Archive for the ‘Tours’ Category

An experience like no other…

Friday, December 18th, 2015

The Space Shuttle in Flightgear

After Vostok-1 and the X-15, Flightgear now adds the third craft capable of reaching space – the Space Shuttle.

Thanks to the fact that virtually all material NASA has on the Shuttle is in public domain, the simulated flight dynamics and the resulting capabilities of the Shuttle are based on a large body of NASA wind-tunnel and actual mission data and hence highly realistic. For instance, the response to aerosurfaces is not only simulates as a function of surface deflection and dynamical pressure, but also as function of Mach number, AoA and the deflection of other surfaces, giving full account of the changing dynamics during entry, cross couplings between controls and non-linearities in the control response.

The Shuttle is under heavy development – the systems and avionics simulation is not quite on the same level of sophistication as the aerodynamics, but even in its present state, the full profile of a mission can be flown, including realistic entry guidance and support for abort modes such as RTLS (return to launch site).

Even under rather off-nominal conditions such as an RTLS, the simulated Shuttle behaves as described in the crew operations manual, i.e. procedures can literally be flown ‘by the book’.

Launch

“The launch looks kind of slow to an observer, but when you’re inside, there ain’t nothing slow about it. I mean, you feel all seven million pounds of that thrust. It’s like this giant hand grabs the orbiter and throws it into space.”

“It’s like being strapped to the front of a freight train going down the track at a hundred mph. You feel all that mass and force behind you.”

“Right before the main engines cut off, you feel like you have a bear sitting on you. It’s three g’s. Then, at the moment of engine cut-off, you go from three to zero g’s instantaneously. The bear jumps off your chest, and you see the seat belts floating upward, which is kind of cool. Then, a couple of seconds later, you hear this big clang and the fuel tank comes off.”

For a bit more than eight minutes, the Space Shuttle is a true rocket ship, capable of a maximal acceleration above three g and highly maneuverable by thrust vectoring. During that time, it needs to reach a stable orbit.

What’s little known is that aerodynamical stress is in fact much stronger during ascent than during entry – the dynamical pressure peaks at almost twice the value reached during entry about thirty seconds into the flight. This makes launch a busy phase – after liftoff from the pad, the Shuttle needs to be rolled to a launch course, than steered safely on a steep ascent path through the lower atmosphere before it finally levels off and races to orbital speed outside the atmosphere.

The simulation takes a lot of the structural limits of the Shuttle into account, so it’s quite important to steer the ascent trajectory through a safe region in order to avoid a catastrophic failure followed by an explosion of the external tank.

Arrival in orbit

“As a commander, you try to figure out how much extra time you need to add to the schedule, based on how many rookies you’ve got, how they’re doing and so on. But when you stick your head down in the mid-deck for the first time, you never know how your crew will be doing Sometimes guys are semi-Velcro’ed to the wall, throwing up, while the folks you least expected to be heroes are just chugging along, executing the plan.”

We can’t really simulate the effect of weightlessness for the user in FG, but there’s a lot that can be simulated in space.

Once the main engines cut off, the flight characteristics of the Shuttle change drastically – it becomes a sluggish object which can just be nudged into slightly different attitudes or somewhat different orbits. The Shuttle doesn’t need to be flown any more – once in orbit, it will stay there for a while even if you do nothing. Instead, it needs to be operated – its systems need to be activated for the space environment.

The simulation includes ready-for-orbit procedures like the propellant dump, umbilical door and start tracker door operation, payload bay door operation, Ku-band antenna deployment and pointing and thermal management. Also, the propellant flow of the orbital engines can be fully controlled, allowing procedures such as RCS to RCS, OMS to RCS or OMS to OMS crossfeed.

There is in fact a complete simulation of the thermal environment running, including radiation fluxes from Sun and Earth, heat radiated into space, heat conduction between different parts of the Shuttle, equipment as sources of additional heat and flash evaporators, spray boilers and radiator as heat sinks connected to the freon cooling loop. In order to keep the Shuttle operational, you need to do heat management by operating the radiator and bringing the Shuttle into a good attitude relative to the Sun. Also, heating elements need to be used to keep thrusters from freezing up.

Orbital operations

“For a pilot, the response of the Shuttle is totally different from that of an airplane. You pulse the jets, then you wait for a response. It takes a while for the input to take effect. Say you want to move directly above the Space Station. You’d do some ‘up’ pulses with your hand controller, as well as some pitch change pulses to rotate the vehicle as you move up. You do a set number of pulses, predicting what the response will be. Then you wait 45 seconds or so to see what happens. Then you correct it. It’s not agressive control, it’s patient, timely inputs.”

The flight characteristics of the Shuttle changes drastically from mission phase to mission phase. Over the course of a mission, controls change from thrust vectoring of main engines and boosters via the reaction control system (RCS) thrusters and thrust vectoring of the orbital maneuvering system (OMS) engines to airfoils for the final glide through the atmosphere.

A host of digital autopilots (DAPs) connects the control inputs with the various ways to control the Shuttle. In orbit, they are augmented by extra functions – there’s inertial and local horizon attitude holding modes, rate controlled moves, pulsed modes, the low thrust Vernier engines for fine attitude control – and practically all aspects of the DAPs are configurable during the mission.

The need to support all these different modes makes the flight control system of the Shuttle easily the most complex of all Flightgear aircraft.

“If you position your body to face away from the windows, and put your arms out as if you were flying with the Earth above your head, you feel like you’re swimming underneath the Earth.”

Often the question comes up whether Flightgear as a flight simulation is any good in simulating spaceflight. It turns out that the flight dynamics solver underlying the Space Shuttle, JSBSim, is quite capable of orbital dynamics, in fact it has been benchmarked against several test cases in a 6-DoF simulation code comparison by NASA.

JSBSim simulates Earth not as a point mass but as a true realistic mass distribution, and there’s even a simulation of differential gravity – the distance from Earth to the upper edge of the Shuttle is slightly larger than the distance to the lower edge, and that corresponds to a tiny residual force which acts like a torque on the Shuttle.

So never believe for a moment that the dynamics of the Shuttle in orbit is simplified or arcade-like because FG is a flightsim – it is not – it is in fact realistic down to effects you never really thought of. JSBSim is not a game engine, it is a professional simulation code that just happens to be OpenSource.

The Shuttle includes a detailed simulation of the remote manipulator system (RMS) arm operation – the arm can be driven in various modes, grab a payload from the bay and release it into space after it has been unlatched.

In addition, there is a simple spacewalk view which allows to step out of the Shuttle and float around, using small thrusters, in essence simulating the operation of a manned maneuvering unit.

