Readers letters
There was an article on the “silent aircraft initiative” which raised considerable concerns. As someone who was involved for many years in aero-engine design, testing, certification and installation I was continually being astounded by highly intelligent specialists not being able to appreciate the real world. Aerodynamicists have pointed out the advantages of boundary layer control for many, many years. The potential fuel saving from this, combined with the environmental arguments about noise, logically suggests, to the uninitiated, engine intakes above the wing designed to swallow the boundary layer.
Unfortunately it is not that simple. Boundary layer air is particularly unsuitable for engines. Its turbulent flow, low energy characteristics dramatically affect engine efficiency and introduce high fatigue stresses into already highly stressed engine compressor components. Engines capable of working in this installation, if possible at all, would be heavier, of lower specific power and lower overall efficiency, with losses more than outweighing any gains achieved from reduced boundary layer.
Even if engine manufacturers manage to ameliorate these penalties somewhat, there is another problem. Designing the system to ensure that the engines will swallow the over-wing boundary layer means that they will also swallow anything which happens to be on the upper surface of the wing. Engine certification requires testing in extreme rainfall, but in evenly spread droplets. No engine will accept asymmetric rivers of water shedding from a very large wing surface. Neither will they be able to accept lumps of accreted snow and ice. Ingestion of this sort will potentially cause failure of all engines simultaneously. Attempts to protect the intakes by physical means would not only negate any boundary flow advantages but introduce additional icing hazards, disrupt the wing's lifting capacity and induce even worse distortion factors on engine intake flow.
The Iceland volcano demonstrated the disruption which can be caused by airborne contaminants in a small sector of the airline network. Imagine the problems if aircraft were regularly grounded because of forecast rain, snow or icing conditions. Over-wing engine installation is acceptable, and does reduce the noise below the wing but, for operating, economic and safety reasons, they have to be mounted well outside the boundary layer.
M P Newman, St Albans, Herts
Next letter: Send out an engineer please