Flight Model Tweaks

1st Official Post

    Jet-Pack (IPACS) Hello there！It is long time from I sand post in this forum. I was busy with my studies and other things recenttly, and have no time to devote to flight simulator. Now I'm trying to learn to modify aircraft aerodynamics. I found that in the new version you added the following aerodynamic code.


    <[float64][GroundEffectFactor] [1.2]>
    <[float64][GroundEffectScale] [0.68]>
    <[float64][GroundEffectCm0] [-0.06]>
    <[float64][GroundEffectCmFlap] [-0.08]>

    I'd like to know what exactly this code does for the ground effect of the aircraft. I am modifying the parameters of the B777 to make its landing performance more realistic.:)

    Thanks a lot for your reply！

    • Official Post

    These values change the pitch nose down or nose up effect when the aircraft is in ground effect.

    Cm0 is the moment that is added in ground effect regardless of flap and CmFlap is proportional to flaps deflection.

    This makes it possible that with full flaps, close to the ground the aircraft could either pitch up or down on its own. In this case the negative values cause a slight pitch down effect, so that it required more elevator to flare.

    Jet-Pack (IPACS) When modifying the ground effect of the B777-300ER, I encountered the following issues:

    1. I adjusted the value of GroundEffectScale, and it seems to have an effect. However, I'd like to understand its numerical relationship with altitude and whether it correlates with other state parameters.

    2. It is known that the value of GroundEffectFactor affects the magnitude of the ground effect force on the aircraft. However, I want to know specifically whether it changes the magnitude of the force or the effective area of the force.

    3. During the modification process, I observed that when the GroundEffectFactor value increases to a certain level, deviations in approach speed lead to significant differences in stress upon entering ground effect. Consequently, the aircraft exhibits different landing performances at similar but different speeds, such as:

    • Approaching at 147 knots, 20 feet pitch up, resulted in a soft landing.

    • Approaching at 145 knots, 30 feet pitch up, resulted in a hard landing.

    Is there another function that can reduce the strong coefficient relationship between GroundEffectFactor and landing speed? Or are there alternative solutions available?

    Code
    <[float64][AspectRatioMultiplier][2.0]>
<[float64][GroundEffectFactor]   [1.18]>
<[float64][GroundEffectScale]    [0.725]>
<[float64][GroundEffectCm0]      [-0.06]>
<[float64][GroundEffectCmFlap]   [-0.08]>
<[float64][BetaSlope]            [1.0]>
<[float64][BetaMax]              [1.0]>
    • Official Post

    The ground effect works by reducing the downwash and thus reducing induced drag locally. It's not a simple formula. The scale is the height at which you start noticing the effect, normalized by roughly the wing span. So at 1.0 if you drop below about one wing span the ground effect starts to kick in. The closer you get to the ground the more it becomes noticeable.

    There are many factors when it comes to a soft landing and many things affect the touch down speed. The ground effect factor, scale, cm0 and flap all contribute a little bit. But so does the CG, flap angle, flap and slat configuration, abruptness of the flare, amount of wing flex (wing further away from ground), etc.

    Indeed, Aerofly continues to be an advanced flight simulator. While modifying the aerodynamics of the B777-300ER, I found the overall aerodynamic model of the aircraft to be very well-developed. Most of the data aligns closely with the performance described in the FCOM (Flight Crew Operations Manual).

    After some adjustments, I strongly believe that the B77w now exhibits closer-to-reality landing aerodynamics. I've uploaded it to Flightsim, and now you can enjoy the landing process of the B77w.

    https://flight-sim.org/filebase/index…ormance-closer/

    You can fully reference the approach phase operations from the actual cockpit, including flare timing and stick movements. Manual throttle control during landing also becomes very easy! Moreover, the touchdown behavior is quite similar to what you'd observe with the 77w at an airport.

    Here are my modifications:

    1. Adjusted induced drag of horizontal stabilizers to improve trim.
    2. Modified angle of attack lift to minimize excessive changes during speed deviations, aligning with real heavy aircraft aerodynamics.
    3. Tweaked landing ground effect parameters to enhance the feel of leveling during landing, resembling real heavy aircraft and PMDG777 aerodynamics. (Soft landings become more controllable.)
    4. Simultaneously adjusted throttle selection speeds in the landing phase .tmd file to match approach speeds based on the same parameters as the actual aircraft.
    6. Adjusted some display parameters in the CDU to align with the data described in the .tmd file.

