OK the new engine modeling for X-Plane 11 is great, but what good is an engine to us pilots without a propeller?

X-Plane has historically done an excellent job of estimating the THRUST of propellers, typically to within just a few percent… but what about the SPIRALING SLIPSTREAM? This has been an area where X-Plane has been much weaker… I just don’t see any good solid references for determining the spiraling slipstream angles for propellers…

and it’s a real shame because the spiraling slip-stream hitting the vertical stab is so responsible for the left-turning tendency in single-engine props.

BUT, can we do better? How would we estimate the slipstream angle, exactly?

Well, as it turns out, there is a pretty darn cool way to do it, which is going into X-Plane 11 Beta-4: A spinning prop is just a spinning pair or trio or quartet of wings (as X-Plane has long understood) and those wings have LIFT and DRAG.

The LIFT from the propeller blade is referred to as THRUST. The DRAG on the propeller blade is what opposes rotation and makes them so darn hard to TURN.

Now, X-Plane has already PRECISELY determined the lift and drag on each little bit of each propeller blade, in the 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock positions to get all the right p-factor and other effects. Now imagine one of these propeller blades spinning for a second putting out both LIFT and DRAG. What happens to the air in this case?

Clearly, the LIFT acts as THRUST, pulling the plane forwards (and kicking the AIR AFT!)

Clearly, the DRAG opposes ROTATION, retarding the rotation of the propeller… but in so doing it is guaranteed to DRAG THE AIR ALONG WITH IT DUE TO FRICTION BETWEEN THE PROP AND THE AIR!

So, how MUCH does the prop drag the air with it as it rotates, forming this spiral stream of wash behind the prop?

Here is my theory: The if the PROPWASH is proportional to the LIFT from each propeller blade, then the SPIRAL is proportional to the DRAG from each propeller blade!

Put another way, if the propeller is putting out TEN pounds of lift and ONE pound of drag,

then the equal and opposite reactions from the air will be in equal proportion: TEN knots of propwash for every ONE knot of spiral!

You see the ratio here? If LIFT pushes the AIR AFT (conservation of momentum! every reaction has an equal and opposite reaction!), then DRAG drags the air along behind the prop in a spiral pattern… and the ratio of drag spiral speed to the propwash is the same as the same ratio of drag to lift of the prop (whew!).

Equal and opposite reaction happens just the same for drag as for lift!

Another example: A small bit of a propeller blade puts out 20 knots of lift (thrust) and 2 pounds of drag (opposing rotation). That’s 20 pounds of lift for every 2 pounds of drag.

Now, due to conservation of momentum, we might prove that this prop has 50 knots of propwash. So what will the side component of propwash, or the spiral sideways motion of the air be? 5 knots, because the drag is 2/20ths, or 1/10th, of the lift, so the sideways drag on the air 2/20ths, or 1/10th, of the propwash!

If I am understanding conservation of momentum here, then I think that this is the key to understand spiral slipstream from the prop: The more drag on the prop (the less efficient it is) the more the spiraling slip-stream! The less drag on the prop (the more efficient it is) the less the spiraling slip-stream! Specifically the ratio of spiral to propwash is the ratio of drag to lift on each bit of the prop, since that is simply the direction and magnitude of the forces on the prop, and the displacement-rate of air that MUST exist to cause those forces! (EQUAL and opposite reaction!)

So, in X-Plane, we of course break the prop down into tiny little pieces and add up the effects from all of them to get a weighted average of the spiraling-slipstream speed for the entire prop, and scale that rotational speed from zero right at the axis of the prop hub to maximum out at the prop tip, and bang, we got a spiral slipstream with math that I believe proves that we are making a very good approximation. (And, doing the math in this new way results in propeller spiral slipstream that runs about 45% higher than the previous model… a positive indication since the spiral slip-stream was previously under-represented!)

So, now we have better propeller spiraling slip-stream, with the needed rudder effects, and it feels GREAT to fly.