storm_999_

x plane 11, b737 rudder rolls plane

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Hi;

I have been flying with MS FX for long time. A few days ago, when i was looking for an alternative I have seen X-Plane 11 and I installed it.
It has got very good graphics but it disappointed me a lot.

I use logitech Extreme joystick. It works very well. I checked all sliders in calibration options. Also there is no problem with cessna. 
As you know it has got b737-800 in default. When I use rudder, it extremely rolls the plane as if i move ailerons. To be able to use rudder, I have turn aileron in opposite side, even it is not very effective. This experience is too much far away from Microsoft Flight Simulator. How come? 

Is anything wrong with my configuration or is that very normal??

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How can only rudder create that impact? If that is very normal then why microsoft flight simulator is far away from this?
 

 

 

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2 hours ago, storm_999_ said:

How can only rudder create that impact? 

That is exactly what one would expect to happen. 

One of the first lessons any pilot learns is "effects of controls". To give a very quick overview, there are three primary flight controls - the elevator, aileron and rudder.

Each control has a primary effect: the elevator controls pitch around the aircraft's lateral axis (think a skewer through the aircraft horizontally, parallel to the wings), the ailerons control roll around the longitudinal axis  (another skewer running nose to tail) and the rudder controls yaw around the vertical axis (a skewer running straight up and down through the middle of the aircraft).

However, the ailerons and the rudder also have secondary, or further effects. When we roll the aeroplane using the ailerons alone, it will 'slip' 'downhill' in the direction of the lower wing. The result is that some airflow will strike the fin (vertical stabiliser) from the direction of the lower wing and 'push' the tail in the direction of the upper wing. Thus the aeroplane will start to yaw in the direction of the roll. The further effect of aileron, therefore, is yaw.

If we push on the rudder pedal in the way that you have here, without any aileron input, the nose yaws  (in your case to the left).

However - think about what is happening to the wings. The wing on the outside of the turn must be travelling faster through the air than the wing on the inside because it is travelling a longer distance around a greater radius. The faster a wing moves through the air the more lift it creates. So if one wing is travelling faster than the other it must also be creating more lift than the other. This causes the aeroplane to roll in the direction of the yaw, and this is the further effect of rudder. 

On a swept-wing aeroplane like the Boeing 737 this roll as a result of yaw is especially powerful. Indeed, there were a number of accidents in real life some years back involving so called rudder "hard overs" on the 737 where the rudder suddenly moved to full deflection uncommanded and the aircraft did almost exactly what you see in your video - rapid uncontrollable yaw leading to an uncontrollable roll inverted, spiral drive and crash.

This effect is certainly present in MSFS, as I have demonstrated it to my students in basic training sessions many times! 

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I have not experienced that degree of roll from rudder input on my setup, although I rarely touch the rudders during flight. On large commercial aircraft, the rudder is primarily used for tracking the centerline during the takeoff roll and f kicking out the crab angle after a crosswind touchdown. It is also used in the event of abnormal situations such as asymetric thrust due to an engine failure. The yaw damper takes care of coordinated turns and turbulence induced yaw. On most aircraft, rudder input will initiate some roll in the same direction due to the resulting slip, but I doubt it would be to the extent you are experiencing on your setup. Having said that, you would never want to use full rudder deflection on a large commercial jet during flight. Bad things will happen and have happened in the real world.

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5 hours ago, skelsey said:

That is exactly what one would expect to happen. 

One of the first lessons any pilot learns is "effects of controls". To give a very quick overview, there are three primary flight controls - the elevator, aileron and rudder.

Each control has a primary effect: the elevator controls pitch around the aircraft's lateral axis (think a skewer through the aircraft horizontally, parallel to the wings), the ailerons control roll around the longitudinal axis  (another skewer running nose to tail) and the rudder controls yaw around the vertical axis (a skewer running straight up and down through the middle of the aircraft).

