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Request: fix the 772 fdm!

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Request: fix the 772 fdm!

Postby Flightgear_captain » Thu Jan 06, 2011 9:03 am

As many have noticed before, the 772 fdm is very unrealistic. Did you know that you can take off from tncm with half of the runway available? With NO flaps? :shock: Please, make it less noob friendly!
Screenshot coming soon.

note: i am only posting this because I don't want to go around waking dead topics and complicating everything.
Last edited by Flightgear_captain on Thu Jan 06, 2011 9:30 pm, edited 1 time in total.
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Re: Request: fix the 772 fdm!

Postby isix » Thu Jan 06, 2011 1:11 pm

You can contact Syd Adams et al. here http://gitorious.org/syd-s-flightgear-content/777-200er .
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Re: Request: fix the 772 fdm!

Postby skyop » Thu Jan 06, 2011 11:25 pm

At least the latest 772ER has an "incorrect takeoff configuration" alarm.

But the FDM is still a piece of work...
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Re: Request: fix the 772 fdm!

Postby nickyivyca » Fri Jan 07, 2011 12:24 am

It's always been one of the things I wanted to work with, but I never found the time. I also don't fly the 777 much because of the AP that doesn't seem to really work.
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Re: Request: fix the 772 fdm!

Postby Flightgear_captain » Fri Jan 07, 2011 4:39 am

Ok, as I promised there would be a screenshot, so here it is.

Image

i DIDN'T use flaps!!! :P
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Re: Request: fix the 772 fdm!

Postby ThorstenB » Sat Jan 15, 2011 1:06 pm

It takes someone who is into FDMs to fix this. I personally don't know (and don't care) too much about FDMs.
But here are some hints maybe motivating someone to have a closer look. As far as I can see:
- the empty airplane mass is correct (315000lb) (matches the 777 spec).
- the two engines are configured to 93400lb of thrust each (with a mass of 13889 lbs) - both match the spec of 777's Rolls-Royce Trent 877 engines.
- the property /yasim/gross-weight-lbs property shows perfectly reasonable values: weight between 315000 and 670000lbs depending on fuel/payload (max 777 T/O weight is 656000lbs).
- properties /engines/engine[0/1]/thrust-lbs show correct values when powering the engines.

So the basic parameters influencing acceleration (mass vs thrust) look correct to me. I still agree that the 777 seems to be accelerating too fast. So there either has to be another issue (maybe drag/friction is too low?) or there is a fundamental problem with the YASim FDM physics.

Again, I don't know much about FDMs - so it's just meant as a teaser for some else to have a look into what could be wrong. If anyone came up with a reasonable suggestion of how and what to change exactly to make it more realistic, I'm sure Syd would approve.

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Re: Request: fix the 772 fdm!

Postby skyop » Sat Jan 15, 2011 5:36 pm

I would suggest looking at the approach parameters. They control pretty much every aspect of a YASim aircraft's handling.

Another thing about the 772: the ailerons are too weak, especially since one jolt of the rudder sends the thing spinning.
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Re: Request: fix the 772 fdm!

Postby nickyivyca » Wed Jan 19, 2011 8:16 am

Try this. It's a bit hard to land because of the elevators, but it flies like an airplane.

Code: Select all
<!--777-200 ER yasim

Specs:
2 - 90,000 lb PW4090 turbofans
or  93,700 lb GE90-94Bs
or  93,400 lb Rolls-Royce Trent 877s

Typical Cruise = 0.84 Mach / 35000 ft
Max Speed = 0.87 / 35000 ft


Wing span: 60.93m
Length: 63.73 m
Tail Height: 18.51m
Diameter : 6.19 m
Wing Area: 427.8m2
Max Fuel :45,220 US gal
Max TO weight: 656,000 lbs

TO speed : 145 kt
Landing speed : 131 kt

Leading edge MAC = -3.132

Vr 137
V2 147

Datum 92.5 in forward of nose - 2.3495 m
MAC 278.51 in - 7.074154 m
-->
<airplane mass="315000">

<!-- Approach configuration -->
<approach speed="139" aoa="0" fuel="0.1">
    <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.35"/>
    <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.35"/>
    <control-setting axis="/controls/flight/flaps" value="1"/>
    <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
    <control-setting axis="/controls/gear/gear-down" value="1"/>
    <solve-weight idx="0" weight="350"/>
    <solve-weight idx="1" weight="80000"/>
</approach>

