Terrain Texturing

Author: Thorsten Renk

Regional and procedural texturing

It’s perhaps not a big secret that the default Flightgear World Scenery does not look stunning everywhere in the world. Yet, with regional texturing in Flightgear 2.8 and easy to configure procedural texturing in the current development version 2.9, two techniques have arrived which have the potential to rapidly change this. But precisely what are these techniques?

Short of addons such as fgphotoscenery, Flightgear has never used aerial photographs for texturing. Instead, the terrain is described in terms of landclasses, and each landclass has an associated texture. Up to 2.8, these texture definitions were the same all over the world, Yet in reality, this is not true – cities in the US for instance tend to be organized in rectangular grid patterns which are completely uncommon in Europe, Irrigated crops in Asia are most likely rice terraces, whereas rice terraces are not a common sight in the US Mid-West. Regional texturing allows to define texturing schemes for specified geographical regions and allow to overcome these problems – European cities can now defined to look different from US cities.

Procedural texturing is an even more powerful technique. In the default rendering scheme, the terrain of a certain landclass is painted with a pre-defined texture, then the light is computed and this is what we see on the screen. Procedural texturing does not use a pre-defined texture, but computes the texture as part of the rendering process. This powerful technique allows textures to be sensitive to the environment and hence simulate wet or dusty terrain, to create the actual texture as a mixure of various overlay textures which change dependent on how steep the terrain is or to add snow cover with any density on the fly. Procedural texturing has been part of the shader effects in Flightgear 2.8, for instance in terms of the wind-dependent wave patterns of water, or the snowline settings, but in 2.9 it gained many additional options and most important is configurable without any knowledge of OpenGL rendering by just a few lines of xml code.

Procedural texturing is best illustrated pictorially – here is a scene (China Lake Naval Air Weapons Station (KNID), California) in default texturing. The visible terrain is mostly shrubland, and there is a pronounced tiling effect – the texture pattern is seen to repeat in the scene, leading to regular structures which become even more prominent from higher altitude.

The same scene in procedural texturing looks much more appealing – the random mixture of different base texture removes the tiling for good, and a thin dust effect creates the impression of dry terrain as appropriate for the near-desert location.

Unfortunately, procedural texturing does not come for free – computing textures on the fly creates a significant drain on framerate, thus procedural texturing is only suitable for modern graphics cards.

The structure of Flightgear Scenery

The combination of regional and procedural texturing is extremely powerful and allows to make dramatic improvements to the world scenery at the simple expense of few lines of xml code. Let’s look at an example location:

Canaima National Park in Venezuela is one of the world’s most fascinating mountain regions with table-mountains like Auyantepui towering over jungle terrain, featuring the world’s tallest waterfall, Angel Falls (3,287 ft). The scenery offers steep near-vertical cliffs hundreds of meters high, rugged and inaccessible plateaus atop the table mountains and lush tropical forest with winding reivers at their feet. The best place to access the park is Canaima airport (SVCN).

Yet, in the Flightgear default rendering scheme, Auyantepui is shown like this:

There is… something wrong here. In order to understand what goes wrong, let’s take a short look at the structure of the Flightgear scenery.

The basic ingredient of the scenery is the terrain mesh, containing the elevation data for all mesh points and the information what landclass the terrain between grid points is. The terrain mesh is created by a tool called TerraGear from public geodata. The output of this stage contains the altitude information of the terrain, and for instance the information that the terrain represents tropical forest (the so-called ‘landclass’).

Upon loading the terrain once it is used by Flightgear, the landclass is associated with a texture. At the same time, random objects such as buildings or trees are created and placed upon the terrain mesh where appropriate. Thus, the tropical forest landclass would at this step be associated with a forest texture and be populated with a large number of trees. At this stage, also shader effects are associated with a particular landclass, for instance water receives a reflection effect, whereas urban terrain may receive the urban shader effect.

In the last step, static (unique and shared) objects are added to the scene. These are objects which appear always at a given location, for instance airport terminals or special landmarks, and they are found in the Flightgear Scenery Database.

Armed with this knowledge, let’s analyze the above scene to find out what goes wrong: We can see that large parts of the table mountain get an agricultural texture. Visiting the scene with the ufo and using ctrl + alt + click (only in 2.9) on the offending terrain reveals that the mesh is here classified as ‘DryCrop’. This isn’t completely unreasonable, as the top of the table mountain is a rather barren grassland – but DryCrop becomes automatically associated with Europen-style agriculture textures – which look just plain silly in a place which in reality is utterly inaccessible, despite the valiant effort of the shader effect to change the agriculture to brown earth on steep slopes. Similarly, the nearby tropical forest is classified as ‘EvergreenForest’ (which is technically correct) – but EvergreenForest is associated with needle forest textures and needle trees.

Editing scenery texturing

There are various possibilities how this could be addressed. For instance, using TerraGear the landclasses in the scenery could be changed to something closer to reality. But to do this requires some learning, TerraGear is not a trivial tool. In this case, it is also unnecessary: The basic elevation mesh is in good order, the landclasses are not unreasonable, just the way textures and random objects are assigned to them is not working, and thus we need to change this.

The mapping of landclasses to textures and various other properties is controlled by a file called materials.xml. The regionalized version of it is found under $FGRoot/Materials/regions/materials.xml. In this file, for each landclass, a block of definitions exists. The idea is then to just copy the block for ‘DryCrop’ and edit the copy to contain an alternative definition valid for a particular geographical region, then change the texture to something more suitable. Plenty of nice textures already are in $FGRoot/Textures/Terrain/ and $FGRoot/Textures.high/Terrain/, so usually we don’t even need a new texture. While we’re at it, we might as well add two more lines to the etxture declaration specifying the overlay texture for procedural texturing. And that’s all it takes – next is EvergreenForest – we repeat the procedure and in addition change the tree texture from evergreen needle trees to tropical trees.

After just about an hour of editing materials.xml (the whole procedure is described in detail here), Canaima National Park looks like this:

Much better – isn’t it? Now all that’s missing is Angel Falls – we’re going to need a static model for this. The Particle System of Flightgear is going to be our friend here…

Canaima Sightseeing

After adding the model of Angel Falls using the ufo, Canaima National Park is ready for a sightseeing Flight (Flightgear 2.9 users can already enjoy it like this!) – once the landclass assignment is okay, procedural texturing takes care of the rest:

Steep cliffs and sheer drops flying over Auyantepui enroute to Angel Falls:

Table mountain tops reaching above the clouds:

Angel Falls seen from high altitude:

The barren top plateau of Auyantepui:

Tropical rainforest on return to Canaima airport:

Don’t wait for someone else to fix the terrain you want to explore – it’s easy, the tools are there and in many cases it’s more work to create a single model of a building than to make terrain texturing in a vast region look good!

Vertical takeoff and landing

The Harrier in Flightgear

Author: Thorsten Renk

The VTOL concept

Quite early on in the history of jet fighter aircraft, it was realized that a main vulnerability of jets is their reliance on an airbase and a runway, targets which can comparatively easily be taken out or temporarily disabled in a war, especially as the operational range of most fighters is quite limited and hence the base has to be relatively close to the front. Vertical takeoff and landing (VTOL) ability was seen as a way to overcome this problem in the 1950s, since a VTOL fighter could operate from basically anywhere.

