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PSA regarding the AJS 37's afterburner


renhanxue
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It has come to my attention that some people consider stage 2 of the Viggen's afterburner to be lagom. That's a very Swedish opinion to have, but it is also wrong and offensive to me and I will fight you about it. So, let's talk about afterburners. Many aircraft have them. They are useful, since in aircraft of the Viggen's era they tend to give about a 50 to 60% increase in thrust over what's available at full military power (to be clear, I'm talking nominal static thrust here), and in some cases reaching up to a 70% increase over dry thrust, and things like the MiG-21's emergency afterburner can go up to 75% if I'm not mistaken.

 

Now, on dry thrust the AJ(S)37 is slightly underpowered compared to its contemporaries, especially the twin engined ones like the F-111 and the F-4. It has a max takeoff weight (MTOW) of just under 19 tons, while the RM8A has a nominal dry static thrust of 65.6 kN, which means around 3.5 kilonewtons per ton. In other words, that's a thrust to weight ratio of about 0.35:1, where the contemporaries tended to approach 0.4 and had a bigger payload margin to boot.

 

But what about the afterburner? Yes... The Viggen's afterburner, on the ground, at a standstill in a test rig, tops out at a nominal thrust of 115.6 kN, which is a 76% increase over max dry thrust, and unlike on the MiG-21 which reaches this neighborhood only while on a rapidly ticking overheating clock, the Viggen's afterburner will stay at this throttle level for as long as you have fuel to feed it (which, granted, isn't very long, but we'll get there). It doesn't stop there, though. Let us look at a thrust (and thrust to drag) chart, because everyone loves charts, right?

 

LPDNBts.jpg

 

Standard atmospheric conditions (ISA), at sea level. On the vertical axis here, we have thrust in kilonewtons - if you don't speak metric, 10 kN is 2248 lbf, or approximately the force required to lift one metric ton at sea level. On the horizontal axis, Mach number. The afterburner stages are the shaded areas labeled zon 1, 2 and 3, with max dry thrust being the black line that follows the bottom of stage 1. The curved black lines marked 1-4 are total drag in level flight with four different external stores loadout "templates" where 1 is lightest/least draggy (drop tank plus something light/small like AIM-9's) and 4 is heaviest/most draggy (16x120kg bombs plus drop tank, basically). Similarly, the line marked R is total drag with a clean aircraft. Where those lines cross a particular thrust setting, that's where the equilibrium airspeed for that loadout and thrust setting is - in order words, that's the speed where the aircraft won't accelerate further because thrust pushing you forward is balanced out by the drag holding you back.

 

Now, a jet engine works according to the principles of Newton's third law, or the conservation of momentum. It takes in air, accelerates it and shoots it out the back, and the equal and opposite reaction pushes it forward (a jet engine was called a "rea(ktions)motor" or "reaction engine" in Swedish in the 1940's and 50's). As your airspeed increases, the relative velocity of the air you're shooting out behind you decreases, and so we see what's happening in the bottom of the chart, at dry thrust and min zone 1: by the time we're up to Mach 0.55 (distance-economical airspeed at sea level), dry thrust in the real aircraft is down to under 50 kN from the 65 we had at a standstill in the test rig. In the transonic region (Mach 0.9 and above), dry thrust - and thrust in zone 1 afterburner - starts dropping sharply.

 

However, in the upper part of the chart, there's something else going on. Recall that the RM8A is based on an early 1960's airliner turbofan that powered things like the DC-9, Boeing 727, and early 737's. That means it has a relatively high bypass ratio for a fighter engine, very close to 1:1, which in turn means that the afterburner has a lot of cold, fresh air to burn and accelerate. The faster you go, the more air it has to work with, and the more thrust it produces. From the chart we can see that in zone 1, as airspeed grows, this is just enough to outweigh the loss of thrust from the regular engine. In zone 2, the added thrust from the afterburner grows faster than the dry thrust drops, and total thrust now increases slightly with airspeed. In zone 3, though, and especially at max zone 3, that's where the fun begins. At sea level and max zone 3, increasing your airspeed by 0.1 Mach also increases your total thrust by ~5-7% throughout the entire speed envelope. The test rig maximum of 115kN is passed at around Mach 0.52, and at Mach 0.9 you're up to around 145 kN, which is over three times the dry thrust you'd have at that airspeed. Remember that figure of a typical increase of 50-70% over dry thrust? That was a in the test rig on the ground, so the numbers aren't really comparable, but it's still amusing to note that max zone 2 gets you 70% over dry thrust at Mach 0.5. Max zone 3 in that situation is like adding another afterburner on top of the regular afterburner, because max zone 3 gives you another 60 percentage points, topping out at 130% over dry thrust at that same Mach number.

