Krueger Flaps

[Aelipse]

Alright, so playing KSP got me interested in basic principles of aerodynamics and aviation, and while the most basic things like production of lift or flight stability are easy to look up and learn, I haven't been able to find anything comprehensible on Krueger flaps.

In case of slats, I understand their function to be redirecting the airflow above the wing and forcing it along its upper surface at high angles of attack, thus preventing a stall (without the help of slats, the air stream would just separate from the wing and create turbulent flow). Please correct me if I got this wrong.

Spoiler

Wikipedia's image of slats:

When it comes to Krueger flaps though, I am lost. Their design in my mind doesn't allow for the air stream to be forced anywhere even close to the upper surface of the wing.

Spoiler

Wikipediea's image of Krueger flaps:

I remember some sources claiming the extended Krueger flaps increase the apparent camber of the wing, which I understand to be the wing's "thickness". How that increases the critical angle of attack and prevents stall, however, is still beyond me.

[Shpaget]

It is my understanding that they significantly increase wing camber, which is beneficial in high angle of attack flight phases.

[PakledHostage]

Like @Shpaget said, kreuger flaps  increase camber which increases lift at a given angle of attack. They also increase the chord length slightly when they extend forward of the wing's leading edge.

They are used on the inboard wing sections of many of the more modern Boeing aircraft, in part because they have the side effect that they help ensure that the inboard wing will stall before the outboard wing. This helps improve stability because the aircraft's nose will tend to pitch down when the wing root stalls first, allowing for stall recognition and recovery before the outer wing sections reach the stall.

[mikegarrison]

The explanations other people have given here are correct.

Kruegers are not slotted. They do increase the wing camber (and to a small extent the wing area).

What you might not realize is that slats are not always gapped from the wing. At small slat extensions there is no gap. They just increase wing area and camber (much like Kruegers do). At larger extensions the slats extend away from the wing and fully become "slats" per se.

One of the biggest effects most high-lift devices do is to increase the effective (and also physical) wing area. Lowering the wing loading makes the airplane better at tolerating high lift and angle of attack at lower speeds. This is why Fowler flaps translate back as well as down.

Gulfstream jets are designed with no leading edge devices. They are also designed with very large (relatively speaking) wing areas. These two facts are strongly related to each other.

[RCgothic]

I'll just add that camber isn't aerofoil thickness, it's aerofoil asymmetry.

An aerofoil with zero camber gives zero lift at zero angle of attack.

But a wing with camber can generate lift at zero angle of attack. This is important, because if the whole plane flies with an angle of attack then the body will be producing lift as well, and body lift is very draggy. You want to minimise the angle attack of the plane body.

 

But sometimes you do want to fly with high angle of attack - e.g. on landing. Adding additional camber with flaps can make the wing more resistant to stalling at high angle of attacks.

[mrfox]

Kruger flaps generally have inferior performance aerodynamically, but they are vastly simpler structurally and hence lighter - especially when considered in conjunction with anti-ice systems that uses hot bleed air in the leading edge (most large civil aircraft excluding the 787). With slats, you need to run hot telescoping metal pipes through a ice cold air gap to deice them. Which makes for a heavy and somewhat unreliable system, with very dangerous failure modes (hot bleed leak near fuel, hydraulics, wing structure...)

[Guest]

Going a bit more in-depth as to how they work, take a gander at the equation for lift force:

•  is the lift force
•  is the air density
•  is the velocity or true airspeed
•  is the planform (projected) wing area
•  is the lift coefficient (varies with angle of attack, Mach, Re and object geometry)

The leading edge flap increases airfoil (wing section) camber, which generally increases the lift coefficient for a given angle of attack. If we double the lift coefficient, we double the lift force.

Others have mentioned the fact that the wing will have a larger area and thus give a greater lift force. That is true. However, when designing an aircraft, it's common practice to use a single, non-varying reference area (this is to reduce complexity and errors), thus, the effect of having a greater wing area is actually included in the lift coefficient - the wing reference area is constant.

Edited June 8, 2020 by Guest