A possible reason for the torque roll, and other "features"...

[jcomm 5,890]

Ok,

 

here we go again, but this time on the defense, rather than attacking :-)

 

There are two things we know about X-Plane10 that most of us don't like to feel because we think they feel unrealistic:

 

- torque-induced roll

- too much wind veering during ground operations (taxi / takeoff)

 

Now, if I look at the FM display (default CTRL-M - see attached image, where the longer vectors on the left side of fuselage / wing root are obviously denoting the asymmetric effects of the propwash/slipstream on various aircraft surfaces...) we can easily see that the FDM accounts for the forces that might counter the torque-induced roll ( vectors near the root of the left wing on a CW rotating prop, and yellow( horizontal ) vectors on the right side of the fuselage and vertical stab )

 

Also, if we remember the first versions of xp9, and the complaints of users regarding it being very difficult to land aircraft ( compared to the other sim... ) when there was a x-wind component ( requiring correct amounts of "aileron into the wind"), we may try to explain why Austin chose to make it the way it is in XP10 (and later versions of XP9 too).

 

1) People wouldn't complain that much regarding ground handling under x-wind and the tendency of the upwind wing to raise...

2) People using turn coordination ( i.e. the mouse to control the aircraft ) can more easily control the aircraft and respond to toque and x-wind effects...

 

So, while I still look forward for TKiller's explanation of why torque roll is the way it is in X-Plane10, I believe this could possibly be the reasons (?)

 

OTOH, I found that the edited C172 that comes with 10.21 has suffered deep rewriting of the prop dynamics, including prop disc, density and geometry!!! Most people use default prop data, which may well be completely out of sync with the aircraft they are fitting those props with!

 

And... I can easily also solve the problems with taxiing and taking off under up to moderate x-winds, as well as torque-induced roll, and make my prop aircraft fly a lot more close to reality when I am not using the mouse for control input.

 

I would gladly post here an example of a mod ACF for the default C172, but I believe I could be violating copyrights, so, should any of you like to try it, PM-me :-)

[airlinejets 709]

Good writeup jcomm. I really do hope Laminar and developers take note.

[Pascal_LSGC 298]

I'm with Geofa here, I'm sure something has changed with the latest version. There must have been something wrong in some versions before, because the propwash effect didn't show up in the force vectors. Now it does. I have to reinstall an older version to verify, because I'm not 100% sure.

So, what has changed ?

At least, now, it's possible to have it right. I didn't test with the default planes, but the latest iteration of the Carenado 172 and 152 are nearly perfect in that department.

 

At last but not least, concerning the physical phenomenon, I think this doc you provided us finally says it all:

 

some of the rotating propwash hits the top of the right wing and the bottom of the left wing. This tends to reduce the amount of roll — but it can never reduce it to zero or cause a roll to the right. Similarly, any air intercepted and “straightened out” by the tail reduces the rolling moment somewhat. Using Newton’s law again, we see that if any air escapes while still rotating down to the right, the airplane will roll to the left.

 

The only way to restore equilibrium is to take a corresponding amount of air and throw it down on the left. Airplane designers have long since learned about this propeller drag rolling moment, and they take steps to compensate for it. For instance, they set the left wing at a slightly higher angle of incidence than the right wing. This is called, unsurprisingly, asymmetric incidence. It is especially useful to apply this trick to the part of the wing that flies in the propwash, so that the effect increases as engine power increases. On a Piper Cherokee, the roll-wise trim is easily adjustable on the ground — in the flap extension mechanism for each flap there is a turnbuckle that allows the flap to be raised or lowered until the roll-wise trim is just right.

 

 

Pascal

[urthgental 181]

all i can say is that the carenado c152

v2.1 still has some noticeable roll moment during flight - depending on throttle settings.

 

this behaviour does not exist with the real f152s in our club. they yaw and always need input to stay on track, but there is no rolling...

[Pascal_LSGC 298]

all i can say is that the carenado c152

v2.1 still has some noticeable roll moment during flight - depending on throttle settings.

 

this behaviour does not exist with the real f152s in our club. they yaw and always need input to stay on track, but there is no rolling...

I flew the 172 yesterday, was nearly perfect. I'll try the 152 again as soon as I find some time.

Also, in real aircrafts, the last drops of roll tendency are probably absorbed by positive horizontal stability. X-Plane is not yet perfectly right in that department. (probably because of fuselage aerodynamics, which are very basic).

[jcomm 5,890]

I've been tweaking the default C172. It is tweaked already, in 10.21, and I do not know when it started, but much better results can be achieved if we play, with caution, with the RoGs and control phase-out. Just that. To overcome the problems with wind on ground, during taxi or takeoff, some editing can also be done, and will not arm you normal flight envelope.

