Mesae and stek79,

As dick Hanson pointed out that there is not much, if any, thrust generated near the hub. It is air moving over the cowl louvers that is “sucking” air into the intake. We are back to the Bernoulli Principal again.

8178's experiment with the fan points to a sort of “hollow tube” effect, but dick Hanson’s pusher props indicated a tight a tight spiral downstream. This got me to wondering. If you look at the spinning prop near the hub, it is acting something like a centrifugal pump. It is rotating and flinging incoming air coming outwards (centrifugal force is not really an accurate term, but lets not get into that, yet). Farther out, the prop is acting like a flying wing with an airfoil and creating “lift,” which is moving air back, as well as rotating it some because of drag. The tips of the propellers are moving at high velocity.

Since props have an airfoil like a wing, there is a pressure differential across the surfaces. The result is, for lack of a better term, a spiraling vortex coming off the prop tips, similar to the way wing tips produce a spiraling vortex. This is more acceleration of the air, but this air is not accelerated around the propeller’s axis of rotation as much as into high velocity eddies that have an axis of rotation perpendicular to the prop’s axis and parallel to the arch of the prop.

My point is that not only is the air accelerated because it is rotating and moving back, but it is also getting whipped up into a froth of high velocity eddies. In other words, the prop is producing all kinds of acceleration in the air The air behind the prop is a virtual “bubble” of high velocity, low pressure air, as compared to the surrounding air. This bubble would be somewhat larger than the prop diameter, and it would exist in front of and behind the prop. The question is, what happens to it?

Most “bubbles” tend to pop, but this is a low pressure “bubble.” The high velocity eddies are going to slow down quickly because of friction. Somewhere behind the propeller, this mass of low pressure air is going to collapse. When that happens, if it has even a small rotation left in it, it is going to gain angular velocity (conservation of angular momentum). It is going to act like water going down the drain in a bath tub. The rotation of the earth is enough to cause a spiraling vortex of water in a bathtub drain. It should not take much rotation in the air to lead to a tight spin as the diameter of the low pressure mass decreases to nothing.

I think it is reasonable to expect that the air ought to tighten up and spin somewhere behind the prop. That is the hypothesis, anyway.