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Inertia?


jbearnolimits

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I was just thinking about how inertia effects the turns of an aircraft. Am I correct in assuming that a plane can roll out of a turn to straight and level flight but still have a slight "drift" in the direction of the turn for a few seconds?

 

Or does the rudder cancel it out if the turn was coordinated? Just wondering because I noticed even with autorudder in fsx it seems there is still that slight drift for a moment but it seems to be less if I bank the other direction for a second and then return to straight flight.

 

I have also noticed that even autopilot seems to have this issue with the larger planes.

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You have a good eye -- congratulations, you are one of the very few people who seem to notice that problem. You are noticing a phenomenon unique to FS. Aircraft do NOT drift in that fashion, though all the default aircraft and many (most??) add-ons do so. A friend and I call it the slip/skid problem, though it's something that can be alleviated by adjusting some of the yaw parameters in the .air file (AirED is good for this). However it does take experimenting, since no two aircraft react the same to the adjustments.

 

Section 1101 (Primary Aerodynamics) has various yaw parameters which you can try. I'd suggest, for the C-172 in FSX, that changing:

 

Cl_da - Airleron (Control) to -500

Cl_P - Roll Rate (damping) to -1200

Cmo to -140

Cm_de - Elevator (Control) to -1800

Cn_beta Sideslip to 300

Cn_dr Rudder (Control) to 200

Cn_R - Yaw Rate to -1000

Cy_Beta Sideslip to -1400

Cy_dr Rudder to 100

 

would be a good start. Be certain that you save a copy of the .air file as is before you start changing anything. Do note that most (not all) of the values I list above are negative numbers, so pay attention to sign. The above will also make a few other slight changes, so check out the handling and see what you think. Remember, save a copy of the original .air file before making any changes.

Edited by lnuss

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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No the intertia of the turning moment is way too weak. But there is "adverse yaw", an aerodynamic effect that yaws the aircraft in the opposite direction of the roll. You find more information about this in the net.

Adverse yaw is not what he's talking about. Adverse yaw is the reaction of the nose in yaw to application of aileron, while he's talking about after the wings are level and controls are neutral. See my post above.

 

Though this behavior is always there in aircraft so affected, it's easiest to see this behavior from the spot view (roughly at 6 o'clock) in slow flight, not too high off the ground, perhaps 500 feet or so.

Edited by lnuss

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Adverse yaw is not what he's talking about. Adverse yaw is the reaction of the nose in yaw to application of aileron, while he's talking about after the wings are level and controls are neutral. See my post above.

 

Though this behavior is always there in aircraft so affected, it's easiest to see this behavior from the spot view (roughly at 6 o'clock) in slow flight, not too high off the ground, perhaps 500 feet or so.

 

Also seems easier to see in the larger jets. I do remember asking before about this and you let me know about it. But I wasn't sure if it was also happening to the larger planes or if it was inertia effects for them.

 

I had made the changes to the c172 you recommend a while back and it did help with that one. I just needed to know if inertia played a part in it at all. Thanks again!

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You have a good eye -- congratulations, you are one of the very few people who seem to notice that problem. You are noticing a phenomenon unique to FS. Aircraft do NOT drift in that fashion, though all the default aircraft and many (most??) add-ons do so. A friend and I call it the slip/skid problem, though it's something that can be alleviated by adjusting some of the yaw parameters in the .air file (AirED is good for this). However it does take experimenting, since no two aircraft react the same to the adjustments.

 

Section 1101 (Primary Aerodynamics) has various yaw parameters which you can try. I'd suggest, for the C-172 in FSX, that changing:

 

Cl_da - Airleron (Control) to -500

Cl_P - Roll Rate (damping) to -1200

Cmo to -140

Cm_de - Elevator (Control) to -1800

Cn_beta Sideslip to 300

Cn_dr Rudder (Control) to 200

Cn_R - Yaw Rate to -1000

Cy_Beta Sideslip to -1400

Cy_dr Rudder to 100

 

would be a good start. Be certain that you save a copy of the .air file as is before you start changing anything. Do note that most (not all) of the values I list above are negative numbers, so pay attention to sign. The above will also make a few other slight changes, so check out the handling and see what you think. Remember, save a copy of the original .air file before making any changes.

 

By the way, just so I get a good idea of WHY it works what is it about these particular settings that makes it help? I mean, why does adjusting Cmo to -140 help? and what is Cmo?

