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.

25 comments on “X-Plane 11 Propeller Modeling

  1. Great thinking, Austin! Only one possible caveat that I can think of…
    If you are calculating the spiral slipstream forces at the point that they are generated, that is at the propeller location on the airplane, won’t some of that force be lost by the time it affects the vertical stabilizer? The loss would be due to air friction as a function of the distance between the prop and the empennage, and also the current speed of the airplane.
    I’m certainly not an aeronautics engineer – just a thought from a laypilot…
    I am loving XP 11 Beta! Thanks!

    1. I was thinking something like that, but not loss as a function of distance between the prop and the empennage but because of the prop proximity to the airplane nose (the Cherokee comes to mind).

  2. Interesting to see the way that programming efficiencies are blended together to effect a reasonable approximation. One additional trick, of course, would be to let X-Plane know whether or not there’s a vertical fin/rudder in the slipstream. How this is handled for certain multi-engine aircraft will add sauce to the goose. A Beech Baron would be handled differently than a Lockheed P-38, for example, and no doubt fin/rudder area would be a component as well. Quite the mental pretzel, to be sure. Thanks for sharing!

  3. Another possible consideration with the above, is the mounting of the engine. I thought I read some where that some engines or even the props are mounted to help combat the slipstream effect. Wouldn’t this with the above be a “minor” but important detail to account for? could it be accounted for?

  4. Awesome stuff, can’t wait to put it to the test.
    Thank you Austin for getting into this.

  5. Thank you so much Austin to you and your team, for all the work!

    Is it, or will it be possible, when performing 360 turns in a prop plane, to fly through your one prop wash (when executing the turn properly) and feel it (at least visually, if not using force feedback device (some sort of wobble))?

  6. Austin, I am glad you have been posting your thoughts here as well. I realize it takes time to write a post but it is much appreciated and I hope it continues during the XP11 run!

    I take it these changes will make it into beta 4?

    1. Yep, 4th paragraph down:

      “Well, as it turns out, there is a pretty darn cool way to do it, which is going into X-Plane 11 Beta-4…”

  7. The prospect of increased P-factor is interesting if worrying. I found P-factor to be greatly exaggerated vs the real world, particularly around the roll axis..

    1. Maybe you need to recalibrate your joystick. I haven’t noticed any P-factor or spiraling slip stream in the current beta. Maybe you are thinking about engine torque ? Which seems to be spot on.

  8. Very interested in this and need to understand it all better……especially how things would be affected by a motor mounted with right thrust built in to counter torque factors/slip stream effects, so on.

    I fly competition with model airplanes so motor mounting, left/right thrust, up/down thrust, prop types and efficiency, slip stream, and all of the mixing done in the radio to account for all of these is very important and exciting.

  9. >>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!<<

    It's great to see someone trying to work things out on his own, from first principles, which seems the way you like to do things. And your theory might even be correct. However, just as an efficiency measure, as a way of getting more good stuff into each release, wouldn't it be better to simply consult with an aeronautical engineer? (Or any engineer–also thinking here of the abysmal, sliding-across-runways-on-takeoff problem, neat theory notwithstanding.)

    What you're having fun (and that's great) discovering for yourself, almost certainly was known decades ago.

    Just a practical suggestion, and not a call for you to tamp down your enthusiasm.

    1. Oh man. A marvellous stuff like X-plane only could be created by a sick person. A person with a disease called ‘PASSION for engineering’.

      It’s a very rare disease…and in a high degree of confidence , especially if patients suffer this disease since a youth age, they are engineers.
      There are no treatment available.

      Flight Model is amazing, and this upgrades are marvellous, but in my opinion, there are secondary at this moment because only a few simmers will see it. Base customers have other priorities and other demands.
      That’s a common symptom of the disease, focus in a very tiny engineering detail and forgot other areas not related with engineering.
      I suffer it too, but in a minor degree than Austin who has very severe symptoms.

  10. As long as you model my Ercoupe and it’s twin rudders out of the prop-stream, I’m happy. 🙂

  11. Very interesting.

    I have to read carefully and put my neurons to work while doing so, to correctly understand what you’re considering to implement – but it looks promising as I and others ( Murmur, Andy Goldstein et al ) have long “complained” about less yaw due to prop effects and specially the asymmetric slipstream hitting different areas of the aircraft ( wings, tail, fuselage… ) then there should be, specially before the “torque bug” was finally solved thx to Murmur’s finding… Looking forward to test it in Beta 4. Thx for sharing your thoughts Austin!

  12. Very thankful for your interesting posts here, Austin! Thanks a bunch for providing us with the best flight sim!

  13. Not to muddy up the water here, but I would expect different effects depending on the amount of “slip” the propeller(s) have with respect to the airstream and the effect of the residual propwash from adjacent propellers (2,3,4,5,6-blade). There are some hairy effects that engineers must encounter when deciding on how many blades, blade pitch and such. Consider the design changes over time of the C-130 props. airspeed….

    Most of that is likely to be inconsequential with X-Plane modeling, but it’s a real world headache.

  14. Updated to beta 4 and tried c172 in a clear calm (no wind at all) weather.
    Unfortunately the plane seems lost the tendency of turning left or rolling opposite to propeller turning direction quite dramatically than before. Especially couldn’t see the rolling tendency during liftoff. Only saw the plane gradually drift left to the runway
    Centreline without any roll.

    1. Correction: it was beta 5 and second test flight seems to show the effect though I don’t know how strong it is in reality. My thanks to LR people for nice hard working!

  15. This is my thought based on experiences as a ship propeller designer.

    The swirl of the air is clearly the source of efficiency loss by energy waste but not a drag nor proportional to drag. It appears rather like a sort of constant deducted from the total energy which is wasted in accelerating the air flow.

    With regard to the so called spiraling slipstream, there are many illustrations explaining vapour trails attached to the propeller blade tips as a sign of spiraling slipstream.

    But, indeed they are just trace of propeller tip vortices visualized by condensation of vapour in the air due to the low pressure created at the tip vortex core. The tip vortices are stationary but because the plane moves forward it looks like it travell downstream and that is why such trail looks as if it penetrates any interfering structure on its way. Of course, some mass of air rotates together with propeller blades but the spiraling flow does not go so far downstream (the viscosity of air is very low) and then the slipstream gets almost aligned to the fuselage. Youtube search by ‘propeller spiraling slipstream’ will bring several videos showing how the actual slipstream flows by tufts on the fuselage of an RC plane.

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