In aircraft research, the reduction of the structural mass is a core aspect since it helps in decreasing the fuel consumption and thus the CO2 emission. The structure itself is designed to withstand the loads on the aircraft. Hence, if the loads can be reduced, the structure can be built lighter. However, in doing so, the requirements of the certification specifications still have to be fulfilled.
Requirements for the aircraft structure
According to the certification specifications for large aircraft, the structure has to withstand among others loads as much as 2.5 times of the aircraft weight as well as very severe turbulence and gusts which are likely to occur once in an aircraft’s service life (approx. 30 years, depending on the aircraft type and mission) [1]. Those are two main factors which evoke the largest loads on the wing and thus drive the wing’s weight.
Maneuver load alleviation
An aircraft wing generates lift with a more or less elliptical distribution, see Figure 1. Loads as high as 2.5 times the aircraft weight emerging during pull-up maneuvers pose a severe strain for the wing. To reduce the latter, the lift of such maneuvers can be redistributed or shifted towards the wing root through coordinated control surface deflections, see Figure 2. With the lift redistribution, the wing bending moment – which is an important quantity in the design – is reduced. This function is known as maneuver load alleviation (MLA).
Figure 1: Lift distribution of an aircraft wing without maneuver load alleviation
A similar principle is applied for turbulences. A sensor at the aircraft nose measures the angle of attack, and in case a gust or turbulence is detected, the flight control computer deflects the control surfaces dynamically to reduce the maximum wing loads, see Figure 3. This function is known as gust load alleviation (GLA).
Figure 3: Comparison of the wing bending moment during a gust encounter with and without gust load alleviation (GLA)
If MLA and GLA are already included in the design process of an aircraft, some material saving can be achieved, which would result in a weight reduction. In the case of two mid-range configurations, one with a backward and another one with a forward swept wing (see Figure 4 and Figure 5), mass savings of the wing box of 2.8% and 6.1%, respectively, are obtained. At the first glance, they might not seem to be much, nevertheless, those percentages are equal to 130.5 kg or 410.4 kg, respectively.
Figure 4: Geometry of the D150 configuration, a typical narrow-body airliner [2]
Beside the maximum loadings which drive the structural weight, fatigue loads on aircraft also play an important role. The latter pose the factor which limits the aircraft’s service life (to about 40000 flights; the number may vary depending on the aircraft type and mission). The main drivers for fatigue are cyclic loadings due to ground-air-ground cycles and turbulences. Regarding the former: the wing generates no lift on the ground, and during take-off, lift as much as approx. 1.3 times the aircraft weight is generated. Regarding the latter: during typical flights with light or moderate turbulences, the structure is indeed not severely strained. However, the number of load cycles caused by the turbulence is high. With MLA and GLA, the structural loading can be reduced in both cases, and on the reference aircraft, the service life is improved by 28% and 12% respectively, on top of the mass savings.
