Leading edge flaps/slats

[tornadoflyer]

Hi,

I recently read an older post about questions asked during Cathay Pacific's interview process. One of questions was directed to describe the difference between leading edge flaps and
leading edge slats. I would appreciate it if another forum member can answer this question for me.

Thank you in advance.

[SMOC]

LE Slats are on the 777, A330/340/346
http://en.wikipedia.org/wiki/Slats

LE Flaps are on the 747, inboard of engines 2 & 3 use Krueger Flaps (http://www.hq.nasa.gov/office/pao/History/SP-468/ch10-5.htm) and outboard of engines 2 & 3 are variable camber flaps. (http://www.airliners.net/open.file/352855/L/)

[e&e]

Referring to the powered slats;
Powered - the slat extension can be controlled by the pilot. This is commonly used on airliners.

This type of high lifting device allows you to have a higher Clmax. or max. angle of attack than in clean config. The problem with it is that it gives you a very high attitude angle during descend and icing formation can occur on this area.It doesn´t affect the zero lift angle of attack.

On the other hand, l.e. flaps can be either Krueger or Betz.
Basically both increase the Clmax. and the zero lift angle of attack while not varying the angle of the polar curve(Cl/Cd curve)

hope it helps

[JediDude]

The basic difference is that LE Slats leave a gap between the slat and the wing allowing high pressure air to bleed from the bottom of the wing to the top. LE Flaps have no gap and function purely to increase the lift of the wing.

[Swedish Steve]

Lets not forget the Leading Edge DROOP on the original Tridents. They came out like slats, but there was no gap.

[Old Smokey]

Leading Edge Flaps and Leading Edge DROOP provide an immediate increase in camber, and therefore lift. The resultant stall speed is reduced, allowing for reduced Takeoff and Landing speeds.

Leading Edge Slats have no effect upon lift, in fact, at low angles of attack may reduce lift. Their effect is to delay separation of the airflow at higher angles of attack, thus stall speed is reduced, allowing for reduced Takeoff and Landing speeds, exactly as for Leading Edge Flaps.

The main operational difference is that the 'slatted' wing will stall at a considerably higher Angle of Attack and Pitch Attitude, necessitating rotation to a higher Pitch Attitude on Takeoff, and a similarly higher Pitch Attitude on Approach.

The main performance difference is that there is much less Drag associated with Slats, than with Leading Edge Flaps / Droop.

Regards,

Old Smokey

[wonderboysteve]

Old Smokey,

I think it is misleading to say that LE Slats have no effect on lift; they have a dramatic effect on maximum lift, but little effect at fixed angle of attack. Amongst other things, the presence of the main element suction peak immediately aft allows off-surface pressure recovery from the slat, which more efficient. The main element itself is also highly loaded in this configuration, although the suction peak is reduced by the opposing circulation from the flap, suppressing separation to higher angles of attack by reducing the adverse pressure gradient.

[john_tullamarine]

Another simplistic way of visualising the picture is

(a) the Handley-Page slat (as it was originally known .. being of good transported convict blood I have to give the Brits due credit where appropriate) takes the basic CL by alpha curve and extends the curve to higher values of both parameters. The shape of the post-stall section may vary a little between the basic and slat configurations but, in essence, it is just an extension of the basic curve.

(b) the nose flap takes the basic curve and moves it up the CL axis.

(c) the two technologies, in combination, result in a combination of the two effects.

A useful picture for showing this can be found in many texts.

Try, for instance,

Aircraft Stability and Control for Pilots and Engineers
B. Dickinson
Pitman and Sons
p104 in my copy .. God knows how old it is .. can't readily see an edition reference .. but it cost me about five quid a long time ago. Kermode, beloved of many of the PPRuNe readership gets to say a few words in the front ...

There should be plenty of copies of Dickinson about in the libraries as it was a pretty standard textbook when I were but a lad. Dickinson was CGI at ETPS and his book is eminently readable provided that you don't mind having to wade through some maths here and there. The average pilot interested in finding out about lots of things could do far worse than picking up a copy in the secondhand bookshops ....