Earth from Space

“Make a memory somewhere during the mission, put your nose up a window and look out and make a memory. And don’t take a picture of it, because you would be disappointed when you get home. Plant it in your brain and don’t ever let it go, and you’ll have it with you always, and nobody can take it from you.”

The default Flightgear terrain rendering engine is not optimal for views from high altitude – in fact, it will be hard-pressed to show anything from a 300 mile orbit. Instead, FG comes with the Earthview orbital rendering engine which can be used from some 80.000 ft and above and displays the planet as a textured sphere. Earthview is fairly sophisticated and renders even correctly placed cloud shadows if a cloud sphere is used.

However, in a default FG installation, only relatively low resolution textures are shipped (which can be procedurally enhanced) – not really enough for photorealistic impressions. More detailed textures can be obtained from the NASA visible Earth page – for the visuals used here, the maximum resolution available has been used – individual texture sheets are 16384×16384 pixels large. At this resolution and converted to dds to allow fast loading, the Earth textures are about 2 GB – which would triple the size of the FG repository – which is the reason they’re not available as default option.

Also the atmosphere contributes much to the view from orbit. Currently only one of the three rendering schemes available in Flightgear (Atmospheric Light Scattering) generates compelling visuals of the upper atmosphere, the other two (Rembrandt and default) do no.

Due to the lack of an atmosphere, lighting in space is fairly hard – there is little light on surfaces not facing the sun. Dedicated GLSL shaders specifically written for space flight manage this transition from the lower to the upper atmosphere.

The cockpit

There’s (literally) hundreds of switches and gauges in the cockpit of the Space Shuttle. At present, only a small fraction of them is fully functional and animated, although many are already part of the internal systems simulation. The following screenshots are intended to provide an impression of what final result we’re aiming at.

The data processing system (DPS) of the Shuttle is controlled via keypads on the central console. Extra switches are used to select which screen a given keypad is currently talking to.

With all screens in the cockpit used, the Space Shuttle can show an unbelievable amount of information at the same time.

At night, the simulation includes the backlighting of instruments, casting a diffuse glow onto the cockpit interior.

Avionics

The data processing system (DPS) of the Space Shuttle was groundbreaking when it was introduced, but looks definitely odd when compared with modern computers. The memory of the computers actually is not large enough to hold the programs for all mission phases at once, hence there is the need to transit between different operational sequences during a mission.

Interaction with the spacecraft is simulated as in reality. It requires to type short instructions sequences such as OPS 201 PRO or ITEM 7 + 4.1 EXEC on a keypad which triggers the appropriate functions.

Since the avionics is fairly complex, some learning curve is required to operate it properly, but there are multiple rewards for the patient user. The Flightgear Space Shuttle is capable of numerous automatically controlled operations – it is possible to do an automatic orbital insertion or deorbit burn by entering PEG-7 targets, there are inertial pointing routines, tracking routines to keep the Shuttle pointed towards a certain celestial body or towards Earth or automatic thermal management routines.

Using the systems management software, one can in detail follow the operations of APUs, the RCS and OMS engines or the payload bay doors, override failed hardware switches, initiate emergency fuel dumps, re-configure the digital autopilots or control the antenna.

Finally, the avionics provides the original guidance and navigation tools for atmospheric entry, in particular the ranging during the aerobraking phase and the final TAEM phase.

Entry

“When you hit the top of the atmosphere, you begin to really slow down, and that’s when the heat rises and you ionize the atmosphere. It’s like being inside a neon light bulb, with a pinkish-purplish pulsing light.”

“What you see out of the overhead window seems like disorganized fire, but it’s not when you look at the way the shockwaves organize the flame. There’s so much flame you cannot believe you are not consumed instantly.”

From a piloting point of view, the entry phase is by far the most interesting. Here, the transition from a spacecraft on a ballistic trajectory in vacuum to an aircraft that glides through the lower atmosphere towards the runway takes place. While at the beginning of the entry, the Shuttle is completely controlled by RCS thrusters, in the end it is fully flown by airfoils like a plane. In-between is a hypersonic phase in which the Shuttle has aerodynamical lift and flies, but is controlled very differently from an aircraft.

As in reality, the simulation provides automatic helpers for trajectory management. In particular, an AoA auto-hold can be activated to help keeping the Shuttle in a thermally safe regime, and the rate-controlled DAP makes the Shuttle respond crisply and controlled to each input, regardless of whether in near-vacuum, in hypersonic descent at Mach 16 or in the lower atmosphere at Mach 2. In fact, while the Shuttle is yaw unstable and needs to be constantly nudged back to zero sideslip, you’d be hard-pressed to observe any trace of this.

In fact, the controls feel almost arcade-like. However, if you’re up for a challenge, you can use a different DAP (for purely educational purpose) which lets you control airfoils directly – and then you’ll feel directly just how much the conditions change from Mach 24 down to Mach 2.

Coming home

“I’m sitting in a chair, my forearms on my knees, and I reached down to take my shoes off. But before I did, I released my cup without giving it a thought, because I expected it to float. And of course, as soon as I did, it fell right to the floor.”

Landing the Shuttle is quite different from landing an airliner. With its hypersonic outline, the Shuttle has the aerodynamics of a brick. It plummets down with 10.000 ft/min, has a glidepath on final of 15-17 degrees and aims for a touchdown at 214 kt – quite a bit higher than for most aircraft.

Yet, for these final few minutes, it is possible to exploit Flightgear’s strength to the full – try any weather or visibility and see whether you can still make the approach. But always remember – there’s just one chance.

And one thing is certain – you’ll never forget your first descebt from orbit right down onto a runway.

Getting the Shuttle

In order to run the Shuttle, you’ll need at least the 3.6 release candidate or a higher version of FG. There are two options to download the Space Shuttle:

A flight-tested version is kept on the FGAddon repository. For this version, someone has made sure that all mission phases can in fact be flown without triggering a bug. This version typically lags behind the current development.

The current development version is kept in its own repository. It contains all the most recent features but is not flight-tested, i.e. simply may not work as expected.

The Shuttle will also be part of the next FG stable release – currently forseen in late winter/early spring 2016.

Get involved

If you’re interested in contributing to one of the most fascinating spacecraft, you’re more than welcome. Ranging from texture art to 3d modeling of payloads and site-specific buildings (think Kennedy Space Center), from data-gathering to refining the aerodynamics and avionics, there’s lots of things that can be done.

You can always get in touch via the Shuttle’s forum thread – or via the devel repository.

Final words

All quotes describing the Space Shuttle experience are from the astronauts, taken from the book Space Shuttle: The First 20 Years (DK Publishing).