<!-- Cruise configuration -->
<cruise speed="490" alt="35000" fuel="0.8">
    <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.8"/>
    <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.8"/>
    <control-setting axis="/controls/flight/flaps" value="0.0"/>
    <control-setting axis="/controls/gear/gear-down" value="0"/>
    <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
    <solve-weight idx="0" weight="350"/>
    <solve-weight idx="1" weight="80000"/>
</cruise>

<cockpit x="22.8" y="0.5" z="0.75"/>

<fuselage ax="25.36" ay="0" az="-0.64" bx="-20.0" by="0.00" bz="0.0"
    width="6.2" taper="0.0" midpoint="1.0"/>
<fuselage ax="16.231" ay="0" az="0" bx="-20.0" by="0.00" bz="0.0"
    width="6.2" taper="1.0" midpoint="0.5"/>
<fuselage ax="-20.0" ay="0" az="0" bx="-37.0" by="0.00" bz="1.2"
    width="6.2" taper="0.0" midpoint="0.0"/>
<fuselage ax="4.0" ay="9.8" az="-3" bx="-1.8" by="9.8" bz="-3"
    width="1.9" taper="1.0"/>
<fuselage ax="4.0" ay="-9.8" az="-3" bx="-1.8" by="-9.8" bz="-3"
    width="1.9" taper="1.0"/>

<wing x="-3" y="3.095" z="-1.5" length="8.5" chord="13.7" sweep="20" taper="0.6" dihedral="9" camber="0.1" incidence="3" twist="-1.5">
    <stall aoa="12" width="12" peak="1.5"/>
    <flap0 start="0.00001" end=".6" lift="1.9" drag="1.3"/>
    <slat start="0.12" end=".65" aoa="2" drag="1.2"/>
    <spoiler start="0.03" end="0.67" lift="0.2" drag="2.0"/>

    <control-input axis="/controls/flight/flaps" control="FLAP0"/>
    <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
    <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>

    <control-output control="FLAP0" prop="/surface-positions/flap-pos-norm"/>
    <control-output control="SLAT" prop="/surface-positions/slats"/>
    <control-output control="SPOILER" prop="/surface-positions/speedbrake-norm"/>

    <control-speed control="FLAP0" transition-time="78"/>
    <control-speed control="SPOILER" transition-time="5"/>
</wing>

<mstab x="-5.969" y="10.984" z="-0.404" length="21.8" chord="8.22" sweep="27.2" taper="0.23" dihedral="6.5" camber="0.05" incidence="1.5" twist="-.5">
    <stall aoa="12" width="12" peak="1.5"/>
    <flap0 start="0.00001" end=".52" lift="1.4" drag="1.3"/>
    <flap1 start=".52" end=".96" lift="1.4" drag="1.2"/>
    <slat start="0.00001" end=".99" aoa="2" drag="1.2"/>
    <spoiler start="0.03" end="0.48" lift="0.2" drag="2.0"/>

    <control-input axis="/controls/flight/flaps" control="FLAP0"/>
    <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
    <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>
    <control-input axis="/controls/flight/aileron" control="FLAP1" split="true"/>
    <control-input axis="/controls/flight/aileron-trim" control="FLAP1" split="true"/>

    <control-output control="FLAP1" side="left" prop="/surface-positions/left-aileron-pos-norm"/>
    <control-output control="FLAP1" side="right" prop="/surface-positions/right-aileron-pos-norm"/>

    <control-speed control="FLAP0" transition-time="78"/>
    <control-speed control="SPOILER" transition-time="5"/>
</mstab>

<hstab x="-31.8" y="1.35" z="1.2" chord="6.5" length="10.8" taper=".45" sweep="30" dihedral="6.5" effectiveness="1" camber="-0.01">
  <stall aoa="15" width="5" peak="1.5"/>
  <flap0 start="0" end="1" lift="1.2" drag="1.1"/>
  <control-input axis="/controls/flight/elevator" control="FLAP0"/>
  <control-input axis="/controls/flight/elevator-trim" control="FLAP0"/>
  <control-output control="FLAP0" prop="/surface-positions/elevator-pos-norm"/>
</hstab>

<vstab x="-29.8" y="0" z="2.7" chord="8.4" length="11.7" taper="0.34" sweep="34">
  <stall aoa="14" width="5" peak="1.5"/>
  <flap0 start="0" end="1" lift="1.5" drag="1.2"/>
  <control-input axis="/controls/flight/rudder" control="FLAP0" invert="true"/>
  <control-input axis="/controls/flight/rudder-trim" control="FLAP0" invert="true"/>
  <control-output control="FLAP0" prop="/surface-positions/rudder-pos-norm"
          min="1" max="-1"/>
</vstab>

