======================================================
= 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
**************************************************