DCS: AJS-37 Viggen Discussion

[drPhibes]

Just pull the yellow/black handle?

[renhanxue]

SFI AJS 37 part 2, chapter II, page 38 (page 41 in the PDF):

 

Quote

24.4 Uncontrolled attitudes

24.4.1 Superstall and spin

If the angle of attack exceeds the permitted limits, some yaw disturbances appear around alpha 25-28°, and at alpha 28-30° there are weak pitch-up tendencies. If the stick is moved forward at pitch-up, the aircraft returns to normal alpha, possibly after overshooting up to alpha ~50°. Note that the angle of attack instrument only shows the area -4° to +26°.

If the stick movement forward at the pitch-up is too small or is made too late, so that the angle of attack does not immediately decrease, the aircraft enters superstall or spin.

Superstall is characterized by:

• indicated alpha 26° (actually around 60°)
• low rotation speed (< 40°/s)
• stable or oscillating nose attitude

 

Two types of spin may be encountered:

Oscillating spin (most common)

• indicated alpha 26° (actually around 60°)
• moderate to high rotation speed (> 40°/s)
• moderate to powerful oscillations in pitch and/or roll

Flat spin

• indicated alpha 26° (actually around 70°)
• high rotation speed (up to 120°/s)
• small or no pitch or roll oscillations

The sink rate in a superstall or spin is around 100 m/s.

If the pitch-up occurs without aileron input, the maneuver usually results in superstall. If the pitch-up occurs with any aileron input active, the aircraft yaws in the opposite direction and the likelihood of a spin increases.

[...]

The oscillating spin may after ≥15 s diverge such that the aircraft rolls inverted, from which position it usually recovers by itself with control surfaces neutral (but see also the section on "inverted plunging spiral" below).

In superstall or spin the pitch authority is good, which eases recovery. Aileron input results in adverse yaw, that is to say rolling right gives a yaw to the left and vice versa. Rudder authority is negligible.

Recovery from superstall and oscillating spin is accomplished by moving the stick to a position somewhat forward of the neutral pitch position, with ailerons and rudder neutral.

To recover from a flat spin, the yawing rotation must be stopped first, which is accomplished with neutral pitch and full roll input in the direction of the rotation ("stick into the spin"). When the rotation has just about ceased, recovery is accomplished with neutral ailerons and the stick somewhat forward of neutral, just like when recovering from superstall and oscillating spin.

 

If the center of gravity is particularly far aft (aircraft with heavy loads on V7 (wing pylons) and no load on S7 (fuselage pylons)) the pitch-up tendency is more pronounced than with normal CoG positions. To return to a normal angle of attack a stronger pitch down input is required; the stick must be moved close to its fully forward position.

These characteristics are not affected to any significant degree by fuselage mounted loads. The characteristics with both fuselage and underwing loads are not known but are assumed to be the same as with only fuselage loads.

Stalled inverted attitudes have not been possible to achieve in flight tests. Moving the stick fully forward gives alpha about -30°, there are no pitch-up (pitch-down?) tendencies and when the stick is moved back to neutral the aircraft returns to a normal angle of attack.

 

24.4.2 Plunging spiral

In certain adverse dynamical scenarios the aircraft can enter an uncontrolled attitude of the autorotating type, here called plunging spiral. The plunging spiral, which can be either right side up or inverted, is considered to be the potentially most dangerous form of uncontrolled flight that has been discovered during the spin tests of aircraft 37.

 

24.4.2.1 Inverted plunging spiral

The most common form of the plunging spiral is the inverted one. The following attitudes/maneuvers repeatably result in an inverted plunging spiral:

• Somersault into inverted position from oscillating spin (for example while attempting to recover from a spin with the stick fully forward)
• Stalling the tailfin through so-called "knife flying" (TN: knife edge?)

The inverted plunging spiral is characterized by the following:

• Negative load factor (-1 to -3)
• Low nose
• Rapid rotation around the roll axis (≥ 200°/s)
• High sink rate (≥ 150 m/s)

Moving the stick back and/or aileron input to either side tends to increase the roll rotation speed. The rotation can be stopped by moving the stick fully forward with no aileron input. When the rotation has ceased, the stick is moved back to neutral pitch, and the aircraft returns to controlled flight.

 

24.4.2.2 Non-inverted plunging spiral

The aircraft only entered a non-inverted plunging spiral on a handful of occasions during the spin tests. It has not been possible to define any repeatable attitude or maneuver that results in a non-inverted plunging spiral.

