Most of us have been taught that the propeller slipstream wraps around the fuselage, strikes the tail on the left side, producing a left-turning force. This has always been presented as dogma without physical evidence. In this article, we present both the physics and physical evidence that the slipstream does not rotate. At the end of the article, we will address some of the misconceptions that have supported this dogma. We ask you to read this section with an open mind.
In this article, we will demonstrate:
• A rotating slipstream is a clear violation on Newton’s first and third laws.
• No angular momentum is transferred from the propeller to the slipstream.
• A left-hand turning propeller produces a non-rotating, right-hand twisted helical slipstream going straight back.
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 propeller produces. But only three real left-turning properties are created by the propeller: torque (actually a rolling force), 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.
Angular Momentum
Angular momentum is the rotational equivalent of linear momentum, describing an object’s tendency to keep spinning or orbiting. Newton’s First Law of Motion states, in part, that an object in motion will continue in a straight line unless acted upon by an external force. Let’s consider an example of angular momentum and the forces involved: the Moon orbiting the Earth. Angular momentum is rotational momentum, and the Earth-Moon system is clearly rotating. The inward force that curves the Moon’s orbit, keeping it from flying out into space, is the gravitational force of the Earth. Newton’s Third Law of Motion states that if object A puts a force on object B, then object B puts an equal and opposite force on object A. So, for every force there is an equal and opposite force. In this case, it is the Moon’s pull on the Earth. This gives us the tides. A second example is when one swings a rock on a string. The string puts a centering force on the rock, and the rock puts an equal and opposite force on the string, which one feels in their hand. Refer to Figure 2.
The concept of a spiraling slipstream is an exact analog to the rock on the string, but nature forgot the string! Without the string, there are no forces, so like the Moon without gravity, the air must move in a straight line. There is another problem; we need two objects. Object “A” is the air, but what is object “B”? Certainly not the fuselage. Thus, the idea of a rotating slipstream is short two forces and one object.
Figure 3 shows the propeller slipstreams of a 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 propeller slipstream of a Corsair. The condensation from the propeller tips is analogous to the wingtip vortices.
Note that the condensation helices of the two airplanes are uniform in diameter as they go back. Later, we will show that the condensation helix is just the outer edge of the propeller’s slipstream.
There can be confusion in that Figures 3 and 4 do show helices (not spirals) from the propellers. The air accelerated by the propellers is making a helix, but the helices are not rotating. The helix is due to the air being pulsed from the propeller. To help visualize this, consider Figure 5, which shows a left-hand turning propeller from above. When the propeller is at 3 o’clock (T1), it accelerates the air back. Later (T2), when the propeller is at 12 o’clock, it accelerates the air back, and the accelerated air from 3 o’clock is farther back. And when the propeller is at 9 o’clock, the air from 12 o’clock and 3 o’clock has each gone farther. It is easy to see that a rotating propeller produces a slipstream that is a non-rotating spiral. Note that the left-hand rotating propeller produces a right-hand spiral. This will be discussed in greater detail below.
Those who object to what has been presented so far claim that the angular momentum of the propeller is transferred to the slipstream. The Wright brothers developed the modern propeller after realizing that a propeller is a rotating wing. Wings, helicopter rotors, ceiling fans, and propellers all accelerate air perpendicular to the direction of motion. Since the physics of propellers and wings are the same, we will discuss the more familiar of the two: the wing.
A wing in straight and level flight accelerates air straight down; a lot of air. Figure 6 shows the airflow over a wing. By Newton’s second law, even a 1-ton airplane accelerates a few tons of air per second to produce the necessary lift. Clearly, that much air cannot have a physical connection with the wing. In fact, other than the thin stream of air drawn over the wing by the low pressure, the only air that touches the wing is due to impact with the air. This produces the parasite drag of the wing.
The same is true with a propeller. The tons of air accelerated to produce thrust do not physically interact with the propeller except forthe horizontal impact of the blade with t he air. This causes the parasite drag. The induced drag is created by the wing producing reduced pressure on top of the wing. The slipstream goes straight back without added angular momentum.
Some argue that since the propeller has angular momentum and accelerates the air, the air must have angular momentum due to the conservation of angular momentum. This is wrong. No angular momentum is transferred to the air. Yes, the propeller does have angular momentum, which is held constant. The crankshaft applies torque to the propeller. The propeller applies an equal and opposite force to the crankshaft according to Newton’s Third Law of Motion. Therefore, angular momentum is conserved. The force applied by the propeller is the result of parasite and induced drag. The propeller is doing a lot of work accelerating the slipstream. Thus, there is a substantial torque on the propeller shaft. This is compensated for by keeping the wings level or by the tail rotor of a helicopter. Counter-rotating propellers or helicopter rotors do no have a net torque since the propellers or rotors compensate for each other. The confusion that the propeller adds angular momentum to the slipstream goes away with counter-rotating propellers.
The propeller’s slipstream is the air accelerated by the propeller to produce forward thrust. It is a straightforward phenomenon. Air is accelerated essentially perpendicular to the blade’s rotation. The actual line will be a few degrees off perpendicular (approximately 4-7) and is equal to the effective angle of attack (eAoA) of the propeller or rotor, producing a forward thrust. The same phenomenon is demonstrated by a helicopter hovering over the water in Figure 7. Note that the helicopter’s blades accelerate the air straight down, and then the air spreads out radially without rotation. If a propeller caused a rotation slipstream, imagine how powerful a helicopter’s rotor slipstream rotation would be. Watch the heavy-lift Erickson Skycrane video below. 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.
