Prop wash ruins the entire ground handling in MSFS

[robert young]

I'm sure some of you have noticed that ground steering in almost all MSFS aircraft, but especially the small aircraft, is a total disaster. I've been under the assumption that it is the friction model that over "grips" the nosewheel and results in ridiculously squirmy steering once you pass around 15 knots. This is despite trying to tune the small number of parameters linked to the ground steering. But it is not the steering at all! It is the PROP WASH. Like those old Volvo cars which kept their side lights on whatever you did to switch them off, Asobo's absurdly exaggerated prop wash makes runway take off steering a nightmare, and it is hard coded. You cannot switch it off. I've tried. Positive and negative values from 0.0001 up to 10000, + and -. Nothing happens. The prop wash is not editable in any practical sense.

Experiment: Just to check and identify exactly what is going on I applied a ZERO steering angle to the nosewheel of several of the smaller MSFS aircraft. This means that you cannot turn AT ALL. But wait, you can! As soon as you exceed around ten knots with full power, the rudder becomes a super-steering superman, flipping the entire aircraft 180 degrees if you apply full rudder. Someone at Asobo actually DESIGNED this deliberately. It wasn't an accident and it is not a bug. Someone actually CHOSE to apply this "feature".

It means that unless you opt for a rudder that is set to an angle of less than around 3 degrees max, and you set yaw authority almost to zero, you are going to squirm your way down the runway. Furthermore, there is no separation between what happens on the ground and what happens in the air. If you have a rudder that can achieve the mildest sideslip you are also going to have a rudder that is going to squirm its way down the take-off run. In LEGACY mode this does not happen. In legacy mode there are several variables that can adjust prop wash on roll, pitch and yaw. NOT SO in MSFS "modern" flight model. There are two variables, or three if you count the dubious " torque on roll". Both of them invite you to enter a value. But after entering a huge array of values, NOTHING makes the slightest difference.

I'm afraid this is yet another indicator of the fact that whoever designed the flight modelling in MSFS has absolutely no idea how aircraft fly. They might be clever with maths and equations, but it is clear they have never actually sat in an aeroplane and thought with any depth about how aircraft respond.

The reason I say this is not to be unkind or provocative, but already end users are complaining about a host of mods, freeware, payware and other addons, as though the authors of these additions are responsible for the flaws within. In most cases they are not. They are dealing with a system which claimed it was a revolution, or at least partly revolution in flight dynamics. It isn't. It is a CUT DOWN version of what was in FSX, but this time without enough user control and with important parameters removed or hidden, to absolutely no purpose. We don't need "1000" lift points. We need INTELLIGENT controls and parameters designed by an intelligent and aware person who understands how aircraft fly.

 

[jcomm]

I have to, unfortunately, totally agree with your view Rob :-/

 

[Lagaffe]

Hi Rob,

This is a very interesting and concluant analyse or the situation. I agreed totaly with you.

 

[jcomm]

There is, nonetheless, a pair of tuning parameters, but it's not very useful. It should act on propwash effect on rudder, and can be set between 0 and 1 in flight_dynamics.cfg.

"yaw_moment_yaw_propwash"
"yaw_moment_delta_rudder_propwash"

plus that "rudder_scaling_table" with p vs scaling coef.


Higher than 1.0 values can probably be used as well ( ? )

I like to see that bursts of power in a taildragger can be used to bring the tail alive, like in my FDM reference simulators, and IRL ( DCS and IL-2 which are light-years ahead of anything MSFS-derived, or even X-Plane which does a much better job in this area ).

 

[Delta558]

Well I've spent most of the day fiddling around with the various rudder / yaw settings on the Savage Cub and have to agree with Robert - it's currently impossible to get the propwash effect working correctly! From my Cub days, even the basic J3 could spin the tail when stationary with a good blip of the throttle and a bootful of rudder, let alone anything more powerful. It was also very controllable, predictable and smooth. Making adjustments to the propwash settings achieves little / nothing, to get the aircraft even slightly controllable in yaw I had to decrease the rudder effect by a factor of six and then you can begin to see just how far out-of-whack the whole thing is!

 

[Geoff_D]

Very interesting, and actually a big relief -- I was beginning to think I could not steer straight !!!

There appears to be some "odd rules" programmed into the planes when they are on the runway, that just do not make any sense.
That coupled with fact, that when the plane is stationary, wind has NO effect on the plane, not even a 100 knot wind gusting 150 !!
Start to move, and then you get it -- !!!

Then once you get into the air, the Dynamic Pitch stability is just WRONG !!!, making the plane almost impossible to trim, and to oscillate in pitch, if disturbed from its steady state condition.

Looks like there is a lot of work still needed, and at a time when Asobo has more than enough other issues to sort out.

Going to be an interesting 6+ months

 

[pontiac51]

I have stumbled on this post and have to agree. It seems rudder input has to be greatly increased to handle a taildragger on the ground. But that way it’s nearly impossible to keep it straight on a runway once up to speed on takeoff or landing. It is also true for the amphibians. While the Icon has a water rudder that helps a little, the recently released Goose does suffer greatly as it uses differential thrust to turn.

Would it make sense to create a ModelBehavior or XML gauge that limits the rudder to a certain movement above 10kts via rudder deflection or rudder scalar variable? Or vice versa increase the rudder when below a certain speed on ground?