The problem of designing a VTOL aircraft is however obvious – such an aircraft needs a thrust to weight ratio above one to lift from the ground with thrust vector pointed downward during takeoff and pointed backward during normal flight. Early designs involved planes landing on their tail (such as the Lockheed XFV-1 or the Ryan X-13 Vertijet), but these planes were difficult to control. Other designs experimented with auxiliary, downward-pointed engines, but their extra weight was found to be impractical in a fighter jet. For a long time the only truly successful design was the Harrier family achieving VTOL due to thrust vectoring nozzles. The Lockheed Martin F-35B is expected to continue the concept of a VTOL fighter in the next millenium.

In the Harrier, the jet exhaust passes through four vectoring nozzles surrounding the center of gravity of the plane. These nozzles can be vectored from zero degrees (to the rear) up to 98 degrees (down and slightly forward for deceleration in hover flight). Since there is no airstream in hover flight across any of the control surfaces, the plane is equipped with a reaction control system with a set of extra small thrusters.

All in all, the VTOL ability comes at a price – engine maintenance is difficult (the wings have to come off), the plane is difficult to fly and pilots have described it as ‘unforgiving’ and the accident rate has been comparatively high. Nevertheless, the Harrier has been considered a successful fighter design.

Vertical takeoff

Let’s explore the Harrier in Flightgear! For any VTOL design, weight is a critical consideration. The plane will lift only if the thrust-to-weight ratio is above one, thus with a full fuel and weapons load, the plane is too heavy to lift. For this reason, whenever feasible, the plane is actually used in STOL (short takeoff and landing) mode with thrust only partially vectored down and lift provided partially by aerodynamical lift and partially by downward thrust. In this case, we’d like to do a VTOL takeoff though. With full fuel loadout and two AIM-9L missiles, the plane is still able to lift from the deck of USS Carl Vinson.

After doing my preflight checks, I vector the thrust about 83 degrees down (the Harrier sits on the gear with the nose pointed up, so if I vector 90 degrees down the plane moves backward on takeoff, which is very dangerous). After releasing the parking brakes, the thrust is slowly increased and the thrust vector corrected such that the plane doesn’t move – now thrust points exactly down. I then increase the thrust until the plane lifts from the deck (this means almost full throttle for the takeoff load), and then, a few meters above the ground vector the thrust very slightly backward to accelerate.

The Harrier has a tendency to lift the nose at this point, so I am very careful to push the nose down early on. As the plane accelerates, I vector the thrust more and more back and retract the gear, and within a few seconds the plane accelerates to above 100 kt and less and less downward thrust is needed. At around 240 kt, I vector the thrust completely back and the Harrier reacts like a normal fighter jet.

The Harrier in flight

It is important to remember to reduce thrust at this point – the Harrier has a very powerful engine due to the need to lift, but it is also a very fuel-consuming engine, and in horizontal flight with full thrust it won’t go anywhere before the tanks are empty.

Once in the air, the Harrier is a fairly typical older-generation fighter jet – it has a high roll rate, a fairly small turn radius and can climb quickly to high altitude. Lacking an afterburner, it is (despite the powerful engine) not a supersonic plane. Also, without thrust vectoring the plane doesn’t handle too well at slow airspeeds and cannot compete with swept-wing designs like the F-14b. However, thrust vectoring can be used in maneuvering to suddenly decelerate the plane by vectoring the thrust forward or to achieve a very tight turn radius.

The Harrier can also land like a conventional fighter jet, in this case vectoring the thrust about 45 degrees down acts like extra flaps – the plane slows down as the backward thrust is reduced and gets extra lift from the downward thrust component.

The current cockpit of the Flightgear Harrier could clearly use some attention, it has rather basic texturing, not all instruments are implemented and all in all it tends to be the least realistic visual element in the scene.

In flight planning, it is important to remember that unlike a conventional landing, a vertical landing involves a prolonged period of full thrust, and thus (especially during practice of the VTOL approach) about 20 to 25%% of the total fuel load should be available for the landing.

Not a helicopter

Despite some similarities to helicopter flight, it should be remembered that the Harrier is not a helicopter and reacts somewhat differently. First of all, torque generated by the main rotor is a big issue for helicopters and needs to be compensated for, but torque is absent for a jet engine – the Harrier does not in itself develop a tendency to yaw when lifting off the ground.

However, the roll stability is dramatically different in hover flight. One can think of a helicopter as the mass of the helicopter body hanging underneath the lifting rotor. Thus, when the body of the helicopter starts to roll, it has a tendency to swing like a pendulum underneath the rotor, but the roll doesn’t grow by itself. In contrast, the Harrier is a mass balanced upon a column of lifting thrust, so any roll tendency will not lead to a pendulum motion but will be self-reinforcing, and if it is not corrected will lead to an unstable condition.

An unstable situation however is worse in the Harrier than in a helicopter since a helicopter pilot has more options – since a helicopter pilot can use the cyclic control to tilt the rotor to the side and well as forward/backward (and so also fly the helicopter sideways or backward). The Harrier can vector thrust only backward or down, but not to the side, i.e. it can not easily be flown sideways and has limited control over unstable situations.

Finally, on a more prosaic note, the view down is much worse in the Harrier than in many helicopter cockpits. For all these reasons, it is safer to land with a small forward velocity (which can quickly be reduced on the ground) than to touch down actually without any forward velocity.

Vertical landing

I fly pretty much a conventional approach till about 10 miles distance to the carrier. At this point I reduce the airspeed to 250 kt and start to get flaps out. I put throttle to idle and vector the nozzles down to 90 degrees. As the plane slows down due to drag, about below 200 kt the aerodynamical lift reduces significantly and I keep increasing throttle to compensate. Below 150 kt, I extend the gear. As the carrier gets closer, I aim to reduce the airspeed to about 50 kt – since the Carrier moves with about 15 kt, that gives me some 35 kt relative motion to the carrier, enough to keep the approach stable. I vector thrust slightly forward and backward from the 90 degree position to adjust airspeed and monitor throttle to control my descent rate.

At 50 kt airspeed, there is no significant aerodynamical lift left, so the plane hovers under almost full thust slowly towards the carrier. It is important to monitor both airspeed and descent rate at this point – if the airspeed drops too much, the reduction of the small remaining lift component means that I descend too fast and get below the flight deck. In addition, in this stage of the approach wind gusts are felt quite badly and can ruin the whole approach if the plane does not have enough forward motion.

Compared to a carrier landing in the F-14b, it feels as if the Harrier approaches the flight deck centimeter by centimeter, although at this point there are about 20 kt relative motion. I keep the nose of the Harrier level with the horizon and pull it up to 8 degrees only after I am above the flightdeck – this effectively vectors thrust forward and kills my remaining airspeed, this in turn reduces lift and combined with a slight decrease in throttle lets me touch down with a forward motion of less than 10 kt, which I kill by applying brakes while I push the throttle quickly to idle to let the plane settle down firmly and avoid being thrown by a sudden gust.

As can be seen here, the Harrier rests in a slightly unusual configuration with the nose pointing upward.

Clearly, the Harrier is not one of the most detailed aircraft available in Flightgear. However, it provides a good, solid hands-on understanding of the advantages and problems of the VTOL concept. Other versions of the Harrier can be found in the Flightgear UK hangar. The above screenshots have been made with the development version of Flightgear using lightfield shading and the environment-sensitive detailed water shader.

Mountain-flying in the French Alps

Author: Thorsten Renk


If you are up for some challenging approaches and tricky landings while you like to enjoy spectacular scenery, here is a good suggestion for a destination – try visiting the French altiports.

