Reducing Skin-Friction Drag by Laminar Flow

In a previous post I introduced the concept of skin-friction and pressure drag, and discussed the contradicting aerodynamic conditions to minimise either of the two types of drag. Overall the minimum resistance of slender shapes (such as aerofoils) to a fluid is attained with an attached laminar boundary layer over the entire surface. However, at some point from the leading edge the boundary layer will naturally transition to turbulent flow (see example of cigarette smoke), and any curvature in the shape will induce an adverse pressure gradient that can cause boundary layer separation. Consequently, laminar flow is generally restricted to a small percentage of the wing around the leading edge. For aircraft wings considerable research has been conducted to come up with mechanisms that maintain laminar flow over large parts of the wings and therefore reduce drag, fuel consumption and increase flying speeds.

One of the the first aircraft to attempt to take advantage of laminar flow was the WW II fighter P-51 Mustang. During the War the Americans and British developed a very slender aerofoil shape, now known as NACA 45-100, with the point of maximum thickness about half-way along the camber line in order to reduce the effects of the adverse pressure gradient. With the maximum camber in the middle it was thus possible to maintain a larger percentage of laminar flow over the wing. In 1938 wind-tunnel tests on the aerofoil recorded a drag coefficient of .003 which was about half of the lowest ever recorded for an aerofoil of similar thickness [1]. On the aircraft however the results of the controlled laboratory tests were never achieved. Laminar flow is a sensitive phenomenon and the slightest roughness of the aerofoil surface roduced by splattered insects, protruding rivets or imperfections in machining will cause premature transition to turbulent flow before the design condition. Furthermore, the air passing through the propeller produces a highly turbulent slipstream which is exacerbated by the vibration of the entire fuselage.

The North American XP-51 Mustang was the first aircraft to incorporate an NACA laminar-flow airfoil. (Photo credit: Wikipedia)

In order to improve on this early design NASA has conducted an array of flight tests on aircraft designed for natural laminar flow (NLF). To protect the leading edge from insect contamination one concept features wrapping the leading edge with paper during take-off, which is then torn-off at higher altitudes. A rather resource wasteful solution! Another solution using wire and felt pad scrapers, to as the name suggests, scrape dead insects from the surface of the wing. Furthermore, covering the leading edge with a curved deflector plate known as a Krüger nose-flap has been investigated on various aircraft. The drawback of these designs is that they disturb the streamlined profile of the aerofoil and therefore induces parasitic drag that outweighs the improvements of maintaining laminar flow. The Krüger flap concept is nowadays incorporated in high-lift devices but only used during landing and take-off, which only accounts for a fraction of the full flight time

Tests on an experimental F-16XL aircraft were used in a NASA programme to assess laminar flow on aircraft flying at supersonic speeds. The main aim was to assess the merit of swept-wings for future high speed civil aircraft. The swept delta-wings used active perforated titanium “gloves” attached to the surface featuring tiny holes through which most of the boundary layer was drained-off by an internal suction system. The panels covered 60% of the wing’s leading edge perforated with about 10 million microscopic size laser-cut holes. Through these holes the suction system in the wing drew away a significant portion of the slower fluid in the boundary layer close to the surface, thereby expanding the extent of laminar flow across the wing. The Supersonic Laminar Flow Control (SLFC) successfully achieved laminar flow over large portions of the wing up to supersonic speeds of Mach 1.6 [2].

The concept of using suction wings to maintain laminar boundary layers has thus far been the most researched and promising solution. Before these technologies can be applied issues such acceptable reliability, maintainability and operational characteristics have to be resolved and the long-term technical and economic viability of the technology demonstrated. The current legislative framework requires the development of novel aircraft design in the near future in order to meet the ambitious fuel economy requirements. Perhaps advances in micro-machining, nanotechnology and smart-material technologies will lead to LFC devices becoming integral parts of revolutionary new aircraft.