During the spin tests the non-inverted plunging spiral only occurred on the following two occasions (not repeatable):

• When recovering from an inverted superstall
• When recovering from an oscillating non-inverted spin

The non-inverted plunging spiral is characterized by the following:

• Positive load factor (+1 to +3)
• Low nose
• Rapid rotation around the roll axis (≥ 200°/s)
• High sink rate (≥ 150 m/s)

In a non-inverted plunging spiral, aileron inputs have no effect. Instead, the roll rotation must be stopped by pulling gently back on the stick until the rotation ceases. When the rotation has ceased, the stick is moved forward to the neutral pitch position and the aircraft returns to controlled flight.

The aerodynamics compendium also mentions that pitch stability is "basically undisturbed" all the way up to alpha 25° or so.

 

Other than that: Swedish speakers, how would you translate "flygläge"? I chose "flight mode" here but it's clunky. Flight regime isn't quite right either, or is it? Flight position?

Edited August 23, 2023 by renhanxue

[addde]

Other than that: Swedish speakers, how would you translate "flygläge"? I chose "flight mode" here but it's clunky. Flight regime isn't quite right either, or is it? Flight position?

Put it in a full sentence please.

[renhanxue]

Put it in a full sentence please.

Kapitelrubriken "Okontrollerade flyglägen"

 

"Överstegrade inverterade flyglägen har inte kunnat erhållas vid prov."

 

"Endast vid ett fåtal tillfällen under spinnutprovningen hamnade fpl i en rättvänd störtspiral. Något repeterbart flygläge/manöver som resulterar i en rättvänd störtspiral har ej kunnat definieras."

[addde]

Kapitelrubriken "Okontrollerade flyglägen"

 

"Överstegrade inverterade flyglägen har inte kunnat erhållas vid prov."

 

"Endast vid ett fåtal tillfällen under spinnutprovningen hamnade fpl i en rättvänd störtspiral. Något repeterbart flygläge/manöver som resulterar i en rättvänd störtspiral har ej kunnat definieras."

 

I would simply say "attitude", as in uncontrolled attitudes. Or maybe "Aircraft state/states".

Edited October 14, 2016 by addde

[microvax]

Yee would agree flight attitude.

 

Fluglage in german. )))

[renhanxue]

It's certainly better than what I had. Changed. Thanks!

[addde]

It's certainly better than what I had. Changed. Thanks!

 

Glad to be of service to the great AFM translator ;)

[renhanxue]

Haha, glad you appreciate it. :)

Please tell me if you see anything else that looks dumb in the English text. The language in these manuals is rather hard to just read in the first place, and making it make sense in aeronautical English is pretty challenging. Figuring what exactly "tätning" (literally "sealing", as in making something airtight) referred to wasn't trivial, but Google eventually turned up a report from Statens Haverikommission where it was actually explained. It means "rapid pitch-up". Explain how that makes sense, anyone? Then again, is that what "departure" refers to in English as well? That makes more sense but it's still kinda odd.

[addde]

Figuring what exactly "tätning" (literally "sealing", as in making something airtight) referred to wasn't trivial, but Google eventually turned up a report from Statens Haverikommission where it was actually explained. It means "rapid pitch-up". Explain how that makes sense, anyone? Then again, is that what "departure" refers to in English as well? That makes more sense but it's still kinda odd.

 

Tätning = rapid pitch-up?

[renhanxue]

Yes. See this SHK report about the crash of a Gripen back in 1999, page 13:

 

Förarens redogörelse för händelseförloppet har inledningsvis varit en betydande källa till information, vilket väglett den första delen av utredningsarbetet. Resultatet av ett antal intervjuer med föraren kan sammanfattas med följande: Han upplevde själva störningen som kort och intensiv och beskrev den som en "duns" i flygplanet och en mindre sättning, d.v.s. lastfaktortillväxt. Störningen vred flygplanet en aning åt vänster samtidigt som den negativa tippvinkeln på ca 70 º ökade till nära nog vertikal dykning. Den efterföljande flygplanrörelsen upplevdes inte som en tätning, (en uppåtriktad, hastig tipprörelse). Under hela sekvensen noterades korrekt flyglägespresentation i siktlinjesindikatorn (SI).

 

(emphasis mine)

[SK37]

I think departure normally means departure from controlled flight, e.g. stall, spin etc..

[JanTelefon]

Could "flygläge" be translated into flight situation or flight position perhaps? From the sentence it seems the text is refering to how the plane is situated.