    Jet-Pack (IPACS) During the modification process, I encountered a few issues that I hope you can help me resolve:

    1. Currently, the performance data displayed in the CDU's approach interface doesn't match the data described in the real 77w's FCOM. For example:
    *Current weight: 230.1t
    *FCOM reference:
    VREF at 30° flaps: 143 knots
    VREF at 25° flaps: 151 knots
    VREF at 20° flaps: 161 knots
    *Aerofly CDU displays:
    VREF at 30° flaps: 139 knots
    VREF at 25° flaps: 143 knots
    VREF at 20° flaps: 146 knots


    I understand that you've used certain formulas to calculate approach speeds. However, I hope these formulas can be refined to match the parameters provided in the FCOM.

    2. According to the parameters provided in the FCOM, with 30° flaps, for every 10-ton decrease in payload, the approach speed should decrease by 3 knots. When I adjusted the payload weight in the .tmd file, I noticed that a 10-ton reduction didn't significantly impact the final approach speed. It seems the issue lies within the aircraft's momentum function or the relationship between lift and speed. I wonder if you can assist me in refining these aspects.

    3.I also want to modify the autopilot panel in the takeoff.tmd to adjust the speed selection to 145 knots (VR). However, I've noticed that no matter how I modify the values, the game doesn't seem to apply them. Please provide me with some guidance.

    Code
                // autopilot
            <[autopilot_advanced][Autopilot][]
                <[bool][InitFlightDirectorsOn]      [true]>
                <[bool][InitAutoThrottleOn]         [true]>
                <[bool][InitManagedHeadingArmed]    [true]>
                <[bool][InitVerticalNavigationArmed][true]>
                <[bool][InitApproachArmed]          [false]>
                <[string8][SelectedAirspeed]        [74.6]> //77.2 to 74.6


    I also hope you can check whether my aerodynamic modifications in the .tmd file have any significant issues. Compared to aircraft makers on forums, I'm just a passionate beginner when it comes to the B77w. I can assure you that my modifications aim to mimic real performance, but I'm unsure if some adjustments might conflict with your defined aerodynamic functions in the game engine. If so, I'd greatly appreciate it if you could point them out! Lately, I've been organizing information from the Aerofly Wiki and translating it. I plan to publish it in the Chinese community to invite more interested players to engage in Aerofly's scenery and aircraft creation.


    In terms of current flight simulation colleagues, I believe that Aerofly's performance with the B77w surpasses many projects of similar scale. At least for me, despite its shortcomings, I'm quite satisfied. Shortcomings primarily stem from incomplete avionics logic and updated navigation data, which are Aerofly's drawbacks. I sincerely hope that Aerofly's Boeing aircraft series continues to improve!

    • Official Post

    Keep in mind that the parameters in the base 777 tmd are not the ones actually used by the aircraft when the sim is loaded. There is an additional parameters.tmd which changes the aerodynamic parameters for the 777-300er. The underlying base aircraft is actually a -200lr with the -300er adjusting some values. This will make it easier for us to add new variants but right now it might be confusing with just one aircraft variant available.

    The flap speeds are derived from lift coefficient per flap, wing area and gross mass. If you look for VReference speeds you find where the lift coefficient for that flap is set. You can adjust that value to adjust the airspeed. The rest is just the lift equation...

    The takeoff mass and gross mass should be correct im the sim.

