Wings are the key components of any flying vehicle, as they produce almost all of the lift. Generally speaking, a larger aircraft will have larger wings to support its weight. The Cessna C-172 has a wing area of 174 sq. ft., while the Airbus A-380 has a wing area of 9,096 sq. ft.; but in this article, we will try to explain that weight is not the only deciding factor in designing a wing.
Lift and drag are the two aerodynamic forces important in the performance calculation of aircraft. In steady and level flight, the lift requirement is equal to the aircraft weight, and drag force is balanced by the thrust of the engine or propeller. This steady condition is called the trim condition and is shown in the figure below:
Importance of Wing Area
The decision about wing design is dependent on many factors, such as weight, flying speed, and the aerodynamic efficiency of the airfoil. The airfoil is the cross-section of the wing facing in to the free-stream velocity. Aerodynamic performance of the air-foil section is measured in terms of aerodynamic lift (CL) and drag (CD) coefficients. (More on the aerodynamic coefficients can be found here)
Wing Sizing Parameters
For a given selection, airfoil performance and flying speed work as design criteria, and wing loading (weight/wing area) and power loading (weight/power) are the parameters that are set to achieve the desired performance. For example, the RQ-9 reaper and MOJAVE are the major attack drones, and both have a design takeoff weight of 4760kg and 3175kg, respectively. MOJAVE has a lower wing loading despite its lower take-off weight; this is because of its low flying speed and short field take-off requirements. A brief comparison of the design parameters of these two UAVs is shown below:
It is evident that MOJAVE has relatively low wing loading, that makes it able to take-off within 300 ft. On the other hand, the Reaper UAV has low power loading and high wing loading, making it suitable for high-speed, long-endurance flights.
Sizing Trade-off Studies
The optimal way to ensure that the aircraft wing size will meet all requirements is to plot the constraint diagram. A constrained diagram with all requirements plotted will have a region of intersection of all regions where all the requirements may be fulfilled. For example, maximum speed, maximum altitude, rate of climb and stall speeds are plotted in figure below:
It is worth mentioning that desired performance can be achieved by various combinations of the sizing parameters and the design space could be anything from a design point to a region. Most of the time, the propulsion system is already selected; in such cases, the design space is reduced to a power plant constraint line. Another is to perform a conservative sizing; in this case, a lower value of wing loading is selected so that uncertainty in weight estimation (which happens most of the time) can be accounted for. One drawback of this approach is its higher material cost and sometimes reduced maximum speed. On the other hand, an optimistic sizing approach is to select the highest power loading, which will reduce the propulsion system size and cost and the highest wing loading in design space, which can reduce the construction cost. This approach shall be followed by a risk management strategy.
Finally, wing sizing is not all about the wing area but wing dimensions as well. Aspect ratio (AR) is an important parameter in making the wing more efficient; it is defined as the ratio of Wing Span to Cord Section. Aspect ratio is an aerodynamic efficiency parameter and not directly a sizing parameter; therefore, more discussion on AR can be found in the articles on induced drag.