 

There are alot of these tricky words in swedish flight manuals, like flare (before landing) is translated as "övergångsbåge". I guess that could be a mouthfull to non-native Swedish speakers. :D

[renhanxue]

"transition arc" sounds great when translated literally though

[JanTelefon]

It does doesn't it. Feels very descriptive of what is actually being done.

[renhanxue]

I wrote a little piece on the rb 04E based on the SFI's. Figured people might be interested.

 

 

Robot 04E (with "robot", abbreviated "rb", being military Swedish for "missile") was the AJ 37 Viggen's signature weapon: a radar-guided, sea-skimming anti-ship missile, developed from the rb 04C which had originally entered service in the 1961 on the A 32 Lansen. The E version entered service in 1975, with 315 missiles produced. Let's have a look at how it works.

 

Rb 04C or D on a A 32 Lansen.

 

Missiles on the assembly line at the air force's Central Aircraft Workshops in Arboga.

 

First, some background on the doctrine and use case that shaped the design of the missile. The Swedish armed forces expected the Warsaw Pact to attempt to secure a beachhead on the Swedish coast with a D-Day style invasion: a massive fleet of hundreds of ships with surface combatant screens protecting a core of various landing craft. The AJ 37's raison d'être was to attack a fleet like this. The rb 04E was mainly intended to be used against the screening combat ships, since if their AA was silenced the Viggens would be able to go to town on the vulnerable landing craft with less expensive weapons like bombs, autocannons and unguided rockets. In order to achieve saturation of the defenses and a reasonable chance to actually sink mutually supporting surface combatants, the plan was to deploy at least four but preferably six or more full squadrons in each attack wave (one squadron in the air was two flights of four aircraft, so six squadrons would be 48 aircraft). Since the plan involved launching up to close to a hundred missiles at the same time (or slightly less - some aircraft would be carrying countermeasures instead of missiles), getting the missiles to spread themselves out between different targets and not collide with each other or lock on each other was a very real concern, which will be apparent when we get into discussing the seeker.

 

Onwards to the technical details!

 

 

The missile's about four and a half meters long (14 ft 9 in), weighs around 625 kg total (1378 lbs), has a shaped charge warhead that weighs about 200 kg (441 lbs) and is powered by a solid rocket motor that produces a nominal thrust of 195 kp (1.9 kN, 430 lbf) for a nominal burn time of 65.5 seconds (can vary between 60 and 75 seconds depending on propellant temperature). The control surfaces are pneumatically actuated. The seeker is a frequency hopping monopulse radar with a parabolic receiver antenna located under the radome in the front of the missile (the text "TRYCK EJ HÄR" on the radome means "do not press here"). The antenna sweeps horizontally only, 28 degrees to each side. The missile cruises at an altitude of 10 meters above sea level, which it maintains by the use of a radar altimeter.

 

The AJ 37 can carry two rb 04E's on the inner underwing pylons. When pre-flighting the missile, the mechanic had a panel with five switches and a knob available to him for programming the missile - there really isn't much the pilot can configure from the cockpit. The panel looks like this:

 

 

The switches are intentionally only labeled with numbers for opsec reasons - the seeker electronics were highly classified and conscripts were not allowed to know much about how it worked. Switch 1 ("balkläge") is the missile's position on the aircraft; V (vänster, left), C (center) or H (höger, right). The centerline pylon © was initially planned as a possible launch position on the AJ 37 but the electronics to actually launch the missile from there were never implemented. The rest of the switches we'll cover when we get to the functionality they affect.

 

The missiles can be launched one by one or both together - in the latter case there's an automatic delay of about two seconds between the two, to avoid collisions. Targeting is simple: the pilot simply points the entire aircraft at the desired target, guided by the head-down radar screen, on which either a PPI or a B-scope is presented together with a wind-compensated aiming line (wind speed is taken from the aircraft computer, where it is either doppler calculated by the radar altimeter system or taken from the weather forecast as input during pre-flight procedures). The presentation looks like this:

 

B-scope and PPI, respectively.