<jet x="1.16" y="9.89"  z="-2.995" mass="13889" thrust="93400" spool-time="3"
egt="840" tsfc="0.32 " n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
  <control-input axis="/controls/engines/engine[0]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
  <control-input axis="/controls/engines/engine[0]/reverser" control="REVERSE_THRUST"/>
    <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[0]"/>
    <control-speed control="REVERSE_THRUST" transition-time="4"/>
<actionpt x="-2.7" y="9.89" z="-2.995"/>
</jet>

<jet x="1.16" y="-9.89"  z="-2.995" mass="13889" thrust="93400"  spool-time="3"
egt="840" tsfc="0.32" n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
  <control-input axis="/controls/engines/engine[1]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
  <control-input axis="/controls/engines/engine[1]/reverser" control="REVERSE_THRUST"/>
    <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[1]"/>
    <control-speed control="REVERSE_THRUST" transition-time="4"/>
<actionpt x="-2.7" y="-9.89" z="-2.995"/>
</jet>

<!-- nose -->
<gear x="19.28"  y="0.00" z="-5.55"
      compression="0.7" spring="1.2" damp="1" sfric="1.5" dfric="1.0">
  <control-input axis="/controls/flight/rudder" control="STEER" square="true"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[0]/position-norm"/>
  <control-speed control="EXTEND" transition-time="19"/>
</gear>

<gear x="-6.39" y="5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
  <control-input axis="/autopilot/autobrake/left-brake-output" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[1]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-6.39" y="-5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
  <control-input axis="/autopilot/autobrake/right-brake-output" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[2]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-7.8" y="5.6" z="-6.22" compression="3.0" spring="1.2" >
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[3]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-7.8" y="-5.6" z="-6.22" compression="3.0" spring="1.2">
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[4]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<tank x="-4.3" y="5" z="-1.5" jet="true" capacity="64000"/>
<tank x="-4.3" y="0" z="-2.5" jet="true" capacity="174900"/>
<tank x="-4.3" y="-5" z="-1.5" jet="true" capacity="64000"/>

<weight x="22.0" y="0" z="0" mass-prop="/sim/weight[0]/weight-lb"/>
<weight x="-4.3" y="0" z="0" mass-prop="/sim/weight[1]/weight-lb"/>

<ballast x="-30" y="0" z="4" mass="-23000"/>

<thruster x="19.28"  y="0.00" z="-5.55" vx="1" vy="0" vz="0" thrust="20000">
  <control-input axis="/sim/model/pushback/target-speed-fps" control="THROTTLE"
  src0="-1" src1="1" dst0="-1" dst1="1"/>
  </thruster>

</airplane>

I'll provide a change list later, because it'll be very long.
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Re: Request: fix the 772 fdm!

Postby ThorstenB » Thu Jan 20, 2011 9:16 pm

Ok, thanks nickyivyca for looking into this! It certainly changed a lot!
But is the lift really correct now? Or maybe too low now?
Looks as if it stalled at about 170kts with full flaps. Normal landing speed should be 130kts, but the FDM just dives/drops at such low speeds. Could also be a balancing issue.
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Re: Request: fix the 772 fdm!

Postby Tuxklok » Thu Jan 20, 2011 11:35 pm

ThorstenB wrote in Thu Jan 20, 2011 9:16 pm:Ok, thanks nickyivyca for looking into this! It certainly changed a lot!
But is the lift really correct now? Or maybe too low now?
Looks as if it stalled at about 170kts with full flaps.

Yikes, that is too high a speed for a stall with full flaps at any weight. :)

According to flight manual, at maximum landing weight of 460k lbs, vref at full flaps (vref30) should be 138 knots. As low as 116 knots at 320k lbs, and only as high as 166 knots at 660k lbs. Vref30 is also the maneuvering speed for full flaps, which says "provides adequate margin above stall at any weight"...not sure how much is considered adequate exactly..but I'd guess at least 10 knots or so.

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Re: Request: fix the 772 fdm!

Postby nickyivyca » Fri Jan 21, 2011 1:15 am

I didn't exactly do much with it-just one simple test flight. If I have some more time I'll try to fix stuff like stall speed and such.

By the way I'll try to make a list of changes here:
-Wing geometry changed (dual-wing set)
-Engine tsfc changed
-Elevators messed with
-Approach, cruise modified
-Overall YASim lift ratio down from ~220 to ~125.

Of course, it still needs to be made flyable, but that's something that I can do. Something that I noticed was that brakes weren't working, though that might just be my computer itself.

