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.

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!

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.