However, the ailerons and the rudder also have secondary, or further effects. When we roll the aeroplane using the ailerons alone, it will 'slip' 'downhill' in the direction of the lower wing. The result is that some airflow will strike the fin (vertical stabiliser) from the direction of the lower wing and 'push' the tail in the direction of the upper wing. Thus the aeroplane will start to yaw in the direction of the roll. The further effect of aileron, therefore, is yaw.

If we push on the rudder pedal in the way that you have here, without any aileron input, the nose yaws  (in your case to the left).

However - think about what is happening to the wings. The wing on the outside of the turn must be travelling faster through the air than the wing on the inside because it is travelling a longer distance around a greater radius. The faster a wing moves through the air the more lift it creates. So if one wing is travelling faster than the other it must also be creating more lift than the other. This causes the aeroplane to roll in the direction of the yaw, and this is the further effect of rudder. 

On a swept-wing aeroplane like the Boeing 737 this roll as a result of yaw is especially powerful. Indeed, there were a number of accidents in real life some years back involving so called rudder "hard overs" on the 737 where the rudder suddenly moved to full deflection uncommanded and the aircraft did almost exactly what you see in your video - rapid uncontrollable yaw leading to an uncontrollable roll inverted, spiral drive and crash.

This effect is certainly present in MSFS, as I have demonstrated it to my students in basic training sessions many times! 

Here is Microsft flight simulator. Same aircraft, same airport but different impact.

 

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2 hours ago, storm_999_ said:

Here is Microsft flight simulator. Same aircraft, same airport but different impact.

Hmm. Probably says more about the flight modelling of the 737 you are flying in MSFS than anything else! 

In any event, in real life I would expect an outcome closer to what you saw in XP.

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1 hour ago, skelsey said:

Hmm. Probably says more about the flight modelling of the 737 you are flying in MSFS than anything else! 

In any event, in real life I would expect an outcome closer to what you saw in XP.

Alright! Thank you! Then I will continue flying with X Plane. 
Is it the most realistic home simulator now? 
Or would you suggest anything  else. Thank you in advance. 

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14 hours ago, storm_999_ said:

As you know it has got b737-800 in default. When I use rudder, it extremely rolls the plane as if i move ailerons. To be able to use rudder, I have turn aileron in opposite side, even it is not very effective. This experience is too much far away from Microsoft Flight Simulator. How come? 

Is anything wrong with my configuration or is that very normal??

As Simon said, it's a normal behaviour. Aircrafts like the B737 have a strong yaw/roll coupling. There have been multiple real life accidents in which B737s experienced rudder runaways due to a design flaw, and in these cases the pilots were not able to maintain control even using full right aileron and crashed: https://en.wikipedia.org/wiki/Boeing_737_rudder_issues

The behaviour of the defaul B737 in FSX is apparently way too tame, while the XP one seems closer to reality. The real B737 has a speed called "crossover speed", above which full aileron deflection can overcome full opposite rudder deflection.

Try deflecting full rudder and full opposite aileron in the XP B737, but at high speed: if you are not able to maintain bank control, then the yaw/roll coupling in the XP B737 is probably too strong (contrasted with the one in FSX which is too tame). If you can maintain bank control instead, then it's probably in the ballpark of the real aircraft.

12 hours ago, skelsey said:

However - think about what is happening to the wings. The wing on the outside of the turn must be travelling faster through the air than the wing on the inside because it is travelling a longer distance around a greater radius. The faster a wing moves through the air the more lift it creates. So if one wing is travelling faster than the other it must also be creating more lift than the other. This causes the aeroplane to roll in the direction of the yaw, and this is the further effect of rudder.

Just a little nitpick, the main cause of the yaw/roll coupling is usually due to wing geometry (dihedral, sweep, high/low wing), while the difference in wing speed is usually only a secondary effect adding to it.

In theory, an aircraft could be made to have negative yaw/roll coupling, by using negative wing dihedral and/or forward swept wings, but then it would have undesirable flight characteristics because it would be unstable in roll.