Altiports are small airfields for small aircraft and helicopters located high up in the mountains, often serving a ski resort. The runway is usually short and quite steeply sloped (in the case of Courchevel, the slope is a solid 18.5%), and all landings are done uphill with no go-around procedure. Since the alitports are by no means in the mountain summit region, the approaches must be done in the confined regions of a valley, which means they are usually curved and somewhat dangerous. As a rule, no navaids are available, thus altiports can only operate in good weather – all this adds up to a challenge. In fact, the History Channel program ‘Most Extreme Airports’ ranks Courchevel as the 7th most dangerous airport in the world, and once you do your first approach, you will quickly discover why.

Flying the French Alps is significantly more interesting by using the highly detailed custom scenery which is available for free under a Creative Commons Attribution-ShareAlike 3.0 here. There are six altiports in the French Alps, L’Alpe d’Huez, Courchevel, La Rosiére, Mégève, Méribel and Valloire, along with a number of airfields in the valleys. The first two of these, L’Alpe d’Huez (LFHU) and Courchevel (LFLJ), have been modelled in detail for the custom scenery and are available from the PAF team hangar here. This package also contains a detailed model of Grenoble Le Versoud Aerodrome (LFLG) which is a good place to take off for a first look at Courchevel or L’Alpe d’Huez.

Here is a picture of the layout of L’Alpe d’Huez as it appears in Flightgear:

The challenges of mountain-flying

Beginners are probably better off with a powerful turboprop STOL aircraft like the de Havilland DHC-6 ‘Twin Otter’ which has the climb rate to pull out of dangerous situations, but the real challenge is better experienced in a small aircraft like the Robin DR-400. With a constant pitch single propeller and no retractable gear, this plane is, especially when passengers and baggage are on board, seriously taxed to climb over the high mountain ridges which in many cases reach above 11.000 ft.

The fuel and payload menu item allows to adjust both the fuel level in the tanks as well as any additional weight on board. Asymmetric weight distribution in JSBSim is in fact taken into account properly, if for instance the copilot weight is reduced in flight, the plane starts to roll.

Here, a DR-400 is lined up for takeoff in Courchevel with L’Alpe d’Huez as destimation. Since the airport is at an altitude of about 6500 ft, it is important to adjust the mixture properly to the altitude in order to get the full engine power for takeoff.

Once the engine is running at full power, the brakes are released and the plane accelerates quickly down the steeply sloped runway, becoming airborn halfway. Departing from Courchevel, it becomes readily apparent why the altiport is challenging and why there is no go-around procedure available.

On the direct way, the high ridges of the Vanoise National Park with elevations well above 9000 ft have to be crossed – with the fully loaded DR-400, this is a slow climb over snow-covered mountains.

Snowcover in Flightgear can be generated by shader effects with a user-controlled snowline. Since the shader effect does not place snow on steep slopes, the outcome looks very compelling.

However, also the lower valleys during the descent to L’Alpe d’Huez have a lot to offer.

This is the final approach, aiming between rocky cliffs. At this point there is still a chance to break off.

The final moments – we are committed to a landing now. The trick is to aim low and reach the threshold in level flight, then pull up to follow the slope of the runway, let gravity develerate the plane and let it touch down softly (if you try a normal approach on a runway with such a steep slope, you will break the gear) and not throttle back engine too fast, because the plane still needs to reach the top of the runway, and once the plane comes to a halt on the slope, the engine often is not powerful enough to get it moving again.

All went well – time to enjoy the company of the other pilots and have a coffee before heading back to Courchevel.

In bad weather, things can be much worse. If the valley is cloud-filled, there is no choice but to turn back if no good view of the airport is possible early on. And in gusty crosswinds, hitting the runway just right is a challenge on its own.

The beauty of mountain flying

However, once one masters the challenges of high-altitude flight and navigating in the confines of valleys, flying the Franch Alps in nice weather is primarily a good way to see spectacular scenery. Let’s head back to Courchevel!

Here, the DR-400 accelerates down the sloped runway of L’Alpe d’Huez – going down, airplanes accelerate much faster than one is used to on level runways, so we can get airborn in just a few moments.

Leaving L’Alpe d’Huez, the village and ski resort becomes visible. The winding road up from the valley is actually a popular mountain stage of the Tour de France.

The vicinity of L’Alpe d’Huez has deep valleys, spectacular cliffs and Canyons and steep, rocky mountain faces – one can fly through the valleys or high above the mountain ridges.

Enroute to Courchevel, we leave the high ridges behind and cross to the Vanoise Park in the vicinity of Modane.

Descending again, Courchevel becomes visible (just to the left of the screenshot) while the lower valleys vanish in afternoon fog.

Final impressions

Of course, one of the must-see destinations in the vicintiy is Mt. Blanc, towering at 15.781 ft above most cloud development. Its rocky lower slopes and steep cliffs make for some really spectacular scenery, and especially at sunrise or sunset, the view from the summit is spectacular.

As the sun goes down, the last clouds light up over L’Alpe d’Huez which is to be closed over the night. High time to get back to Grenoble.

Advanced Weather v1.4 in Flightgear 2.6+

Part I: Convective clouds

Author: Thorsten Renk

The screenshots shown in the following use shaders, textures and scenery which are for various reasons (incompatible license, too recent development,…) not part of the official Flightgear 2.6 release. However, these are available for download and every feature works with Flightgear 2.6. The following packages need to be installed in addition to get to see the same: lightfield shader package v1.1, Juneau custom scenery, and textures from regional textures v0.1.

An integrated weather system

Without a doubt, clouds, haze and fog are the most easily noticed features of weather in a flight simulation, followed by winds. Advanced Weather v1.4 is however more than a tool to draw clouds and set wind parameters – it is a system with a (limited) understanding what the weather which it currently tries to render is, and it aims to simulate features of atmospheric physics.

This means that different weather phenomena tie together – winds and terrain influence the way cloud formation is taking place, cloud formation and the formation of thermal updrafts is connected, and weather is always understood as part of a large-scale weather pattern involving high and low pressure systems.

At the same time, clouds and atmospheric haze also influence the atmospheric light (and now also the scenery) in an essential way – strong fogging changes the color of sunrises to a blue-grey, wave patterns on the ocean follow the wind strength and direction and rain causes visibly wet runways. Let us have a look at how this works in fair weather.

The formation of convective clouds

Fair weather is typically characterized by convective cloud development: The sun heats the terrain and the air layer just above, thus warm air rises up in ‘bubbles’ and forms thermals, as the air rises, it expands and cools and eventually the moisture condenses into droplets, forming the characteristic, cauliflower-shaped Cumulus clouds. Cumulus clouds are the most common example of clouds formed by pronounced vertical motion of air.

As every glider pilot looking for thermals has to learn quickly, the formation of convective clouds depends on many different factors. The terrain type is crucial – while rock or concrete surfaces heat well in sunshine and may easily lead to well-developed thermals and cloud formation, open water or ice is much less likely to heat up in sunshine and seed Cumulus formation. High points in the terrain mark the spots where the bubbles of warm air are most likely to lift off the ground. Another important factor is the time of the day: The sun needs sufficient time to heat the terrain, therefore Cumulus formation is densest around noon, but the thermal updrafts are strongest in the afternoon, and while pronounced Cumulus clouds are unlikely to form in the morning, the thermal energy accumulated over a day may still give rise to well-developed clouds in the evening.