I have tried to find example code for such a thing, but was not successful. Maybe you can help me out with that? I do know how to write an XML gauge, but how do I change Paramus from the aircraft.cfg with it?

 

[Roy Holmes]

The program is full of mistakes. I do not fly props but the single stage turbojet I'm working on has 99% N1 and 50% N2. It has the correct N1 to N2 data entry but that seems to be ignored. If the airplane is moving with the throttle shut idle RPM is 70% which means you cannot slow down in the air below about 250 kts and some of the bigger jets with poor brakes can barely be brought to a halt on landing.

Roy

 

[mariomotta]

How not to agree with Roy ?
MSFS 2020 is currently a nice game, eye-candy, but far from being a flight simulator, still a long way will have to do Asobo to please the experts simmer, I would recommend the team to hire some pilot as a consultant

 

[Delta558]

Would it make sense to create a ModelBehavior or XML gauge that limits the rudder to a certain movement above 10kts via rudder deflection or rudder scalar variable? Or vice versa increase the rudder when below a certain speed on ground?

There are tables in the aircraft editor (as there were in FSX/P3D) which limit the effect of the control surfaces at given mach numbers. The surfaces would be fully deflected, but the effect of each surface could be set to give less effect at lower speeds - the rudder is an obvious example given the discussions, at lower speeds you should be able to use full displacement either side and see no effect (propwash aside). The effect of the rudder should gradually build with increased speed.

The point at which the rudder really starts to 'bite' obviously varies from aircraft to aircraft (I think the Canberra is about 60kts, the Vulcan about 90kts) so a 'one size fits all' solution would be inappropriate.

Unfortunately, I have been unable to get any of the control surface effectiveness v Mach tables to work in the new sim, so it appears you have either full effectiveness even at the lowest speeds OR there is something written into the core sim to address this but it is not noticeable!

 

[pontiac51]

I have solved the issue with a phantom water rudder, which seems to work just fine for my purposes.

 

[jcomm]

Using fake / phantom structures and control surfaces has also been a widely used method in X-Plane, to control some of the quirks that plague, for instance but not only, taildraggers...

Anyway, while trying to play with castering tailwheels in MFS, I did some tests with the Extra and the Pitts, as well as the Cub, and it appeared to me that modelling of rudder effectiveness was there. At low taxi speeds one can widely wiggle it without much effect ( tailwheel unlocked ) unless we give it a burst of propwash. It looked consistent to me .

As "q" builds up, the rudder comes alive and more effective, just like IRL. So, I really don't see your points here ?

I could also played around with some of the parameters, and they appeared to work too ?

"yaw_moment_yaw_propwash"
"yaw_moment_delta_rudder_propwash"

plus that "rudder_scaling_table" with p vs scaling coef.

One test you can make to check that indeed it's not * only * overdone rudder authority that responds for some effects, is to modify, say, the Extra, making it a much heavier aircraft, like say a P-47 D, adjusting the MoI s, and then make it's tail wheel free castering.

Go to the runway, set wind to nill, and give a burst, then idle the throttle and wiggle the rudder... I don't have MFS installed right now for testing, but I will ask Alexis to run this test for me... I believe the response will be very poor, and you will actually not be able to steer unless you use differential braking.

Also, try giving a burst of prop, you will get authority for a while, but not enough to counter a stronger x-wind. Only when fulll takeoff power is set will you be able to steer properly again...

 

[Alexismmef]

Doing the Experiments Jose asked me to accomplish, I left with some very strange Results, which I wanted to share with you guys, and see if anyone can explain to me what is happening.

First, I used the Default Extra E330 from Asobo, and with Debug showing SimForce, I put Full Power to see the Force Vector being generated From the Rudder Surface, due to Propwash, to check if MFS2020 is correctly simulating it, which it was in my opinion.

Now, Following JComm's suggestion, I increased Empty Weight from 1200lbs to 12000lbs, and used the new MOI value calculated by MFS2020, nothing else. Engine had the same output, didn't touch Surfaces, nothing. The same Experiment, using Full Power, the same Debug SimForce showed me much smaller Force Vector from the Rudder?

Is MFS2020 Propwash Variable with the Weight of the Aircraft, which to me makes no sense? Or is this Debug Mislabeled, and it's not actually showing Force Vector, but something else derived from it?

 

[jcomm]

Excellent finding Alexis, thx for having tested !

Probably instead of the lift force the vectors are instead showing the actual acceleration / net resultant (?)

 

[Bazzar]

The program is full of mistakes. I do not fly props but the single stage turbojet I'm working on has 99% N1 and 50% N2. It has the correct N1 to N2 data entry but that seems to be ignored. If the airplane is moving with the throttle shut idle RPM is 70% which means you cannot slow down in the air below about 250 kts and some of the bigger jets with poor brakes can barely be brought to a halt on landing.

Roy

It's not just jets Roy. I have a warbird under way capable of close to 400 mph. The engine details and parameters are all correct and against the "bible"on the type. My latest build attempt followed the SDK to the letter (just for fun) Apart from not being able to get it anywhere close to 400 mph, ( 180 at best) if I close the throttle and even use flap, I can't get it to slow below 120 knots. Not very safe for landing a mid-weight tail-dragger....