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View Full Version : Reverse Thrust: Kreuger & V.C.L.E retract, Why?


Betsy & Nikki
22nd May 2007, 08:50
I came across this video while preparing for an interview.

http://youtube.com/watch?v=dqCsyFPIAZQ

It shows a Virgin Atlantic 747-400 on final approach and landing. I noticed that the inboard Kreuger flap and Variable camber leading edge flap (midspan only) retract when reverse thrust is selected and then extend when de-selected.

My question is why does this happen?

Skip to 5 mins and 50 seconds to see what I'm talking about.

B&N

Nepotisim
22nd May 2007, 08:57
They retract to prevent damage to the l/e flaps during reverser operation and blowing stuff in there. Thats all.

Flight Detent
23rd May 2007, 02:30
That's right on the money!

The B747 Classic does the same thing sometimes, from memory, someone may be able to help me here, the Classic does that, except with one particular engine make fitted, but I can't remember which one!

Cheers...FD...:\

411A
23rd May 2007, 02:41
The same is true on the very first Boeing civil jet type...the B-707, with the exception that all leading edge devices retract, then extend once again, once reverse thrust is canceled.
All engine types as well, I believe, RB.508 Conway's included.

A well thought out design, IMO.

Stuck_in_an_ATR
23rd May 2007, 12:02
Kreuger flap and Variable camber leading edge flap


What's the difference between those two anyway? On the video they seem to work the same way...

Betsy & Nikki
23rd May 2007, 13:48
Thanks for the replys guys,

Could another reason why the kreuger flaps and the variable camber leading edge flaps (midspan) retract when reverse thrust is selected, is to reduce lift over the inboard section of the wing, hence put more weight on the wheels and therefore allow more effective breaking?? Just a thought.

I don't fly the B744 however my understanding of the difference between the two are:

Physical difference:
Kreuger Flaps have no slot (they increase the camber of the wing and are designed to smooth out the section between the inboard engines and the fuselage, therefore reduce drag, increase lift)
V.C.L.E Flaps have a slot (designed to accelerate airflow over the wing and therefore delay the separation point at high AoA's). Also known as a boundary layer control device. They also increase the camber of the wing.

I haven't been able to find a convincing answer to what kreuger flaps actually do :ugh:

Anyway, these pictures may assist in understanding the 744 leading edge:

http://en.wikipedia.org/wiki/Image:Undercarriage.b747.arp.jpg

http://www.airliners.net/open.file/0228836/L/

B&N

john_tullamarine
24th May 2007, 03:25
.. is to reduce lift over the inboard section of the wing

On the contrary, I suspect that they provide a better situation for lift and drag extended .. I'm guessing that the initial answer is more likely.

LEDs are not a great deal of interest to lift at low alpha.

CV880
24th May 2007, 03:30
Original 747 with JT9D engine's had turbine reversers and the L/E devices retracted to stop them from being buffeted by the hot reverser flow and also to keep the hot and somewhat corrosive and dirty, sooty gases out of the leading edge cavity. When they gave up trying to keep the turbine reverser operative and issued an SB to delete/deactivate them permanently the auto retract function on reverse was deleted as well. Later, Boeing reinstated the auto retract on reverse as I think there were still fatigue issues from buffeting with some engine types.
707 retacted the L/E devices for same reason - keep the soot and hot gases out of the leading edge cavity.

company_spy
24th May 2007, 09:32
Quote:
Kreuger flap and Variable camber leading edge flap
What's the difference between those two anyway? On the video they seem to work the same way...

In essence a Variable camber is "slotted", a Kreuger is not.

You want your wing root to stall before your wing tip.

lefthanddownabit
24th May 2007, 21:12
In essence a Variable camber is "slotted", a Kreuger is not.


You can have slotted Krueger Flaps too. The real difference is that the Variable Camber Flaps change camber during extension, the flexible panel bending around the four bar linkage underneath. Essentially they are Krueger Flaps that have a curved surface. When retracted the VCF is flush with the relatively flat wing lower surface, but when extended the panel is cambered like a LE Slat.

Look at the photographs and video and you can see this happening. The VCFs have a curved surface, but the Kruegers remain flat.

Both VCF and Kruegers change the camber of the wing too of course.

