Re: 727 inboard leading edge devices

Date:         21 Dec 97 02:32:53 
From: (Mike Hore)
Organization: JAM Software
References:   1 2
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[reposted since I think Karl's disk ate it]

Simon Ellwood <> wrote:
> In article <airliners.1997.2831@ohare.Chicago.COM>, Andrew Goldfinger
> <> writes
> >     The inboard leading edge devices on the 727 do not deploy by sliding
> >forward as do the devices on most other aircraft.  Instead, they rotate
> >downward and outward pivoting near the leading edge.  Intuitively, this
> >bothers me.  With a "sliding" device, the change in wing shape during
> >deployment seems more continuous to me.  Do the 727 devices do strange
> >things to the aerodynamics when partially deployed?  Why were the devices
> >made in this way?  Is there a name for this type of device?
> The inboard devices are known as Kruger flaps, whilst the outer devices
> are standard slats. The Krugers are normally flexible and form a LE
> shape during deployment. Aircraft such as the 707 & 747 have only these
> Krugers - no slats.

That's nearly right - and BTW, a little nitpick -- the "u" in "Kruger"
has an umlaut over it, or alternatively, it can be spelled "Krueger"
as is usually done in English-speaking countries...

The Kruegers on the 727 don't flex -- they just stay flat.  The
bending during extension was introduced with the 747.

> Both devices increase lift by increasing the curvature and camber of the
> wing at the LE. However, when the slat is fully deployed, it leaves a
> slot between itself and the LE of the wing. This slot effectively
> accelerates flow over the wing LE, increasing the energy in the boundary
> layer flow.

Agreed.  But on the 747 the Kruegers do form a slot.  And with the
bending, they're pretty much equivalent to slats on other designs.
As I remember, they were used in preference to slats since the
wing LE is very sharp, and a slat which formed such a LE wouldn't
have such an ideal shape.  There's also very limited room for
stowage, and the bending Krueger was a very nice answer.

Later aft-loaded wing designs have a less-sharp LE, so slats are
fine on these A/C.

> The upshot of all this is that the stall angle of the slatted part of
> the wing is increased, and thus on an aircraft like the 727, the wing
> roots stall before the tips. This has the advantages of :-
> 1) Preventing tip stall and roll when the aircraft stalls.
> 2) As the root stalls before the tips, and the 727 has a sharply swept
> wing, a strong pitch down is induced from the "still flying" tips which
> are a fair way behind the CG.
> Has anyone noticed that on the 727 the no.s 2,3 and 6,7 slats deploy
> first at a lower flap setting than the rest of the slats and Kruger's ??
> Can any of you Boeing engineers tell us why ??

I've heard that this partial deployment in the flaps 2 setting
reduces wing loading.  The same sort of thing is done on the 747
with the flaps 1 setting, which in later 747's leaves the
trailing-edge flaps completely up.  And there's also a difference
between the -400 and Classic 747's in which LE panels participate.

Having seen all this going on, though, I too would be very interested
in a more authoritative explanation.

Cheers,  Mike.

Mike Hore