Let’s follow the development of convective clouds during a day in Juneau (Alaska). At sunrise, only very few clouds form, and they are transient, whispy phenomena (click to enlarge images):

Later in the day, the cloud formation is somewhat stronger. Note how clouds tend to form over mountain peaks, but do not form over open water. Also, no strong cloud development occurs over Taku glacier in the upper left, despite its high altitude, as the ice surface does not heat well in the sun.

At noon, the thermal updrafts become stronger and consequently the clouds become more well-defined. While in the morning the upward motion of air rarely exceeds 0.5 m/s, around noon this becomes rather 1 m/s, to strengthen even more in the afternoon.

Yet a few hours later, the number of clouds decreases again as the thermal irradiation by the sun weakens, but then typically fewer but stronger thermals with larger cap clouds are found.

Towards sunset, there is still significant thermal energy left to lead to sizeable cloud development, although the number of clouds as well as the typical strength of thermals is decreasing again. During the night, the development of convective clouds breaks more or less down completely as there is no thermal energy from the sun available.

Interaction of convective clouds, wind and the terrain

Wind meeting a terrain barrier corresponds to an upward-moving airflow, and hence is able to alter the development pattern of convective clouds in an essential way. Consider the following scene above Maui (Hawaii) in the absence of winds. Clouds rim the peak of Haleakala, but do not actually reach all the way up to the mountaintop (note also that due to the different latitude of Hawaii, there is far more thermal energy coming from the sun than in Juneau, leading to a much higher overall density of clouds):

With 20 kt winds coming from the north, the picture changes quite drastically: clouds are now pushed up all the way to the summit of Haleakala by the rising air, whereas the falling air south of the crater creates a lee effect in which convective clouds disappear. The vegetation pattern of Maui reflects these prevaling conditions – while moist air is carried up all the way to the summit of Haleakala, it rains off and irrigates the northern slopes of the mountain, leading to a bright green forest belt. The southern slopes on the other hand see typically falling air and dissolving clouds, and are consequently much drier.

Convective clouds in flight

If the appropriate option is selected, thermals are automatically generated along with Cumulus clouds so that thermal soaring is possible. Combined with the effect of ridge lift, this can make for rather realistic mountain soaring conditions in which a good degree of skill is required to stay in the air.

But the detailed interplay between convection and the terrain leads to interesting scenes also in other planes. Around noon, the peaks of high mountains are often covered in rather dramatic clouds piling up against the slopes.

Further down, the altitude of the cloudbase is no longer determined by the terrain but by the air layers.

And yet, the terrain elevation and the change between land and water imprint a pronounced pattern onto the distribution of density, shape and size of convective clouds.

Convection may also occur due to vertical instabilities in upper air layers, leading to the development of Altocumulus clouds, or at even higher altitudes Cirrocumulus clouds. Here’s an example of the development of Altocumulus fields at 15.000 ft. At such high altitudes, the clouds are no longer influenced by the terrain underneath, but rather by the properties of the air layers between which the Altocumuli form. For instance, Altocumulus development may be caused by the instabilities associated with an approaching cold front, and may thus signal the danger of severe thunderstorms in the near future. The Advanced Weather offline weather generation automatically includes this and other rules of large-scale weather patterns.

Next: Layered clouds, haze, fog and precipitation

What would it be like to fly a rocket into space?

Do you want to earn your astronaut’s wings?

Author: Thorsten Renk

Real spaceships aren’t actually piloted into orbit. The risk that a human being, strapped to his acceleration seat and under a crushing acceleration of 4 g for a prolonged period of time is unable to fly with the precision required to reach orbit is far too great, and real spacecraft reach orbit on autopilot.

But what would it be like? Welcome to a scenario in which a Russian Vostok spacecraft has been acquired by the USA and fitted for a manually flown mission.

This is the launch vehicle assembled at Edwards Airforce Base. The actual capsule is hidden under an aerodynamically formed protective cover. Below it is the third stage of the rocket, with its exhaust nozzle visible. All this is mounted on top of the huge first and second stage. Unlike many US rockets, which use sequentially burning stages, the first stage of the Vostok launch vehicle consists of four boosters which burn along with the long, cylindrical second stage.

The inside of the spacecraft is a very small place. There are no big windows (and currently the protective cover blocks the view outside in any case), so there is not so much to see except the instruments. In front of me is the main instrument panel, and to the right is the stage control panel, left of it the control handle.

Here’s a closeup onto the main instrument panel. Since I won’t be able to see anything of the outside during much of the ascent and the descent, this is what I will have to navigate with. The most important instruments are in the lower half of the panel – altimeter, inertial speed indicator, vertical speed indicator, dynamical pressure, orientation and acceleration. This isn’t enough to fly with any precision, say to rendezvous with ISS – but that’s not what the Vostok is for, it’s made to carry a human into orbit and back, and this is what I will do.

One of my most important aids however is a handwritten cue sheet which tells me roughly at what altitude, velocity and orientation the rocket should be at a given time. Without such reference, it is very hard to gauge whether the rocket is on a good ascent path or not.

Unfortunately, the ‘not being able to see too much’ is also a technical limitation. The Flightgear rendering engine is not designed to handle views from low Earth orbit, and even with the cutting edge development high altitude and extreme visibility rendering I’m using in the following, the view doesn’t really measure up to real views of Earth from orbit.

After igniting the engine, the thrust takes a few seconds to ramp up, but the Vostok rocket delivers a solid 2 g initial thrust with first and second stage burning, so I lift off quickly. After the first few seconds, I rotate the rocket around its main axis such that I am facing my launch heading. To make use of Earth’s rotation, launches are done eastward. As soon as I reach the desired heading, I push the ascent path out of the vertical along my launch vector to about 60 degrees with the horizon. During ascent, I will thus be more and more hanging face-down in the capsule, facing Earth at all times. Which is very reasonable, because in case of any instrument malfunction, this gives me at least a rough visual reference. Of course, the actual forces in the capsule are nothing like hanging face-down, the acceleration always pushes me back into the seat.

After passing about 20.000 ft, the dynamical pressure starts growing large, and I have to throttle back to avoid damage to the rocket. After all, a rocket is little more than a thin shell around fuel tanks: For instance, the second stage weighs roughly 100 tons at liftoff, but its empty weight is a bit over 7 tons. The air thins rapidly, however, and thus the dynamical pressure decreases quickly and I go to full thrust again. Once above the pressure peak, I push the nose of the rocket further down to 30 degrees with the horizon and start building up forward velocity while Edwards AFB vanishes below.

The full power of the JSBSim flight dynamics and atmosphere model affects this part of the ascent, and so the interaction between rocket and atmosphere is as realistic as the available data on the Vostok can make it.

After about 90 seconds, the fuel of the first stage boosters is almost spent, and the reduced mass of the launch vehicle ramps up the acceleration to 4 g and beyond. Once again, I throttle back to stay below 4 g to avoid damage to the rocket. At about 120 seconds, the first stage is out of fuel, and I separate the boosters. I am now high enough that air friction is negligible, and so I also blast the protective cover off the capsule and can take the first look outside (nothing much to see though). The second stage is still heavy at this point, and so the thrust goes back to about 2 g as we climb the 100 km altitude limit into space.

The whole flight dynamics changes quite drastically during a mission from the initial launch vehicle to the re-entry of the capsule. Also the weight of spent fuel is a significant factor. All these effects are quite distinctly felt during ascent to orbit.