More information on the Flightgear Space Shuttle:

* FG Wiki page for the Shuttle
* FG Space Shuttle project overview including a gallery and more technical information
* Development repository on SourceForge
* The crew operations manual – read this to get most of the experience

The new Cessna 172p

Monday, December 7th, 2015

by Gilberto Agostinho

FlightGear’s default aircraft, the Cessna 172P, went through a major makeover. This post will show some of these improvements.

The aircraft exterior is now much more detailed, with new higher resolution liveries (as well as several cockpit and interior themes).

The cockpit is now fully textured and fully functional. All switches, buttons and levers are operable (try pulling out some circuit breakers!). When using ALS, the new interior shadow effect is very immersive.

There are six variants avilable now (default, two bush tire variants, amphibian, pontoon and skis) as well as two types of engine (160 HP and 180 HP). Below, the amphibian variant at San Francisco bay.

The cockpit has now a glass effect, making the windows reflective. The instruments can also be illuminated if the sunlight is getting weak.

If the conditions are just right, the cockpit glass will get foggy or display some frost, as in the image below. To control that, use the air vent and air heat levers (as well as cracking the windows!).

Pre-flight inspection has now been implemented. It’s possible now to add and remove tie-downs, wheel chocks and the pitot tube cover. On top of that, one has to keep an eye for the oil level and possible fuel contamination. The plane has a tutorial explaining how all these new features work.

The aircraft can now get damaged. Land too hard and the front wheel will collapse; dive and pull the yoke at once and the wings will break. But there is no need to worry if your plane gets damaged: a repair button has been added to the aircraft menu.

Regarding sounds, some major improvements have been done: these include not only new cockpit sounds (switches, levers) but also environment sounds: water sounds for the amphibian and pontoon variants, new wind, rain and thunder sounds for all of them.

The aircraft comes with two flashlights (one white, one red) for night flights, allowing one to start the plane from cold and dark regardless of the amount of sunlight.

Among other improvements, we have now a better flight dynamics model, with better stall and spin behaviour, and new hydrodynamics for the float variations.

If you run the FG development version and want to track the latest developments for this plane, you can find the development repository here. There is also a forum thread for discussion and feedback.

Simulating the ever-changing scenery

Saturday, February 21st, 2015

The secrets of the environment settings

If you look at aerial imagery of a region every day for a year, it never changes. Yet if you would fly over the same region in reality every day, it would almost never look the same twice. In reality, nature is a dynamically changing environment, and what you see from a cockpit reflects this.

Some of these changes have to do with weather – on a cloudy day, the light is different from bright sun, the shadows are muted, the amount of haze may change so that faraway terrain looks fainter… and these are readily captured by the weather simulation.

Yet there are more subtle effects. For instance, snow may linger on the ground even on a sunny day with temperatures above freezing if the original layer was thick enough. Snow may fall, but not remain on the ground if the ground is warm enough. In essence, whether you see snow or not depends not so much on how the weather is now, but how it has been the last days, weeks or even months.

Such changes to the scenery in FG are taken care of by the environment settings which control how the terrain is shown. You can find the menu as an entry under Environment.

Currently, the full range of environment effects is only implemented for the Atmospheric Light Scattering (ALS) framework starting from medium quality settings, however the snow effect is available for all rendering frameworks.

Let’s explore some of the things this can do:

Seasonal changes

This is how the default terrain is shown without any environment effects – a summer day in Grenoble:

Moving the season slider somewhat to the right brings autumn coloring into the scene – deciduous tree patches change colors to orange-red, fields and grass appear yellowish:

Changing to a yet later season causes deciduous trees to shed leaves and changes most of the vegetation to a dull brown:

Modifying the snow line and thickness allows to add a sprinkle of snow to the valleys, simulating the first snowfall of late fall:

Finally, adding more snow changes the whole scene into deep winter:

In coastal regions, the appearance of water can also be changed. Here is the coast of Norway near Bergen in summer:

Using the snow and ice sliders allows to simulate winter with lots of drift ice in the sea:

Using a combination of the season and snow settings, it is hence possible to simulate a lot of the seasonal changes during the year. But that’s not all.

Dust and greenery

Have you noticed how colors fade during a long spell of dry weather, to be restored only when rain washes the dust away? Or how a desert might look green for a few weeks after rainfall, to change to its usual dusty appearance later? The environment system also provides those options – let us take a look at the Sierra Nevada. This is how the chain appears from China Lake (with a good measure of snow added to the peaks):

Using the dust slider makes all the colors fade and lets the scene appear dry:

Using the vegetation slider instead gives a fresh green touch to the desert as if after a rainfall:

Changes may be subtle and affect more than just color. Consider this close-up of a dry runway:

The environment settings allow to make it wet (this will happen automatically when the weather predicts rain, but terrain can be wet without current rainfall). This creates puddles and alters the whole reflectivity of the surface – look at how the light changes:

Finally, adding snow covers the runway partially in snowdrifts:

Why can’t this happen automatically?

The environment subsystem just renders as it is told, it is hence easy to misuse it – think snowfall and ice cover on Caribbean islands for instance. Sometimes, the question gets asked why this is implemented that way, and why parameters aren’t just set automatically.

The answer to that is – based on what should they be set? Flightgear does not include a global climate simulation as would be needed to determine how likely it was that there was e.g. snowfall during the last days or weeks, or that there was a dry summer and hence everything should look dusty.

The idea is that the user can adjust these settings, either based on how the scene currently looks at a location, or based on what the user wants to experience (it’s a simulation after all – there’s nothing wrong with simulating a tropical day in Hawaii on a bleak winter day).

If used with some care, the environment settings offer a chance to experience the same scenery in a hundred different ways, each time subtly different.

If misused, the settings deliver weird to crazy results of course.

For the sake of completeness, for low-performance systems which are unable to run shader effects, using the commandline option –season=winter offers at least the choice between the default summer textures and a snow-covered set of textures, although no control over snowline and thickness.

The magic of light and haze

Wednesday, December 17th, 2014

The ‘Atmospheric Light Scattering’ (ALS) rendering framework

Have you ever admired the beautiful colors of a sunset? Have you maybe wondered why sometimes sunsets show a fantastic palette of glowing red and golden colors in the sky and sometimes a rather muted blue-grey? Have you observed distant hills fade into blue haze while the glittering reflection of the sun on water shifts color to a yellow-orange and asked yourself where the difference comes from? Have you wondered why there’s sometimes a halo visible around the moon?

All these phenomena and more are related to light scattering in the atmosphere. Actually, most of what we see looking out of a cockpit from 36.000 ft is not scenery but light scattered somewhere on haze, clouds or air molecules. To create a realistic impression of a scene during flight, we can’t think of haze being something simple that obscures the scene, instead we have to invest as much attention to rendering haze properly as to the more prominent scene elements. In Flightgear, that’s what the ALS framework is doing.

