Aviation Photo #0341524 Bombardier DHC-8-401 Q400 - Horizon Air

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Can anyone tell me what those weird swirling vortices off of the props is caused by?
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    Royal S King
    Horizon Air
    Bombardier DHC-8-401 Q400
    Bombardier DHC-8 Dash 8
    Bombardier DHC-8 Dash 8
    Seattle / Tacoma - International
    November 2002
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Photo Added: April 19, 2003

Comments (1)

11 years ago
The vortices coming off the props are caused by the same thing that causes wingtip vortices: A pressure differential where greater air pressure exists on the rear-facing side of the prop and less air pressure exists at the forward facing side of the prop blade.

It should at this point be noted that because air is accelerated around both sides of the propeller blade, there is a drop in pressure on both sides, but because there is a greater acceleration on the forward facing side, the pressure drop on that side is greater and therefore the propellor blade has a tendancy to move into this partial vaccuum (bernoulli theory). One could also say that because there is greater air pressure on the rearward facing side of the blade, that the blade is forced into the vaccum by the air itself (newtonian theory). Either way, the blade moves forward, and so does the plane.

But a byproduct of this happening is the vortice. Nature abhors a vaccuum, and to fill the vaccuum, air from the backside of the prop will travel around the outside of the tip of the prop and rush to fill the empty partial vaccuum on the forward-facing edge of the prop, and what results is a spinning vortice left behind by the tip of the prop.

The reason this vortice is visible is quite simple. As the velocity of the air inside the vortice increases, the pressure of the air decreases, and with that the temperature also decreases, occaisionally there is enough moisture in the air that the temperature will decrease below the ambient dewpoint, and the water vapor will condense into liquid droplets in the center of the vortice, causing it to be visable.

Does that answer your question Royal?

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