EDIT:

Code: Select all
<!--777-200 ER yasim

Specs:
2 - 90,000 lb PW4090 turbofans
or  93,700 lb GE90-94Bs
or  93,400 lb Rolls-Royce Trent 877s

Typical Cruise = 0.84 Mach / 35000 ft
Max Speed = 0.87 / 35000 ft


Wing span: 60.93m
Length: 63.73 m
Tail Height: 18.51m
Diameter : 6.19 m
Wing Area: 427.8m2
Max Fuel :45,220 US gal
Max TO weight: 656,000 lbs

TO speed : 145 kt
Landing speed : 131 kt

Leading edge MAC = -3.132

Vr 137
V2 147

Datum 92.5 in forward of nose - 2.3495 m
MAC 278.51 in - 7.074154 m
-->
<airplane mass="315000">

<!-- Approach configuration -->
<approach speed="139" aoa="3" fuel="0.1">
    <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.35"/>
    <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.35"/>
    <control-setting axis="/controls/flight/flaps" value="1"/>
    <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
    <control-setting axis="/controls/gear/gear-down" value="1"/>
    <solve-weight idx="0" weight="350"/>
    <solve-weight idx="1" weight="80000"/>
</approach>

<!-- Cruise configuration -->
<cruise speed="490" alt="35000" fuel="0.8">
    <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.8"/>
    <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.8"/>
    <control-setting axis="/controls/flight/flaps" value="0.0"/>
    <control-setting axis="/controls/gear/gear-down" value="0"/>
    <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
    <solve-weight idx="0" weight="350"/>
    <solve-weight idx="1" weight="80000"/>
</cruise>

<cockpit x="22.8" y="0.5" z="0.75"/>

<fuselage ax="25.36" ay="0" az="-0.64" bx="-20.0" by="0.00" bz="0.0"
    width="6.2" taper="0.0" midpoint="1.0"/>
<fuselage ax="16.231" ay="0" az="0" bx="-20.0" by="0.00" bz="0.0"
    width="6.2" taper="1.0" midpoint="0.5"/>
<fuselage ax="-20.0" ay="0" az="0" bx="-37.0" by="0.00" bz="1.2"
    width="6.2" taper="0.0" midpoint="0.0"/>
<fuselage ax="4.0" ay="9.8" az="-3" bx="-1.8" by="9.8" bz="-3"
    width="1.9" taper="1.0"/>
<fuselage ax="4.0" ay="-9.8" az="-3" bx="-1.8" by="-9.8" bz="-3"
    width="1.9" taper="1.0"/>

<wing x="-3" y="3.095" z="-1.5" length="8.5" chord="13.7" sweep="20" taper="0.6" dihedral="9" camber="0.1" incidence="3" twist="-1.5">
    <stall aoa="12" width="12" peak="1.5"/>

    <flap0 start="0.00001" end=".6" lift="1.6" drag="1.15"/>
    <slat start="0.12" end=".65" aoa="2" drag="1.2"/>
    <spoiler start="0.03" end="0.67" lift="0.2" drag="2.0"/>

    <control-input axis="/controls/flight/flaps" control="FLAP0"/>
    <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
    <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>

    <control-output control="FLAP0" prop="/surface-positions/flap-pos-norm"/>
    <control-output control="SLAT" prop="/surface-positions/slats"/>
    <control-output control="SPOILER" prop="/surface-positions/speedbrake-norm"/>

    <control-speed control="FLAP0" transition-time="78"/>
    <control-speed control="SPOILER" transition-time="5"/>
</wing>

<mstab x="-5.969" y="10.984" z="-0.404" length="21.8" chord="8.22" sweep="27.2" taper="0.23" dihedral="6.5" camber="0.05" incidence="1.5" twist="-.5">
    <stall aoa="12" width="12" peak="1.5"/>

    <flap0 start="0.00001" end=".52" lift="1.4" drag="1.15"/>
    <flap1 start=".52" end=".96" lift="1.4" drag="1.2"/>
    <slat start="0.00001" end=".99" aoa="2" drag="1.2"/>
    <spoiler start="0.03" end="0.48" lift="0.2" drag="2.0"/>

    <control-input axis="/controls/flight/flaps" control="FLAP0"/>
    <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
    <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>
    <control-input axis="/controls/flight/aileron" control="FLAP1" split="true"/>
    <control-input axis="/controls/flight/aileron-trim" control="FLAP1" split="true"/>