    Jet-Pack (IPACS) Thank you for the help! Some minor data errors have been fixed now. I fine-tuned the aerodynamics again, and the performance is great! You can modify the aircraft's payload in parameter.tmd and synchronize it in the CDU to calculate VERF. The approach speed will be a reference! Also, the autoland feature will be spectacular; it's truly beautiful!;)<3

    I've encountered a small issue though:
    I adjusted the formula for calculating VERF, and it's working fine. The VREF displayed on the PFD is derived from the data after my modification. However, the speed for the 25° flaps hasn't been affected by the modification. It still maintains the calculation result from the previous formula. This often causes overlap with VREF.
    Specifically, as shown in the image:

    Hope you can provide some help. After fixing this issue, I'll publish it again to FlightSim. Perhaps you can synchronize this data in future updates. At least, I believe its performance is exceptional, surpassing Infinite Flight and RFS on mobile.^^^^

    Reporting another issue:
    The spoilers on the B77w should only deploy when all six wheels of the main landing gear are fully on the ground (compressing the main landing gear hydraulic struts).


    Currently:
    The spoilers extend when the lowest wheel touches the ground, causing a disruptive lift effect and making the aircraft drop.

    I don't know how to modify the aircraft's logic system at the moment, so I hope this issue can be addressed. Thanks again for the help!<3

    Regards,

    Orange Dog

    Quote from Jet-Pack (IPACS)

    I would have to dig into the manuals to confirm when the aircraft deploys the spoilers. Often it's a mix of gear compression and wheel spin up and radar altimeter in some cases.

    Hope it gets better. There is also the display issue of PFD speed above. Perhaps some input data needs to be modified. This is the last problem I've found so far.:)

    Quote from orangedog433

    The spoilers on the B77w should only deploy when all six wheels of the main landing gear are fully on the ground (compressing the main landing gear hydraulic struts).


    The spoilers extend when the lowest wheel touches the ground, causing a disruptive lift effect and making the aircraft drop

    That's only partially correct and even more complex.

    Even with all 6 wheels on the ground only 4 panels extend and not to their full extension angle.

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    As Jet-Pack mentioned it's a combination of various factors


    The spoilers extend when the lowest wheel touches the ground, causing a disruptive lift effect and making the aircraft drop.

    That's what happens with most airliners IRL, since spoiler extension is mainly tied to wheel spin up. If the touchdown is too smooth, there's no strut compression, but enough wheel spin up to make the spoilers fully extend. Furthermore on many swept wing planes the spoiler extension causes pitch up. The 767 with it's, compared to the 777, 'reversed' bogie is difficult to grease on, but IF you can achieve it, this can happen very easily.

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    My airplane art:

    Bernt Stolle - Art for Sale | Fine Art America

    Jet-Pack (IPACS) I wonder that what's the meanings of these codes. I am still modifing 77w so that it can perform much more realistically.:) I adjusted the force acting on the landing gears. It looks good , but I still don't know if my action is logical. So please explain some roles of these codes which seems don't be mentioned in wiki.

    Thanks a lot!<3

    Code
    <[float64][LimitSpring]      [11000000.0]>
<[float64][LimitDamping]     [  200000.0]>
<[float64][ForceCompressed]  [ 600000.0]>
<[float64][DampingCompressed][  300000.0]>
<[float64][ForceExtended]    [    2500.0]>
<[float64][DampingExtended]  [    1500.0]>
    • Official Post

    These are spring constants and damping constants in N/m and N/(m/s). For the oleo damper the values vary between the fully extended position and fully compressed position. The limit spring and damping values come into play once the damper has reached the fully compressed position and hits the hard stop.

    The force extended and damping extended are most important when you do a soft touchdown and when you lift off. The gear then moves from the full extended towards being compressed or the other way around. The more the gear compresses during the touchdown the more the spring and damping values increase until the spring/damper produces enough opposing force to bounce the aircraft back up. Fast motion is damped with damping constant, strong force is stopped with the spring force constants.

    Just try and manipulate one of the values like DampingExtended and ForceExtended.
    Keep in mind that they also have to work for a fully loaded aircraft at 2G touchdowns.

    Quote from Jet-Pack (IPACS)

    These values change the pitch nose down or nose up effect when the aircraft is in ground effect.

    Cm0 is the moment that is added in ground effect regardless of flap and CmFlap is proportional to flaps deflection.

    This makes it possible that with full flaps, close to the ground the aircraft could either pitch up or down on its own. In this case the negative values cause a slight pitch down effect, so that it required more elevator to flare.