A little bit of theory

To first approximation, the normal lighting situation of a scene during daytime is that the sun is high in the sky and illuminates an object, from which reflected sunlight falls into the eye. There are thus two light rays – the illumination ray (I-ray) goes from the sun to the object and the observation ray (O-ray) from object to the eye.

In vacuum, that’s all there is to it, and pictures from the surface of the Moon illustrate this – objects remain visible no matter how far away, and any surface which is not in direct light is pitch black.

In an atmosphere, light scattering can affect both the I-ray and the O-ray, and there can be in-scattering and out-scattering. In-scattering corresponds to light from somewhere else in the scene being scattered onto the object (or into the eye), out-scattering corresponds to light from the sun being scattered away from the object or light from the object being scattered away from the eye. I-ray in-scattering causes ambient (non-directional) light – shadows are no longer pitch black but receive still some kind of illumination. Under a thin overcast haze layer, there is for instance strong I-ray in-scattering – while there is lots of light available, it comes from almost everywhere in the sky and no shadows are cast onto the ground. O-ray in- and out-scattering both cause objects being shrouded by haze, but in-scattering causes a bright haze, out-scattering a dark haze effect.

To complicate matters, there are three basic physical scattering mechanisms which can take place: Rayleigh, Mie and diffuse scattering.

* Rayleigh scattering occurs on very small particles – the air molecules themselves or fine dust (‘dry haze‘). It has no preferred direction, but is much stronger for blue light than for red light.

* Mie scattering occurs on larger particles – usually water droplets (‘wet haze‘). It has no color dependence, but is much stronger at small angles than at large angles, i.e. Mie-scattered light almost keeps its original direction.

* diffuse scattering isn’t really a distinct elementary process but the effect of multiple scattering processes over which direction and color specific dependence is blurred, hence diffuse scattering has no color or directional dependence.

Any real scene is hence a mixture of Rayleigh, Mie and diffuse in- and out-scattering on O-ray and I-ray (which makes for a total of 12 scattering channels, out of which 11 are modeled in at least some approximation by ALS – only Mie in-scattering on the I-ray is not considered since it is not very important in practice).

Wet and dry haze

Since most flight-relevant fog is wet haze, in FG the amount of wet haze is directly linked to the reported visibility. In many weather situations, fog is densest in the lowest convective air layer and the air is much cleaner above. ALS hence allows to render a lower layer of volumetric fog in addition to much less dense haze in the upper atmosphere. Seen from above (as in the scene showing morning fog at the foothills of Nanga Parbat), wet haze appears a bright white during the day, but when entering the fog, its color gradually changes to a dark blue-grey as diffuse out-scattering blocks the light.

The amount of dry haze (or Rayleigh scattering) relative to the wet haze is controlled by the air pollution slider in the weather configuration. Since Rayleigh haze is stronger for blue light, at large visibility O-ray in-scattering dominates (driven by the strong light coming from above) and far objects appear shifted towards sky-blue in color (such as the Sierra Nevada chain seen from China Lake below):

However, if the visibility is poor and/or the incident light from above is blocked, O-ray out-scattering is dominant, and all objects appear shifted to a dirty yellow – in other words, high air pollution makes objects appear in smog (here, downtown San Francisco):

The sky in low light

During the day, the I-ray is typically much shorter than the O-ray because the sunlight crosses the atmosphere vertically. The density of the atmosphere is variable in altitude, but effectively the whole vertical extent correspondsto a length of perhaps 10 km, whereas on a clear day objects 200 km distant can easily be seen. This is why Rayleigh out-scattering for the incoming light is not dominant while the sun is high.

In low light however, the I-ray passes a long distance through the atmosphere, the blue light is scattered out, and hence the direct light of the scene illumination is shifted to red (the indirect light however is driven by Rayleigh in-scattering and hence is shifted to blue). This can be seen here where the sun is below the horizon and illuminates the clear air close to the horizon whereas the lower haze layer is only visible in blue indirect light:

Without a lower haze layer, the whole scene appears in blue indirect light as seen here in the predawn Himalaya

As soon as the sun comes above the horizon and touches the highest peaks, strikingly beautiful contrasts appear between the blue indirect and the red direct illumination, leading to the phenomenon known as Alpenglow:

Looking away from the sun in low light, the clear atmosphere takes a deep violet color:

At very early predawn, just the far fringes of the upper atmosphere are illuminated. In clear air, the colors of dawn are muted:

In contrast, here a strong dry haze component leads to a sizable shift of the light to a red-golden color which lets the low wet haze layer glow brightly in the early morning light. While the light illuminating the wet haze is driven by Rayleigh scattering, the wet haze itself is a Mie scatterer – it glows most close to the sun, and the colors get more muted away – this is most evident from the thin clouds in the scene:

The combination of wet and dry haze can lead to nice and subtle color variations in low light:

The atmosphere seen from above

The following screenshots have been rendered with the EarthView orbital rendering option of FG in combination with ALS.

The characteristic electric blue glow of the atmosphere which is so prominently visible from low earth orbit is predominantly driven by Rayleigh scattering in the upper atmosphere.

Where the bulk of earth blocks the incoming light, Rayleigh scattering can no longer take place and the glow of the atmosphere gradually fades out:

The terrain itself is illuminated by light which has suffered Rayleigh out-scattering. In the dawn zone, this gives it again a color shift, here just slightly towards the yellow in this late afternoon impression of clouds hanging above the coast of Florida:

It is quite possible to observe the shadow earth casts into the atmosphere from lower altitude – here is an impression of it from 36.000 ft above an overcast cloud layer:

Clouds

Although they are rendered with rather different techniques, clouds physically are wet haze – thin translucent clouds are Mie scatterers, and thick clouds are diffuse scatterers. Thus, thin clouds light up very brightly in a halo when the sunlight is seen through them due to O-ray Mie in-scattering, whereas thick clouds appear dark and hide the sun due to O-ray diffuse out-scattering.

In predawn light, low clouds appear dark since they are yet in the shaded part of the atmosphere, but high Cirrus cloulds can already receive some sunlight:

As the sun comes up, this can lead to a dramatic play of light and shadow, with bright high-altitude clouds seen through a dark lower layer:

Again, the light that reaches the clouds at low sun is subject to I-ray Rayleigh scattering and its color depends on the amount of dry haze. In clean air, the colors of a cloud layer appear more muted

whereas for a high air pollution value the colors are much more strongly shifted towards the red-golden.