    <control-output control="FLAP1" side="left" prop="/surface-positions/left-aileron-pos-norm"/>
    <control-output control="FLAP1" side="right" prop="/surface-positions/right-aileron-pos-norm"/>

    <control-speed control="FLAP0" transition-time="78"/>
    <control-speed control="SPOILER" transition-time="5"/>
</mstab>

<hstab x="-31.8" y="1.35" z="1.2" chord="6.5" length="10.8" taper=".45" sweep="30" dihedral="6.5" effectiveness="2.5">
  <stall aoa="15" width="5" peak="1.5"/>
  <flap0 start="0" end="1" lift="1.38" drag="1.1"/>
  <control-input axis="/controls/flight/elevator" control="FLAP0"/>
  <control-input axis="/controls/flight/elevator-trim" control="FLAP0"/>
  <control-output control="FLAP0" prop="/surface-positions/elevator-pos-norm"/>
</hstab>

<vstab x="-29.8" y="0" z="2.7" chord="8.4" length="11.7" taper="0.34" sweep="34">
  <stall aoa="14" width="5" peak="1.5"/>
  <flap0 start="0" end="1" lift="1.5" drag="1.2"/>
  <control-input axis="/controls/flight/rudder" control="FLAP0" invert="true"/>
  <control-input axis="/controls/flight/rudder-trim" control="FLAP0" invert="true"/>
  <control-output control="FLAP0" prop="/surface-positions/rudder-pos-norm"
          min="1" max="-1"/>
</vstab>

<jet x="1.16" y="9.89"  z="-2.995" mass="13889" thrust="93400" spool-time="3"
egt="840" tsfc="0.32 " n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
  <control-input axis="/controls/engines/engine[0]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
  <control-input axis="/controls/engines/engine[0]/reverser" control="REVERSE_THRUST"/>
    <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[0]"/>
    <control-speed control="REVERSE_THRUST" transition-time="4"/>
<actionpt x="-2.7" y="9.89" z="-2.995"/>
</jet>

<jet x="1.16" y="-9.89"  z="-2.995" mass="13889" thrust="93400"  spool-time="3"
egt="840" tsfc="0.32" n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
  <control-input axis="/controls/engines/engine[1]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
  <control-input axis="/controls/engines/engine[1]/reverser" control="REVERSE_THRUST"/>
    <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[1]"/>
    <control-speed control="REVERSE_THRUST" transition-time="4"/>
<actionpt x="-2.7" y="-9.89" z="-2.995"/>
</jet>

<!-- nose -->
<gear x="19.28"  y="0.00" z="-5.55"
      compression="0.7" spring="1.2" damp="1" sfric="1.5" dfric="1.0">
  <control-input axis="/controls/flight/rudder" control="STEER" square="true"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[0]/position-norm"/>
  <control-speed control="EXTEND" transition-time="19"/>
</gear>

<gear x="-6.39" y="5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
  <control-input axis="/autopilot/autobrake/left-brake-output" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-left" control="BRAKE"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[1]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-6.39" y="-5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
  <control-input axis="/autopilot/autobrake/right-brake-output" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
  <control-input axis="/controls/gear/brake-left" control="BRAKE"/>
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[2]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-7.8" y="5.6" z="-6.22" compression="3.0" spring="1.2" >
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[3]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<gear x="-7.8" y="-5.6" z="-6.22" compression="3.0" spring="1.2">
  <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
  <control-output control="EXTEND" prop="/gear/gear[4]/position-norm"/>
  <control-speed control="EXTEND" transition-time="20"/>
</gear>

<tank x="-4.3" y="5" z="-1.5" jet="true" capacity="64000"/>
<tank x="-4.3" y="0" z="-2.5" jet="true" capacity="174900"/>
<tank x="-4.3" y="-5" z="-1.5" jet="true" capacity="64000"/>

<weight x="22.0" y="0" z="0" mass-prop="/sim/weight[0]/weight-lb"/>
<weight x="-4.3" y="0" z="0" mass-prop="/sim/weight[1]/weight-lb"/>

<ballast x="20" y="0" z="0" mass="15000"/>

<ballast x="-30" y="0" z="4" mass="-28000"/>

<thruster x="19.28"  y="0.00" z="-5.55" vx="1" vy="0" vz="0" thrust="20000">
  <control-input axis="/sim/model/pushback/target-speed-fps" control="THROTTLE"
  src0="-1" src1="1" dst0="-1" dst1="1"/>
  </thruster>

</airplane>

Try that. Flies MUCH better than the old one. I was able to land it by hand, with the main gears touching down first as well.
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Re: Request: fix the 772 fdm!

