Archived

This topic is now archived and is closed to further replies.

jcomm

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

Recommended Posts

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 :-)

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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...

Share this post


Link to post
Share on other sites

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).

Share this post


Link to post
Share on other sites

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...

Share this post


Link to post
Share on other sites

 

 


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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

 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 ?

Share this post


Link to post
Share on other sites

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)

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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...

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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....

Share this post


Link to post
Share on other sites

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.

 

Ok, this is interesting information. In this case, a step is effectively taken to help propwash compensate for torque. Now we have to understand why this engine deaxing helps.

 

 

 

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.

 

You're confusing the effect in reality, and what can be done in X-Plane to simulate it. Given the idea to change the wing angle locally, I suggested tweaking the airfoils, because I'm not sure the wing's chord can be locally tweaked.

 

 

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.

 

And this is exactly why we have this discussion. Torque, induced by a spinning prop, physically exists, this is a given fact. So, something has to compensate for it, and compensate relatively to the power deployed by the prop. More prop power = more torque = more compensation needed. The only aerodynamic effect that can be related to power IS propwash ! Furthermore, propwash effect on the plane has to be optimised, in a way or another, in order to fully compensate for torque.

 

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.

 

It's difficult to debate with you, you always confuse the symptoms and the causes. The object of this thread is not to object that real aircrafts don't generally have the roll tendency. The object of this thread is to understand how to mimic in X-Plane models what you as a pilot, observe in reality. We see that in reality, builders have to tweak the airframe, or as you said offset the engine to the right. So, we have to do this kind of tweaking in X-Plane as well, because a perfectly symmetric aircraft will have the roll tendency.

Share this post


Link to post
Share on other sites

For Pascal. I read that the explination for engine offset...... is to move more airflow over the inboard left wing, which of course creates more lift on the left. Lift would also change with power changes. I like that explanation

Share this post


Link to post
Share on other sites

For Pascal. I read that the explination for engine offset...... is to move more airflow over the inboard left wing, which of course creates more lift on the left. Lift would also change with power changes. I like that explanation

Great, that would explain it very well !

I always misinterpreted engine cant as a change in axis angle, but apparently there's a real shift. Apparently the figures given above by Jcomm, from planemaker, are an angle.

@Jcomm: what happens if we actually shift the engine axis in planemaker ? Does X-Plane take this into account ?

Share this post


Link to post
Share on other sites

Just an observation. The only reason I stayed with xplane 10 initially is that the Carenado aircraft flew so well (yes never perfect in a sim but good enough for training purposes, and actually nicer imho than most in fs). Something happened about 6 months later that caused this horrible rolling and non ability to trim, so much so that I had to leave the sim. Then something happened after 10.20 that again made them fly great.....

 

I just did a flight in the Carenado V35-calm winds-and was able to trim the aircraft (pitch only) and watch it fly stabilized for about 10 seconds before needing minor corrections-very realistic.

 

I also downloaded a freeware aircraft today-twitchy controls, non stability, and it was retired no sooner than downloaded.

 

Therefore I can only conclude whatever Carenado is doing is the right approach, along with the changes to some of the default aircraft that has also improved them considerably. 

Share this post


Link to post
Share on other sites

Great, that would explain it very well !

I always misinterpreted engine cant as a change in axis angle, but apparently there's a real shift. Apparently the figures given above by Jcomm, from planemaker, are an angle.

@Jcomm: what happens if we actually shift the engine axis in planemaker ? Does X-Plane take this into account ?

 

Yes, it does Pascal. This was actually the tecnique we used in MSFS some years ago :-) because in that sim, and derivates, there is simply no way to cant the thrust axis... In X-Plane we can do a lot more, including variable thrust axis ;-)

Share this post


Link to post
Share on other sites

Yes, it does Pascal. This was actually the tecnique we used in MSFS some years ago :-) because in that sim, and derivates, there is simply no way to cant the thrust axis... In X-Plane we can do a lot more, including variable thrust axis ;-)

oh great ! I'll have to dive into planemaker one day ;)

Share this post


Link to post
Share on other sites

Yes, it does Pascal. This was actually the tecnique we used in MSFS some years ago :-) because in that sim, and derivates, there is simply no way to cant the thrust axis... In X-Plane we can do a lot more, including variable thrust axis ;-)

Not actually true... and let's not get into a long debate of one sim vs another... just want to clarify this...

In FSX you can define the x/y/z position (which will locate the engine) as well as the p/b/h (which will rotate the engine).  There is also variable thrust axis, though I believe its use is in turbines and not props.

 

Now, back to your discussion. :smile:

Share this post


Link to post
Share on other sites

 as well as the p/b/h (which will rotate the engine).  

 

You're ABSOLUTELY RIGHT!  As a matter of fact, just found an old post of mine, on a thread where, among others, I and my great friend and "teacher" ( RIP ) Ron Freimuth, were debating this and other aspects new to FSX :-) where I wrote:

 

The cant subject I am referring to is...ThrustAnglesPitchHeading.0 = 0,0 //Thrust pitch and heading angles in degrees (+pitch down, +heading right)a new Engine parameter one can use to align the thrust axis in directions other than the usual long axis of the aircraft.If you make the 2nd parameter equal, say 90.0, you're defining a thrust axis perpendicular to the aircraft longitudinal (z in MSFS) axis.We could use a bit of XML, or even better, "C" programming to make the engine interact with collective setting in the helicopters, for better torque yaw effects.

 

(memories.... I'm getting old... that's what it is...)

 

Here it is :-)

Share this post


Link to post
Share on other sites