What drives the mechanical challenges which make trailing-edge flaps an (almost) ubiquitous solution vs leading-edge flaps?

Question

(Apologies for the mouthful of a title.)

I was thinking about canard aircraft and how trailing edge flaps (TEF) are actually counterproductive for them. Leading edge flaps (LEF) could theoretically counter the problem, by unloading the canard instead of further loading it. Great!

But aside from on massive aircraft, LEF are rarely, if ever, seen. I'm making an assumption that this is because of the complexity of developing LEF, an assumption which gets some traction when comparing the 747's leading edge flaps to the DC-3's trailing edge split flips.

Assuming my assumption is correct, what is it about aerodynamics and structures which makes LEF solutions so much harder/more complex than TEF ones?

^{Please note: I'm not asking about leading-edge slats, which are sometimes confused with leading-edge flaps.}

Answer 1

Leading edge flaps are less efficient than trailing edge flaps. Efficiency here means the ratio of change of lift to change in flap angle.

With every flap deflection, both camber and angle of attack of a surface change. While, for example, a nose-down deflection decreases angle of attack while increasing camber such that both effects work against each other, the changes in angle of attack and camber of a trailing edge flap work in the same direction.

Also, a leading edge flap needs to be connected to the wing at its rear end which is a poor choice for a hinge line: The added forces due to deflection will drive the flap to larger deflections. This means it is unstable, always attempting to run into the stops at maximum deflection. To keep the flap under control will require heavy actuators. At the trailing edge the hinge line is ahead of the added lift forces and this will drive the flap to a neutral position regarding tail surfaces while the interconnection of ailerons will keep both sides equally loaded. Trailing edge flaps are inherently stable.

Leading edge devices are only helpful to increase the maximum angle of attack: While the whole airplane will adjust angle of attack to compensate, the flap will reduce the suction peak at the leading edge so flow separation is delayed. This is particularly helpful when you combine slotted flaps inboard with regular ailerons outboard. Without a suitable leading edge flap ahead of the ailerons, the circulation added by the inboard flaps will make the outer wing stall before the inboard wing reaches its full potential. This leads to rather unpleasant stalling characteristics, something to avoid especially during approach and landing.

Answer 2

More than mechanical challenges (if any) it's actually basic aerodynamics that

makes trailing-edge flaps an (almost) ubiquitous solution vs leading-edge flaps.

From this answer:

The main purpose of a leading edge and/or trailing edge device is to change the camber of the airfoil. Without entering in physical/mathematical details, it can be shown that, going from the leading edge toward the trailing edge of an airfoil, the contribution of each piece of its camber to the aerodynamic force is proportional to the following plot/equation (my own plot of the function $\sqrt{\frac{x}{1-x}}$):

There it can be seen that the contribution to the aerodynamic forces of the forward part of the camber is basically negligible while it is the 30-something% most rearward portion of it which almost entirely contributes to the aerodynamic forces. This explains why spoilers, ailerons, rudders, ... whatever surface used to change the aerodynamic forces is placed toward the end of the airfoil and not at its leading edge.

This also tells us that, whatever their name is, any high-lift device placed on the leading edge (like slats or Krüger flaps) does not alter significantly the aerodynamic forces: they are indeed used to improve/extend stall characteristics at high AoA but have almost no impact on the (slope of the) lift or on the pitching moment, as clearly visible in the following plot from this Boeing report:

Answer 3

Mechanical challenges - not so much, they can be solved as many existing aeroplanes demonstrate. Flaps and slates have slightly different functions, as shown in the graph below, from this answer.

• Trailing edge devices increase lift at a given Angle of Attack (AoA), which is desirable - but they reduce the critical AoA.
• Leading edge devices increase critical (stall) AoA.

Particularly aeroplanes with a high subsonic cruise speed have a large disparity in $C_L$ requirements: low $C_L$ in cruise due to the large airspeed, high required $C_L$ during approach and landing. Landing is the critical phase, most high subsonic jets do require the additional $C_L$ and AoA safety margin that the leading edge provides.

Again, mechanical complexity is not really the driver here, aerospace engineers are usually pretty competent at solving complex problems 🙂. The triple slotted trailing edge flap on the Boeing 727 was mechanically pretty complicated...

---Edit---

Please note that the leading edge Krüger Flap functions the same as other leading edge devices, as for instance cited in my go-to book on aeroplane design, page 256:

KRUEGER FLAPS perform in the same way as slats, but they are thinner and more suitable for installation on thin wings. Krueger flaps are often used on the inboard part of wings, in combination with outboard slats, to obtain positive longitudinal stability in the stall.

So it seems that the Krüger flaps are called flaps because they flap outwards when deployed, not because they function identically to trailing edge devices. Also, the above Torenbeek citation identifies another function of the Krüger flap: it creates a nose-up moment, partially compensating for the nose-down moment of the trailing edge flaps.