 

All of this comes at the price of massive fuel consumption, of course (over 15% of internal fuel per minute is easily achievable), but now I'm finally starting to get to the point. I've seen some people limit themselves to zone 2, presumably because they think that this will save them fuel. In many cases this simply isn't true. Yes, zones 1 and 2 use less fuel per minute, but you gotta ask yourself why you're throttling up. A lot of the time, it's simply because "I want to go faster". Well, zone 3 also wants you to go faster, as indicated by the fact that it gives you more fun (thrust) the faster you go, and if you are nice to the aircraft she will also be nice to you. Let's have a look at an acceleration chart:

 

LKDbPHT.jpg

 

Standard atmospheric conditions, sea level, loadout in group 3 (roughly speaking, four rocket pods and a drop tank), full fuel minus 275 kg to account for takeoff and initial acceleration up to Mach 0.55. On the horizontal axis (way down at the bottom), Mach number. Vertically, we have three different graphs, where the lines marked "MAX SL" is max dry thrust and the other three are max afterburner zone 1/2/3 respectively. At the top, time taken to reach the given Mach number, in minutes. In the middle, fuel used, in percentage points (full internal fuel is around 100-106%). At the bottom, distance covered over the ground, in kilometers.

 

We can tell two important things from this diagram. First, acceleration in max zone 3 is hilarious. Even with this relatively heavy loadout (and full fuel, too) you go from M 0.55 to M 0.8 in about 15 seconds. Additionally, the Mach number/time relation is almost linear all the way up to the transonic region because the thrust increases almost as fast as the drag does. Well, we already kinda knew that. Second, and more importantly, accelerating in max zone 2 is no more fuel efficient than doing it in zone 3. In fact, it rapidly becomes less fuel efficient. Zone 1 (or even max dry thrust) can be more fuel efficient if you're going slow and the desired difference in speeds is small, but if all you're thinking is "man I wish I was going faster right now", just kick it straight into max zone 3 - the shorter time needed for the acceleration outweighs the higher fuel consumption. The differences are smaller on a lightly loaded aircraft, but zone 2 is insignificantly more fuel efficient for acceleration even on a clean aircraft. On heavier aircraft or on a hot and humid day, the differences are bigger, in zone 3's favor. At higher altitudes, say 5-6km and above, max zone 3 is almost always the most fuel efficient (exception for small speed changes with light loadouts). If you reach the desired speed and want to maintain it, then you obviously throttle down, but for getting there just use max zone 3.

 

The same thing goes for climbing, but there you also have to consider why you're climbing. If you just want to get high, right now, max zone 3 is almost always the most fuel efficient (or insignificantly less fuel efficient than the other options) and it gets you there really fast. If you want to climb because you're going to do a long ferry flight and want to take advantage of the better fuel economy per kilometer at high altitude, it's better to climb dry or in zone 1 because you'll spend about the same amount of fuel as in zone 3 but also cover a long distance while doing it.

 

The exception to this rule is takeoff, where zone 2 is generally a good compromise between thrust and fuel consumption, except with very heavy loadouts. Well, as long as you have runway to spare, that is...

