Matt's Blog

Anniversary

Well, its been over a year since my first post, but I didn’t notice. Never mind, there’s always next year

November 6th, 2006

Misleading

The BBC have posted yet another “silent aircraft” article with some improved pictures and buzzwords from the one they posted more than two years ago. I wonder why the guys at Boeing and Airbus haven’t tried this yet? Lets take a more detailed look at the claims.

As turbulent airflow, generated by irregular surfaces, causes noise, the designers tried to make the airframe as smooth as possible.

Wow! I guess they don’t teach that in elementary fluid mechanics… oh, hang on, they do.

The aerofoil shape of the body means that it also contributes to the aircraft’s lift, meaning it can make a slower approach on landing, again reducing noise.

On the other hand, the aerofoil shape of the body means it has a higher stalling speed. Its well-known that a delta-wing has trouble maintaining attached flow at high angles of attack (i.e: low speeds), whereas a long, thin, non-blended wing can maintain attached flow much longer. Gliders are a good example: they go really slowly and have long, thin wings, not blended delta-wings. Also, separated flow creates a lot more noise than turbulent flow, particularly if the re-attachment point is unsteady.

The engines of the SAX-40 are embedded within the blended wing design with the air intakes on top. This means that the upper surface of the aircraft shields people on the ground from engine noise.

It also means that when flow over the upper surface detaches, the engines are quite likely to re-breathe their own exhaust, causing them to flame out. Also, embedding the engines within the body means that vibrations are transmitted more easily, making the cabin more noisy. Engines within the body are much harder to inspect and remove for maintenance. Finally, the proximity of the engine to passengers means the airframe will need more shielding in case of flame out, rotor burst, birdstrike, etc… causing the engine to explode.

To reduce [landing gear noise], the SAX-40 would have fairings to cover the wheels and braking systems, creating as smooth a flow of air as possible. This could reduce the noise from the landing gear by up to 7dB.

Genius! I wonder why no-one else has thought of that? Oh, yeah, they have…

To minimise [trailing edge noise], the SAX-40 would have “trailing edge brushes”, a series of long, thin protrusions off the back of the wing.

Well, the energy has to be dissipated somehow. Depending on the placement of the “brushes” (i.e: regular or irregular) I suspect the noise will simply be shifted into lower or higher frequencies.

In summary: this sounds like some academic is getting a lot of free publicity and therefore bigger grants, while not acheiving very much.

November 6th, 2006