What is Spiral Propwash?

Spiral propwash is an aerodynamic effect that causes most single engine prop planes to yaw when they are flown at high power and low speed (takeoff and climbout, for example.) The prop doesn't blow the air straight back, but gives it a bit of a twirl as well.
It all derives from Newton's third law. You know about basic propwash - the prop acts as a fan and blows the air backwards, so that the relative airspeed behind the prop is greater than the plane's actual speed through the air. It's easy to compute this - the propwash delta is simply the prop thrust divided by the mass flow rate through the prop. (Just another form of F = M*A.)

But... the prop is also exerting torque on the air, to be precise, the engine torque. This force also has to be accounted for by a corresponding acceleration of the air passing through the prop. So the cylinder of air flowing through the prop doesn't go straight back, but rotates as well. Again, the rotation speed comes straight from Newton's third law, and is torque divided by mass flow rate and prop diameter. An informal way of looking at it is that the prop is dragging the air around with its rotation.

There's been some debate over whether there's really a "rotating cylinder" of air behind the prop, or whether there are just little ribbons of moving air behind the prop blades. It terms of working out the overall effect on the plane, the distinction is unimportant - Newton's third law says there is a certain average spiral component to the airflow. The easiest way to model it mathematically is as a rotating cylinder, with a considerable degree of turbulence added by the prop blades. The spiral component does eventually dissipate into the surrounding air, but keep in mind that for a plane in flight, it takes less than a second for the air to travel from the prop to the tail, so the rotating cylinder model holds up pretty well.

Spiral propwash has a significant effect on single engine planes, notably during takeoff and climbout, where you're fying at high power and low airspeed. The reason is that the vertical stabilizer (in most single engine configurations) sits in the upper half of the propwash. So with your typical CW prop rotation, the spiral propwash component hits the vstab from the left, pushing it to the right, causing the plane to yaw left. This is the primary reason you need right rudder during takeoff and climbout. (There is also P-factor, caused by the prop shaft not being lined up with the airflow, which works in the same direction. However, with most light planes, spiral propwash is the dominant factor.)

X-Plane does a pretty decent job of modelling this as long as you have enough elements in the wing and stabilizer areas that are exposed to propwash. Turn on the graphical flight model display (hit the "/" key a couple times) and you'll see different length lift vectors in the wing surfaces in propwash.
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