Blade Jet

Why does the engine of a jet aircraft appear to spin back and forth at startup?

I don’t know much about the anatomy of a jet engine, funny because I took private pilot flight school. But my question is – why, for lets say a 737 – does the engine fan blades (?) appear to move in each direction before finally spinning in full rotation? Also, what is this white spinny type paint in the middle of the fan?

It is an optical illusion. Your eye transmits a series of still images to your brain at a rate between 24 and 60 times per second. Everyone’s eye is slightly different, so I couldn’t say what your eyes’ “refresh rate” is.

Essentially what happens is that as the rotational speed of the engine increases, it will eventually reach a point where the fan blades of the engine are rotating past the same point at the exact same time as your eye refreshes the images sent to your brain. If that exact speed were maintained, the engine would appear to be stopped. However, since the engine continues to accelerate it’s rotational speed, the fan blades appear to start rotating in the opposite direction for a while, then begin rotating forwards again. You can sometimes see the same thing on car wheel hubs where it looks like the hub is rotating backwards (not talking about those silly spinners).

The reason there is paint on the central hub of the engine (sometimes a swirl, sometimes just a straight line) is to enable the ground crew to instantly see if the engine is operating. If that were missing, a quick glance at the engine may not be sufficient to see if the fan is spinning. However, if that mark is just a blur, the ground crew immediately knows to stay well clear of the front of the engine. With hearing protection in place and aircraft operating all around you, it can sometimes be difficult to tell by sound alone if the engines on the aircraft you are working on are running.

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