At this stage, I have to start watching my ascent casefully. The second stage separation should bring me roughly to my orbital altitude with about zero vertical speed so that the third stage burn just keeps me at this altitude while accelerating me to orbital velocity of a bit more than 28.000 km/h. However, the second stage reaches more than 4 g thrust towards the end of its burn, while the third stage starts with barely 0.5 g thust, so any mistake I make at this stage will at best take very long to correct with the 3rd stage burn, at worst be unrecoverable. Thus, I control the pitch angle very carefully and monitor altitude and vertical speed.

About 5 minutes after launch, the second stage burns out and I separate it as well and ignite the third stage. Flying a rocket is very different from flying an airplane – while an airplane reacts to its immediate surroundings and doesn’t remember much of what was five minutes ago, the rocket’s current state is pretty much determined by what happened the last five minutes. If the ascent to this stage was bad, there’s nothing much I can do to correct it now. But my altitude and speed after 2nd stafe separation are within reasonable parameters, and so I continue build up speed while keeping my altitude with the half g thrust the third stage provides.

Another five minutes later, close to reaching orbital velocity, I have to throttle down. The speed must be reached quite accurately, otherwise I might go into an elliptical orbit rather than an almost circular orbit. And this is problematic, because the TDU has even less thrust than the 3rd stage, so if the 3rd stage brings me too high, I might not be able to de-orbit at all.

There are also technical reasons – Flightgear currently isn’t designed to handle an altitude above 150 km, so I have to reach an orbit below 150 km and above 100 km where the atmosphere is thin enough.

I watch the perigee indicator carefully, and as it starts rapidly climbing, I separate the 3rd stage – I am in orbit! Apogee and perigee indicators read 128 km and 140 km, so while not completely circular, this is reasonably good.

Flying to this stage isn’t easy – only three Flightgear pilots have to my knowledge reached a stable orbit with the Vostok spacecraft. You have to work for your astronaut’s wings!

From this point, I only have the minimal thrust of the TDU available to turn the spacecraft and decelerate. Rather than aerodynamical controls, I now have to fire thrusters to change my attitude, so the spacecraft handles once again completely different.

JSBSim handles the attitude control thrusters just as well as the aerodynamical controls, and the spacecraft handles again very plausibly at this stage of the mission.

There’s not much to do while drifting along in the orbit. Look out and watching the sunrise is nice though.

The cutting-edge development experimental lightfield shader brings out the Earth shadow moving across the terrain, the stark shadows in low light and the differential light dependent on altitude very nicely.

To de-orbit, I turn the spacecraft around and fire the TDU main engine to use up the remaining fuel. This lower my perigee such that it intersects with the atmosphere – the friction will have to take care of the rest. Then I separate the TDU as well. At first, the first gentle touches of the atmosphere lead to a tumbling motion of the capsule, this then stabilizes as the drag increases, and I start falling faster and faster.

If you though the 4 g during ascent where tough, then you haven’t experienced re-entry yet. As the capsule finally reaches the lower atmosphere, a deceleration force of 8 g and more brutally brings me from orbital speed to a few hundred km/h. I simply black out during this stage.

Flightgear optionally simulates blackout and redout due to extreme acceleration at set limits.

By the time I get conscious again, I have an altitude of about 10 km and most of the speed is gone. Time to get the brake parachute out and kill the rest of the forward motion. After the braking parachute has done its job, I get the main parachute out, and once my vertical motion has slowed down, the final task is to activate the soft landing sensor.

Close to the US west coast, I gently splash into the ocean. Nothing to do now except to sit tight and wait for the recovery crew to pick me up…

Soaring with the ASK-13

Soaring in Innsbruck

Author: Thorsten Renk

Innsbruck, 6 am on a warm summer morning just after sunrise. The first members of the aeroclub vacationing with their gliders get out of the tents and observe the weather. There is thin Cirrus coverage overhead, and the first weak Cumulus clouds are forming already. Winds are a steady 10 degrees from East – it looks like a promising day for soaring. I get the ASK-13 out and ready.

(In reality, we would not have the plane out like this in the morning – it’s dangerous to let light planes with excessively long wings lie around in 10 kt winds – either it would have a hangar space, or it would be disassembled in a trailer – but there are some things even Flightgear doesn’t simulate.)

The Local Weather subsystem of Flightgear has a fair amount of knowledge of convection and thermals. For instance, it includes the time into the decision what thermals are formed, and the strength of early morning thermal activity is thus a good guide for the strength later in the day.

Around noon, the thermal activity has significantly increased and thick Cumulus development is seen over the mountain ridges. I’m getting ready for a winch launch in the ASK-13. The ASK-13 is a rather old glider with a glide ratio of 28 optimum (i.e. it covers 28 m distance while sinking 1 m), thus it is not suitable for covering large distances, but it has very good slow-flying properties, and thus it can circle in very narrow thermals.

There’s no need to make a flightplan – soaring can’t be planned. I’ll simply get up there, have a look around and see what possibilities there are, and if I don’t catch a thermal quickly, I’ll land back on the field and do a second winch launch. Soaring in most locations is about finding the thermal updrafts beneath Cumulus clouds and circling them to get altitude. In the mountains, there is additionally the upward deflection of wind by mountain ridges, called ridge lift, which also leads to vicious sink in the lee of the mountains. Flying a glider in the mountains is difficult – often you may have to circle in a confined valley, you may unexpectedly get into a lee and lose a lot of altitude, or you may be forced to land in an unsuitable location.

Flightgear simulates both thermal and ridge lift and (in a very experimental way) in principle also wave lift. Ridge lift is dependent on the wind, terrain roughtness and the local terrain slope, whereas thermal lift is dependent on factors like terrain type and terrain elevation – just as in real life, strong lift does not form over open water but rather over surfaces which heat up in the sun. So, looking for rock surfaces or elevated terrain where a thermal is more likely to form are meaningful in Flightgear as in real life.

A winch launch gets us up into the air quickly – with a good climber like the ASK-13, 500 m altitude are possible (European gliders measure the altitude in meters, not in feet). Winch launch takes a bit getting used to, as it leads to quite rough acceleration and climb more resembling the performance of a rocket than of a plane, but is fun after a while.

The altitude gained in Flightgear winch launches is a bit generous when compared with reality. What launches are supported depends on the plane – several gliders allow aerotow in addition to winch launch. Aerotow allows to get to significantly higher altitude and basically any position, but is in reality much more expensive than winch launch.

Just after disconnecting from the winch, all of Innsbruck lies before us. I’m now turning south, as there are some promising clouds on the slopes there. I have to find a thermal quickly – there are none on the valley floor, so the actual altitude reserve I have are not 300 m above valley floor with 200 m for a safe landing, but far less since I have to look above elevated terrain.

And we enter the first thermal! It’s even a fairly decent one, with a net lift of more than 1.5 m/s. While strong thermals can give lift of 3-4 m/s, starting out low I have to use what I can get – this is not the time to be picky. Properly centering a thermal when entering from below requires some skill – there is no visual reference provided by the cap cloud, so this has to be flown by instruments.

A thermal in Flightgear is not just a uniform area of lift – it has a fairly complicated structure. There is a rim of turbulent air, and an outer layer of sink, the lift is strongest in the center of thermal, the whole column of rising air is slanted and a bit wasp-waisted, so the radius entering a thermal low is smaller than right beneath the cap cloud. Optionally, thermals also have a time evolution, i.e. they form and die off after a while.