    This is a key point that Jet Pack has mentioned. Traditionally, an aircraft will pitch down as it enters ground effect. This along with the 5 knot reduction crossing the threshold will require some back pressure to maintain the approach pitch and trajectory. One key observation is how fly by wire aircraft handles this. Since FBW would automatically trim out this ground effect impact, flaring a FBW aircraft would be difficult. To tackle this issue, FBW systems will start to trim the nose down at the height the aircraft would enter ground effect. This forces you to counter the nose down trim with back pressure, thus making the flare conventional. An aircraft will enter ground effect at about half of it's wingspan. So, a wingspan of 100FT starts to enter ground effect at 50FT AGL.

    Jet-Pack (IPACS)

    it seems that something went wrong with 77w after newest update.

    When I want to use A/P to land. Its nose will pitch up and down repeatedly by control from A/P, also can`t flare correctly. Maybe the A/P logic works incorrectly! It just kept normal last version.;(

    Also, I noticed that NO VSPD will be shown on PFDs when the plane just touch down. As far as I know, NO VSPD wouldn`t display until 777 lands on the runway and reduces the true air speed brlow 45 knots. Just like what it performed before update. I can`t find references on FCOM at the moment. But the real planes represented in videos on YTb can prove me. PMDG also make the same miskates on their 777s. But I think it shouldn`t appear here!:S

    [

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    Also I want to ask if it`s possible to apply my modifications after tested and proved by IPACS so that I wouldn`t modify .tmds after every updates? I spend many times for a more real 77w performance and flying experience. I can send all my modification through E-mai. If IPACS do not accept such requests. Just pretend I didn't say it.:thumbup:

    • Official Post

    Due to the timing it's currently quite difficult in incorporate external changes.

    To keep your custom changes safe from any Steam updates consider collecting all your changes in your user documents folder in a new folder b777_300er. Inside this folder create a new subfolder called something like "physics_mod". In this folder you can store your manual changes in form of parameters.tmd file or systems.tmd files. Then just add an options.tmc file with the type "default". This makes it so that after your normal B777 is loaded Aerofly FS also loads your custom modifications and applies them on top of the base parameters and the b777_300er parameters. Perhaps you will need to copy the parameters.tmd file from inside the b777_300er Steam folder but hopefully you just need to add your changes that should be loaded on top of that default file.

    Please let me know if this works, I have not modified the aircraft like that. But in theory it should work.

    Regarding the V-speeds, I think right now it's just a time delay of 30 seconds on ground, then it resets the system. Could also be a different logic depending if you perform a full flight or if you started the flight on (short/long) final approach.

    Quote from Jet-Pack (IPACS)

    Due to the timing it's currently quite difficult in incorporate external changes.

    To keep your custom changes safe from any Steam updates consider collecting all your changes in your user documents folder in a new folder b777_300er. Inside this folder create a new subfolder called something like "physics_mod". In this folder you can store your manual changes in form of parameters.tmd file or systems.tmd files. Then just add an options.tmc file with the type "default". This makes it so that after your normal B777 is loaded Aerofly FS also loads your custom modifications and applies them on top of the base parameters and the b777_300er parameters. Perhaps you will need to copy the parameters.tmd file from inside the b777_300er Steam folder but hopefully you just need to add your changes that should be loaded on top of that default file.

    Please let me know if this works, I have not modified the aircraft like that. But in theory it should work.

    Regarding the V-speeds, I think right now it's just a time delay of 30 seconds on ground, then it resets the system. Could also be a different logic depending if you perform a full flight or if you started the flight on (short/long) final approach.

    I will try it tomorrow. The update also make some unknown crash on my laptop, but my desktop works normally. I have to delete and redownload Aerofly so that I wouldn't stuck in loading page for half an hour.:(:?:

    Why do the A380 spoilers slightly raise during engine startup and flap deployment?

    Kind regards and safe flying.

    -Thekingbuji333

    Jet-Pack (IPACS) The method you mentioned above work well! Most of my applications are effective. But my modifications on CDU VREF don`t work. It can take effect if I modify system.tmd directly, but can`t on parameters. These are my codes. Please help me check them.:):)

    I think it might lose some codes to describe it exactly?:(