 

The number 60 shown in the bottom right means that the range of the display is set to 60 km. The two short, curved lines on the PPI represent the ranges 12 and 24 km respectively, while the line marked "raktframlinje" is the wind-compensated aiming line. Originally, the 12 and 24 km lines represented minimum and maximum firing ranges for the missile, but at some point the procedure was improved to calculate the engagement envelope dynamically based on air pressure, temperature and speed of the launching aircraft (later manuals recommend a max launch range of about 20 km). The pilot can select if the missile's seeker should be in single ("ENKEL") or group ("GRUPP") targeting mode. In single target mode, the missile will simply lock on the first detected target. In group mode, the target selection process is more involved and we'll get back to it in a little bit. The missile can be launched at altitudes between 50 and 425 meters above sea level and airspeeds between Mach 0.7 and 0.92. The aircraft's radar does not need to be radiating to launch the missile, since the targeting is done just by pointing the aircraft the right way. In fact, the missile can be launched completely "blind" - this was particularly desirable on the Lansen, which did not have a radar in every aircraft. The flight lead could do the radar thing and the rest of the flight just launched when he did - a tactic that was also technically usable on the AJ 37. Once launched, the missile is completely autonomous and can no longer be controlled in any way by the launching aircraft.

 

When the launch signal is given, the missile activates its internal batteries, releases its gyro from being slaved to the aircraft's attitude gyros, unlocks and pressurizes the aileron actuators, and when the batteries have reached full power (after about 0.6 seconds), it separates from the aircraft. 0.7 seconds after separation, the elevators and rudders are pressurized and the missile immediately starts diving at an angle of about 7 degrees. About 1.1 seconds after separation, the missile starts yawing either 2.5 or 7.5 degrees to either the left or the right - which direction and by how much is determined by the position of the knob (marked 6, "kurstillskott") on the switch panel on the missile. After 8 seconds, the missile returns to the launch course. The reason for this is to separate the missiles horizontally.

 

When the missile's radar altimeter detects that the missile has had an altitude under 120 meters above sea level for more than 100 milliseconds, the automatic 7 degree dive stops and the missile instead follows a descent profile that takes around 10 seconds to reach its cruise altitude of 10 meters. Missiles launched from the right pylon ignite their rocket engine when descending below 130 meters, while missiles launched from the left pylon ignite it upon reaching the cruise altitude, to further separate them in time and in altitude.

 

 

When the cruise altitude is reached, the seeker starts scanning for targets; the scan area (and lock envelope) is shown above. When a possible target is detected, the seeker activates a function called "three-view logic", which means that the ranging function continues seeking forward about 80 meters. Then, the antenna sweep is reversed and the ranging seeks about 250 meters backwards, then the sweep is reversed again and the ranging seeks about 300 meters forwards. If the seeker gets a return again during the first or second reversed sweep, the target is considered valid. If no return is received during the first or second reversed sweeps, the target search continues. On the other hand, if the seeker gets another return immediately after the first indication, caused by the size of the target, the three-view logic function is blocked and the seeker accepts the target immediately.

 

When the seeker has locked on a target, the range to the target is monitored. The range should be decreasing, since the missile is approaching it. If the closing speed is too low, for example because the seeker has locked on another missile flying in the same direction, the missile releases the lock and starts a new search. The seeker will not lock on targets that are located such that the missile cannot be maneuvered to hit them, either.

 

In group targeting mode, the seeker will assume that the target ships are traveling in columns, and can be programmed to lock on a target in the first, second or third row as seen from the attacking aircraft, using the target selection switch (marked 5, "målval") on the switch panel on the missile. In order for the missile to lock in group mode, two or more targets have to be detected in the same range sweep, and they have to be a maximum of 2700 meters from each other (this number looks arbitrary, but it's just about 1.5 nautical miles). In order to allow for at least some flexibility in the line up, the seeker performs a fictional widening of the antenna lobe by copying detected targets and considering them for the next range sweep as well. This is all perhaps best explained with a picture:

 

 

The "angle jump" function, which can be enabled on the switch panel using the switch marked 4 ("vinkelhopp") makes the missile skip the first possible target it sees and lock on the next one instead, if one is found before the antenna sweep reaches the end position and turns around.

 

The missile also has an additional targeting mode, called "active + passive", which can be selected on the switch panel (switch marked 3, "följemod"). When this is selected, the missile is basically home-on-jam - if it detects it is being jammed, it will lock on the jammer after one full horizontal sweep. While locked on the jammer (passive targeting mode), the antenna is kept pointed at the signal source and the missile tracks the bearing to it. The range search stays active during the passive target tracking and if a target is detected in the jammer's direction, the missile will lock on that. If the jammer stops transmitting, the missile will keep going "blind" for two seconds; after that it resumes active targeting.

 

Rb 04E seeker unit.

 

The seeker keeps the missile pointed straight at the target until it is less than 4000 meters away, at which point the missile starts accounting for the target's speed and leads it. The seeker keeps tracking the target until it has closed to 250 meters, then the missile flies blind the last distance. If the warhead does not detonate when the target is passed, the missile re-starts targeting and simply locks on the first thing it sees (disregarding the single/group target selection and any previous considerations).

