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Do the holes in splitter plates reduce the boundary layer, or do they serve a different function such as acoustic/vibration damping?

Many splitter plates have a series of holes drilled into the surface closer to the engine side of the intake. Suction is applied to these holes, further reducing the boundary layer.

This bit is from Wikipedia, it's marked Citation Needed, and I'm unable to corroborate it.

Taking the Eurofighter example:

enter image description here
(Source)

If it is true, does it only work at supersonic speeds? The way I see the channel above the intake is that it is diffuser shaped. So a reduction in pressure above the intake would only work at supersonic speeds, is that correct?

A similar system can be [barely] seen on the F/A-18E.

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Do the holes in splitter plates reduce the boundary layer?

Yes. Suction is applied to remove the slow-moving layer of air close to the surface, so the air entering the intake has as uniform a speed distribution over the intake cross section as possible. That this air is used for cooling before being dumped overboard is sensible, but if cooling were the single purpose for sucking it in, then better and more efficient designs exist.

The boundary layer which has built up on the forward fuselage is removed by the splitter plate itself. The perforation is only for removing the fresh boundary layer which has built up over the upstream length of the splitter plate.

On some spike intakes the designers even implemented boundary layer suction on the intake cone, ducting the flow through struts in the intake to the outside of the engine. Even this complex arrangement seems to be worth it because it improves the efficiency of the intake and allows to widen its operating limits. From this article of the Airspacemag web site:

The SR-71 inlet rids itself of boundary layer air by sucking it in through slots on the spike and passing it through ducts that exit the nacelle.

The airflow schematics in the picture below call this particular flow "centerbody bleed". Note how it is reversed at low speed in order to suck more air into the engine while it is used to dump the boundary layer air as soon as intake pressure rises above ambient pressure at higher flight speeds.

SR-71 engine airflow patterns

SR-71 engine airflow patterns (picture source)

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  • $\begingroup$ Do you know if the suctioning is active at low speeds (e.g. take off) and high AOA? The boundary layer shouldn't be that big in a convergent zone like the one in the Eurofighter. $\endgroup$
    – Gypaets
    Commented May 20, 2017 at 9:22
  • $\begingroup$ @Gypaets: I'm sure there is some scheduling over speed, thrust and angle of attack but don't know the details. $\endgroup$
    – Peter Kämpf
    Commented May 20, 2017 at 12:01
  • $\begingroup$ @PeterKämpf - That should explain the MiG-21 nose too, very nice. Regarding the EF example and the area above the plate, is my assumption true or false for working at only supersonic speeds? $\endgroup$
    – user14897
    Commented May 20, 2017 at 13:57
  • $\begingroup$ @ymb1: I am very sure that it also works at transsonic speed, but cannot prove it. $\endgroup$
    – Peter Kämpf
    Commented May 20, 2017 at 15:29
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The ones in the F/A-18E are for fuel cooling. From the flight manual:

A fuel/air heat exchanger is located above each engine inlet, near the leading edge. When the heat exchanger bypass valve is open, the heat exchanger uses inlet air to provide additional fuel cooling. Air is drawn from the inlet through several banks of small pin holes (bleed plates) and is exhausted through the spoiler opening on the upper surface of the LEX/fuselage.

I couldn't find sources, but this doesn't mean that they are not used for boundary layer reduction too.

Suctioning the boundary layer to improve the homogeneity of inlet air and to cool down fuel seems like a good way to kill two birds with one stone.

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  • $\begingroup$ Very nice find, thank you. $\endgroup$
    – user14897
    Commented May 20, 2017 at 13:58

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