 

 

tl;dr: zone 1 is useful for making noise and keeping formation at high subsonic speeds, zone 2 is rarely useful, zone 3 is the FARA zone


Edited by renhanxue
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As usual, awesome and informative post Renhanxue! :)

 

These first generation afterburning turbofans were really interesting engines (IMHO), namely in how they contrasted with the late-generation turbojets and how their appearance on paper widely differed from the experience of operating them. There were only a few combat aircraft that went operational with this type of higher-bypass ratio afterburning turbofans: F-111, F-14A, and the Viggen. The A-7 used a nonafterburning version of the TF30 also, but thats another story.

 

I don't know if many Draken pilots transitioned to the AJ-37 Viggen (given their different roles), but F-4 pilots transitioning to the F-14A had some interesting stories about how engine performance varied from their expectations. BTW, the TF30 is really similar to the RM8A in terms of general design, performance characteristics, and limitations.

 

F-4 pilots transitioning to the F-14A were expecting more thrust (which happened) and generally better performance in acceleration and climb (which also happened) along with much improved fuel economy. However, one of the most common early complaints/concerns about the TF30 was it's ravenous thirst for fuel! The engineers were quoting a 30% improvement in thrust specific fuel economy (military power) and much better loiter and cruise performance. However, in full afterburner, the TF30 burned nearly twice as much fuel per hour as the J79 (58,000 lbs/hr vs 33,000 lbs/hr). This wouldn't seem to be a big problem, except these newly transitioned F-4 pilots were using afterburner all the time!

 

In the Phantom, pilots were used to running around the skies at high subsonic speeds (even loaded) with just military power. According to what I've read, Mach 0.8-0.9 was quite feasible. However, the Tomcat needed at least zone 1 to maintain those speeds during maneuvers. Furthermore, the seemingly low thrust of military power means that afterburner was needed for just about any energy maneuver, even if relatively brief. However....at low altitudes or high speeds (or both :D) the power was mighty impressive.

 

With the Viggen, I can imagine a similar response where pilots head out with this new, ultra-sleek, ultra-modern fighter with a massively powerful engine only to be slowly climbing in steps to maintain their optimal cruise speed of Mach 0.55....a bit of a surprise. A pilot used to something like the Draken might have thought that someone misplaced a decimal on the thrust output. But then at zone 3, the aircraft accelerates faster than anything they imagined and zoom climbs to high altitudes while still accelerating. At those altitudes and speeds with the high-ish bypass ratio, the engine is almost functioning a bit like a ramjet. Which also means that if you have the audacity to unlight the burner at those high altitudes....the engine flames out.

 

Of course, you can't try this amazing feat for long as the RM8A can burn up to 65,000 lbs/hr in zone 3! I find this kind of operational dichotomy so interesting, the RM8A is almost two different engines between military and afterburner operations with diametrically opposed strengths and weaknesses.

 

I can't wait to thoroughly stress test Leatherneck's rendition of this impressive and personality packed engine. And from what is written in the manual, plenty of personality will be included. :)

 

Only 6 days left!

 

Thanks again for the great post Renhanxue, it was another great reminder of how much fun we will be having soon!

 

-Nick

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Glad you liked it :]

 

For a more relevant and correct comparison with other aircraft of the period: according to the F-14D flight manual, the GE-F110-400, which is almost 20 years younger, starts at 61 kN static dry thrust installed in the aircraft. The RM8A cranks out 65.6 kN in the test rig and does 55 kN installed in the aircraft at M 0.3, so eyeballing the thrust diagram let's say 60kN-ish static in the aircraft, which is a very conveniently close figure. Going from there to Mach 0.9 at sea level in full afterburner, the F110's thrust more than doubles to 134 kN, but in the same conditions the RM8A is now up to 145 kN. It's pretty interesting to see that the mid-60's airline engine conversion not only keeps up but actually edges ahead as far as thrust goes, but the F110 is a bit lighter and smaller and I bet it has advantages in other areas (like avoiding compressor stalls at high alpha, for example...). And of course the F-14 is cheating and has two of the things while not even coming close to twice the Viggen's mass.


Edited by renhanxue
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With the Viggen, I can imagine a similar response where pilots head out with this new, ultra-sleek, ultra-modern fighter with a massively powerful engine only to be slowly climbing in steps to maintain their optimal cruise speed of Mach 0.55....a bit of a surprise. A pilot used to something like the Draken might have thought that someone misplaced a decimal on the thrust output.

