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I've read that NASA's boundary layer suction system on the wings can reduce drag by removing the boundary layer through the suction holes. If this can reduce a lot of drag (hence improving fuel efficiency), then why can't the same idea be used in commercial aircraft?

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    $\begingroup$ What makes you think it can't be used? In aviation, as in many industries, many ideas are not implemented not because it can't be but because of costs, complexity, maintenance, weight...and other drawbacks. $\endgroup$ – Manu H Nov 20 '19 at 8:20
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Boundary layer suction systems have been considered for a very long time, since they provide significant advantages.

However, their main disadvantage, which has never been satisfactorily overcome, is their propensity to clog with dirt and biological matter. The maintenance costs to ensure the system is operational would outweigh the gains in aerodynamic efficiency.

Additionally, this is not the kind of system that could be left to its own devices (good if it works, fine if it doesn't); it would need to operate consistently because the critical angle of attack would depend on it.

Furthermore, this system would require power to operate the pumps, and it is not obvious that the efficiency gains would make up for the power requirements in all scenarios.

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Boeing did implement a passive hybrid laminar flow control system using boundary layer suction for the 787-9 on the horizontal and vertical tails. The passive flow control implementation appears to apply US Patent No. 7,866,609B2, which does not require a pump.

There are many practical issues that prevent a flow control device and/or method from being used:

  • performance degradation due insect contamination, roughness, steps and gaps compromising any drag benefit, e.g., Krüger-type flaps are typically used for insect protection, so flow control devices would need to integrate with such high lift devices,
  • any use of mechanical pumps increases weight, lifetime costs due to maintenance, and a reliability risk if the implementation substantially affects stability and control characteristics,
  • high energy requirements, e.g., active flow control actuators tend to consume a lot of power, so considering the whole point is to save energy, there should be a net gain.
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