After a good ten minutes work, we reach the cloudbase with a good 1500 m more on the altimeter. Now it’s time to go fly some mountains!

The ASK-13 is a twin-seater – a view from the back seat position (taken either by a passenger or an instructor) as we head south into Stubaital – Widdersberg just ahead slightly yo the right.

The ASK-13 can actually take a passenger (sitting on a different computer) via dual control in the backseat position.

Some gorgeous views of the Alps!

Here, I’m using the Innsbruck Custom Scenery, which received a lot of attention also by model developers in recent years, and is hence rather spectacular (see http://www.flightgear.org/forums/viewtopic.php?f=5&t=5350 in the Forum).

Since I’m now hugging a western slope, there is some amount of ridge lift available. The wind is not strong enough for me to really gain altitude, but by going close to the slope I can almost compensate my sink and hence make distance without losing altitude. The downside is that the wind is definitely strong enough to create substantial sink on leeward slopes, so I have to be very careful where I fly.

Heading into a small valley. Soaring is all about realizing possibilities – here the cloud looks very promising, and if I get a good strong thermal, I might even be able to cross the slopes to the right into the next valley. But it will not be easy – the terrain will restrict my circles, so I may not be able to center a thermal properly. I decide to give it a try.

This isn’t going well at all… First, there is some leeward sink coming in, reducing my altitude. Then, the thermal is there, but it is too weak and too small to really lift me up. In the center, I get half a meter of lift, but I can’t keep the plane stationary there, and circling the thermal my net lift is down to almost nothing. Moreover, I am running out of time – the cloud drifts towards the western slope, and I can’t follow it with the altitude I have left – I decide to abandon before conditions become unsafe.

As in real life, a promising cloud doesn’t guarantee a strong thermal. There is some randomness in the correlation between cap cloud size and thermal lift, and even a strong thermal may be in practice not flyable because its radius is too small. The thermal system is not designed to make soaring easy, but to make it a realistic experience, and disappointments are part of the experience. Soaring is not just heading to the next cloud to catch the next lift – one also has to be prepared for the case that no lift can be had there.

Turning around, the situation isn’y actually dangerous yet (in the event, I just need a 50 m strip of relatively level grass to land, which is easy to be had), but it’s not good either. I don’t really want to land in the countryside, because my team might otherwise face a long drive with the trailer to get the plane back. First, I have to head back into Stubaital.

And here we are – plenty of fields to use as landing sites just in case. And a very long way to go back to Innsbruck – all the way to the end of the valley. It looks impossible, but… there is still the ridge lift. Unfortunately, no thermals to help us out of this – the only visible clouds are on leeward slopes, and that’s a very bad idea to try. So I decide to fly very close to the slope to catch most of the ridge lift and try to get back to Innsbruck.

And… it works just fine – with just 50 m altitude loss I make it back – since the valley floor now drops, I have plenty of altitude to spare for my approach to Innsbruck.

Safely back at Innsbruck, and time for the next person to get into the plane and enjoy flying the Alps. I had hoped to fly a longer trip (I did manage to climb above the Habicht once with spectacular views into Italy), but then, soaring can’t be planned, and especially in the mountains, the conditions are often difficult.

A trip to Tenzing-Hillary Airport

A trip to Tenzing-Hillary Airport

Author: Thorsten Renk

One of the most dangerous airports in the world, Tenzing-Hilary Airport, also known as Lukla airport, hugs a small plateau in the Himalayan foothills. It is the gateway for trekkers into the Sagarmatha national park and climbers trying to reach the summit of Mt. Everest. The runway has a length of 460 m and a 12 degree slope – it needs aircraft with STOL (short takeoff and landing) capacities to operate from it. Usually DHC-6 Twin Otter or Dornier Do 228 aircraft, weather permitting, connect Kathmandu and Lukla. Today we will make the trip with the Beechcraft 1900D, a more modern commercial twin-engine turboprop which is also up to the task ahead.

Preflight preparations begin at Kathmandu airport. We will take off before dawn and experience the sunrise in-flight over the mountains. The route is about 70 miles due east from Kathmandu, just along the main mountain range. In good weather, several major summits are visible. Lukla itself is not equipped for instrument approaches, so we have to approach in VFR flight.

Many airports in Flightgear have a set of night textures, making the models visually appealing not only during day but also when it’s dark. Similarly, for many airplanes the cockpit lighting is modelled in some detail. Here, I have switched on the main panel light to illuminate my cockpit during flight preparations. The aircraft models also usually have strobe, nav, taxi or landing lights simulated.

Today, we have broken cloud cover over Kathmandu. As we climb, dawn approaches and the sky brightens, outlining the towering mountain ranges. Kathmandu has an elevation of 4300 ft, Lukla of about 9300 ft, but even this altitude is not even halfway up to Mount Everest with a bit above 29.000 ft. In fact, since the B-1900D is only certified up to 25.000 ft, we wouldn’t even reach the summit at top altitude.

At sunrise and sunset, Flightgear models the different level of light available on the ground and in the air. While it may still be dark on the ground, more light reaches the plane at higher altitude.

A few minutes later, the sun comes above the horizon and sky and cloud lights up while the terrain is still in deep shadow.

For this flight, I am using a development version of Flightgear which experiments with an improved modelling of atmospheric haze layers and shading of the terrain during sunrise and sunset. The result are quite impressive views of the sky. Presumably, this feature will become available with the regular release of Flightgear 2.6.

As we reach Lukla valley, the sun is up and some morning fog hangs in the lower foothills of the mountain ranges.

Now we turn into the approach, and Lukla valley is right before us. There is some fog in the upper valley, but the airstrip itself is clear (it can barely be seen just below the left windshield wiper).

We fly close to the left valley edge to have more space for the final approach. This means crossing some ridges at low altitude and sets off terrain warnings.

Many planes in the Flightgear world have instrumentation which warns about insufficient terrain clearance or potential collisions with incoming traffic.

Now it’s time to turn right into the 060 final approach. The wind is bad – it comes almost right from the rear, but as you’ll see shortly, the approach to runway 30 isn’t exactly available.

Here we are, lined up with the runway. Time to get the gear out and to decelerate a bit.

Some wind drift as we come in – last minute corrections. Lukla is not an airport for missed approaches or second chances – there is a solid rock wall right behind the runway and no chance to pull up. We have to hit the runway now, no matter what happens.

This fairly detailed model of Lukla is an addon to the official Flightgear scenery.

And… here we are, braking real hard.

Welcome to Tenzing-Hillary airport. We hope you enjoyed the flight with us!

In case you find the idea that Air New Zealand would operate in the Himalaya a bit odd, Flightgear offers The Livery Database where many more liveries from all over the world can be found for popular aircraft.

Reconnaissance with the SR-71 Blackbird

Mach 3.2 at 85.000 ft – the SR-71 in Flightgear

Author: Thorsten Renk


A flight in the SR-71, or the ‘Habu’ as the crews call it, starts long before you enter the cockpit. With the aircraft making Mach 3, you can’t simply fly where you like or take a wrong turn. The Blackbird goes half a mile in the time it takes you to say ‘Oops’, it can be 20 miles in enemy territory by the time it takes you to check a map, and the turn radius is more than hundred miles. This means that basically anything you do needs to be planned in advance.