As an additional verification that the slipstream does not rotate around the fuselage as depicted in many handbooks and courses, refer to Figures 8 and 9. Both airplanes are producing a smoke trail that travels straight back along the bottom of the airplane. If the spiraling slipstream myth were true, the smoke trail would be dragged and wrapped around the fuselage and empennage.
Another argument against a rotating slipstream is that the claimed direction of rotation is always in the opposite direction. Look at Figure 1. The left-hand turning propeller is producing a left-hand rotating spiral. That is the way it would be if the propeller were transferring angular momentum to the slipstream. Now look at Figure 5. It shows that a left-hand rotating propeller produces a slipstream that is a right-hand spiral. This is also shown in Figure 10, which is a simulation of a right-hand spiral slipstream from a left-hand rotating propeller. The coup de gras is Figure 11, which shows a Corsair taking off. It clearly has a left-hand turning propeller producing a right-hand spiral.
One point should be noted. If angular momentum were added to the slipstream, and since there are no confining forces, the slipstream would rapidly expand. Any energy expended, not straight back, is wasted energy and would be seen as an inefficiency of the propeller. The peak efficiency of an airplane’s propeller can approach 90%. This does not leave much for angular momentum losses. In addition, Figure 1 shows the spiraling slipstream as a neat ribbon rotating around the airplane, striking the tail. In fact, the fictitious slipstream would be generated from all angles of the propeller rotation. So, there wouldn’t be a nice ribbon but a cylinder striking the wings and the tail on the bottom and sides.
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!
One important aspect of the spiraling slipstream myth is that, despite people examining 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 to that of the propeller’s rotation. It is a slipstream of a propeller rotating in the opposite direction. Figure 9 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!
Common (and not so common!) misunderstandings of the propeller slipstream physics.
- Argument: the propeller “drags” the air in its direction of motion, creating a spiraling slipstream. Response: Even the Wright brothers knew that a propeller is a rotating wing. So, if a propeller dragged the air in the direction of motion, so would a wing. That is not how a wing works. It certainly would mean flight would be high-drag and very inefficient because the propeller’s energy would be directed outward at an angle, vs forward. Aircraft designers would need much more powerful engines.
Argument: the propeller slipstream spirals due to the conservation of momentum. Response: The angular momentum of the propeller is constant, and no significant angular momentum is transferred to the slipstream. The engine has to overcome the propeller’s parasite drag and induced drag. The induced drag is the work done to accelerate the air back. Both drags are in the plane of the propeller and do not add angular momentum to the slipstream. In classical propeller disk theory, there is no rotation in the slipstream.
Argument: if a torque is applied to the propeller, the air leaving the propeller must possess an angular momentum that corresponds to the applied torque, i.e., it must be rotating in the same direction as the propeller. Response: The air leaving the propeller does not have angular momentum because there are two forces that the propeller must overcome: parasite and induced drag. Parasite drag is the resistance of any object moving through the air. Induced drag is the force associated with the acceleration of the air. Neither type of drag transfers angular momentum to the air. Parasite drag is in the plane of rotation. Induced drag is essentially perpendicular to the plane of rotation (it actually occurs at the propeller’s effective angle of attach – eAoA – which is only a few degrees).
- Argument: The left-turning yaw must exist because airplanes turn left. Response. If a rotating slipstream existed, the mythical rotation would be intercepted by the wing and disrupt the rotation. To accept the rotating slipstream hypothesis, one must accept all of the effects that would be related to it. The hypothesis is that a rotating slipstream contacts the left side of the vertical stabilizer, thereby causing a left-turning force (tendency). A rotating slipstream, however, when contacting the vertical stabilizer, would result in a right rolling motion because the contact is offset from the aircraft’s longitudinal axis. Additionally, a rotating slipstream would contact the left wings with an upward motion and the right wing with a downward motion. This would cause another right rolling motion. Added to that would be the increased Angle of Attack (AoA) of the left wing. and the decreased AoA of the right wing, further enhancing the right tolling motion. Finally, the spiraling slipstream would contact the horizontal stabilizer, causing a right rolling motion. Taken in aggregate, these right rolling forces would be quite noticeable. However, no one has ever indicated that a right rolling motion exists. If a rotating slipstream causes no right rolling motion, does it truly create a left-turning tendency? How can it be that it causes one without the other?
- Argument: The propeller drags the air into a spiral (yep, another dragging misunderstanding). Response: There is no dragging of the air because there is no connection, i.e., no force, between the propeller and the air accelerated through the propeller disc that could “drag” the air. As already noted, no connection yields the airflow flowing in a straight line. However, for the sake of argument, let’s say that this occurs and the propeller continues to affect the accelerated air after the air is accelerated through the propeller disc. How far aft of the propeller will this effect occur? 1 cm? 10 cm? 100cm/1 meter? 10 meters? 100 meters? This is important because once the propeller is no longer affecting the air, the air will continue in a straight line by Newton’s 1st Law of Motion. For the hypothetical/mythical spiraling slipstream to cause a left yawing of an airplane, the propeller’s effect must be a significant distance from the propeller disc, especially for larger single-engine aircraft. A Pilatus PC-12’s vertical stabilizer is about 18 meters (54’) aft of the propeller. The result is that the propeller would be very inefficient because a lot of power would be needed to continue rotating a large cylinder of air (propeller diameter wide and length of the distance from the propeller to the vertical stabilizer) aft of the propeller disc, plus doing the necessary work of accelerating air through the propeller disc to produce thrust. Another propeller inefficiency of rotating the spiraling slipstream is found in Newton’s Third Law of Motion. If the propeller is rotating the air, there is an equal and opposite force on the propeller. The result is that the propeller’s thrust would be angled outward rather than forward.