Yes, and it's also mentioned somewhere (I think in the aerodynamics compendium?) that the AJ 37 is considerably draggier than the Draken is in most situations but especially in level flight (not surprising if you look at a Draken from the front - it's really thin), so you slow down faster when coming out of afterburner too.

 

A clean Draken can easily break the sound barrier in level flight on just dry thrust at altitude, something that is definitely impossible on the Viggen. Then again a Draken without drop tanks at military power has an endurance of maybe 40 minutes, and that's at high altitude...

 

edit: also, to clarify, Mach 0.55 is distance-economical cruise speed at sea level. It gradually increases with altitude up to Mach 0.9 around 7-8km MSL or somewhere around there.


Edited by renhanxue
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Glad you liked it :]

 

For a more relevant and correct comparison with other aircraft of the period: according to the F-14D flight manual, the GE-F110-400, which is almost 20 years younger, starts at 61 kN static dry thrust installed in the aircraft. The RM8A cranks out 65.6 kN in the test rig and does 55 kN installed in the aircraft at M 0.3, so eyeballing the thrust diagram let's say 60kN-ish static in the aircraft, which is a very conveniently close figure. Going from there to Mach 0.9 at sea level in full afterburner, the F110's thrust more than doubles to 134 kN, but in the same conditions the RM8A is now up to 145 kN. It's pretty interesting to see that the mid-60's airline engine conversion not only keeps up but actually edges ahead as far as thrust goes, but I bet that the F110 has advantages in other areas (like avoiding compressor stalls at high alpha, for example...). And of course the F-14 is cheating and has two of the things while not even coming close to twice the Viggen's mass.

 

Yes, the RM8A had a mighty output for it's time (even surpassing the huge output of the J75) and even for today. The significant increase in thrust with speed is also an interesting characteristic that the TF30 shares, it nearly matches the F110s output at Mach 0.9 and sea level despite having about 30% less static thrust.

 

But the F110s real benefit was the less glamorous stuff: much, much better reliability/stability, much more military thrust (with less thrust loss at altitude), much quicker spool up times, and much improved fuel economy in afterburner (it actually has the same max fuel flow as the TF30 while providing more thrust). Many pilots found that operational improvements were even bigger than expected since mil thrust was so much better during normal operations.

 

The transition to lower bypass ratio turbofans (among other technological changes) helped to close the gap between the first turbofans and turbojets - quick spool up times and a smaller gap between mil and afterburner thrust.

 

Still, those early engines were outstanding for going very fast and performed quite well at that task. It was the other parts of the operating range that demanded improvements and led to the reliable/responsive engines of today. If my life depended on it, I'd much rather have a newer engine. But the entertainment value of these old and fussy engines is hard to beat. For me, it's another strong selling point of the Viggen module and how it offers a unique experience among other modern-ish fighters in DCS. :thumbup:

 

-Nick

 

PS - It's not cheating, how else do you get 40,000 lbs of thrust in 1974? :)

 

PPS -

Even higher fuel flow: SFI manual states 8 000/h basic engine and 60 000l/h to the afterburner totalling at 68 000 liters/hour. (120 000 lbs/h)

Max fuelcapacity is 6775 with external tank so all can be gone in just about 6 minutes.

 

Whoa! It burns as much fuel as 2 TF30s in zone 5?(!) That's a serious external combustion engine! :D


Edited by BlackLion213
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That's the max fuel pump capacity, not quite the same as actual fuel consumption. Actual fuel consumption:

 

CWf8qNF.png

 

Standard atmosphere on the left, standard atmosphere +10° C on the right. At Mach 1.1 on the deck, ~19% of internal fuel (4476 kg as per Leatherneck's manual) works out to about 850 kg/minute or 51 metric tons per hour. That's only 112,400 lbs/hour.

 

Fuel consumption is lower at higher altitudes; doing Mach 1.6 at 8km MSL it's down to only 12%/minute.