However, faraway cloud banks at the horizon can also reduce or alter the incident morning light by I-ray scattering. For thin, scattered clouds, this reduction is small and mornings appear bright

but if the cloud cover gets stronger, the light is reduced

and color shifted

to the point that a sunrise appears no longer red-orange-golden but blue-violet underneath a thick layer:

Diffuse haze in the atmosphere acts on the incoming light just the same way as a well-formed cloud layer – the light illuminating the clouds is reduced, and as the direct light is filtered out, the blue indirect I-ray Rayleigh in-scattering becomes more important, shifting colors to violet.

Compare the subtle play of dawn light shining through a cloud for a clear day

with a very hazy day with poor visibility aloft to appreciate the flattening of the color distribution:

Artificial light

At night, artificial light sources contribute a lot to the illumination – think of the orange glow of haze above a well-lit airport or a city. In principle, artificial light follows the same principles as sunlight, except that the intensity is usually far less, and so the paths through the atmosphere are smaller and effects are only visible in fairly dense fog. Then, often Mie-scattering can be observed, creating halos around lights seen through the fog.

Most of these effects are currently not included in ALS, however the Mie-scattering halos for runway lighting and the illumination of dense fog by landing lights are features already implemented:

Final thoughts

All the different scattering phenomena described above only scratch at the surface of what nature really does. In a real sunrise, clouds may cast shadows onto each other. There’s multiple scattering processes – a brightly illuminated haze layer high above may scatter lots of light down onto lower layers. There’s genuinely colored hazes like dust in a sandstrom which change the colors of dawnlight in yet different ways. There are effects of the human perception which make the eye see very faint or very bright light in colors different from what they actually are (which is why moonlight, despite being actually white light, appears as blue). While ALS tries to capture some of these processes, nature still does infinitely more, and sometimes one wonders how nature manages to get it all done in real time.

But even thinking about some of the phenomena causing it, you will never look at the play of haze and light the same way as before – be it in Flightgear or in reality.

All the screenshots above are rendered with the current development version of Flightgear (FG 3.3) out of the box. On a modern gaming laptop, in flight they typically render with 30+ fps (mainly dependent on visibility and LOD settings and the usage of hires scenery).

The ALS framework itself takes some 10 atmosphere-related input parameters to generate the visuals of the sky and of hazes, and this leads to an almost infinite variety. Unfortunately the majority of parameter combinations can not occur on Earth (ALS as such is quite capable of rendering a Martian sky), hence the raw input parameters are largely not under user-control. What limits the visuals ALS generates out of the box in practice is the actual range of parameters passed to the renderer by the weather simulation. Here, Advanced Weather using the offline weather engine is somewhat more faithful in generating reasonable light propagation models in the lower atmosphere than Advanced Weather in METAR mode, which is in turn better than Basic Weather, but even Advanced Weather currently exhausts just a fraction of the possibilities ALS really offers.

Modeling a compelling haze distribution and the resulting light attenuation in real time is a genuine challenge, since it is impossible to actually do the scattering calculations (which involve nested integrals) in anything resembling real time, so in every case, fast yet faithful approximations have to be found.

To experiment some with sunrises, try various weather scenarios and play with the lower haze settings and the air pollution on the Advanced Weather options panel.

The F-14b is back

Tuesday, November 18th, 2014

Ready to launch?

Thanks to Alexis Bory and Enrique Laso, the F-14b has been for a long time one of Flightgear’s most impressive 3d models, with a highly detailed cockpit and a large number of modeled systems.

But it just got even better – are you ready for a ride?

New flight dynamics

Richard Harrison has added a detailed JSBSim model for the flight dynamics based on a number of aerodynamical data sources which makes especially the behaviour at low airspeed very close to the real airplane. This also includes an accurate modeling of stall and departure into spin or flat spin and high alpha control reversal. Wing sweep can be controlled manually and affects the behaviour of the plane,

All of the plane’s control systems are implemented in JSBSim rather than in Nasal (which means they are computed at a much higher rate than the framerate), making the response of the plane more fluid, especially at framerates below 30 fps. All in all, the detailed JSBSim FDM adds quite a lot to the flight experience,

Improved systems modeling

The 3d cockpit has received a number of additions, among them a master warning panel with working indicators, an engine control panel and a master generator control panel. Other switches, such as the fuel cutoffs on the glareshield panel, are now functional, such that an engine startup/shutdown procedure from the cockpit is now possible.

Here is an example of the cockpit view in low-level flight:

And the RIO view:

The full range of operations

Just like the previous YaSim version, the new JSBSim F-14b supports a full range of military operations. The plane is fully aircraft-carrier capable (due to the improved modeling of low airspeed behaviour, carrier landings are somewhat more difficult than with the YaSim version though).

The plane also has a detailed radar with several different modes, capable of tracking targets, and the operation of the AIM-9M sidewinder missile is modeled as well as the M61A6 Vulcan gun.

Full air-to-air refueling capability from e.g. the KA-6 is also modeled:

Enjoy the new F-14b along with many exciting new features on current GIT (3.3) or with the forthcoming stable release 3.4!

(All features presented in the screenshots (bluish atmosphere haze, details on the Vinson flightdeck, improved appearance of water,…) are available in the current development version and will be part of the 3.4 release. The screenshots have been taken off the coast of Corsica and over Nevada, both in the default 2.0 World Scenery.)

Pushing the boundaries – the X-15 story

Monday, February 3rd, 2014

Suborbital flight with the X-15

Going to the edge of space… and back!

Operational history of the X-15

The North American X-15 was a rocket-powered, hypersonic research aircraft operated from 1959 to 1968 by the US Airforce and NASA. During that time, it set a number of records and greatly expanded the knowledge about conditions in the upper atmosphere and in hypersonic flight, thus ultimately laying the foundations upon which the Space Shuttle was built.

The X-15 reached Mach 6.72 on October 3, 1967, which is still today the official world record for the highest speed ever reached by a manned aircraft. In ballistic flight, it reached a top altitude of 354,200 feet (107.8 km) on August 22, 1963, crossing the boundary of space as defined by the Fédération Aéronautique International and making the X-15 the worlds first spaceplane. The 100 km altitude was only crossed on one other flight, but since the USAF defined the criterion for spaceflight by reaching an altitude of 50 miles, 13 different flights met this criterion and qualified the pilots for astronaut status.

Technical data

The X-15 is powered by the XLR-99 using ammonia and liquid oxygen as propellants, giving the plane a thrust of 70,400 lb and a thrust/weight ratio of 2.07. The rocket engine would only burn for about 80 seconds, the smallest part of the whole flight profile, but this would be sufficient to fling the plane on a high reaching ballistic trajectory or to accelerate it to tremendous velocities. It was the first man-rated rocket engine that could be throttled.

The plane has a thick wedge tail for stability at hypersonic flight conditions, however this produces a lot of drag at lower speeds. This means that the glide slope in the unpowered approach back to base is rather steep, and once back in the lower atmosphere, the X-15 sinks rapidly.

