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Howto:Aerial refueling: Difference between revisions
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== What is possible == | == What is possible == | ||
[[Image:F-8E_refueling_behind_KA-6.jpg|thumb|270px|A F-8E receiving fuel from a hose-equipped KA-6.]] | |||
At present, there are two tanker [[aircraft]] and several receiving aircraft capable of in-air refuelling: | At present, there are two tanker [[aircraft]] and several receiving aircraft capable of in-air refuelling: | ||
{| class="prettytable" border="1px" cellspacing="0" cellpadding="2" | {| class="prettytable" border="1px" cellspacing="0" cellpadding="2" | ||
!Aircraft | ! style="background:#efefef" |Aircraft | ||
!Can be refueled by | ! style="background:#efefef" |Can be refueled by | ||
|- | |- | ||
|[[Douglas A4 Skyhawk]] | |[[Douglas A4 Skyhawk]] | ||
|rowspan="5"| | |rowspan="5"|[[Grumman KA-6]] | ||
|- | |- | ||
|[[English Electric Lightning]] | |[[English Electric Lightning]] | ||
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|- | |- | ||
|[[Fairchild A-10]] | |[[Fairchild A-10]] | ||
|rowspan="4"|[[Boeing KC-135E | |rowspan="4"|[[Boeing KC-135E]] | ||
|- | |- | ||
|[[F-117 Nighthawk]] | |[[F-117 Nighthawk]] | ||
| Line 33: | Line 34: | ||
Implementing aerial refueling capability to other aircraft is pretty straight forward, as explained in [[Howto: Implement aerial refueling capability|this article]]. It does not require much coding-skills and can be done by the average FlightGear user. | Implementing aerial refueling capability to other aircraft is pretty straight forward, as explained in [[Howto: Implement aerial refueling capability|this article]]. It does not require much coding-skills and can be done by the average FlightGear user. | ||
== Necessary preparations == | ==Receiving fuel== | ||
Like the [[aircraft carrier]]s, AAR is implemented as an AI scenario. Selecting these | === Necessary preparations === | ||
Like the [[aircraft carrier]]s, AAR is implemented as an AI scenario. Selecting these requires an extra command in the commandline (eg. <tt>--ai-scenario=refueling_demo_1</tt>, check <tt>[[$FG_ROOT]]/AI/</tt> to see what scenarios are available), or selecting the corresponding scenario from the [[FlightGear Launch Control|launcher/FGRun]]. | |||
The A4F, Lightning or T38 are special cases, as they should automatically load a scenario containing a tanker (without the need for extra commands). | |||
Depending on the scenario, you might see the tanker crossing overhead when the sim starts; if not, don't worry. | Make sure that "AI models" are enabled and start at [[San Francisco International Airport]] (KSFO, the default airport). Depending on the scenario, you might see the tanker crossing overhead when the sim starts; if not, don't worry. | ||
== In the cockpit == | === In the cockpit === | ||
Perhaps the first thing to do after starting the engines if necessary is to select the appropriate TACAN channel if your aircraft is so equipped (the A4F and Lightning both are). | Perhaps the first thing to do after starting the engines, if necessary, is to select the appropriate TACAN channel if your aircraft is so equipped (the A4F and Lightning both are). Enter the channel using the relevant dropdown boxes in the radios dialogue (<tt>Equipment > Radio Settings</tt> or press Ctrl-R). Note that you should pick the channel of the tanker-type that you aircraft can receive from ([[#What is possible|see the table above]]). | ||
{| class="prettytable" border="1px" cellspacing="0" cellpadding="2" | |||
! style="background:#efefef" |Tanker | |||
! style="background:#efefef" |TACAN | |||
|- | |||
|KA6 | |||
|050X | |||
|- | |||
|KC-135 | |||
|040X | |||
|} | |||
You should now see the current bearing to the tanker indicated in the nav display of the A4 or the TACAN indicator (green needle) in the Lightning. If the tanker is within range, it will also appear on the radar display of the T38 or Lightning. | |||
Now, take off. | |||
Turn to an appropriate heading, guided by the TACAN bearing (you should try a "leading" approach to close in on the tanker) and look for the tanker on the radar or nav | === In the air === | ||
==== General procedure ==== | |||
Turn to an appropriate heading, guided by the TACAN bearing (you should try a "leading" approach to close in on the tanker) and look for the tanker on the radar or nav screen. Around 5nm away, you should reduce your speed to around 20kts faster than the tanker (these fly at 280 kts TAS). The KC-135 will be visible from about 10nm, the KA6-D, being smaller, just over 1 nm. Use airbrakes to keep control of your speed should you find yourself overshooting. | |||