I haven't been able to find a convincing answer to what kreuger flaps actually do
In operation, Krueger Flaps work like drooping leading edge flaps, but without a lower surface. The lower surface airflow detaches at the leading edge of the Krueger and re-attaches on the wing lower surface, forming a separation bubble. Therefore a lower surface is not really missed. Kruegers also make good speedbrakes.

They increase wing area and camber in a crude but effective way. With the flap extended the stall angle of attack is increased, just like any leading edge device.

GeoAC
25th May 2007, 00:36
Get a grip!! Who knows what the conditions were pre-approach? All you couch jockeys always seem to be ready to crap on the man in the seat without knowing the odds!!

Dan Winterland
25th May 2007, 01:40
None of the LE devices in the 747s are slotted. they are simple devices which just increase the camber. I suspect the designers went for the more basic Kugers on the inboard sections because that section of the wing is already heavily cambered. The flatter outer parts of the wing benefit from the greater camber given by the variable camber devices.

These are not the same as the Airbus LE devices which are slotted slats and vary position with different flap settings.

Spanner Turner
25th May 2007, 09:47
Dan Winterland said......"None of the LE devices in the 747s are slotted."

Well, I beg your pardon, if your Honour shall allow me to object!

Come on guys. None of this stuff is rocket science! This aircraft type (or shall I say derivative so as to include -100, -200, -300, -400, SP, E4B, VC-25, Combi, Freighter, Shuttle transport etc, etc, etc, etc, etc) first flew on the 9-FEB-1969 and entered service on 22-JAN-1970. This was over 27 years ago and none of this design is state secret any more (if ever it was). If I may quote from the Boeing "description and operation" section of the 747 maintenance manual:


Krueger Flaps.
Operation
The krueger flaps are positioned by rotary actuators which drive
mechanical linkages attached to the flap. The flaps retain a flat
surface when they extend and there is no gap created to allow
airflow between the leading edge flap and the wing leading edge.
Each flap has a folding nose which is unfolded when the flap is
extended, and restowed above the flap panel when the flap is
retracted. The nose is hinged to the flap and positioned by the flap
linkage.
The flap is supported by two hinges which attach to the wing leading
edge.
Bulb seals are provided at the sides and forward and aft edges of
each flap panel for aerodynamic sealing.


Variable Camber Leading Edge Flaps
Operation
The flaps are positioned by rotary actuators connected to drive arms.
Mechanical linkages curve the flexible fiber glass panels as they
drive. The area of each flap is increased by a folding nose which is
positioned at the flap leading edge as the flap is extended. A slot is
also formed between the flap and the wing leading edge to increase
aerodynamic efficiency. The folding nose section is stowed and the
panel returned to a flat shape when the flap is retracted.



As for retraction when symetrical reverse is applied:


Automatic Leading Edge Flap Retraction
Group A leading edge flaps are automatically retracted during ground
reverse thrust operation in order to improve structural life of the flap
panels. The function is armed when the airplane is on the ground and
the flaps are operating in the pneumatic mode.
Group A leading edge flaps retract when armed and engines 1 and 4
are placed in reverse thrust or engines 2 and 3 are placed in reverse
thrust. The flaps re-extended when the reverse thrust signals are
removed.

NOTE:
Thrust reverser operation is defined as reverse thrust
selected or airmotor brake released or gearbox unlocked
on a symmetrical pair of engines.

Betsy & Nikki
25th May 2007, 13:09
Thanks heaps for the description Spanner Turner :D:D:D

Great info!

B&N

exflyboy
25th May 2007, 13:30
Remember guys it's a Boeing. Everything is designed around plain old common sense. Think of the simplist answer and 9 times out of 10 you'll be right.

Airbus?, well god know what it thinks!!

InSoMnIaC
25th May 2007, 19:16
spanner_tunner: - first flew on the 9-FEB-1969 and entered service on 22-JAN-1970. This was over 27 years ago

yes you're right but it was also over 37 years ago. :)

ERJFO
29th May 2007, 11:37
It's really just as well that the L.E flaps/slats retract with reverser deployment because it acts like a "lift-dump" and unloads the wing a little bit more than just spoiler deployment. I imagine the decrease in drag is minimal and the A/C slows faster with the L.E. devices up.