Discharge characteristics of drilled throttle orifices in fiber-reinforced plastics for an HLFC system


A series of test campaigns at DLR’s large flow meter (LFM) has been conducted to investigate the discharge characteristics of cylindrical throttle orifices, drilled in fiber-reinforced plastics, with respect to their diameter and manufacturing imperfections. In hybrid laminar flow control (HLFC) applications following the simplified so-called ALTTA concept, those throttle holes interfacing a common plenum to multiple chambers play an important role in regulating the local amount of suction through the porous wing surface. The work performed aims at the further development and industrialized application of HLFC in an aircraft environment. This question is tackled by the industry-oriented drilling of throttle holes into coupon-sized flat samples. Drilling parameters allowing a stable and reproducible process were established through optical inspection. The major experimental findings are discussed in this article alongside some general manufacturing recommendations for throttle holes in HLFC applications.

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A :

Orifice area (mm2)

C :

Discharge coefficient (–)

D :

Orifice diameter (mm)

n :

Drilling speed (rpm)

p :

Static pressure (Pa)

t :

Laminate thickness (mm)

v f :

Drill feed (mm/min)

α :

Conicity angle (°)

ρ :

Density (kg/m3)

γ :

Ratio of specific heats (–)


Fiber volume content (%)


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This project has received funding from the Clean Sky 2 joint undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement no. CS2‐LPA-GAM‐2014‐2015‐01.

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Kilian, T., Forßbohm, T., Seitz, A. et al. Discharge characteristics of drilled throttle orifices in fiber-reinforced plastics for an HLFC system. CEAS Aeronaut J 11, 431–439 (2020). https://doi.org/10.1007/s13272-019-00416-y

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  • Hybrid laminar flow control
  • Experimental aerodynamics
  • Manufacturing
  • Carbon fiber-reinforced plastics (CFRP)