 

Oh, and another thing...I think you would be proud of me. I've gotten the hang of hitting center line even with this issue lol.

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By the way, just so I get a good idea of WHY it works what is it about these particular settings that makes it help? I mean, why does adjusting Cmo to -140 help? and what is Cmo?

I don't really know, and I'm not sure anyone can really provide an answer. Also, I didn't work out these settings. A friend of mine, who had much more patience for this sort of thing than I ever would, spent many, many hours of trial and error developing this. FS "aerodynamics" are table driven, rather than trying to be a true physics engine, and so these factors are what control aircraft behavior. Many of these factors work with and/or against each other, thus changing one has effects from other factors too, making things even more complex.

 

Oh, and another thing...I think you would be proud of me. I've gotten the hang of hitting center line even with this issue lol.

You should be proud of making good progress -- I'm happy for you.

 

So make the changes and let me know how it works out for you.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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By the way, just so I get a good idea of WHY it works what is it about these particular settings that makes it help? I mean, why does adjusting Cmo to -140 help? and what is Cmo?

 

Oh, and another thing...I think you would be proud of me. I've gotten the hang of hitting center line even with this issue lol.

 

Messing with the .air file is for experts only. If you have to ask, you're not one. Leave it alone.

`Cmo` is something to do with Coefficient of Restitution in Centre of Mass Frame.

Precisely what is beyond the scope of this forum.

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Messing with the .air file is for experts only.

So long as you save a copy of the original, you can't really hurt anything, just perhaps have some weird experiences "in flight," easily rectified by going back to the original. Since there's a rather strong lack of documentation from MS about what these various factors do, how do you become an expert except for years of experimentation or by learning from someone who did that?

 

Besides, these particular changes are helpful, and are easily done, once you know the desired values. So I wouldn't consider this operation to "need" an expert.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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So long as you save a copy of the original, you can't really hurt anything, just perhaps have some weird experiences "in flight," easily rectified by going back to the original. Since there's a rather strong lack of documentation from MS about what these various factors do, how do you become an expert except for years of experimentation or by learning from someone who did that?

 

Besides, these particular changes are helpful, and are easily done, once you know the desired values. So I wouldn't consider this operation to "need" an expert.

 

Disagree, One becomes an expert by hours of learning and understanding the innate issues between various parameters in the .air file (and the interrelationship between the .air and aircraft.cfg file).

One cannot do it in five minutes (or else you would have answered the question about Cmo and why it's important).

 

I calculated that with `a bit`of aviating knowledge (about five years worth derived from building my own piston single, twin seat Europa) one can have a reasonable stab at air file development but not without help and guidance - random `stabs in the dark` are tantamount to learning how can screw it up.

 

And even then I needed the hand of the late, great Ron Freimuth to put me right in several areas. And where would you suggest one can get the values? Not from Cessna, Piper, Lockheed Martin, Boeing?

X-Plane is a far better place to start with Blade Elemt Theory, which can be worked into a halfway decent flight model in about six months. And that can be done by plugging-in known parameters.

 

There are NO such education resources for the .air flight model - not even who the proprietor of the software was, because it wasn't Microsoft...

All knowledge has come from re-engineering and reverse engineering, and filtering out the fields that don't work with the various elements that variously form FS2004, FSX, and P3D.

 

So who out there claims to be an `expert`? And where are their published works for review by other experts?

 

You can ONLY screw it up by `hitting and hoping`, so advising anyone to go ahead and try in this early stage of their flight simming career leads ONLY to disaster.

But don't let me stop you from confirming that.

FSX has been out for 13 years now, while our new arrival only found flight sim about 6 mths ago. And no evidence to this point that he has `aviating experience` in the real world...

Do you REALLY want to advise him to play with something even we, with many years of experience, can't claim the first thing about..?

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Very interesting, maybe I can answer a bit of the question from real world experience.

I was connected with developing various bits on the GAF/ASTA Jindivik drone on the Aberporth range in the late 80's through til ops ceased in 2004. Having the radar track data & the aircraft telemetry & command signal recordings using the common timing system it was possible to establish that the aircraft track was delayed by about 2 seconds - this for a fixed bank angle rate of 10 degrees per second. Note that 10 degrees per second sounds slow, but it surprised a lot of pilots - possibly because the ailerons on the Jindivik go to max deflection almost instantaneously before reducing to maintain the rate of bank ( rate gyro feed back). If a Rapid roll was selected the instantaneous rate of roll could be 60 to 80 degrees per second, diminishing to zero as the desired bank was achieved, but was only used with demanded bank angles between 60 & 78 degrees.