[[Image:KC-135E.jpg|thumb|270px|The KC-135E aircraft with the boom.]] | [[Image:KC-135E.jpg|thumb|270px|The KC-135E aircraft with the boom.]] | ||
Close to within 50ft of the tanker ( | Close to within 50ft of the tanker (do not get too close, or visual artifacts might hide the boom from view). You should see indication in the cockpit that you are receiving fuel (there is a green light in the A4 fuel gauge, as well as a green light on the right side of the T-38's panel), and you should see the indicated tank load increase. | ||
Getting to this stage is not necessarily easy - it can take a lot of practice. | Getting to this stage is not necessarily easy - it can take a lot of practice. As with carrier landings, this is not an easy manoeuver in real life either and there are additional complications in the sim. The tanker, being an AI model, is unaffected by the wind and flies TAS (True Air Speed), while you are flying IAS (Indicated Air Speed) and are affected by the environment. As in real life, your aircraft will also steadily increase in weight as the tanks fill which will affect the trim of the aircraft. You might find it helpful to use the [[Autopilot#Velocity_control|autothrottle]] to help control your speed (Ctrl-S then Page Up/Down to increase and decrease the set speed). | ||
Once your tanks are full, or you have taken as much fuel as you wish, close the throttle a little, back away from the tanker and continue your intended flight. | Once your tanks are full, or you have taken as much fuel as you wish, close the throttle a little, back away from the tanker and continue your intended flight. | ||
=== Troubleshooting the approach === | ==== Troubleshooting the approach ==== | ||
If you had problems approaching the tanker, then it is useful to understand just what the cause of the difficulty is, so that you know what to practice. In a nutshell, what you have to do is to fly to the right spot behind the tanker, <i>in such a way that the relative velocity with the tanker is zero</i>. The first part, flying to the correct position, is easy - just keep the tanker centered in your visual field and fly till you are there. All the trouble is in matching the velocity. Successful aerial refueling thus requires precision control of your airspeed. | |||
If you had problems approaching the tanker, then it | |||
This, again, would be no problem if the throttle would directly control your airspeed - then you could just set the throttle to the airspeed you need as soon as you reach the right spot. However, the throttle controls thrust, and airspeed results from an equilibrium between thrust and drag which takes some time to reach. Let's assume you have reached the right spot behind the tanker with the correct airspeed, but with insufficient thrust, and see what happens: Since the drag force is now larger than the thrust, the plane starts to settle into a new equilibrium and the airspeed will slowly drop. A few seconds pass before the airspeed gauge reflects this, as gauges usually do not work instantaneously. Thus, but the time you notice the chance in airspeed from the gauge, the plane is already slower than the gauge indicates. Assume you try to correct you increasing thrust a bit. It takes a few moments before the turbine spins up to the new RPM setting (like gauges, they don't react instantaneously), then more thrust is generated. From that moment on, it takes a few seconds for the plane to settle into a new equilibrium airspeed corresponding to that thrust level. By the time that is reached, some 20 seconds may have passed - and you may find yourself already far behind the tanker! | This, again, would be no problem if the throttle would directly control your airspeed - then you could just set the throttle to the airspeed you need as soon as you reach the right spot. However, the throttle controls thrust, and airspeed results from an equilibrium between thrust and drag which takes some time to reach. Let's assume you have reached the right spot behind the tanker with the correct airspeed, but with insufficient thrust, and see what happens: Since the drag force is now larger than the thrust, the plane starts to settle into a new equilibrium and the airspeed will slowly drop. A few seconds pass before the airspeed gauge reflects this, as gauges usually do not work instantaneously. Thus, but the time you notice the chance in airspeed from the gauge, the plane is already slower than the gauge indicates. Assume you try to correct you increasing thrust a bit. It takes a few moments before the turbine spins up to the new RPM setting (like gauges, they don't react instantaneously), then more thrust is generated. From that moment on, it takes a few seconds for the plane to settle into a new equilibrium airspeed corresponding to that thrust level. By the time that is reached, some 20 seconds may have passed - and you may find yourself already far behind the tanker! | ||