For maneuvering in the upper atmosphere where there is no significant air and the control surfaces do not work, the X-15 is equipped with a reaction control system (RCS) using hydrogen peroxide as propellant.

Flight dynamics of the X-15 in Flightgear is based on NASA-TN-D-2532 ‘Flight Measurements of Stability and Control Derivatives of the X-15 Research Airplane to a Mach Number of 6.02 and an Angle of Attack of 25 degrees’.

The RCS is not modeled in the default version of the X-15 available from the Flightgear download page, however an alternative versions of the X-15 with RCS and 3d cockpit are linked below.

Getting ready for suborbital flight

In reality, the X-15 was dropped from a B-52 aircraft at typically 45,000 ft and 450 kt, and then started its engines. This required a lot of preparation, however we also need to prepare the sim for suborbital flight.


Rendering suborbital flight is nothing Flightgear is designed to do, but as it is a very flexible framework, it can still be made to do it. The main problem is opening up the visibility to values which are plausible from the top of a ballistic arc at the edge of space, which amounts to about 400-600 km. This will require a modern graphics card and lots of system memory (the screenshots below were done on a GeForce GTX 670M with 3 GB GPU memory and another 8 GB system memory, this delivered a framerate of ~20 fps at arc top). Trying to open the visibility to large values can have severe performance impacts to the point that FG becomes unresponsive and can crash FG when memory actually runs out – it is recommended to try suitable settings with the ufo before using the X-15.

Some settings need to be tweaked:

* In order for the terrain to be loaded, the LOD range for terrain needs to be set. In the menu, View->Adjust LOD ranges, and set LOD bare to 500000 in order to allow terrain to be loaded up to 500 km distance.

* Loading terrain doesn’t help if the renderer does not display it. The camera of the renderer needs to be instructed not to clip faraway objects. Open the property browser from the Debug->Property Browser menu, and change into /sim/rendering/camera-group/ and adjust zfar to 500000 (or set the property at startup via commandline).

* Finally the weather system needs to be convinced to produce large visibility at high altitude. For Basic Weather, set the visibility at high altitude accrodingly in the mask. Advanced Weather will do it automatically if Max. Visibility in the Advanced Settings is high enough, however the gui doesn’t allow that, hence use the property browser again to set /local-weather/config/aux-max-vis-range to 13.12 (the slider operates on a log scale which is then converted to the actual value).

Switch randon objects, buildings and vegetation off before the flight – you won’t see them, and they will cost a lot of memory which you badly need otherwise. Launching over islands limits the amount of terrain to be loaded, also World Scenery 1.0 with low polygon count works better than he new World Scenery 2.0.

Finally, in the View->Rendering menu, switch Atmospheric Light Scattering on – this will render the atmosphere visuals.

One problem may be that FG can’t load the scenery fast enough. If the OS caches used files, loading the scenery from disk into memory once with an ufo-flight before using the X-15 may help here.

Climbing into space

Start the simulation in air, i.e. using commandline options –altitude=45000 and –vc=450 — this will produce the state of the X-15 just after having been dropped from a B-52. For a semi-historic trajectory, you can start above Nellis AFB (KLSV) and aim at a course of 240 deg which will roughly get you to Edwards AFB and Rogers Dry lake, the historic landing site for the X-15.

Take a few seconds after the drop to stabilize the plane into a shallow descent, double-check all settings and make sure you’re ready. If all looks well, push the throttle forward till the rocket engine ignites.

The XLR-99 delivers significant thrust, and speed will build up rapidly. We’re far too low for this, so pull gently on the stick till the plane goes into a 45 degree climb out of the lower atmosphere.

After a bit more than a minute, the main engine will cut out, but the X-15 will climb on. With increasing altitude, pressure based airspeed and altitude gauge become unreliable, so take a look at their inertial counterparts on the right side of the instrument panel now.

As the ballistic climb continues, the airfoils are losing effectiveness rapidly – time to switch on the RCS! Operate the BAL switch on the right side of the panel, press ‘i’ to grab the stick for RCS control (which in reality would be located on the left side of the cockpit). Think spacecraft now – there’s no damping force left, so operate the thrusters with carefully controlled bursts to stabilize the X-15. Once you have time to look out, you should see a lot of California. And Edwards AFB is really far, far down!

Back to Earth

Now comes the dangerous part — we’re falling down from 330.000 ft, we’re going to be really fast and the deceleration will be hard. The good news is that the view from the cockpit is now quite a bit more spectacular as the planet comes into view.

Stabilize the attitude using the RCS thrusters while high up. If the X-15 enters the atmosphere in a spin or roll condition, you will likely not survive the entry. As the plane gets lower, the airflow should start to build up, and if everything is going well, the X-15 should align its nose with the airflow.

The ailerons may become responsive below 200.000 ft already, start switching back to aerodynamical controls using the ‘u’ key and stabilize roll.

If you’ve been high up, the X-15 is falling really steeply at this point.

As the ground rushes closer, eventually the elevator becomes responsive as well, typically this starts below 80.000 ft. At this point, the plane will be going really fast and the ground approach rapidly. Pull back on the stick gently and watch the g-force. At this speed, even a gentle pull will translate into lots of force. Expect to experience 6-8 g during the pull out and prepare to black out in the worst phase. This is the most dangerous part of the flight.

Of course, if you don’t want to see a blackout simulated, you can always switch it off in the menu.

If everything went well, you should end up somewhere around 30.000 to 40.000 ft in level flight, with Edwards AFB (or whatever your landing site may be) in convenient reach. Now you can start trusting the pressure-based instrumentation again.

From this point, the drag of the stabilizing fins will be felt badly. Glide the plane maintaining about 300 kt. Rogers Dry Lake is a big place, so planning an approach should be reasonably easy.

Skids and gear out for the final approach…

… and a safe landing on the lakebed.

High speed profiles

Historically, the X-15 has not only been flown in high altitude profiles but also in high speed profiles. These are somewhat easier to pilot and control. For a high speed profile, aim at a more shallow climb angle, level off early and try to go horizontal around 100.000 ft, then let the X-15 accelerate and see how fast she will go.

After the engine cuts out, you can simply maintain altitude till the airspeed bleeds off and then slowly descent towards the landing site. Here’s an approach to Edwards AFB from a high speed run, coming in at 60.000 ft now.

Enjoy flying the first spaceplane mankind has built!

Alternative versions of the X-15

B-52 launched X-15 by Enrique Laso Leon (requires startup from historical location and joystick throttle control)

Free launched X-15 based on Enrique’s version, allowing startup at any location and keyboard throttle control, with some sound effects added.