Keith

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Mallcott, I wasn't advising him to play around with unknown parameters, I was offering values that had been worked out by an expert (not me) and, more importantly, were tested to show an improvement in handling, and that would alleviate his specifically stated problem.

 

AND, I reiterate that, so long as he saves a copy of the original, he can't hurt anything, since the original can then be reinstated.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Mallcott, I wasn't advising him to play around with unknown parameters, I was offering values that had been worked out by an expert (not me) and, more importantly, were tested to show an improvement in handling, and that would alleviate his specifically stated problem.

 

AND, I reiterate that, so long as he saves a copy of the original, he can't hurt anything, since the original can then be reinstated.

 

Disagree, again.

 

If one cannot get the results one expects, frustration is the ONLY likely result.

 

Flight dynamics modelling is regarded as an art, not a science, for good reason.

Mixing the correct color palette doesn't make you Michelangelo... it doesn't even make one an aircraft painter!

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Very interesting, maybe I can answer a bit of the question from real world experience.

I was connected with developing various bits on the GAF/ASTA Jindivik drone on the Aberporth range in the late 80's through til ops ceased in 2004. Having the radar track data & the aircraft telemetry & command signal recordings using the common timing system it was possible to establish that the aircraft track was delayed by about 2 seconds - this for a fixed bank angle rate of 10 degrees per second. Note that 10 degrees per second sounds slow, but it surprised a lot of pilots - possibly because the ailerons on the Jindivik go to max deflection almost instantaneously before reducing to maintain the rate of bank ( rate gyro feed back). If a Rapid roll was selected the instantaneous rate of roll could be 60 to 80 degrees per second, diminishing to zero as the desired bank was achieved, but was only used with demanded bank angles between 60 & 78 degrees.

Keith

 

This only raises more questions than answers: Was the track response delayed by the rudder? Aileron? Yaw drag? Control surface response to radio input? Delays caused by radio control? Basic design of the Jindivik drone? Weight/inertia against size of control surfaces?

 

All of these questions require answers before the solution can be proposed...

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Some people have the utmost contempt for anything they don't understand.

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This only raises more questions than answers: Was the track response delayed by the rudder? Aileron? Yaw drag? Control surface response to radio input? Delays caused by radio control? Basic design of the Jindivik drone? Weight/inertia against size of control surfaces?

 

All of these questions require answers before the solution can be proposed...

 

All valid questions, but I no longer have the raw telemetry data to double check, but here goes from memory. Jindivik has no rudder. Aileron movement can be checked relative to control input timing, which at say a control transmission & telemetry return distance of 150 miles & a radio speed that of light is very small (.0016secs). There is a built in delay of 1/3 sec in analogue controlled aircraft to allow throttle, roll & pitch control to be 'trilled' i.e. to effectively allow transmission of two or three commands simultaneously. So yes there are delays if using the command timing, but not if aileron deflection is measured, the aileron deflecting as quoted in my original response. As with all aircraft the basic design along with weight/inertia vs size of control surface affect response, but forward inertia is possibly a greater value in a large passenger aircraft, so possibly delaying change in flight path even more? So if I say that accounting for delays in transmission etc the delay in change of track is in the order of 1.5 seconds, it means that with a bank angle rate of 10 degs/sec the aircraft is at least at 15 degs of bank before a track change can be seen, & this is with a fairly agile small aircraft travelling at 400+ knots. As speed diminishes then one can envisage the track delay to be smaller.

This leads onto an interesting thought about 'fighting' drones flown presumably by pilots in a ground station some many miles away & the delays in that type of situation if using satellites - think of the delays news correspondents suffer......

Keith

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So if I say that accounting for delays in transmission etc the delay in change of track is in the order of 1.5 seconds, it means that with a bank angle rate of 10 degs/sec the aircraft is at least at 15 degs of bank before a track change can be seen, & this is with a fairly agile small aircraft travelling at 400+ knots.