On longer recon flights, we would have tankers waiting for us in certain locations, but today is just a training flight. We will take off from Nellis AFB, Nevada, go north climbing, then turn around and overfly Nevada at 85.000 ft under mission conditions, then descend and head back to Nellis. All the waypoints for this flight have to be entered into the Astro-Inertial navigation system of the Blackbird in advance.

I am using Flightgear’s route manager to define the waypoints. As in reality, the plane is difficult to control at high altitudes manually, so the autopilot will have to take care of the climb to 85.000 ft. The waypoints need to be defined carefully such that the course is even possible to follow – at service ceiling, the plane is not very maneuverable. I could also, using the AI system of Flightgear, arrange for various tankers to meet me at certain points during my mission if I would want to fly a realistic long range mission profile for the SR-71.

When everything is ready, we finally enter the plane and taxi to the runway. The weather conditions are ideal for reconnaisance – it’s a very clear day with dry air and few clouds.

Many airports in Flightgear have a detailed network of taxiways and one can start the simulation on a specified parking position rather than ready on the runway, and an ever-increasing number of airports also is populated in full detail with 3d models showing not only the main buildings, but also other operations currently ongoing. Nellis AFB is one of the most detailed airports, where one can spend literally hours to explore every detail.

Takeoff and climb

With full afterburners, we gain speed and take off.

The two J58 engines with 34.000 pounts of thrust each sure look impressive with full AB thrust engaged – but the Habu is also a rather heavy bird. Moreover, the engines are designed for high altitude operations, so we just have a thrust/weight ratio of about 0.44, nowhere near to a fighter jet, and so even with full AB thrust, the climb is rather slow.

At about 25.000 ft, we go just a little supersonic for the first time. In this regime, wave drag is very high and the engines actually are not powerful enough to accelerate the aircraft any further. Also, in the thin air, the plane becomes increasingly difficult to handle precisely, and I transfer control to the autopilot.

In order to climb out to full altitude, we have to use gravity’s help and perform the so-called ‘dipsy’ maneuver – we climb to 33.000 ft, level off and let the plane go as fast as it can, then do a shallow dive to about 30.000 ft to let gravity accelerate us to Mach 1.25. Now we’re out of the wave drag region, i.e. drag is much reduced and we can climb further.

Flightgear handles the procedure rather accurately, It is not possible to simply hit the afterburners and fly to 85.000 ft, and if you do not reach sufficient speed at a given altitude, you can’t climb any further. The Blackbird reqires the pilot to adhere to the essential procedures. As in reality, in this altitude it is very difficult to control the plane manually with the precision required for the maneuver, but the autopilot can handle it well.

At the edge of space

Under the control of the autopilot, we continue to climb with a constant KEAS (equivalent airspeed) value of 450 kt all the way up to 70.000 ft, and then let the KEAS value drop to 400 kt while we reach 85.000 ft and Mach 3.2. At this altitude, we’re literally on the edge of space, and utterly alone – no other aircraft can reach this altitude.

The view from 85.000 ft is spectacular on a clear day, and at mission altitude the operator in the back seat becomes busy while the pilot can relax a little since the plane does little but fly straight under AP control.

Flightgear has an experimental skydome shader which tries to solve the physics of light scattering in the atmosphere in addition to the default skydome which handles both foggy and clear conditions reasonably well. The more detailed scattering solution is especially suitable for a thin atmosphere, such as at high altitude or on a very clear day, and it can give quite spectacular results under the right conditions.

At this altitude, the difference between indicated airspeed and the actual speed over ground is very pronounced: While we read just about 400 kt in the cockpit, we’re actually going more than 1900 kt groundspeed.

Flightgear has accurate models for the atmosphere at high altitude and effects like ram pressure taking the difference between true airspeed, indicated airspeed and equivalent airspeed, as well as Mach number to airspeed change with altitude into account. For most planes, these effects are not very prominent, but for the Blackbird they show up rather pronounced.

Returning to base

After completing the recon run, we slow down to 350 KEAS and descend again to 20.000 ft where I switch off the autopilot and resume manual control. In evening light, we head back to Nellis through a scattered cloud layer.

Some more dense clouds hang over Las Vegas as we merge into the approach pattern for Nellis AFB.

Cloud formation is tied to some degree to location: clouds are much more likely to form over the sun-warmed city than over cool open water. Also, terrain elevation plays some role.

The Habu is a supersonic bird – at low speeds it handles like a brick. One needs to be very careful not to lose too much airspeed when turning into the final approach. As compared with other planes, the approach is also really fast to retain enough lift – the Habu approaches with about 220 kt and touches down with litte under 200 kt – more than many propeller-driven aircraft will ever make. However, there remains the problem of deceleration… As we turn into final approach, I arm the drag chute, which is automatically deployed as we touch down.

The JSBSim Flight Dynamics Model handles object like the drag chute rather well as external forces. The drag chute has its own aerodynamical properties, it feels the wind and the drag effect is velocity dependent. As in reality, it takes quite a lot of space to decelerate a plane touching down with 200 kt, and in fact without the drag chute it would be a problem to slow down even given the long runway at Nellis.

After a successful training mission, we reach the temporary parking position of the Habu and head for debriefing, before we leave the base for a nice, cold beer in Las Vegas.

Carrier Ops (USS Carl Vinson)

Carrier Operations in Flightgear

Author: Thorsten Renk

Pre-flight preparation

The flight deck of the USS Carl Vinson, 8:30 am Pacific Daylight Time, off the US west coast: an F-14b is made ready for a flight. The weather is rough, 16 kt of winds coming from the open ocean, with gusts reaching up to 20 kt and changing directions. The Vinson has just crossed a patch of rain, but the clouds seem to be breaking up.

While the ground crew takes care of the plane, the pilot and the RIO go through mission briefing. Our flight this morning will be an intercept training – there is an intercept target north of us which we are to identify.

The scenario is set up using Flightgear’s AI system – both the carrier group and the intercept target are defined as AI scenarios which are defined before starting the simulation. Here I am using a simple setup placing a target on a predefined course – but using Flightgear’s scripting language, it would easily be possible to set up a situation completely unknown to me, or an unknown number of targets, or even a scenario which reacts to my presence in a certain way. AI scenarios can be quite complex – the Vinson scenario simulates the movement of a whole carrier group! The weather conditions can come from live weather reports, or be generated by a sophisticated offline weather system. Many planes in Flightgear (such as the F-14b) offer multi-crew support, i.e. in principle I could share this mission with a human as RIO – in this case however, I’m actually flying alone.

Ready to launch!

We enter the cockpit and close the canopy. While the crew arms the plane (we’ll be carrying a light air-superiority loadout), I am busy adjusting the plane for takeoff. Among other things, I adjust my altimeter to the current pressure and enter the TACAN channel of the Vinson into the left console. TACAN (TACtical Air Navigation) will be my guide back to the Vinson across a cloud-covered, featureless ocean. I also check the fuel loadout – due to the somewhat rough weather conditions and gusty winds, I prefer to take a lighter fuel load rather than launch with all tanks full.

After all preparations are done, I taxi the plane to the launch catapult and it is attached to the guiding rail. I set the throttle to full afterburner – we are good to go. Windgusts blow the catapult steam all over the deck.