Postby VaLeo » Mon Jan 24, 2011 9:47 pm

Thanks nickyivyca for your work!

I'm really love 777, this my favorite big plane in FG.

But it really annoying that it cant do sharp turns on the ground - it seems that differential brakes dont work (but for SenecaII it fully working thought).
Is it problem of YASIM (the <control-input control="BRAKE"/> axis is sole straight force), and nothing can be done here, or is it bug in 777' FDM, or in my fgfs-git?
Is anyone here have working diff. brakes (,.-keys) in 777?
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Re: Request: fix the 772 fdm!

Postby nickyivyca » Tue Jan 25, 2011 12:20 am

Code: Select all
    <!--777-200 ER yasim

    Specs:
    2 - 90,000 lb PW4090 turbofans
    or  93,700 lb GE90-94Bs
    or  93,400 lb Rolls-Royce Trent 877s

    Typical Cruise = 0.84 Mach / 35000 ft
    Max Speed = 0.87 / 35000 ft


    Wing span: 60.93m
    Length: 63.73 m
    Tail Height: 18.51m
    Diameter : 6.19 m
    Wing Area: 427.8m2
    Max Fuel :45,220 US gal
    Max TO weight: 656,000 lbs

    TO speed : 145 kt
    Landing speed : 131 kt

    Leading edge MAC = -3.132

    Vr 137
    V2 147

    Datum 92.5 in forward of nose - 2.3495 m
    MAC 278.51 in - 7.074154 m
    -->
    <airplane mass="315000">

    <!-- Approach configuration -->
    <approach speed="139" aoa="3" fuel="0.1">
        <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.35"/>
        <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.35"/>
        <control-setting axis="/controls/flight/flaps" value="1"/>
        <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
        <control-setting axis="/controls/gear/gear-down" value="1"/>
        <solve-weight idx="0" weight="350"/>
        <solve-weight idx="1" weight="80000"/>
    </approach>

    <!-- Cruise configuration -->
    <cruise speed="490" alt="35000" fuel="0.8">
        <control-setting axis="/controls/engines/engine[0]/throttle-lever" value="0.8"/>
        <control-setting axis="/controls/engines/engine[1]/throttle-lever" value="0.8"/>
        <control-setting axis="/controls/flight/flaps" value="0.0"/>
        <control-setting axis="/controls/gear/gear-down" value="0"/>
        <control-setting axis="/surface-positions/flap-pos-norm" value="1"/>
        <solve-weight idx="0" weight="350"/>
        <solve-weight idx="1" weight="80000"/>
    </cruise>

    <cockpit x="22.8" y="0.5" z="0.75"/>

    <fuselage ax="25.36" ay="0" az="-0.64" bx="-20.0" by="0.00" bz="0.0"
        width="6.2" taper="0.0" midpoint="1.0"/>
    <fuselage ax="16.231" ay="0" az="0" bx="-20.0" by="0.00" bz="0.0"
        width="6.2" taper="1.0" midpoint="0.5"/>
    <fuselage ax="-20.0" ay="0" az="0" bx="-37.0" by="0.00" bz="1.2"
        width="6.2" taper="0.0" midpoint="0.0"/>
    <fuselage ax="4.0" ay="9.8" az="-3" bx="-1.8" by="9.8" bz="-3"
        width="1.9" taper="1.0"/>
    <fuselage ax="4.0" ay="-9.8" az="-3" bx="-1.8" by="-9.8" bz="-3"
        width="1.9" taper="1.0"/>

    <wing x="-3" y="3.095" z="-1.5" length="8.5" chord="13.7" sweep="20" taper="0.6" dihedral="9" camber="0.1" incidence="3" twist="-1.5">
        <stall aoa="12" width="12" peak="1.5"/>

        <flap0 start="0.00001" end=".6" lift="1.6" drag="1.15"/>
        <slat start="0.12" end=".65" aoa="2" drag="1.2"/>
        <spoiler start="0.03" end="0.67" lift="0.2" drag="2.0"/>

        <control-input axis="/controls/flight/flaps" control="FLAP0"/>
        <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
        <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>

        <control-output control="FLAP0" prop="/surface-positions/flap-pos-norm"/>
        <control-output control="SLAT" prop="/surface-positions/slats"/>
        <control-output control="SPOILER" prop="/surface-positions/speedbrake-norm"/>

        <control-speed control="FLAP0" transition-time="78"/>
        <control-speed control="SPOILER" transition-time="5"/>
    </wing>