 

I have no idea how much the F110 drinks at full afterburner, but I'd bet a pretty hefty sum it's one heck of a lot less :V


Edited by renhanxue
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Great! I've never seen that chart before. Not as bad then. As at normal combatspeed around 0.8 when you would use the burner in turns you would only be looking at about 15% a minute which would be about 46500 lit/h (82000lbs/h)

This means about 8,5 min in full burn instead! :-)

Really impressed with all the information you managed to dig out of the archives. How do you find the time??

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Single guy with few commitments working a 9-5 desk job, so it's not too hard... I was actually at university last year doing history, and then it was even less hard.

 

Also, digging in the classified archives consists of 99.2% waiting for them to actually declassify things, 0.3% paging through stuff you're not interested in but which happened to be in the same file, and 0.5% highly interesting reading. :V

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Everytime someone brings up fuel economy in regards to zone 3, I will quote TAA 61:

 

Reserving fuel to reach a distant landing base, when such can be found closer, is bad tactic if enemy fighters thereby get an opportunity to attack.

 

If it worked for Lansen, it probably works for Viggen, and Georgia has plenty of strips. Although I think maximum time in zone 3 is limited not by carried fuel, but by how quickly the pumps can provide fuel to the engine (not fast enough it turns out).

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Well! Still impressed. You are doing a great job finding everything out for others!

Wish I also had some sparetime over. Only working 50 % at the moment but my wife does full time studies so I end up babysitting the rest of the time. I'm negotiationg at the moment to buy a new computer to play DCS again. We'll see if it happens. I'm really craving when I see the AJS cockpit. Bet it feels amazing with Oculus Rift.

Got a 9 hrs in the cockpit tomorrow so I should at least be able to spend a bit of time stuying your SFI manuals and the LN manual a bit more.

Benefit of the Airbus is the table to place the ipad on ;-)

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I have no idea how much the F110 drinks at full afterburner, but I'd bet a pretty hefty sum it's one heck of a lot less :V

 

I hope someone can answer this, I ask this for a long time while working the F110-GE-100C and no one could tell me, we only worry about idle (700-1700pph)

To whom it may concern,

I am an idiot, unfortunately for the world, I have a internet connection and a fondness for beer....apologies for that.

Thank you for you patience.

 

 

Many people don't want the truth, they want constant reassurance that whatever misconception/fallacies they believe in are true..

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Speaking of which, I've seen people express disbelief at how the aircraft just keeps accelerating at low altitude (like Jive's comment in his video). That's not a bug in the flight model. Take a look at the first diagram in the OP again and follow the drag curve marked "R" (clean aircraft). Where the diagram ends at Mach 1.1, thrust minus drag in that configuration is still around 25 kilonewtons. Newton's second law says force is equal to mass times acceleration, and a clean aircraft with 40% internal fuel left weighs 12250 kg, so solving for acceleration tells us that at that point you're still accelerating at around 2 m/s², or in other words you're gaining speed at a rate of around 7 km/h per second. On a cold day thrust increases even further.

 

There may be problems associated with going that fast, such as overstressing the airframe or getting compressor stalls (although the JA 37 with its very similar airframe had Vne set 100 km/h higher, at 1450 km/h IAS), but one thing that definitely isn't stopping you at sea level is lack of thrust. In standard atmospheric conditions this is true up to an altitude of around 7 km - above that, you should have an actual top speed limited by drag, topping out at around Mach 1.9 at 11000 meters (Mach 2 on a cold day).

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Step 1: Climb to 10km absurdly fast with zone 3

 

Step 2: Accelerate absurdly fast with zone 3

 

Step 3: Watch as your speed soars up past Mach 2.5 and Mach Tuck kills you

 

Step 4: ????

 

Step 5: laugh

DCS modules are built up to a spec, not down to a schedule.

 

In order to utilize a system to your advantage, you must know how it works.

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I figured that's what happened the first time I tried a high alt top speed run. Felt like the controls reversed and I blacked out, woke up facing a hillside at Mach 1.5 or so (I throttled down before blacking out).

 

 

Sent from my iPhone using Tapatalk

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