Special thanks

The modelers of the X-15 in Flightgear:

Enrique Laso Leon
Jon S. Berndt

World Scenery 2.0

Tuesday, January 14th, 2014

Together with the release of Flightgear 3.0, a new world-wide scenery is now made available!

Flightgear’s world scenery is based on large-scale processing of publicly available and GPL compatible geodata. There is practically no manual intervention involved, which means that the scenery team can’t decide what quality the scenery will have at a certain location, that is only determined by the quality of the available data.

Thanks to the efforts of developers in bringing the processing toolchain up to date, the new official scenery with much better resolution than the previous scenery has now been possible. The new scenery is already available via Terrasync, but it requires a recent version of Flightgear, older versions are not capable of handling the vertex number of the new terrain mesh.

This FlightGear World Scenery was compiled from:
– ViewFinderPanoramas elevation model by Jonathan de Ferranti
– VMap0 Ed.5 worldwide land cover
– CORINE land cover 2006v16 for Europe
– Several custom land cover enhancements
– The latest airports (2013.10), maintained by Robin Peel of X-Plane
– Line data by OpenStreetMap

In general, airport layouts are now improved and updated all over the world, major roads and rivers are drawn to much higher accuracy than previously and the elevation mesh resolution is increased everywhere.

Europe

The most stunning improvements are found in Europe, where in addition to the increased resolution of the elevation mesh, also the CORINE database provides high resolution landcover data. This makes the visuals both in mountain regions as well as plains much more applealing. Combined with regional texture schemes and procedural texturing, an almost photo-realistic effect can often be achieved.

Corsica, France seen from above in morning fog (utilizing Mediterranean texture scheme) :

Details of Corsica, France in low-level flight with the F-20:

Fjell lands in Norway (using Scandinavian texture scheme):

Norwegian fjordlands:

Ouside Europe

In the absence of CORINE data, improvements in the landcover rendering are not as dramatic, which leaves flat terrain largely comparable to the previous version of the scenery. However, mountainous regions benefit enormously from the improved elevation mesh resolution. The rendering of light and shade, transition shader effects and snow effects all key on elevation gradients and allow in essence to render the terrain with much more visual detail despite the lack of detailed landcover.

Desert hill chain near Tabas, Iran, seen from the ground (using Middle-East texture scheme and dust shader effect):

As above, seen from the air:

The Grand Canyon, USA (using dust shader effect):

View of the Grand Canyon, USA from high altitude:

Nanga Parbat, Himalaya, Pakistan seen across the Indus valley:

Himalaya north of Nanga Parbat:

Thanks

Special thanks to the people involved:

John Holden
Olivier Jacq
Vic Marriott
Julien Nguyen
Gijs de Rooy
Christian Schmitt
Martin Spott
James Turner
Markus Metz
Pete Sadrozinski

The art of cloud and weather rendering

Tuesday, June 25th, 2013

Author: Thorsten Renk

Advanced Weather

Advanced Weather is one of Flightgear’s two weather-generating systems. It operates based on a (limited) understanding of atmosphere physics – the user selects a weather situation, either from the menu or via specifying a METAR string, and the system simulates the weather from there. For instance, once the system knows how unstable the lowest layer of air is against convection, it automatically decides on the presence of thermals, turbulence, convective cloud number and visual appearance. In this way, generated weather matches cloud types in the different layers based on what would typically also occur in reality for the given weather situation.

The system renders practically all clouds in 3D. To get close to a real sky appearance, it utilizes a large variety of algorithms grouping cloudlets into layers, streaks or undulatus patterns. Combined with the ability to change the weather as a function of position, endless varieties of weather situations appear, and both in the online and offline weather modes, the sky never really looks the same.

Simply select a basic weather scenario and watch the cloud patterns change from high or low altitude!








Clouds and the terrain

Cloud layer placement in level terrain is a simple exercise, but to render weather properly in mountain areas is a challenge. The weather system continually receives information about the terrain surrounding the plane, from which the distribution of wind and turbulence close to the ground as well as the placement pattern of clouds is computed.

Try flying a mountain rescue helicopter in bad weather to see the weather system in action! Or simply go sightseeing in the mountains with a single-engine plane.




Precipitation and turbulence

Precipitation is rendered beneath overdeveloping Congestus and Cumulonimbus clouds as well as beneath layered clouds. Either via a METAR string or on the advanced options configuration tab, the outside temperature can be specified – and precipitation changes from rain into snow accordingly. Also on the configuration tab, the stability of the convective air layer can be determined. Try combining an unstable convective layer with stronger winds, and watch turbulence evolve and rugged clouds with strong vertical development appear, or select a very stable atmosphere and observe well-shaped, large Cumulus clouds evolve. Or try the thunderstorm scenario, and observe large Cumulonimbus clouds tower over the scene.

Using Environment shader effects, it is possible to add a snowline, wet terrain with gleaming puddles or drift ice into the scene – use this for best effect in rainy or snowy weather.

Try setting up a stormy scenario by adjusting the wind, and watch trees sway in the wind. Can you fly a helicopter in 30 kt winds and torrential rainfall?






Lighting

Advanced Weather is fully interfaced with the Atmospheric Light Scattering rendering framework – which means clouds in low light get differential lighting according to altitude: While cloud bottoms of Cumulonimbus clouds may already be in shadow, cloud tops can still receive light. With the sun behind them, faint clouds glow in bright radiance whereas thick clouds show shadows, making for a beautiful play of light and shade.

The weather configuration tab also contains an air pollution effect – use this to see low light colors of sky and clouds change from clean air to smog.

Try an early morning takeoff before dawn, or flying into the night, and watch the low light illuminating the scene – there’s nothing quite as nice as a sunrise in the mountains.




Advanced Weather for Flightgear – made for pilots who love to watch clouds! All features shown will be available for the next official release!

Fly Hawaii!

Monday, January 21st, 2013

Author: Thorsten Renk

Destination Hawaii

One of the first places available as hires scenery in Flightgear, and also among the first places to receive a dedicated regional texture scheme, the island chain of Hawaii is a very spectacular destination in the Flightgear world. It offers a compelling variety of terrain from dry and barren lava plains to lush tropical rainforest, from the gentle fertile plains to rugged mountains and steep cliffs towering over the sea and from the densely populated island of Oahu to uninhabited Kaho’olawe.

Flying Hawaii can be easy or challenging – there are busy international airports and lone airstrips in remote locations, the altitude of the terrain ranges from sea level all the way up to Mauna Kea towering at 13,796 ft and steep gorges cut into the lava cliffs allow for tricky helicopter excursions.

