Physics wins

All flight textbooks and many sites on the internet describe the propeller slipstream as a rotating spiral—a corkscrew—that wraps and rotates around the airplane, thus putting a yawing force on the vertical stabilizer, as shown in Figure 1. This mythical yawing is considered one of the four left-turning tendencies a propeller produces. But only three real left-turning properties are created by the propeller: torque, P-factor, and gyroscopic precession. The propeller slipstream is accelerated straight back from the propeller disc. Additionally, the propeller tips create a helical shape (not a spiral) in the air, and the slipstream does not rotate. (A helix has a constant radius, whereas a spiral’s radius increases or decreases.) 

The spurious idea of a rotating spiral runs up against Newton’s pesky laws of motion. Newton’s first law states that an object in motion will remain in straight-line motion unless acted on by an external force. The assumed motion of the air is not in a straight line. So, to rotate this helix, a force is necessary to drive the rotation plus an additional force at every point of the correct strength and always pointing toward the center of rotation. See Figure 2. It is hard to conceive a power source for this continually operating force. If that’s not bad enough, Newton’s third law states that if object A puts a force on object B, object B must put an equal and opposite force on object A. Referring to Figure 2, to rotate the slipstream we need a second force to match the magically bending force. Remember, once the propeller accelerates the air, the propeller keeps going forward—it no longer acts on the air it accelerated. Next, we would need two objects. However, we only have one object: the air. The myth runs up against the buzzsaw of basic physics. There is only one object and no forces.

Figure 3 shows the slipstreams of a Lockheed C–130 Hercules, delineated by the condensation from the propeller tips. Notice the condensation helices are continuous across the top of the wings. This is clear evidence that the slipstream is moving straight back and not rotating. Otherwise, they would be disrupted by the wings. Figure 4 shows a similar behavior of the slipstream of a Vought Corsair.

To aid in comprehension, consider a rotating machine gun as shown crudely in Figure 5. It should be clear that the bullets from any orientation of the gun go straight back, and the totality of the bullets form a helical pattern. So, it is with the propeller slipstream.

The propeller’s slipstream is the air accelerated by the propeller to produce forward thrust. It is a straightforward phenomenon. Air is accelerated straight back, perpendicular to the rotation of the blades, producing a forward thrust. The same phenomenon is demonstrated by a helicopter hovering over the water in Figure 6. Note that the helicopter’s blades accelerate the air straight down and then the air spreads out radially without rotation.

One added comical aspect of the spiraling slipstream myth is that, although people have looked at the depiction for decades, no one seems to have noticed that the supposed rotation is going in the wrong direction. The sense of rotation of the slipstream is the same as the propeller’s rotation. Figure 7 shows the cavitation of a boat’s propeller. Looking from left to right, it is clear that the propeller is rotating counterclockwise and the sense of rotation of the cavitation is clockwise.

The takeaway:

1. Air is accelerated straight back from the propeller.

2. The slipstream is a nonrotating helical shape.

3. There is no rotation of the slipstream. Therefore, there is no yaw force created.

4. The widely believed and taught rotating slipstream is a myth. The perceived but nonexistent rotation is an illusion that is easy to accept because the propeller is rotating. But in the end, physics wins. 

David Anderson is a physicist, private pilot, and co-author of Understanding Flight.

understandingflight.com