Calling FDE experts

[Tweek]

As always, and probably like a lot of you, I spend far too much time trying to tweak and improve things in the sim, rather than actually enjoying flying about! So, my latest niggle is with regards to the flight dynamics of certain aircraft...

1. This may be a Piglet specific annoyance, as I don't recall experiencing it on any other aircraft - when rolling, there's a fairly lengthy period between you actually centering the stick and the aircraft stops rolling. This is particularly evident on the F-5A, if you want to try it to see what I mean. It's as if it needs some sort of dampening so when you do centre the stick, the aircraft stops rolling almost immediately (obviously not stopping dead, else it'd be like an arcade game).

2. I'll do my best to try and explain this one - certain aircraft have a tendancy to go into some bizarre twisting maneouvre, again, when rolling. This is mostly evident in the default Extra 300 and Dave Garwood's Hunters. It's as if when they roll, they also apply a bit of rudder aswell, so instead of your nose staying on the horizon (with the correct elevator adjustments, of course), you end up with it bobbing up and down all over the place, unless you roll incredibly slowly.

Sorry if the problems aren't too clear, but they certainly need to be experienced to see what I'm on about. If you need further explaination then I'll try and record a couple of videos to show you what I mean.

So really, I'm just wondering whether there are any values I can change, either in the config or the air file, that'll reduce these effects. Simple as that!

Disclaimer: whether these characteristics are realistic or not, I don't know, but either way, I'd rather just have them suited to my tastes. I'd rather just have it so I actually enjoy flying these aircraft, rather than be annoyed at how they behave!

[JohnC]

1) The main value for this is in Table 1101 (.air), Cl_p or roll damping factor. Increasing the magnitude of this number will provide more velocity (angular) dependent resistance. As this increases, you'll need to further increase the aileron roll moment, or Cl_da. You can make velocity based adjustments to these broad numbers in tables 455 and 453 for Cl_p and Cl_da respectively.

2) This behavior is most likely caused by an aileron yawing moment, Cn_da but could also be roll induced yaw, Cn_p. These can also be found in table 1101, in the yaw section. Decreasing their magnitude would likely achieve the results you want. As with the above values, velocity based adjustments can be made in tables 462 and 465.

2b) If you are really dis-satisfied with a "bobbing" behavior, you can also try to increase yaw stability with the weathervane, Cn_Beta, and the yaw damping factor, Cn_r. Cn_Beta acts like a spring for small Beta angles, providing and increasing force to center with increasing angle and Cn_r acts like a dash-pot and provides an increasing resistance to movement as angular velocity increases. Velocity based adjustment in 463 and 459.

Also, If any of the aircraft you are using have complex stalling characteristics such as spins, changing stability values will very likely mess up those behaviors.

[Matt Wynn]

certain aircraft remember though Tweek will continue to roll with the MOI (Moment of Inertia), you might want to try changing the .cfg first like 'roll stability' the extra real world 'bobs' (in my experience) it's down to the pilot to correct it, the extra is designed to be a handful :icon_lol: usual way to kill it dead is to just flick the nose skywards then roll, that way if/when it bobs you are back on flight plane. kinda shown in my old display video.... watch there, every time i plan a roll theres a bit of a flick, or a bit of nose high to counter it... i'm making a video at the moment that shows how the Default Extra CAN be flown, i hear people saying it can't knife edge in the past, she can... :icon_lol: same video will just be a basic FS Aerobatics video showing what can be done, and what can't...

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[Tweek]

Thankyou, John, I'll have a fiddle with those values and see what I can come up with.

[Tweek]

ignore

[Mr.Mugel]

When the center of gravity is not exactly correct (lets say it is too far in the front), and the plane is trimmed out, to fly straight when not inverted, than it will take the nows way down towards the ground when inverted, as the effect of the wrong center of gravity will work the same way as in level flight pushing the nose down, but the trim will work inverted and will push the plane down additionally.

A colleague from my old R/C club (he was german master in R/C aerobatics) told me how to try out if an aerobatic plane has an exact CoG. You just need to trim the plane for level flight and than push it down heading straight vertical towards the ground, so no CoG is taking effect, if it is correct it keeps going straight down, if the CoG is wrong and the plane only flies level by trim, it will pull up or push the nose even further, going into inverted flight.

Haven´t tested this in FS, but physically it is totally logical in my opinion. Hope it helps!

[JohnC]

"Haven´t tested this in FS, but physically it is totally logical in my opinion" That's sounds a little dangerous to me. I'll see if I can break this down briefly, but please don't hesitate for clarification if something doesn't make sense.

- It's important to note that all rigid body motion occurs about the CoG (translational through, rotational around), so it is impossible to negate it's affect.

- Since speed will undoubtedly increase as the aircraft noses down, this assumes all moments will remain constant relative to each other with increasing speed. Aerodynamics are hugely based on Reynold's number, and will be different for every control surface. Their numbers will remain proportional to each other as speed increases, but the relationships will not necessarily be similar. In addition to this, control surfaces with a significant offset from the CoG (such as a high main wing or horizontal stab) will produce a more substantial change in stability characteristics since their moment arm is relatively large.