    <mstab x="-5.969" y="10.984" z="-0.404" length="21.8" chord="8.22" sweep="27.2" taper="0.23" dihedral="6.5" camber="0.05" incidence="1.5" twist="-.5">
        <stall aoa="12" width="12" peak="1.5"/>

        <flap0 start="0.00001" end=".52" lift="1.4" drag="1.15"/>
        <flap1 start=".52" end=".96" lift="1.4" drag="1.2"/>
        <slat start="0.00001" end=".99" aoa="2" drag="1.2"/>
        <spoiler start="0.03" end="0.48" lift="0.2" drag="2.0"/>

        <control-input axis="/controls/flight/flaps" control="FLAP0"/>
        <control-input axis="/surface-positions/flap-pos-norm" control="SLAT" src0="0.5" dst0="0" src1="1.0" dst1="1.0"/>
        <control-input axis="/controls/flight/speedbrake" control="SPOILER"/>
        <control-input axis="/controls/flight/aileron" control="FLAP1" split="true"/>
        <control-input axis="/controls/flight/aileron-trim" control="FLAP1" split="true"/>

        <control-output control="FLAP1" side="left" prop="/surface-positions/left-aileron-pos-norm"/>
        <control-output control="FLAP1" side="right" prop="/surface-positions/right-aileron-pos-norm"/>

        <control-speed control="FLAP0" transition-time="78"/>
        <control-speed control="SPOILER" transition-time="5"/>
    </mstab>

    <hstab x="-31.8" y="1.35" z="1.2" chord="6.5" length="10.8" taper=".45" sweep="30" dihedral="6.5" effectiveness="2.5">
      <stall aoa="15" width="5" peak="1.5"/>
      <flap0 start="0" end="1" lift="1.38" drag="1.1"/>
      <control-input axis="/controls/flight/elevator" control="FLAP0"/>
      <control-input axis="/controls/flight/elevator-trim" control="FLAP0"/>
      <control-output control="FLAP0" prop="/surface-positions/elevator-pos-norm"/>
    </hstab>

    <vstab x="-29.8" y="0" z="2.7" chord="8.4" length="11.7" taper="0.34" sweep="34">
      <stall aoa="14" width="5" peak="1.5"/>
      <flap0 start="0" end="1" lift="1.5" drag="1.2"/>
      <control-input axis="/controls/flight/rudder" control="FLAP0" invert="true"/>
      <control-input axis="/controls/flight/rudder-trim" control="FLAP0" invert="true"/>
      <control-output control="FLAP0" prop="/surface-positions/rudder-pos-norm"
              min="1" max="-1"/>
    </vstab>

    <jet x="1.16" y="9.89"  z="-2.995" mass="13889" thrust="93400" spool-time="3"
    egt="840" tsfc="0.32 " n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
      <control-input axis="/controls/engines/engine[0]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
      <control-input axis="/controls/engines/engine[0]/reverser" control="REVERSE_THRUST"/>
        <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[0]"/>
        <control-speed control="REVERSE_THRUST" transition-time="4"/>
    <actionpt x="-2.7" y="9.89" z="-2.995"/>
    </jet>

    <jet x="1.16" y="-9.89"  z="-2.995" mass="13889" thrust="93400"  spool-time="3"
    egt="840" tsfc="0.32" n1-idle="45" n1-max="105" n2-idle="70" n2-max="108">
      <control-input axis="/controls/engines/engine[1]/throttle-lever" control="THROTTLE" src0="0.2" dst0="0" src1="1" dst1="1"/>
      <control-input axis="/controls/engines/engine[1]/reverser" control="REVERSE_THRUST"/>
        <control-output control="REVERSE_THRUST" prop="/surface-positions/reverser-norm[1]"/>
        <control-speed control="REVERSE_THRUST" transition-time="4"/>
    <actionpt x="-2.7" y="-9.89" z="-2.995"/>
    </jet>

    <!-- nose -->
    <gear x="19.28"  y="0.00" z="-5.55"
          compression="0.7" spring="1.2" damp="1" sfric="1.5" dfric="1.0">
      <control-input axis="/controls/flight/rudder" control="STEER" square="true"/>
      <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
      <control-output control="EXTEND" prop="/gear/gear[0]/position-norm"/>
      <control-speed control="EXTEND" transition-time="19"/>
    </gear>

    <gear x="-6.39" y="5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
      <control-input axis="/autopilot/autobrake/left-brake-output" control="BRAKE"/>
      <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
      <control-input axis="/controls/gear/brake-left" control="BRAKE"/>
      <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
      <control-output control="EXTEND" prop="/gear/gear[1]/position-norm"/>
      <control-speed control="EXTEND" transition-time="20"/>
    </gear>

