Progress in Aerospace Sciences 38 (2002) 571–600 Aircraft viscous drag reduction using riblets P.R. Viswanath* Experimental Aerodynamics Division, National Aerospace Laboratories, Bangalore 560 017, India Abstract The last two decades have seen considerable research activity on the use of riblets for viscous drag reduction. Experimental results concerning the performance of 3 M riblets on airfoils, wings and wing-body or aircraft configurations at different speed regimes are reviewed; these applications bring in additional effects like pressure gradients and three dimensionality. In addition to drag reduction, aspects of altered flow features due to riblets are discussed based on detailed wind tunnel measurements at low speeds. The available results obtained from wind tunnels as well as flight tests firmly establish the effectiveness of riblets from low speed to moderate supersonic Mach numbers. With optimized riblets, skin friction drag reduction in the range of 5–8% have been measured on 2D airfoils at low incidence and in mild adverse pressure gradients; strong evidence exist at low speeds to indicate that riblets are more effective in adverse pressure gradients. On wings of moderate sweep relevant to transport aircraft, riblets remain effective providing drag reduction comparable to 2D airfoils, as long as the local angle between the surface streamlines and riblet orientation is relatively small (o101). Limited data available on wing-body configurations show that total drag reduction of about 2–3% is likely. Certain suggestions for future research are outlined. r 2002 Elsevier Science Ltd. All rights reserved. Contents 1. Introduction .......................................... 572 2. Some general remarks on the experimental evaluation of drag reduction due to riblets ... 574 3. Airfoils ............................................ 574 3.1. Airfoils at low speeds ................................... 574 3.2. NAL studies on airfoils at low speeds .......................... 576 3.3. Low-speed studies on NACA 0012 and GAW-2 airfoils ................. 577 3.3.1. Selection of riblets ................................ 577 3.3.2. Drag performance with incidence ........................ 577 3.3.3. Boundary layer development on the NACA 0012 airfoil ............ 579 3.3.4. Turbulence intensity profiles on the NACA 0012 airfoil ............ 580 3.3.5. Reynolds shear stress profiles on the NACA 0012 airfoil and quadrant analysis 581 3.4. Airfoils at transonic speeds ................................ 583 4. Swept wings .......................................... 585 4.1. NAL studies on a swept wing with GAW-2 profile ................... 586 4.1.1. Selection of riblets ................................ 587 4.1.2. Drag performance with incidence ........................ 587 *Tel.: +91-80-522-4024; fax: +91-80-522-3942. E-mail addresses: vish@ead.cmmacs.ernet.in (P.R. Viswanath). 0376-0421/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0376-0421(02)00048-9