the 747 (which I have less knowledge of, so Jumbo boy's correct me if I'm wrong!) the kruegers are flat when retracted, but on extension the flat section actually changes in camber as they extend. "

Can't vouch for the Classic 747, but the 747-400 Kruegers (inboards) are as described previously (flat with a folding nose, like a "J").

The outboards and midspans on the other hand, are Variable Camber (They also have a folding nose, but the longer flat section changes in camber when deployed).

Regards.
Q.

[L337]

I believe the -400 wing is different from the Classic. ie "new". If that is the case, why did Boeing stick with Kruger flaps, on the mid span, and outbord sections, and chose not to design the new wing with more efficient Slats?

Or are Krueger flaps, as good as slats, if properly designed?

L337.

[rwm]

It is not so much a matter of which is more efficient, but as was stated earlier, the kruger flap is to allow lift , and also stall at an earlier point than the outboard part of the wing.

[L337]

The 747-400 has Kreuger Flaps all the way along the leading edge. It has no slats at all, only Krueger Flaps.

L337

[gas path]

Sorry... WRONG!!
The 747-400 has kreugar flaps inboard of the inboard engines, as described they help to ensure that in the event of a stall the inboard section of the wing will stall first. Outboard of the pylons 2 and 3 are variable camber leading edge FLAPS.

The only difference between the 747-100/200/300/SP (besides the flap programming) is that the -400 has 1 extra variable camber leading edge flap (per side!!!) on the outboard section due to the increased span.

[L337]

So a variable camber leading edge flap is not a krueger flap.

Still the question, why "variable camber leading edge flaps" and no Slats?

L337

[QAVION]

"Still the question, why "variable camber leading edge flaps" and no Slats? "

It is, perhaps, a case of ease of modification to existing structure (therefore reduction in costs) and ease of certification.

BTW, what do you define as "slats"? Are they substantially more efficient? If so, why? Variable cambers appear to increase wing area significantly.

http://members.ozemail.com.au/~b744er/VariableCamber.gif

Rgds.
Q.

[Intruder]

"BTW, what do you define as "slats"? Are they substantially
more efficient? If so, why? Variable cambers appear to
increase wing area significantly."

From the picture in the link you posted, the variable camber LE flap appears to have the exact same functions as a LE slat -- increase wing area and create a slot at the LE. Only the method of stowage/deployment appears to differ. In my mind, the slats make up the wing LE when stowed, and deploy forward and downward on tracks.

One possible difference is that the slats can deploy partially or fully (as in the aerodynamically-controlled slats on the F-86 Sabre and A-4 Skyhawk); while the variable cambers only appear effective when fully deployed.