# High-Flying Efficiency: Proving the Power of Cost-Effective Riblets for Aircraft Drag Reductions

**Task**

Investigating different Riblet layouts to prove the application strategy for achieving highest performance increases at best cost-effectiveness.

**Solution**

Conducting profound flight tests with different Riblet layouts based on in-depth numerical Riblet analyses and CFD simulations.

**Benefit**

Demonstrating the effectiveness of sharkskin textured surfaces for reducing drag and increasing efficiency at a cost effective low area application.

**Figure 2**. The boundary layer was split in 15 prisms in order to capture the velocity profile close to the wall, which is crucial for an accurate Riblet design. Based on these baseline simulations and the determined wall shear stresses, the ideal Riblet geometries and allocations for maximizing their impact were designed, which can be seen in

**Figure 3**for FL 410.

The chosen method to apply Riblets were plastic foils, which do not enable the adaption of Riblet sizes to every surface point according to the ideal Riblet distribution (this would be enabled by the use of laser technology). Therefore, the Riblet sizes had to be discretized. After analyzing the optimal trade off, a Riblet design was chosen with only two different Riblet sizes represented by two different foil types (in **Figure 5:** Riblet size A : blue; Riblet size B: red). Four different Riblet layouts were examined:

- Foil was applied to the entire possible Riblet area
- Riblets on the wings’ Pressure Sides and Suction Sides
- only on the wings’ Suction Sides and
- no Riblets to obtain representative baseline measurements.

At each of the two flight levels chosen for the test’s measurements (FL410 and FL285), different cruise test points were measured: Each test point held a different parameter constant as follows:

- PLA (Power Lever Angle at maximum) in order to check the maximum achievable Mach number
- KIAS (Indicated Airspeed): 0,54 Mach, 313 knots for FL410; 0,4 Mach, 242 knots for FL 285
- N1 (Fan Speed) at 99.5% of its maximum
- ITT (Internal Turbine Temperature) at 600 [K]

Figure 4: Maximal Mach numbers achieved at FL 410 with different Riblet layouts

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Outcome

**Figure 4**, which shows that the maximal Mach number increases with the applied Riblet area. An almost 3.5% higher Mach number is reachable if the complete aircraft is covered with Riblets. When Riblets are applied only to the Suction Sides, the maximal Mach number increases up to 1.5%.

**Figure 5**shows the Mach number increase divided by the Riblet area. This new parameter gives an insight about the Riblet area efficiency. The smallest applied Riblet area, which is also the most economic layout, clearly shows the highest efficiency gain. The Riblet application affected the fuel consumption by a reduction of up to 3,4%, which is more than expected. However, the comparability of the KIAS tests are not certain, due to variations in the temperature and the flown speed. Therefore, no definite conclusions about the fuel consumption can be made, although a significant reduction is certain.