 

At 250 meters from the target, the missile arms its fuzes. The missile is not intended to actually hit the target - the warhead is a shaped charge that is focused downwards, so it is supposed to be detonated above the target. There are three different proximity fuzes - one magnetometric, one temperature-sensitive and one based on the radar altimeter, which triggers on the sudden altitude change when passing over the target. There are two proximity fuze modes, selected with the switch marked 2 ("zonrör") on the switch panel - in mode 1, only the radar altimeter is active, while in mode 2, any two fuzes both giving the detonation signal is required (presumably mode 2 is for use in rough seas to prevent accidental detonations from high waves). Additionally, there is also a contact fuze in the nose of the missile, which detonates it after a small delay if it should hit the target directly.

 

 

In summary, I find the group mode to of questionable utility since it requires the targets to line up almost perfectly, but I guess they did what they could to try to get the missile to be able to work against large ship formations. In the single target mode though the missile seems to be a pretty nasty piece of business for 1975, especially considering the radio silent mass usage doctrine and the fact that very few aircraft needed to actually radiate to enable a launch. The main weakness was probably that there were so few missiles purchased - about two missiles per AJ 37, total.

Edited October 15, 2016 by renhanxue

[jeffdude]

That is very interesting! I can't wait to employ these bad boys! the entire weapons load is very interesting, some will be revolutionary to DCS player combat

[mattebubben]

Thanks for that very interesting read =).

 

And personally i think i might be more likely to use the RB 04E instead of the RB 15F (Unless very important or highly protected targets) just because i find it to be more interesting and its also a more classic weapon and is so closely tied with the Viggen etc.

 

I also like its "Advanced Simplicity" ^^.

 

But yea i very much look forward to doing anti-ship missions in the Viggen.

 

With RB-04Es against the escorts and then RB05s (Or alternatively Bombs or Rockets) against Transports or Landing Craft.

Edited October 14, 2016 by mattebubben

[Farks]

Fantastic writing renhanxue!

 

If the Falklands Wars and other engagements with ASM's are any indication of their effectiveness, E1 (the AJ 37 squadrons) would have been a major threat even for a military super power. And let's not forget the west-german Marineflieger which was of similair size and had the same mission as E1.

[myHelljumper]

Big thanks for that !!

[Pocket Sized]

Wow! I love the look of the seeker, its just a tiny satellite dish :D

What's the typical top speed on one of these babies? The motor burns very slowly.

 

Also, how does it compare to the jet powered RBS-15 in terms of range? I imagine they're pretty similar in the way they operate.

 

Edited October 15, 2016 by Pocket Sized

[renhanxue]

Wow! I love the look of the seeker, its just a tiny satellite dish :D

What's the typical top speed on one of these babies? The motor burns very slowly.

 

Wasn't there a second anti ship missile? Could've sworn one of them was jet powered...

I haven't found any number on the cruise speed, just "high subsonic". Doing the math though, in order to travel 20 km in the 65 seconds the rocket motor burns, you have to do around 1100 km/h or M 0.9. So somewhere in that ballpark is a reasonable assumption.

 

The rb 15 probably has a considerably longer range since it's turbojet powered, but I don't have any exact numbers.

[Pocket Sized]

I haven't found any number on the cruise speed, just "high subsonic". Doing the math though, in order to travel 20 km in the 65 seconds the rocket motor burns, you have to do around 1100 km/h or M 0.9. So somewhere in that ballpark is a reasonable assumption.

 

The second ASM you're talking about is the rb 15F, which is similar-ish to the rb 04 but has the rocket motor replaced with a turbojet, a new considerably smarter seeker that can be programmed to fly a course with multiple waypoints before starting targeting, and probably a new warhead. It was integrated on the AJS 37 in the early 90's.

 

Wow, that was quick :P

 

Yeah, I found the missile on the first page of the thread and edited my post.

 

However seeing how recent the missile is makes me question if we're getting it or not...

[renhanxue]

I happened to refresh at the right moment ;)

 

The rb 15 has been confirmed to be in :)

I'll see about a write up about that one too.

 

By the way, I was wondering what was up with the 2700 meters figure for the group targeting mode since it seemed pretty arbitrary, but then I realized it's pretty close to two nautical miles (correction, 1.5). I bet that's where it comes from.

Edited October 15, 2016 by renhanxue

[Justin Case]

1 nm = 1852 m

2 nm = 3704 m

 

2700 m = 1,46 nm

 

My bet is on something more arbitrary.