Aircraft in Flightgear allow to customize fuel load, and quite often also the weight distribution of cargo, passengers, or in the case of the F-14, the armamant. All this influences the behaviour the plane will show later in the air, thus this is also an important part of pre-flight preparation. For western fighter jets such as the F-14b, radio navigation is done using the TACAN system. Flightgear has both ‘fixed’ TACAN installations (for instance at airbases) which are part of the scenery, as well as definable TACAN channels to be assigned to AI objects. 

In the air

The catapult launches us forward, and will full afterburners roaring our jet is in the air. For a moment the gusty winds shake us hard, but with rolling friction gone the plane accelerates quickly, and as I retract the gear we can climb steeply into the more quiet air above.

The weather simulation distinguishes between the (usually more gusty) boundary layer winds, and the stronger, but less gusty high altitude winds. The thickness of the boundary layer depends largely on terrain roughness, i.e. it is rather thin – as I pull the plane up, I can leave it quickly.

We keep climbing through scattered clouds into a brilliant morning sky.

At 25.000 ft, I level the plane and turn to the planned intercept course. I could use the autopilot for a while, but I enjoy actually flying myself too much.

Many planes in Flightgear have realistic autopilots. In the case of the F-14b, the AP is carefully limited to what functionality its real counterpart can provide – it is a simple system that can level wings, hold an altitude and hold a course, but it cannot by itself follow radio navigation as the more modern systems of other planes do.

As we go supersonic and race towards the intercept target, the wings automatically fold into their delta configuration to optimize for supersonic flight.

However, today we are in for a disappointment: We do not find the intercept target in time, and racing with full afterburner power, our fuel reserves are quite limited. I decide to abort the chase eventually. To be on the safe side, I ask the Vinson for a tanker.

Tankers could have set up in advance as AI scenario, but Flightgear also has the option to call a tanker for aerial refueling right to your current location – which is what I am using now. 

The KA6 used to refuel the F-14b is quite a small plane and difficult to detect visually, but as we ask for a tanker, we get its TACAN channel to guide us into position. However, I decide to track it on the radar instead (as I would for an intercept) and fly the approach by radar.

The F-14b has a fairly radar that is modelled in quite some detail – it has both a scanning and a tracking mode, it provides information about the target heading and groups targets into different types.

Aerial refueling

Getting fuel from a tanker requires some precision flying – the idea is to approach from behind just a bit faster than the tanker, and then to decelerate without dropping altitude just in the right spot. The trick is to gauge accurately how quickly the plane will slow down once the throttle is pulled back – a mistake there will inevitably lead to oscillations around the right position.

With the probe extended, we approach with just above 250 kt into the sweet spot of the KA6.

and finally start receiving fuel so that we can make it back to Vinson

Aerial refueling, both via probe (as demonstrated here) and boom is implemented in Flightgear. Although many aspects are easier than in real life (there is no turbulence induced by the tanker for instance), it is a tricky enough maneuver to master – especially since the AI tankers fly realistic racetrack patterns, i.e. at some point they start to turn!

Back to Vinson

TACAN guides us back to the Vinson. This time, I fly in the subsonic regime. Another 15 minutes later, we start to descend at the position of the Vinson. Here’s the view from the RIO position as we descend towards a cloud later at around 8000 ft.

We overfly the Vinson and its escort group to get into position for an approach.

Then I slow down the plane, extend flaps, the hook and gear and turn into my final approach. Carrier landings, especially in rough winds, are always more of a controlled crash than a proper landing… but TACAN and the Fresnel Lens Optical Landing System are there to help me align properly in difficult conditions.

However, in this case, the unpredictable crosswinds blows me off course.

Weather in Flightgear can change – gust speed and direction may vary on a short timescale, but winds may also change driven by a new weather report in the live weather system or by the dynamics of the offline weather system. 

At this point I decide to go around, so I switch afterburners back on and retract the gear, blast by the Vinson and come again for a second try. After contacting the Vinson, the carrier turns into a new recovery course.

AI control allows to modify the behaviour of Ai scenarios runtime. In this case, I direct the Vinson to a new course better suited for my landing while I go around.

Caught by the wire on the second attempt…

Missing the approach the first time is not too uncommon with the carrier – it’s always better to try again and hope that things go better than to try to force the aircraft down onto the deck when thing are not going right. Even when touching the deck, it’s not guaranteed that the wire catches, so one should always be prepared to yank the throttle forward.


We get out of the plane…

This time, the mission was a failure – we did not manage to reach the intercept target as planned. But this is as life goes – sometimes things do not work out as planned, sometimes something goes wrong with the plane, sometimes the weather does unpredictable things. The important thing is to be prepared to abort whatever you’re doing if it’s unsafe, and to react to the conditions. It’s always better to stay on the safe side than to end the day in flames.

Flightgear has the option to randomly fail systems with a certain probability. Had I wanted, I could have set up the simulation in such a way that my altimeter wouldn’t work. In several planes, even quite detailed emergency procedures are supported, such as extracting gear without pressure in the hydraulic system, or engine restart in the air after flameout.


Youtube video of Carl Vinson Ops.  (Best viewed by clicking “Watch on YouTube” and then going “Full Screen”)  Seriously FULL SCREEN and CRANK UP THE VOLUME!!!

Predator drone video footage circling the Carl Vinson …

Eurocopter EC-135: Aircraft of the Week

Eurocopter EC-135


The Eurocopter EC-135 comes with a very impressive 3d cockpit with photorealistic texturing – one example of very few aircraft in Flightgear.

Unfortunately, many of the switches are not yet functional, and the procedures to start the engine are very simple. Some work on support for more detailed procedures would be beneficial for the helicopter. Nevertheless, the realistic looks of the cockpit create a very nice feeling of immersion into the simulation.

The exterior model, for which a variety of liveries are available, is likewise very impressive – it makes use of state-of-the-art reflection shaders and has animations for lights, the rotors and the doors.

If the model crashes, the crash is also (partially) animated by showing the broken rotor blades.

Flight characteristics

Lacking any experience with any helicopter in reality, it is somewhat difficult to judge how well the FDM is done. Helicopters in Flightgear are not easy to fly due to the overall high degree of realism. However, compared with other models such as the Bo-105 or the R-22, the EC-135 handles certainly a bit easier and is a suitable helicopter for a beginner to learn the basics of helicopter flight. Also as compared to many other helicopters in Flightgear, the EC-135 has a rather powerful engine and can quickly climb vertically.

The model shows a lot of phenomena characteristic for helicopters: For instance, the rotors generate a lot more lift in forward flight than in hover flight, which needs to be compensated for when approaching for landing. In slow or hover flight, the EC-135 can swing like a pendulum under the rotor – this is a very nasty condition and difficult to deal with. The torque of the main rotor is clearly felt and must be compensated by the rear rotor, although this is not as tricky to balance as with other helicopters. The helicopter can easily be flown backwards or sidewards – it’s however tricky not to lose control when doing so. Another interesting experience is to hover at high altitude, then reduce lift via the collective – the helicopter drops down rapidly, and one can observe the blades spinning up.

My personal wishlist

More functionality in the cockpit and more implemented procedures would be a very nice addition to the model.

Things to experience

There are plenty of heliports in the Flightgear world. One nice tour is to load the Vinson AI scenario, and, starting out from the carrier itself, visit its escort group (provided you don’t mind that it’s not a US Navy helicopter…). Most of the ships have a helipad where you can land and enjoy the view you usually don’t get to appreciate. Also, many buildings have helipads on their roofs. It’s somewhat tricky to land on such a tight spot, but it can be done, and usually results into a good feeling of accomplishment.