    ======================================================
    = Fokker Dr.1                                        =
    = WWI Fighter                                        =
    = for FlightGear with LaRCsim and the UIUC Aeromodel =
    =                                                    =
    = Flight model by:                                   =
    = Michael Selig, et al. (m-selig@uiuc.edu)           =
    = http://www.aae.uiuc.edu/m-selig/apasim.html        =
    ======================================================

    To run, try:

    fgfs --aircraft=fkdr1-v1-nl-uiuc

    Files and directory structure required in $FG_ROOT/Aircraft/ to fly the
    model:

    fkdr1-v1-nl-uiuc-set.xml
    fkdr1/Sounds/uiuc/fkdr1-sound.xml
    UIUC/fkdr1-v1-nl/aircraft.dat
    UIUC/fkdr1-v1-nl/CDfa-03.dat
    UIUC/fkdr1-v1-nl/CLfa-03.dat
    UIUC/fkdr1-v1-nl/Cmfa-03.dat
    UIUC/fkdr1-v1-nl/Cmfade-01.dat

    These files above come with the FlightGear base package.

    To add a 3D external model, read the file:

    ~/Aircraft/UIUC/beech99/README.beech99.html

    as an example to follow.  A Fokker Dr.1 model file that does work is
    fokdr1m2.zip from http://www.flightsim.com.  (The fuselage for this
    model is too wide in the cockpit region.)

    There are several variants of this which can also be used, namely
    these files:

      dr-1cfs.zip
      dr1mp98.zip
      dr1mpcfs.zip
      fkdr1blk.zip
      fokdr-15.zip

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    Model description and updates:

    11/22/2002 - Expanded data to +-180 deg.

    11/10/2002 - First release: v1-nl

    * Motivation: FGFS and the UIUC aero model were used to develop flight
      models for both the Sopwith Camel and Fokker Dr.1 Triplane.  These
      models were then used in another simulation with a collaborator,
      Brian Fuesz.  In that simulation, guns, terrain, villages, multiple
      planes, etc were added to simulate the last flight of the Red Baron.
      This work was filmed for the Discovery Channel show "Unsolved
      History: The Death of the Red Baron" scheduled to first air Dec 18,
      2002.

    * A weights and balance was performed to arrive at an allowable
      c.g. location and from that data, mass moments of inertia were
      calculated.

    * Lift, drag and pitching moment data is modeled from -90 to +90 deg.
      Because the aerodynamics are not modeled from -180 to +180 deg, the
      aircraft will sometimes twitch when coming out of a tail slide as it
      passed through 90 deg.

    * In general, the aerodynamics are modeled using various sources.

    * Apparent mass effects are modeled.

    * Gyroscopic forces caused by engine rotation and aircraft rotations
      are modeled.  For an animation of how a WWI-type rotary engine
      works, go here:http://www.keveney.com/gnome.html
      An example of gyroscopic forces, are those forces produced when one
      tries to rotate by hand a spinning bicycle wheel.

    * Spin aerodynamics are not yet modeled.

    * The simulation starts on the ground.  Throttle up to take off or
      alternatively, use Ctrl-U to jump up in 1000-ft increments.

    * The Fokker Dr.1 did not have brakes.  Application of brakes in FGFS
      will cause the aircraft to promptly nose over.  (I have added a fake
      contact point ahead of the aircraft to avoid completely tipping
      over.)  The c.g. of the aircraft sits almost directly above the
      wheel contact point.  There is a reason for this.  The aft fuselage
      and tail were designed to be very light.  Thus, the tail could not
      support much load, so the weight of the aircraft largely rests on
      the main wheels, which again requires the c.g. to be almost directly
      above the wheels.

    * WWI aircraft engines did not have a conventional throttle (at least
      most did not).  The engines were either on or off/idle using a
      "blip-throttle".  So it is not realistic to fly with a variable
      throttle, which the current model allows.

    * To modelers, I can provide a graphic showing the c.g. location.