For the rest, nice and interesting post as always. 🙂

2 hours ago, storm_999_ said:

Is it the most realistic home simulator now? 
Or would you suggest anything  else. Thank you in advance. 

Both FSX and XP native flight models have their advantages and limitations, so one can't generalize. At the end of the day, the main factor are the skills of the individual aircraft designer. Don't expect extremely accurate flight characteristics from default aircrafts in both platforms, and even add-on aircrafts in both sims can be more or less accurate, depending on the skills of the specific 3rd party developer.

 

 

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Murmur is correct. It is also called yaw induced roll.

Below a certain airspeed (crossover) on airliners, the rudder will overpower the ailerons.  The reason is that at low airspeed maneuvering you need more rudder authority. Like on decrab in crosswind and keeping the aircraft on the centerline on a slippery runway, one engine out scenario, (Vmcg etc).  However you do NOT want this at high speeds as it is very dangerous (as you experienced), so to prevent an accidental hard rudder input at high airspeed, airliners use what is called "rudder phaseout".  The higher the airspeed, the less rudder deflection you will get if you stamp on the pedal.  At low airspeeds, the max rudder deflection on an airliner is around 30 degrees, at cruise it is only a few degrees. But basically the aircraft manufacturer can set the crossover speed to what is necessary and safe.

Have a look at this real accident replay by NTSB, USAir 427, the rudder on the 737 went by itself to full deflection, the pilots countered with full aileron+rollspoilers but didn't have a chance...  We model this pretty close on our 733, within 2 kts if I recall correct.

https://www.youtube.com/watch?v=7CIAXOq9pwI

 

M

Edited by MortenM
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I have just realized another significant difference between XP and MSFX. 

in XP when you set engines idle, pull the stick and set the trim max, b737 doesnt lose its control even if it gives stall warning. It just glides, loses altitude as a level flight.
but if you do same things and MSFX, plane will lose the stability completely and crash down very fast, it wont glide.

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11 minutes ago, storm_999_ said:

I have just realized another significant difference between XP and MSFX. 

in XP when you set engines idle, pull the stick and set the trim max, b737 doesnt lose its control even if it gives stall warning. It just glides, loses altitude as a level flight.
but if you do same things and MSFX, plane will lose the stability completely and crash down very fast, it wont glide.

How an aircraft stalls will vary from type to type and the input from the pilots and weather. Not many have tried this with an airliner, but if you check out the AF447 accident report (A330) and how it went down you will see that it "glided" (wings level) all the way down from cruise in the stall.  So I guess again XP has it right if we use that example :wink:  I can also tell you this is also what happens in the big professional 737 sims I tried

M

 

Edited by MortenM
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4 hours ago, storm_999_ said:

Is it the most realistic home simulator now? 

As Murmur says, it's really very dependent on the flight modelling of each particular aircraft. Personally I am an MSFS/P3D person but are aircraft with very good, realistic flight models in XP just as there are aircraft with very good, realistic flight models in MSFS -- and some with horrendous flight models I am sure in both (certainly in MSFS!).

15 minutes ago, storm_999_ said:

I have just realized another significant difference between XP and MSFX. 

in XP when you set engines idle, pull the stick and set the trim max, b737 doesnt lose its control even if it gives stall warning. It just glides, loses altitude as a level flight.
but if you do same things and MSFX, plane will lose the stability completely and crash down very fast, it wont glide.

In all cases I would caution that when you start getting to the edge of the envelope (stalling/spinning/very high speed/very low speed/high G flight etc etc etc) you can find in any simulator that you start getting weird results, from our desktop products right up to multi-million pound commercial Level D full flight simulators, especially with large transport category aircraft as there is not very much empirical data to program them by and the simulation models invariably start to break down a little bit at the extremes. Simulators of all forms are generally capable of being quite well tuned to behave very realistically under the vast majority of normal flight conditions but don't set your expectation level too high when it comes to extreme manoeuvres!

