Improving Glider Realism: Difference between revisions

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(→‎Variometer: gave formula for TE reading)
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=== Glider external 3D models ===
=== Glider external 3D models ===


This is the best understood bit of glider simulation... designing a decent 3D model. You can tell if it's any good just by looking at it, although performance plays a part too.
This is the best understood bit of glider simulation... designing a decent 3D model. You can tell if it's any good just by looking at it, although performance plays a part too. For the current FG state of the art, here's the ASK21:
 
[[Image:Ask21 external.jpg]]


=== Glider cockpit 3D models ===
=== Glider cockpit 3D models ===

Revision as of 15:56, 22 April 2009

This section lists the areas that have a significant impact on soaring realism. This wiki has additional relevant information in the Soaring area.

Gliders

This section reviews the design issues affecting the user aircraft.

Flight Model

Of course, gliders need a custom-designed flight model.

Glider external 3D models

This is the best understood bit of glider simulation... designing a decent 3D model. You can tell if it's any good just by looking at it, although performance plays a part too. For the current FG state of the art, here's the ASK21:

Ask21 external.jpg

Glider cockpit 3D models

Less well understood, but still not rocket science, gliders need a good 3d model for the cockpit with the panel and the various levers (joystick, flaps, airbrakes, water ballast, landing gear).

Gauges

Unfortunately it is not possible to create a decent glider by simply copying across existing instruments from a power aircraft. The minimum instrument set is an altimeter, air speed indicator, and variometer. The variometer (vario) is unique to soaring.

Variometer

At its simplest, a variometer is a rate of climb indicator. However, this pure (uncompensated) indication of vertical speed is very poor for climbing effectively in thermals as the effect of the vertical movement of the air is swamped by the pilot's actions with the control column (so called 'stick thermals'). Since the 1930's, real gliding variometers have used some method to compensate for the climb rate induced by the pilot pushing or pulling on the control column.

  • Total energy compensation. If the glider is climbing, a factor can be subtracted from the indicated lift if the glider is decelerating, and the reverse during descent. So if the glider is neither accelerating or decelerating the absolute rate of climb (or sink) will be shown.
    • Potential energy = mass x G x height (or height = energy / (mass x G))
    • Kinetic energy = 0.5 x mass x velocity squared
    • if in time period 't' the glider goes from height 'h1'..'h2' and speed 'v1'..'v2':
    • uncompensated vario reading = (h2-h1)/t
    • TE adjustment = height the glider would have gained if it hadn't accelerated
    • = (change in kinetic energy / (mass x G))
    • = (0.5 * mass * v2^2 - 0.5 * mass * v1^2) / (mass x G)
    • = (v2^2 - v1^2) / 2G where G = 9.81 meters per second per second
    • TE reading = uncompensated reading + TE adjustment
    • TE reading = (h2-h1)/t + (v2^2 - v1^2) / 19.62
    • (all units meters, seconds, meters per second)
  • Netto compensation. The design sink rate of the glider at the current airspeed is added to the total energy vario reading, so the variometer actually displays the vertical rate of the air outside the glider. For a perfectly compensated instrument, the vario will show zero in still air regardless of the airspeed of the aircraft

IGC file logger

To compare flights with others, it helps to have a log of your flight in the 'IGC format'. This is a text file with an agreed format, with some header rows and then one-row-per-timestamp for the lat/long/alt.

The specification for the IGC format log file is available on the FAI website.

The full specification has become unbelievably tortuous, but most of the records are optional and an example of a working file would be:

AXXXb21_sim_probe 2.55
HFDTE070608
HFFXA035
HFPLTPILOTINCHARGE: not recorded
HFCM2CREW2: not recorded
HFGTYGLIDERTYPE:DG
HFGIDGLIDERID:B21
HFDTM100GPSDATUM: WGS-1984
HFRFWFIRMWAREVERSION: 2.55
HFRHWHARDWAREVERSION: 2008
HFFTYFRTYPE: sim_probe by Ian Forster-Lewis
HFGPSGPS:Microsoft Flight Simulator
HFPRSPRESSALTSENSOR: Microsoft Flight Simulator
HFCIDCOMPETITIONID:B21
HFCCLCOMPETITIONCLASS:Microsoft Flight Simulator
I013638FXA
B1658174040958N07737022WA0094000940000
B1658214040875N07737069WA0095200952000
B1658254040811N07737136WA0095300953000
... and more B records for the rest of the file
G123456789

IGC files can be uploaded to everytrail.com or can be converted by gpsvisualizer.com for viewing in Google Earth.

Enviroment lift modelling

This section reviews the requirements for the environment modelling, in particular the simulation of the vertical component of air movement on which gliders depend for soaring flight.

Thermals

Ridgelift

A paper on the efficient calculation of ridge lift is available from Ian Forster-Lewis.

Wave

Multiplayer

Solo soaring is all about admiring the scenery, and multiplayer soaring is predominantly about comparing times to complete the same cross-country task.