    <gear x="-6.39" y="-5.6" z="-5.7" compression="0.8" spring="1.0" sfric="1" dfric="0.9">
      <control-input axis="/autopilot/autobrake/right-brake-output" control="BRAKE"/>
      <control-input axis="/controls/gear/brake-parking" control="BRAKE"/>
      <control-input axis="/controls/gear/brake-right" control="BRAKE"/>
      <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
      <control-output control="EXTEND" prop="/gear/gear[2]/position-norm"/>
      <control-speed control="EXTEND" transition-time="20"/>
    </gear>

    <gear x="-7.8" y="5.6" z="-6.22" compression="3.0" spring="1.2" >
      <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
      <control-output control="EXTEND" prop="/gear/gear[3]/position-norm"/>
      <control-speed control="EXTEND" transition-time="20"/>
    </gear>

    <gear x="-7.8" y="-5.6" z="-6.22" compression="3.0" spring="1.2">
      <control-input axis="/controls/gear/gear-down" control="EXTEND"/>
      <control-output control="EXTEND" prop="/gear/gear[4]/position-norm"/>
      <control-speed control="EXTEND" transition-time="20"/>
    </gear>

    <tank x="-4.3" y="5" z="-1.5" jet="true" capacity="64000"/>
    <tank x="-4.3" y="0" z="-2.5" jet="true" capacity="174900"/>
    <tank x="-4.3" y="-5" z="-1.5" jet="true" capacity="64000"/>

    <weight x="22.0" y="0" z="0" mass-prop="/sim/weight[0]/weight-lb"/>
    <weight x="-4.3" y="0" z="0" mass-prop="/sim/weight[1]/weight-lb"/>

    <ballast x="20" y="0" z="0" mass="15000"/>

    <ballast x="-30" y="0" z="4" mass="-28000"/>

    <thruster x="19.28"  y="0.00" z="-5.55" vx="1" vy="0" vz="0" thrust="20000">
      <control-input axis="/sim/model/pushback/target-speed-fps" control="THROTTLE"
      src0="-1" src1="1" dst0="-1" dst1="1"/>
      </thruster>

    </airplane>

It was just a problem with the mappings to the properties, I accidentally mapped both gears to the left brake property.
Personal Fleet: 787-8, CRJ-200, 737-300, MD-81, DHC-3A, Beechcraft Starship
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Re: Request: fix the 772 fdm!

Postby VaLeo » Tue Jan 25, 2011 4:02 pm

Yes, thats I found myself early, but thats no problem here.

Lets try, for example, 747-400.
(Auto)start engines, full throttle, shift-B. Plane begin to move, then press "," and steer to left. Its begin rotate around left gear, with very small turn radius.

Then repeat same with 777.
Its turning only by steer, very big turn radius, and its impossible to drive parkpos<->rwy. Pressing "," or "." simply stops aircraft.
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Re: Request: fix the 772 fdm!

Postby Tuxklok » Tue Jan 25, 2011 5:32 pm

First make sure you are taxiing slowly, no more than 10 knots for turns, around 25 knots straight. In real 777 you will also get help from main gear steering when nose gear steering is greater than 13 degrees. I really doubt that locking one brake and pivoting on it is an approved procedure, or one that would be so effective in real life due to the large number of wheels on the main gear. In the manual it actually mentions differential braking and braking while turning should be avoided under normal conditions. However, differential thrust combined with light differential braking is mentioned in the manual as way to reduce turning radius for tight turns, 180 degree turns in particular.

According to manual, minimum turning radius at the nose (not nose gear) is 110 feet. So that is what the model should be checked against and tuned to achieve, not some arbitrary anticipated behavior.

You could try this procedure from the manual, alluded to above, to get the minimum turning radius of 110 feet.
Code: Select all
Turning radius during 180 degree turns can be reduced, with lower engine
thrust and less nose gear tire wear by following a few specific taxi techniques.
Taxi the aircraft so that the main gear tires are as close as possible to the runway
edge. This provides more runway surface to make the turn. Stop the aircraft
completely with the thrust at idle. Hold the tiller to the maximum steering
angle, release the brakes, then add thrust on the outboard engine. Use only the
outboard engine and maintain 10 knots or less during the turn to minimize the
turn radius. These actions will result in a low speed turn and less runway being
used. Light intermittent brake applications on the inside of the turn will help
reduce the turn radius.


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