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First, <i>never</i> fly by the airspeed gauge in close proximity to the tanker, always react to <i>what you see</i> - if you see the tanker fly away, you are too slow no matter what the airspeed gauge says! If you react to what you see, you gain precious seconds (and that takes care of the wind effects, i.e. the fact that the tanker flys TAS whereas you IAS as well). Second, change thrust before you see the effect. Assume in the above example, you'd give a short burst of thrust, then put the thrust lever back to a slightly higher setting than it was before. If done just right, the airspeed drop gets compensated almost immediately and the new equilibrium is reached quickly without falling far behind the tanker. Finally, use altitude to your advantage instead of fighting it. If you pull the nose up, you'll lose just a bit of airspeed, so if you approach a bit below the tanker being a bit faster than the tanker, you can pull up as soon as you reach the spot, and you will stop just fine in the correct spot. | First, <i>never</i> fly by the airspeed gauge in close proximity to the tanker, always react to <i>what you see</i> - if you see the tanker fly away, you are too slow no matter what the airspeed gauge says! If you react to what you see, you gain precious seconds (and that takes care of the wind effects, i.e. the fact that the tanker flys TAS whereas you IAS as well). Second, change thrust before you see the effect. Assume in the above example, you'd give a short burst of thrust, then put the thrust lever back to a slightly higher setting than it was before. If done just right, the airspeed drop gets compensated almost immediately and the new equilibrium is reached quickly without falling far behind the tanker. Finally, use altitude to your advantage instead of fighting it. If you pull the nose up, you'll lose just a bit of airspeed, so if you approach a bit below the tanker being a bit faster than the tanker, you can pull up as soon as you reach the spot, and you will stop just fine in the correct spot. | ||
== More advanced topics == | == More advanced topics == | ||
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==== Preferences.xml method ==== | ==== Preferences.xml method ==== | ||
Use your operating system's search facility to locate this if you don't know where it is). Open preferences.xml in a text editor (e.g. notepad if on windows) and search for the <ai> </ai> tags. Place a line like <tt><scenario>refueling_demo</scenario></tt> somewhere within the <ai> tags; you should see other scenarios already there too, perhaps commented out: i.e. with <! -- --> | Use your operating system's search facility to locate this if you don't know where it is). Open preferences.xml in a text editor (e.g. notepad if on windows) and search for the <ai> </ai> tags. Place a line like <tt><scenario>refueling_demo</scenario></tt> somewhere within the <ai> tags; you should see other scenarios already there too, perhaps commented out: i.e. with <! -- --> | ||
== Simulation vs. reality == | |||
[[Image:f16-air-air-refuel.jpg|thumb|270px|Approaching the tanker with a [[F-16]] - and receiving fuel already (FG 1.9.1)!]] | |||
There are a few things which are (presumably) easier in the simulation as compared with the real world, but there are also some that are harder. First, the envelope for refueling is actually quite generous - you start getting fuel in a position where in the real world this could not possibly work (see picture). The real refueling position of the F-16 feels a lot less comfortable! It is also mercifully short - usually holding the plane in the correct spot for about 30 seconds is all you need. There is also no turbulence or other change in the airstream induced by the tanker modelled. | |||
What is on the other hand probably easier in real life is flying by eye - in reality, we usually don't have any problems accurately gauging relative motions of a few feet per second. On the screen, with finite resolution and not-exactly-real textures and shadows, a lot of visual cues are missing and gauging a small relative motion becomes difficult. Also, in real life both the tanker and the refueling plane fly in the same airstream and wind effects not being felt by the tanker AI model are not an issue. | |||
==Related content== | ==Related content== | ||