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Surface Coatings and Drag Reduction
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The cost of fuel is, by far, the most significant expenditure when considering total aircraft operating costs. As the price of fuel increases, the percentage of the total cost that it represents increases as well. Operators from all aviation sectors, inclusive of airline, corporate and general aviation, are constantly demanding improved fuel efficiency and aircraft manufacturers and associciated industry are responding with improved designs and new products. Among these products are various surface coatings developed with the intent of reducing aircraft aerodynamic drag.
In aerodynamics, drag refers to forces that oppose the relative motion of an object through the air. In an aircraft, drag is overcome by thrust. To provide thrust, aircraft engines burn fuel. If drag is reduced, the thrust required to overcome it will be proportionally reduced and the required fuel burn will decrease.
During the design phase, aircraft manufacturers invest significant effort and expense in attempting to minimise the drag profile and improve the overall performance of their products. Component weights, aerodynamic shapes, engine efficiency, wing span, winglets and numerous other factors such as the addition of appropriate fairings or seals, and minimising the size and number of exposed components, are all considered during the design process. Many of the design phase decisions result in a corresponding drag component that would not be affected by the subsequent application of a surface coating. However, drag is also caused by skin friction.
Drag from skin friction is created by the disruption of the airflow across aircraft surfaces. Skin friction will increase as a result of surface roughness due to surface or paint imperfections, the adhesion of dirt or dead insects to aircraft surfaces or the presence of contaminating fluids caused by leaks, spillage or deicing. In these cases, surface coatings may provide a degree of drag reduction.
Manufacturers of most aftermarket coatings currently available typically make claims that their product(s) will contribute to all or some of:
- Increased paint life
- Increased surface smoothness
- Corrosion protection
- Decreased adherence of dirt and other contaminants
- Decreased drag and reduced fuel consumption
Application of an aftermarket aircraft coating usually involves a three step process:
- Surface cleaning
- Polarisation (inducing a positive charge) of the surface
- Application of the coating
Many coatings are composed of nanoparticles which are small enough to fill even the tiniest of cracks and imperfections. Manufacturers claim that the extremely smooth surface which results from product application can reduce both contaminate adhesion and aerodynamic drag. British Airways has conducted a surface coating trial with one of their transatlantic route-dedicated Airbus A318 aircraft and report positive results.
As an alternative to aftermarket coatings, Lufthansa Technik AG and Airbus are experimenting with a paint application process that would emulate the drag reduction characteristics of shark skin. Using specialized application, stamping and drying techniques, tiny riblets are formed in the surface of the paint. At high speed, the riblets reduce drag by reducing turbulence perpendicular to the airflow. Further research and development are required before large scale implementation of this process might be practical. However, existing research indicates that specialized paint application processes could result in a one percent fuel savings.
In an article entitled "Surface Coatings and Drag Reduction" published in Aero Magazine, Boeing examines the potential drag reduction of aftermarket coatings against the standard fit and finish of their products at the time of delivery. The article concludes that, against the new aircraft benchmark, there is little advantage to be gained by applying aftermarket coatings save for possible surface protection and potential reduction in the aircraft wash cycle frequency. The article asserts that the best methods of minimising drag include careful management of aircraft loading to ensure optimum centre of gravity and the proper maintenance of:
- Seals and surface fit and fair
- Moveable surface rigging (doors, control surfaces,etc)
- External aircraft cleanliness
Boeing concludes that "the most effective means of reducing drag is to maintain aerodynamically clean airplanes".