Flap Speed Limit

Figure 14.10

Flaps increase CLMAX and decrease stall speed, so when flaps are deployed it is necessary to provide additional protection to avoid exceeding the structural limit load. It can be seen from the V-n diagram in Figure 14.10 that it is possible for a greater load to be applied to the structure at quite moderate airspeeds with flaps down. The limit load factor with flaps deployed is reduced from 2.5 to 2 to give additional protection to the flaps and also the wing structure. If flaps are deployed in turbulence, a given vertical gust can generate a much larger lift force which will subject the structure to a larger load, possibly exceeding the ability of the structure to withstand it, and the structure could fail.

VFE : the Wing Flaps Extended Speed is the maximum airspeed at which the aircraft should be flown with the flaps in a prescribed extended position. (Top of the white arc on the ASI).

Extending flaps for turbulence penetration in the cruise would reduce the stall speed and increases the margin to stall, but the margin to structural limitations will be reduced by a greater amount. Flaps must only be used as laid down in the aircraft Flight Manual.

Limitations 14

Limitations 14

Aerodynamic forces acting on the aircraft produce distortion of the structure, and this distortion produces corresponding elastic forces in the structure (“winding up the spring”). Structural distortion produces additional aerodynamic loading and this process is continued until either an equilibrium condition is reached or structural failure occurs.

This interaction between the aerodynamic loads and the elastic deformation of the airframe is known as aeroelasticity, or aeroelastic coupling.

At low airspeeds, the aerodynamic forces are relatively small, and the resultant distortion of the structure produces only negligible effects. At higher speeds, aerodynamic loads and the consequent distortion are correspondingly greater. Aerodynamic force is proportional to V2, but structural torsional stiffness remains constant. This relationship implies that at some high speed, the aerodynamic force build-up may overpower the resisting torsional stiffness and ‘divergence’ will occur. The aircraft must be designed so the speed at which divergence occurs is higher than the design speeds VD / MD.

Definitions:

Elasticity

No structure is perfectly rigid. The structure of an aircraft is designed to be as light as possible. This results in the aircraft being a fairly flexible structure, the amount of flexibility depending on the design configuration of the aircraft. E.g. aspect ratio, sweepback, taper ratio etc.

Backlash

The possibility of movement of the control surface without any movement of the pilot’s controls.

Mass distribution

The position of the CG of a surface in relation to its torsional axis.

Mass balance

A mass located to change the position of the CG of a surface in relation to its torsional axis.

Divergence

The structure will continue to distort until it breaks.

Flutter

The rapid and uncontrolled oscillation of a surface resulting from imbalance. Flutter normally leads to a catastrophic failure of the structure.

4

3

2

1

FLEXURAL

AC AXIS

Figure 14.11

Refer to Figure 14.11 which represents the view of a wing tip, and consider a vertical gust increasing the angle of attack of the wing. The additional lift force will bend the wing tip upwards from position 1 to 2 and the increase in lift acting through the AC, which is forward of the flexural axis, will twist the wing tip nose-up; this increases the angle of attack further. The wing tip will rapidly progress to position 3 and 4. The wing is being wound up like a spring and can break if distorted too much.

How far the structure is distorted depends on:

•the flexibility of the structure.

•the distance between the AC and the flexural axis.

•the dynamic pressure (IAS).

Methods of delaying divergence to a higher speed:

•The structure can be made stiffer, but this will increase weight.

•A better solution is to move the flexural axis closer to the AC. This can easily be accomplished by mounting a mass forward of the AC. Instead of using a large piece of lead, as in control surface mass balance, the engines can be mounted forward of the leading edge and this will move the flexural axis closer to the AC. (Also see Flutter, page 477).

Limitations 14