Reverse Thrust: Kreuger & V.C.L.E retract, Why?
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 |
They retract to prevent damage to the l/e flaps during reverser operation and blowing stuff in there. Thats all.
|
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...:\ |
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. |
Kreuger flap and Variable camber leading edge flap |
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:U...e.b747.arp.jpg http://www.airliners.net/open.file/0228836/L/ B&N |
.. 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. |
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. |
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... You want your wing root to stall before your wing tip. |
In essence a Variable camber is "slotted", a Kreuger is not. 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 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. |
EMB-145 reverse thrust
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!!
|
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. |
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. |
Thanks heaps for the description Spanner Turner :D:D:D
Great info! B&N |
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!! |
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. :) |
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.
|
All times are GMT. The time now is 02:26. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.