Stuart Buchanan wrote:
The model and FDM are fairly basic, but it is quite fun to fly, and a challenge
to land. Comment and any modifications are very welcome. This is my first YASim
aircraft, so I'm sure the FDM could benefit from some tuning.
Stuart, this is a really nice addition. Taking you at your word, I
spent some enjoyable time last evening tweaking the yasim fdm. Since
this activity can be subject to personal preferences, I am attaching a
diff file and will comment on the changes, and my reasons for each. I
have never flown the real pitts, but have had several extended
discussions with several pilots at my field that fly it regularly.
1. CG location - It seemed to be very tail heavy on my first flight, so
I googled to find where the CG should be. Could not find the plan
numbers, but did find a 1/3 scale RC CG recommendation. So added
ballast to move the CG to x= -1.37 m which should be very similar to the
location recommended for the RC pitts. I always use the 3d model and
$FG_ROOT/bin/yasim to get the CG where documentation says it should be.
This made a very big difference.
2. Approach configuration - In spite of the documentation, this needs
to be the landing stall configuration. I found the landing stall speed
using google and estimated the aoa.
3. Increased the roll rate - more flap0 lift and decreased twist.
Decreasing the twist also makes stalls quicker, more positive which
helps the snap rolls and spins.
4. The real AC snap rolls easily and spins easily (aerobatics). So
decreased the wing and mstab stall widths.
5. Adjusted the cruise and take-off rpms so the engine behaves more
like a fixed pitch prop should (i.e at full throttle, the rpm should
increase as the AC accelerates.
Hope this is helpful,
Dave Perry
? pitts.diff
Index: pittss1c.xml
===================================================================
RCS file: /var/cvs/FlightGear-0.9/data/Aircraft/pittss1c/pittss1c.xml,v
retrieving revision 1.1
diff -p -u -r1.1 pittss1c.xml
--- pittss1c.xml 10 Nov 2007 17:18:47 -0000 1.1
+++ pittss1c.xml 11 Nov 2007 21:38:47 -0000
@@ -9,13 +9,13 @@ YASim aerodynamic model for a Pitts S1C
<airplane mass="720">
<!-- Approach configuration -->
- <approach speed="100" aoa="5">
+ <approach speed="50" aoa="12">
<control-setting axis="/controls/engines/engine[0]/throttle" value="0.5"/>
<control-setting axis="/controls/engines/engine[0]/mixture" value="1.0"/>
</approach>
<!-- Cruise configuration -->
- <cruise speed="160" alt="0">
+ <cruise speed="160" alt="7000">
<control-setting axis="/controls/engines/engine[0]/throttle" value="0.75"/>
<control-setting axis="/controls/engines/engine[0]/mixture" value="0.75"/>
<control-setting axis="/controls/flight/elevator-trim" value="0.1"/>
@@ -27,10 +27,10 @@ YASim aerodynamic model for a Pitts S1C
<fuselage ax="0" ay="0" az="0" bx="-4.22" by="0" bz="-0.0"
width="0.9" taper="0.8" midpoint="0.3"/>
- <wing x="-1.87" y="0.31" z="-0.44" taper="1.0" incidence="0" twist="-3.0"
+ <wing x="-1.87" y="0.31" z="-0.44" taper="1.0" incidence="0" twist="-1.0"
length="2.3" chord="0.9" sweep="0" dihedral="2.7" camber="0.1">
- <stall aoa="22" width="4" peak="1.5"/>
- <flap0 start="0.31" end="0.85" lift="1.3" drag="1.1"/>
+ <stall aoa="12" width="2.0" peak="1.5"/>
+ <flap0 start="0.31" end="0.85" lift="1.6" drag="1.1"/>
<control-input axis="/controls/flight/aileron" control="FLAP0" split="true"/>
<control-input axis="/controls/flight/aileron-trim" control="FLAP0" split="true"/>
<control-output control="FLAP0" side="left"
@@ -39,26 +39,26 @@ YASim aerodynamic model for a Pitts S1C
prop="/surface-positions/right-aileron-pos-norm"/>
</wing>
- <hstab x="-3.91" y="0.1" z="0.04" taper="0.8" effectiveness="1.24"
+ <hstab x="-3.91" y="0.1" z="0.04" taper="0.8" effectiveness="1.5"
length="1.0" chord="1.0" sweep="0">
- <stall aoa="25" width="4" peak="1.5"/>
- <flap0 start="0.5" end="1" lift="1.3" drag="1.2"/>
+ <stall aoa="20" width="4" peak="1.5"/>
+ <flap0 start="0.1" end="1" lift="1.65" drag="1.2"/>
<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>
<mstab x="-1.37" y="0" z="0.5"
- taper="0.0" dihedral="0" twist="-3.0"
+ taper="0.0" dihedral="0" twist="-1.0"
length="2.62" chord="0.9" sweep="6" incidence="0.0">
- <stall aoa="20" width="4" peak="1.5"/>
+ <stall aoa="12" width="2.0" peak="1.5"/>
</mstab>
<!-- rudder -->
<vstab x="-3.9" y="0" z="-0.1" taper="0.8" effectiveness="2.0"
length="0.65" chord="1.0" sweep="30" incidence="0.0">
<stall aoa="30" width="4" peak="1.5"/>
- <flap0 start="0.2" end="0.8" lift="1.2" drag="1.1"/>
+ <flap0 start="0.0" end="0.1" lift="2.0" drag="1.1"/>
<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"
@@ -66,16 +66,16 @@ YASim aerodynamic model for a Pitts S1C
</vstab>
<propeller radius="0.65"
- cruise-speed="150" cruise-rpm="2200"
+ cruise-speed="150" cruise-rpm="2700"
cruise-alt="5000" cruise-power="140"
- takeoff-power="175" takeoff-rpm="2600"
- x="-1.3" y="0.0" z="0.0" mass="160" moment="3">
+ takeoff-power="175" takeoff-rpm="2500"
+ x="-0.3" y="0.0" z="0.0" mass="160" moment="3">
<actionpt x="-0.26" y="0.0" z="0.0"/>
<control-input axis="/controls/engines/engine[0]/throttle" control="THROTTLE"/>
<control-input axis="/controls/engines/engine[0]/starter" control="STARTER"/>
<control-input axis="/controls/engines/engine[0]/magnetos" control="MAGNETOS"/>
<control-input axis="/controls/engines/engine[0]/mixture" control="MIXTURE"/>
- <piston-engine eng-power="180" eng-rpm="2700" displacement="361" compression="8.5"/>
+ <piston-engine eng-power="180" eng-rpm="2700" alt="0" displacement="361" compression="8.5"/>
</propeller>
<gear x="-4.5" y="0" z="-0.48" compression="0.1"
@@ -109,6 +109,7 @@ YASim aerodynamic model for a Pitts S1C
<tank x="-1.5" y="0" z="-0.0" capacity="114"/>
<ballast x="-2.0" y="0" z="-0.0" mass="20"/>
+ <ballast x="-0.5" y="0" z="-0.0" mass="210"/>
<!-- pilot -->
<weight x="-2.4" y="0" z="-0.0" mass-prop="/yasim/pitts/pilot-lb"/>
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