    * Something I have not yet modeled is rudder ineffectiveness on roll
      out and touch down.  When the aircraft is sitting on the wheels and
      tail skid, the angle of attack of the wing is so high that it is
      mostly stalled and the flow off the aft fuselage is also not well
      behaved.  The result is that there is not much dynamic pressure
      (flow speed) on the vertical stabilizer, so there is little rudder
      authority in this condition.  As a point of interest, why would the
      designers settle for this result?  It's because of the rotary
      engine.  The max speed is limited to 1200 rpm because of the
      otherwise higher stresses on the rotary engine parts.  To obtain the
      necessary thrust at such a low rpm, a large diameter ~8.5 ft
      propeller was required.  With such a large diameter propeller and
      short stubby fuselage, the aircraft sat nose high.

    * Interesting flight characteristics to note:

      - Just as with the Sopwith Camel and other WWI vintage aircraft, the
        gyroscopic forces of the engine tend to couple the aerodynamic
        controls.  For example, pulling up will cause the aircraft to not
        only nose up but also yaw to the right, requiring left rudder to
        coordinate.  A more general discussion of this effect can be found
        in the file: ~/Aircraft/UIUC/sopwithCamel/README.sopwithCamel.html

        It should be added, however, that the engine polar moment of
        inertia of the Dr.1 is approximately half that of the Camel.  The
        effect of the gyroscopic forces is not exactly "half" because the
        aerodynamics that resist these motions are somewhat different
        between the two aircraft.  Nevertheless, when flying, the effects
        of the gyroscopic forces are appreciably less than those on the
        Sopwith Camel.

      - I have a sense that there needs to be more yaw damping, but I
        cannot justify it just yet.  So I am sticking w/ the computed
        figures.  Also, the pilot report (URL below) makes mention of the
        fact that one needs to be on the rudder all of the time, which
        suggests poor yaw damping.

      - On takeoff, there is a tendency for the wing to dip and drag on
        the ground.  The same thing happens on landing.  This is not an
        artifact of some modeling approximations, but in fact the Fokker
        Triplane was known to "dip" a wing on takeoff and landing.  The
        solution was to add the "axe handles" or wooden pool skids at the
        tips of the lower wing.  The pilot's solution to avoid it in the
        first place is to use cross controls: right aileron and left
        rudder and vice versa until level again.

      - The ailerons on the Fokker triplane were "heavy", meaning that
        considerable stick force was required to maneuver.  For this
        reason, "elephant ears" were used on the tips of the ailerons for
        mass balance.  The trouble with putting the mass balance at the
        wings tips is that on the side with the aileron down, the upwash
        around the wing impinges on the balance horn and this produces
        high drag.  On the other aileron in the up position, it is more
        aligned with the upwash coming around the wing tip, so there is
        little added drag.  As a result of this rather large difference in
        drag, strong adverse yaw is the outcome.  For instance, a left
        roll with the right wing aileron going down produces high drag at
        the tip of the right wing.  As a consequence, this yaws the
        airplane to the right, opposite the intended direction of the
        turn.  There were three variations on the design of the "elephant
        ears" and I speculate that it was driven by this substantial
        adverse yaw problem.  Smaller "elephant ears" produced less
        adverse yaw, which is good, but they also provided less balance
        and therefore higher stick force, which is bad.  My guess is that
        it was a case of "pick your poison".  Generally, with the adverse
        yaw problem generous use of the rudder is required when banking to
        turn.

    * Some pilot reports based on full scale replicas:
      http://rwebs.net/avhistory/flight.htm
      http://www.airandspacemagazine.com/asm/web/special/ethell/pirep1.html

    * To those having specific knowledge of the Fokker Dr.1, I am
      interested in obtaining the following data:

      [] RPM when the throttle is blipped.  I am currently using 300 RPM.
         The current aircraft uses a 110-hp Uberursel UR-2, which I
         believe is what is Baron von Richthofen (The Red Baron) was
         flying on the day when he was shot down.

      [] Engine mass characteristics on a component-by-component basis.
         This information will help me refine the polar moment of inertia
         that drives the gyroscopic forces.

      [] On a related note, the mass of the propeller which also figures
         into the gyroscopic forces.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    **************************************************
     Prof. Michael S. Selig
     Dept. of Aerospace Engineering
     University of Illinois at Urbana-Champaign
     306 Talbot Laboratory
     104 South Wright Street
     Urbana, IL 61801-2935
     (217) 244-5757 (o), (509) 691-1373 (fax)
     m-selig@uiuc.edu
     http://www.aae.uiuc.edu/m-selig
    **************************************************