 

This is all well and good, but has little to do with the OP's question, which is about a problem in the flight model of the FSX default C-172, which has the airplane sliding sideways (defying laws of physics) for a few seconds after wings level. That same problem will also result in sustained turns with more than a gentle bank (30º plus, for certain) causing the aircraft to start descending after a bit, but back pressure causing the nose to rise (much as if top rudder were added, rather than in the fashion of real aircraft). So one cannot hold altitude (you can, in a real aircraft, including the C-172, in at least a 45º bank).

 

This problem is NOT the same as larger, heavier aircraft taking longer to react, this is an aircraft doing the (real world) impossible. If it makes no sense to you, go back, read our descriptions, then try it yourself. Fly the FSX C-172 in slow flight just a few hundred AGL, then make a turn (yes, try to keep the ball centered, too) such as turning base to final, then level your wings, with NO wind. Do this from the 6 o'clock locked (not rubber band) spot view, and see what happens. Did you slide sideways for a bit?

 

Now get more altitude, then add full power to the C-172 and make a 45º (or even a 30º) banked turn. Hold that turn (use cockpit or spot view here, makes no difference) round and round and round, attempting to maintain altitude. What happens to altitude? What happens to pitch attitude?

 

Now use a modified .air file with the values I supplied above and go through those exercises again. Is there a difference? Check out other facets of handling. Is there any change? For better? For worse?

 

Bye.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Inuss,

Just tried the Cessna 172, but I get annoyed with it because its always left wing low if power is on, but if one throttles back & put it into a glide at 60 kn, then the torque is almost zero & it will fly straight, but is very responsive to almost zero aileron. I had though misread the original question, so sorry for going off at a tangent.

Keith

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so sorry for going off at a tangent.

Not a problem, Keith, I just wanted to get us back on track for the OP.

 

its always left wing low if power is on

Sometimes a fuel imbalance can cause wing low problems in many FS aircraft, often to a greater degree than I ever saw in real aircraft.

 

Larry N.

As Skylab would say:

Remember: Aviation is NOT an exact Science!

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Inuss,

Just tried the Cessna 172, but I get annoyed with it because its always left wing low if power is on, but if one throttles back & put it into a glide at 60 kn, then the torque is almost zero & it will fly straight, but is very responsive to almost zero aileron. I had though misread the original question, so sorry for going off at a tangent.

Keith

 

So, how have you modified the C of G? Off-center weight and balance is exaggerated in FSX.

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So, how have you modified the C of G? Off-center weight and balance is exaggerated in FSX.

 

My initial test was bog standard FSX C172SP, no changes to air file or aircraft.cfg. Since then I have noted that I have two persons on board therefore no offset, neutralised the lateral position of the fuel, modified the air file as per your post, but noted that there is no flight tuning entry for p factor on yaw. So two tests, one without that entry & one with p factor set at 0.2. Both still left wing low although 0.2 value a lot better.

Comparing it with my WIP model which has the same value of prop MoI, & similar engine power & size of prop, my model exhibits zero turn with torque with p factor set at 1.0, wonder why - must compare air file & aircraft.cfg more closely.

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My initial test was bog standard FSX C172SP, no changes to air file or aircraft.cfg. Since then I have noted that I have two persons on board therefore no offset, neutralised the lateral position of the fuel, modified the air file as per your post, but noted that there is no flight tuning entry for p factor on yaw. So two tests, one without that entry & one with p factor set at 0.2. Both still left wing low although 0.2 value a lot better.

Comparing it with my WIP model which has the same value of prop MoI, & similar engine power & size of prop, my model exhibits zero turn with torque with p factor set at 1.0, wonder why - must compare air file & aircraft.cfg more closely.

 

And that is the very reason I answered previously as I did. YOu are now making half-bitten attempts to correct what you perceive as a `a problem` without any understanding of how things interract, nor validation as to whether there actually is `a problem` requiring fixing.

 

You are better off seeking out the Real Air 172 .air and .cfg files and using them, or making use of the corrected flight modelling from Alex Metzger.

Both may be found via google searches e.g.

http://fsdeveloper.com/forum/attachments/a_metzger_fdes-zip.20571/?temp_hash=eb08ba2b3340143106a5b79b03504c75

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Mallcot,

Thank you very much for that link to Alex Metzger - what a totally different aircraft he has produced & much more as I would envisage how the real one would fly. Aircraft.cfg identical to the M$ version, but block 1101 90% different, not compared the other blocks yet. I did notice though that the C of G shows up as being at 15% chord, which seems a bit too far forward.

So thank you again, I shall now go back & enjoy the modified C172.

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