There's not many people that really know how a real B737 behaves in a full stall (in fact, probably only the Boeing test pilots who did the initial certification) so it's hard to say what the correct behaviour is. I would be surprised if there was no tendency at all to drop a wing but who knows? About the best guess we have is the video below from a B737 full flight simulator, which would tend to bear that out but, again, any real pilot will tell you that there are lots of question marks around whether these things are really capable of simulating conditions outside the flight envelope.

 

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This last video also shows something that is missing in FSX and present in XP's 737 - secondary stalls ...

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Something that a lot of people don't appreciate about the 737, is that it has needed quite a few aerodynamic tweaks over the years to make it stable in the yaw axis, and these have been very effective. Despite the evolution of the 737 from its original 100/200, through the 300/400/500 and on to the NGs - and now the Max - seemingly having changed little in terms of shape, there are two rather large alterations to the 737 which become readily apparent when you compare an original 737 to a more recent one.

The tail is somewhat taller on a modern 737 (the original 737-100 was 37 feet tall, the 737 NG is 41 feet tall), but less obviously, the tail of a modern 737 has a considerably larger surface area, courtesy of the addition of a very large dorsal fillet extension at the front of the vertical stabiliser. It needed that, especially when it switched powerplants to the CFM 56 from the JT8D and the newer LEAP 1B found in the MAX version, where the thrust of the engines jumped up from around 20,000lbs to around 27,000lbs.

Boeing were aware of this very early on in the 737's continued development of the Classic 300/400/500. Initially they wanted to simply make the tailfin taller on the newer 737, but several major airline clients expressed their dismay at this prospect, since they had existing hangars built for their large fleets of 737 100 and 200s which would not accommodate a taller 737; this would mean that rather than build new hangars, they'd potentially consider an alternative aeroplane such as the A320 or one of the DC-9's derivatives. Needless to say, Boeing didn't want to lose clients, so they instead made the new 737's fin only a little bit taller, but they still needed to improve yaw control so they gave it a much larger surface area with the addition of a big dorsal fillet. It's not the only time Boeing have done this to one of their successful models, if you compare the early Flying Fortress B17A/B/C variants to the later D/E/F/G variants, that too has a massive dorsal fillet to improve yaw stability, since with warplanes like the Fortress, losing an outer engine in combat whilst up at high altitude in thinner air, making control difficult, was not an unlikely prospect.

But back with the 737 - and this is the really important bit related to this post - beyond simply the additional area of the dorsal fillet making the aeroplane more laterally stable, adding a dorsal fillet has another very important and relevant aerodynamic effect, one which might surprise you: Compare the change in the leading edge angle of the modern 737's tailfin (where it changes from the fillet to the fin) to the very similar change in the leading edge angle of this aeroplane:

https://en.wikipedia.org/wiki/Saab_35_Draken

See how it is similar? Beyond simply looking kind of cool, there is a very good reason to do have that changing angle of a leading edge on a wing or fin, and that is because it generates a vortex which makes the surface hugely more effective than it would otherwise be. This is great for jet fighters because more effective aileron controls means a faster roll rate and thus more manoeuverability, and on an airliner's tail it means a more effective rudder. So if an engine fails on take off, the Vmc (minimum control speed with one engine out) will be more easily achieved, making the 737 a much safer aeroplane. But of course, it also means because the rudder on a 737 is so effective, it will cause a secondary roll effect unless you counter that with the ailerons. That's what you are seeing in your simulator.

It's worth bearing in mind here too, that because the rudder is so very effective on a 737, it's not a great idea to be using a lot of rudder input at high speeds anyway, as not only will it cause a lot of yaw and roll, but it will also put a massive amount of sideways load on the entire tailfin and could potentially over-stress it (not a big deal in a flight sim, but very dangerous on a real aeroplane). That's actually true of most aeroplanes, you should never really give the rudder large inputs at high speeds, unless you want to find out what you plane flies like without a tail fin!

 

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