- Finally, you should exercise a fair amount of caution when applying techniques used in R/C flight to larger aircraft. Almost every R/C aircraft I've seen does not pass the Buckingham Pi Theorem when compared to their larger namesakes.

[Tweek]

Ok, the values in problem 2 corrected it, so that one is sorted. Thanks!

However, in #1, no matter what I do with the values, put them up, down or whatever, the F-5 still wants to carry on rolling after I centre the stick. It seems to take a moment for it to actually start rolling too, so I'm wondering whether there's maybe a bit of lag between actually moving the stick and something happening with the control surfaces? That could also explain why it carries on rolling by itself for longer than it should.

While we're here, I may aswell bring up another one. Some aircraft pull into a very high angle of attack when you make anything more than a small movement of the elevators. What values would I need to change to decrease the angle of attack in a turn (and vice versa - some aircraft don't display this characteristic enough, like the Accel F/A-18)? The problem is mostly evident when you are constantly losing speed when piling on even the slightest amount of G.

[JohnC]

1) If the you'd like to change the rate of positive and negative acceleration, then you'll probably want to take Smoothie's suggestion and decrease the roll MOI.

3) This is more complicated, and I'll need to wait until this evening before I can get into it.

[JohnC]

-- double post --

[Wozza]

However, in #1, no matter what I do with the values, put them up, down or whatever, the F-5 still wants to carry on rolling after I centre the stick. It seems to take a moment for it to actually start rolling too, so I'm wondering whether there's maybe a bit of lag between actually moving the stick and something happening with the control surfaces? That could also explain why it carries on rolling by itself for longer than it should.

Hi
If changing some of the params in section 1101 have no effect, have a look down the end of the airfile for sections that start with !*FS2K.These are individual sections that are used rather than the 1101 entries.
Wozza

[Tweek]

Hi
If changing some of the params in section 1101 have no effect, have a look down the end of the airfile for sections that start with !*FS2K.These are individual sections that are used rather than the 1101 entries.
Wozza

Excellent, thank you very much! Edited the values incorrectly and I ended up with an aircraft that wouldn't stop rolling, so I guessed that must be the problem! Changed the values back the other way and it sorted it.

[fliger747]

Most tactical aircraft will not stop rolling (quickly) unless some reversal of controls is applied as there is not a large positive stability designed in as there might be say in a transport aircraft.

T.

[Tweek]

Most tactical aircraft will not stop rolling (quickly) unless some reversal of controls is applied as there is not a large positive stability designed in as there might be say in a transport aircraft.

T.

Of course, but on the aircraft I've edited, the effect was far too pronounced. With the F-5, if I let the stick go as it levelled out (obviously, you'd do it just before you levelled out usually), it'd end up inverted again. I've merely reduced it so it's not totally uncontrollable when making a sharp, fast roll.

[fliger747]

I once landed at SFO with 117,000 kg of fuel... talk about roll inertia!

Yep, making them "fly" is fun!

Cheers; t

[Wozza]

Yep, making them "fly" is fun!

Cheers; t
Then why do I always end up banging my head against the keyboard and in tears :D

[warchild]

I'm loving all the replies i'm reading in this. Incredible amounts of knowledge here, but one thing i didnt see, and whats missing from a great majority of config files is the entry for adverse ( coupled ) yaw. i can only imagine that one reason no one uses it is because finding a formula for deriving it, is almost impossible, and yet, it's the very thing that will cause your plane to do some really bizzarre ****, including what you mentioned above..
Sadly, it would take a good sized book to explain the physical dynamics of adverse yaw, and worse, although i can visualize it, and understand it, i have an incredibly difficult time putting anything into words that make sense to anyone but me.. a basic starting point for finding it though would be to take the MOI for yaw, and work that into a 3D plane equation ( not sure my teminology is correct )..
Since adverse yaw works within a 3d globe encircling the plane and is the result of the combined forced or pitch roll and yaw creating a fourth vector of motion, you will need to find a way to determine the directional force of the adverse yaw and work it into your equation..

Another area that gets freaquently abused is pitch moment. In general, it's easy to make this value 9 in table 1101 of the air file ) way way too high, requiring an unbelievable amount of counter force to keep the plane from rocketing up or down. One gentleman went into that above though so i wont try and reinvent the wheel here.. he did better than i ever could at explaining it..

best of luck..

Pam

[jmig]

Most tactical aircraft will not stop rolling (quickly) unless some reversal of controls is applied as there is not a large positive stability designed in as there might be say in a transport aircraft.

T.

I support T's statement. The F-5's wings are very small and the aircraft has a tremendous roll rate. I can't remember if I ever went beyond the center position to stop a fast roll in the T-38 or F-4? (never looked) I do know that it took a few attempts to stop the T-38 at wings level when doing a rapid aileron rolls.

Looking at videos of acrobatic pilots, it looks to me that they go beyond the center when stopping a roll.