 

Further editing can, I believe ( because I haven't tested it yet  ), give a less effective response to control inputs (already partly achieved through RoG editing).  I am also trying to find out what is required to better translate the prop effects into yaw instead of roll, but beware, this is also a problem in MSFS/P3D, FlightGear (Yasim or JSBSim), etc...

 

Again, I do believe this stems more from an option made by Austin than from a limitation of the X-Plane fdm, but I look fwd for Tom Killer's notes...

[Pascal_LSGC 298]

 

 

I am also trying to find out what is required to better translate the prop effects into yaw instead of roll

 

Roll: If you read again the quote above, this is also a problem in real aircrafts, and we must take steps to compensate. For instance: assign a slightly different airfoil for the inner portion of the left wing. Is it possible in planemaker ?

 

Yaw: again, the culprit here might be the lack of a proper fuselage aerodynamic model.

[LAdamson 240]

More than not, engines are angled on their mounts to compensate for torque. Different incidence in the wings is very rare. Adjustments such as the Piper , or adjustable cams on a Cessna high wing's rear spar, are to account for any tendencies of a heavy wing, caused by slight differences in the build. When you make adjustments to the wing to account for torque, it's going to want to roll in the opposite direction, when at higher airspeeds and less power during a cruise speed descent. But in reality, we end up with some left rudder, and not changing to an opposite aileron trim.

[Pascal_LSGC 298]

 If I understand well, Larry, you claim that this statement:
 

 

some of the rotating propwash hits the top of the right wing and the bottom of the left wing. This tends to reduce the amount of roll — but it can never reduce it to zero or cause a roll to the right.

 

 

and this one:

 

Airplane designers have long since learned about this propeller drag rolling moment, and they take steps to compensate for it.

 

 

are not accurate ?

[jcomm 5,890]

Regarding the differences between 10.21 and 10.05r2, the attached images (with the default C172 on each X-Plane10 version) show that in fact in 10.21 the asymmetry is even less than in 10.05r2 (2nd img)

[LAdamson 240]

It's interesting to spend hour after hour reading pilot reports on various aircraft. Notice that referrences to trimming and re-trimming aileron for torque, are coming from the "flight sim" world, MU2 included. Aileron trim, if the aircraft has it, since many don't....... Is used for the perfect balance of fuel, passengers, and mid-rigging. It's not required to get rid of a noticeable tendency to roll, such as you'll feel with rudder and elevator forces, when not trimmed out. With the MU2, aileron trim is used for perfectly leveled wings, so that the spoilers used roll can lay flat on the wing during level flight.

[LAdamson 240]

I'm having a tough time including quotes on my IPhone sometimes. For Pascal....

 

Designers learned that offsetting the engjinr to the right, for clockwise prop rotation from the cockpit view, is good way to offset torque tendencies with varying amounts of engine power. Most single engine planes have this offset built into the mount. Some Reno racers keep a straight mount to use left roll as an advantage, since turns are always to the left.

 

 

 

 

 

 

and this one:

 

 

are not accurate ?

We're getting off the right track here, if we start thinking that designs need a different portion of airfoil design to compensate for torque. Just remember, that what you design in, to compensate for an engine under power, will have the opposite effect, when power is pulled back, and airspeeds will now cause a roll in the opposite direction. This is not good.

[jcomm 5,890]

The default C172 in X-Plane10 has it's engine/prop canted since the first releases, 0.2º down and 0.2 right.

 

The biggest differences in 10.21 are: 

 

- they added control phase-out to soften the effects of control inputs in the absence of any force feedback/hing forces...

- they tweaked the prop disc / geometry ---> very important!

- control forces were also tweaked...

 

I don't know if anything changed with the airfoils...

[LAdamson 240]

We can put me on the attacking side of the line. It's not a case of "feeling" that torque induced roll is unrealistic. It IS unrealistic. I've flown enough airplanes to know different, and so many other pillots I've disussed this with, agree. Austin even talks about eliminating any roll during the test phase of his Lancair. And a real pilot of a XPlane simulated turboprop twin commuter aircraft , certainly made it known that the torque roll, and need to aileron trim, shouldn't be present. This was at X-pilot. The need to trim, irratated him, as it does me. It shouldn't be termed as a "pilot challenge", and then many XP users come to believe that the effect is real.

[jcomm 5,890]

Larry, of course the way it is it is not right, but my point with the OP, and given that the FDM even accounts for (some of) the forces resulting from the asymmetric splipstream interference with different aircraft surfaces, it looks to me more like an option to simplify the use for those user flying auto-coordinated, without joystick or rudder pedals....

 

The same applies to the effects of wind on ground.

 

For me, it was an option, rather than a limitation. Of course we can debate that option as a good one if it is confirmed....