High Lift Systems
A wing designed for efficient high-speed flight is often quite different
from one designed solely for take-off and landing. Take-off and landing
distances are strongly influenced by aircraft stalling speed, with lower
stall speeds requiring lower acceleration or deceleration and correspondingly
shorter field lengths. It is always possible to reduce stall speed by increasing
wing area, but it is not desirable to cruise with hundreds of square feet
of extra wing area (and the associated weight and drag), area that is only
needed for a few minutes. Since the stalling speed is related to wing parameters
by:
It is also possible to reduce stalling speed by reducing weight, increasing
air density, or increasing wing CLmax . The latter parameter is the most
interesting. One can design a wing airfoil that compromises cruise efficiency
to obtain a good CLmax , but it is usually more efficient to include movable
leading and/or trailing edges so that one may obtain good high speed performance
while achieving a high CLmax at take-off and landing. The primary goal of
a high lift system is a high CLmax; however, it may also be desirable to
maintain low drag at take-off, or high drag on approach. It is also necessary
to do this with a system that has low weight and high reliability.
This is generally achieved by incorporating some form of trailing edge flap
and perhaps a leading edge device such as a slat.
The triple-slotted flap system used on a 737 is shown below.
Below is a double-slotted flap and slat system (a 4-element airfoil). Here,
some of the increase in CLmax is associated with an increase in chord length
(Fowler motion) provided by motion along the flap track or by a rotation
axis that is located below the wing.
Modern high lift systems are often quite complex with many elements and
multi-bar linkages. Here is a double-slotted flap system as used on a DC-8.
For some time Douglas resisted the temptation to use tracks and resorted
to such elaborate 4-bar linkages. The idea was that these would be more
reliable. In practice, it seems both schemes are very reliable.
Current practice has been to simplify the flap system and double (or even
single) slotted systems are often preferred.
Flaps change the airfoil pressure distribution, increasing the camber of
the airfoil and allowing more of the lift to be carried over the rear portion
of the section. If the maximum lift coefficient is controlled by the height
of the forward suction peak, the flap permits more lift for a given peak
height. Slotted flaps achieve higher lift coefficients than plain or split
flaps because the boundary layer that forms over the flap starts at the
flap leading edge and is "healthier" than it would have been if
it had traversed the entire forward part of the airfoil before reaching
the flap. The forward segment also achieves a higher Clmax than it would
without the flap because the pressure at the trailing edge is reduced due
to interference, and this reduces the adverse pressure gradient in this
region.
The favorable effects of a slotted flap on Clmax was known early in the
development on high lift systems. That a 2-slotted flap is better than a
single-slotted flap and that a triple-slotted flap achieved even higher
Cl's suggests that one might try more slots. Handley Page did this in the
1920's.
Tests showed a Clmax of almost 4.0 for a 6-slotted airfoil.
Leading edge devices such as nose flaps, Kruger flaps, and slats reduce
the pressure peak near the nose by changing the nose camber. Slots and slats
permit a new boundary layer to start on the main wing portion, eliminating
the detrimental effect of the initial adverse gradient.
Today computational fluid dynamics is used to design these complex systems;
however, the prediction of CLmax by direct computation is still difficult
and unreliable. Wind tunnel tests are also difficult to interpret due to
the sensitivity of CLmax to Reynolds number and even freestream turbulence
levels.
Navier Stokes computations of the flow over a 4-element airfoil section
(NASA)