All flight textbooks and many sites on the internet describe the propeller slipstream (also sometimes referred to as prop wash) 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 propellor produces. But only three real left-turning properties are created by the propellor: torque, p-factor, and gyroscopic precession. We will show that the propeller slipstream is accelerated straight back from the propeller disc. Additionally, the propeller tips scribe a helical shape (not a spiral!) 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 those pesky Newton’s Laws of Motion. Newton’s first law states: 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 (not a spiral!), 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, i.e., it no longer acts on the air it accelerated. Next, we would need two objects. However, we only have one object: the air. The bottom line is 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 C-130 Hercules, delineated by the condensation from the propeller tips. Notice the condensation helixes 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 Corsair.
To aid 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 though people have looked at the depiction for decades, no one seems to have noticed that the supposed rotation is going in the wrong direction. Refer to Figure 1. The sense of rotation of the slipstream is opposite that of the propeller’s rotation. It is a slipstream of a propeller rotating in the opposite direction. 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. If the slipstream rotated, which it doesn’t, it would cause a right – not a left – turning force!
If a propeller caused a rotation slipstream, imagine how powerful a helicopter’s rotor slipstream rotation would be. Watch the Erickson Skycrane video. Notice the air is accelerated perpendicular to the rotor’s plane of rotation and when the accelerated air contacts the water, it spreads out radially, i.e., there is no rotation.
The Takeaway:
- Air is accelerated straight back from the propellor.
- The slipstream is a non-rotating helical shape because the propeller only covers part of the area at a time.
- The propeller tips scribe a helical shape but there is no rotation of the accelerated air; therefore, no yaw force is created.
- The widely believed and taught rotating slipstream is a myth. The perceived but non-existent rotation is an illusion that is easy to accept because the propeller is rotating. But in the end, physics wins the prize!