Currently the scenery is only available via TerraSync and not by direct download from the website, presumably this will change with the next release of world scenery. While the release preparations for Flightgear 2.10 are underway, this article provides a first glimpse into some stunning new features which are currently being developed for the 3.0 release in summer 2013 – high resolution terrain texturing for closeup scenes.

Aeronautical charts for the whole of Hawaii are available online at skyvector.com, see for instance here for all charts relevant for Honolulu International Airport.

Hawaii ‘Big Island’

With a total area of 4,028 square miles, Hawaii is by far the biggest island of the archipelago, exceeding the size of all other islands taken together. It is also the youngest of all islands, dominated by the gentle rising cones of the five massive shield volcanoes Kohala, Mauna Kea, Hualalai, Mauna Loa and Kilauea, with the last two still being active.

The central part of the island is occupied by the twin cones of Mauna Kea (foreground) and Mauna Loa (background) which both reach above 13,000 ft and consists of extended lava fields, while the coastal region is somewhat more fertile.

The first destination reached however when arriving from the Honolulu region is Upolu Point, a region of eroded volcanic rock and spectacular gorges.

A flight to Hilo, the main city of the island, can pass between the two major shield volcanoes and requires a climb from sea level to more than 7,000 ft, which requires some adjustment of the mixture in a single-engine propeller plane. The climb to the pass is mainly above arid grasslands.

At higher altitudes, the spectacular lava fields of Mauna Loa dominate the scene.

Here is yet another view on Mauna Kea from the pass – often the volcanoes reach above the cloud layer.

Seen from the pass, Hilo seems close, but the slope of the terrain is so gentle that it is very easy to underestimate the true distance. Towards the coast, forests and fertile ground dominate the scene again.

Maui

Maui is perhaps the island with the most diverse terrain. Its eastern part is dominated by the mighty cone of Haleakala, reaching just above 10,000 ft. The middle part is a fertile valley, whereas the western part features the rugged West Maui Mountains, which are considerably lower than Haleakala, but certainly make up for that with steep cliffs and deeply cut valleys.

Since the prevailing winds come from the northern side, air rises on the flanks of Haleakala, leading to fertile and overgrown northern slopes, whereas the southern slopes of Haleakala look completely different and show rather different weather.

Flightgear’s Advanced Weather is actually capable of simulating the resulting distribution of clouds from this effect – in fact, Haleakala has been an inportant test case in the development of the weather system.

Closely grouped in the vicinity of Maui are also the islands Lanai, Molokai and Kaho’olawe, easy to see in clear weather, thus Maui is an ideal starting point for island-hopping adventures.

Approaching from east, the scenery is dominated by Haleakala, here the more arid southern slopes are seen.

Maui is substantially older than Hawaii island, and so the volcano has started to erode quite significantly when compared to Mauna Loa – as a result, the fertile land extends much higher up. Haleakala crater however remains a rather impressive sight.

When approaching from the west, the cliffs and gorges of the West Maui Mountains are the first feature to become apparent.

On a clear day, the surrounding islands (here Molokai in the background) can clearly be seen:

The West Maui Mountains themselves contain quite some impressive sights – it is especially worthwhile to explore the various canyons and cliffs with a helicopter.

Yet another flyby view from the F-14b RIO position on the West Maui Mountains:

Oahu

Going west, the geological age of the island chain increases, and thus terrain features become more gentle as the volcanic rock erodes and changes into fertile soil. The island of Oahu is where the majority of the Hawaiian population lives and where the capital Honolulu is located. This is also where Honolulu International Airport, the most busy of all Hawaiian airports is found, and the home of famous sights as Pearl Harbour. Honolulu was envisioned as an emergency landing site for the space shuttle, and in fact the ‘reef runway’ (shared, as the rest of the airfield, with Hickam Air Force Base) used to be designated for this purpose.

Oahu stretches between two mountain ridges, which rise up to an elevation of just over 4000 ft. Here is a view of the island from the west.

Central Oahu is flat and largely in agricultural use. In the background, Honolulu and Pearl Harbour can be seen.

One of the most scenic spots on the island is Kailua beach on the north-eastern coast, offering a spectacular constrast of steep cliffs, long beaches and lush tropical vegetation.

The hires ground texturing scheme for Oahu has been carefully designed to display the contrast between lush vegetation and the red volcanic soil.

The other islands – Lanai, Molokai, Kauai, Kaho’Olawe and Niihau

Lanai is a fairly arid and sparsely populated island south-west of Maui with a single airport. It is dominated by a single mountain ridge reaching just above 3000 ft, with some valleys carved by erosion.

Molokai is, like Maui, a fairly diverse island – its eastern part consists of steep and towering cliffs whereas its western part is mostly flat and gentle landscape. Kalaupapa airport (PHLU) is built on a peninsula just beneath the cliff faces.

Kaho’Olawe is a small, uninhabited island. It has no airport and can only be reached by helicopter.

Its surface is mostly composed of arid stretches and lava fields.

Kauai, the garden island, is one of the nicest bits of scenery in the Hawaiian islands. It features the spectacular Na’Pali coast and Waimea Canyon.

Sadly, the scenery in Flightgear is currently a bit of a let-down – the terrain shows some errors in Kauai, and neither the Na’Pali coast nor Waimea come anywhere close to the originals.

Here is a scene close to Hanalei:

Finally, the island of Niihau is not part of the high resolution scenery package, and thus not really worth visiting.

Some Hawaiian airports

Hilo International Airport (PHTO) is located on the eastern side of Hawaii island at the coast – in a vert scenic location close to the town of Hilo. It is one of the two major airports of the archipelago and with a runway length of 9,800 ft large enough to admit essentially all airplanes.

Kona International Airport (PHKO) is located in the lava fields at the western coast of Hawaii island. Three million pounds of dynamite have been used to flatten the lava flow on which it was constructed. It offers a single 11,000 ft runway which is second in length only to Honolulu International Airport.

Waimea-Kohala Airport (PHMU) is a not very busy public airfield at 2,600 ft altitude in the western drylands of Hawaii island. It offers a single 5,197 ft runway.

Princeville: (HI01) is a small private airport close to Hanalei on the garden island Kauai. It is only suitable for smaller aircraft.

Lihue: (PHLI) is the main airport of Kauai. It has mainly connections to Honolulu, but also some long-distance traffic to the US mainland.

12 Days of Flight Tips (Season 2)

Wednesday, January 2nd, 2013

Last year, Oscar (youtube user: osjcag) created a series of short “howto” movies called the 12 Days of FlightGear Tips.  This year he is producing Season #2!  Each day he releases a new tip in honor of the twelve days of Christmas. Make sure you check back each day for the new tip!  Even “seasoned” FlightGear pilots may pick up a new trick or two.  Enjoy!