C-101 and L-39: Trainer Comparison

[Bearhedge]

Hi team,

Typically for me, instead of making a nice polished guide like some of our more distinguished members, I’m going to do a WIP thread for my DCS trainer comparison work. This way others can hop in to comment and even if I don’t get around to doing a ‘proper’ write-up in the end, people can potentially make use of the interim findings.

I’m putting together some numbers and features comparisons between the C-101 and L-39 and will create a few missions to try out the differences in practice.

I’m starting out with a little slideshow of the two aircraft - photos taken from an imaginary arms show where both manufacturers were putting their best foot forward for a prospective gulf nation buyer:

First observations and rough numbers to follow soon.

[Bearhedge]

General notes / initial observations

The two aircraft share many features / purpose:

-both powered by a single turbofan engine of relatively modest output by modern standards
-2-seaters with instructor functions
-low wing design
-high subsonic speed
-designed for jet training / light attack / COIN (COunter-INsurgency)

The L-39 is the older design of the two, having first flown in 1968, whereas the C-101 first flew in 1977.

Externally the first visual differences one notices are the empennage, the placement of the intakes and the L-39’s fixed wingtip tanks:

The aircraft dimensions are relatively similar:

	L-39	C-101
Dimensions (m)
Overall length	12.132	12.245
Wing span	9.461	10.6
Height	4.72	4.25
Wing area (sqm)	18.8	20

The C-101 has a slightly larger wing and the L-39 stands a little taller. Wing loading in a typical mission weight is relatively similar also, around 238 kg/sqm in the L-39 and 242 kg/sqm in the C-101.

Fuel capacity

While the aircraft are similar in size, the C-101’s fuel capacity is notably larger. The maximum internal fuel capacity of the C-101 is 4,148 pounds (1,882 kg) while the L-39’s is only 2,161 pounds (980kg).

This difference is due to specific design considerations - the C-101 had to be able to fly from Spain to the Canary Islands. In 1975 when work on the C-101 started, the Western Sahara was still in Spanish hands, and the Aviojet was expected to conduct support missions in that territory.

The fuel consumption figures of both aircraft are relatively similar, so the C-101 has a lot longer loiter time than the L-39. I don’t know how often DCS pilots will find themselves requiring such fuel capacity, though.

Cockpit

Sitting in the cockpit, the differences between the aircraft are quite stark. I’ve flown the L-39 a lot more than the C-101 so I’m at home in it but I have to say the large, clear gauges and the overall layout of the C-101 cockpit is very pleasant and makes for an easier instrument scan.

The below shots are taken trimmed for level cruise at 7,000 ft at 230kts to compare the view from inside the cockpit:

C101CC:

L-39ZA:

Visibility from both aircraft is generally good, however the more modern HUD of the C-101 allows a less cluttered view ahead. The C-101 rim of the cockpit sits low at cruise, giving a good view forward and down.

[Victork2]

This is really interesting! I hope you continue with it.ive been thinking of getting a trainer so this is perfect reading at the moment

[Aginor]

Nice read so far!

[wheelsup_cavu]

Looking forward to your next observations.

Wheels

[Bearhedge]

Engine / powerplant

This section will be a tough one for me to write, as I can’t say I really properly understand jets / turbofans - I can pretend to be able to chat about them but basically it’s magic to me: air and fuel go in, magic happens, thrust comes out. So take the following with that in mind.

First, some high level notes. The L-39 is powered by a single Ivchenko AI-25TL turbofan. The C-101 is powered by a single Garrett TFE-731 turbofan (the C-101EB has a lower output TFE-731-2-2J and the C-101C a higher output TFE-731-5-1J). Both are turbofan engines with relatively similar thrust (we’ll get to that) and weigh about the same (333-350kg dry weight, according to Wikipedia - couldn’t find a better source).

Interestingly, neither engine was developed for military use: the Garrett engine design was based around the core of a DC-10 APU (TSCP700) and was first used in Learjet 35/36 and Dassault Falcon 10 (corporate / business jets). The Ivchenko engine was designed for short-haul use in the Yak-40 regional jet / VIP transport.

Engine	Type	Length	Diameter	Maximum Thrust
Garrett TFE 731-2-2J	Turbofan	127 cm (50 inches)	100 cm (39 inches)	16.45 kN (3,700 lbf)
Garrett TFE 731-5-1J	Turbofan	127 cm (50 inches)	100 cm (39 inches)	20.90 kN (4,700 lbf)
Ivchenko AI-25TL	Turbofan	335.8 cm (132 inches)	61.6 cm (24 inches)	16.9 kN (3,800 lbf)

The engines don’t appear very similar, though. An aerospace engineer’s approach would probably be more scientific - alas, I am not one. Therefore my first question was, why is one short and fat and the other long and skinny? (no, this isn’t a sexist Welsh pub joke - keep reading)

The top photo is the Garrett engine and the bottom one the Ivchkenko:

From the Wikipedia article on geared turbofans (I know - terrible source, don’t lynch me):

In a conventional turbofan, a single shaft (the “low-pressure” or LP shaft) connects the fan, the low-pressure compressor and the low-pressure turbine (a second concentric shaft connects the high-pressure compressor and high-pressure turbine). In this configuration, the maximum tip speed for the larger radius fan limits the rotational speed for the LP shaft and thus the LP compressor and turbine. At high bypass ratios (and thus high radius ratios) the tip speeds of the LP turbine and LP compressor must be relatively low, which means extra compressor and turbine stages are required to keep the average stage loadings and, therefore, overall component efficiencies to an acceptable level.

In conventional turbofans the fan tips exceed the speed of sound causing a characteristic drone, requiring sound deadening. Geared turbofans operate the fan at sufficiently low rotational speed to avoid supersonic tip speeds.

The Garrett engine is a geared turbofan, meaning that there is a planetary reduction gearbox between the fan and the low pressure shaft. This allows the LP shaft to run at a higher rotational speed thus enabling fewer stages to be used in both the LP turbine and the LP compressor, increasing efficiency and reducing weight. It appears that the Ivchenko is a more conventional twin-shaft turbofan that doesn’t have a planetary reduction gearbox.

The cutaway shows how the L-39 engine has a 9-stage high pressure compressor, whereas the C-101 engine only has a single stage high pressure compressor.

The Garrett engine also has a higher bypass ratio (2.82:1) than the Ivchenko (2.0:1).

In terms of specific fuel consumption, the Garrett engine is more efficient at stated 0.477 lb/h/lb versus Ivchenko’s stated 0.600 lb/h/lb.

The C-101 engineers saw it appropriate to keep the pilot informed of the fuel flow rate, whereas their czech mates didn’t really think that was a high priority. At sea level, the following fuel flow figures were shown at full throttle:

The higher output TFE 731-5-1J gets through just under 2,000 pounds per hour (907 kg), whereas the EB sips away at a more modest 1,700 pounds per hour (771 kg) at maximum thrust. In the absence of a fuel flow gauge or specific information, the only reference I have to the L-39 is the following from warbirdalley.com:

At climb power settings and low altitude, the IA-25 turbofan burns about 330 gallons per hour (GPH), but this rapidly decreases to about 160 GPH in cruise at Flight Level 180.

330 gallons is 1,250 litres, i.e. something around 2,215 pounds of jet fuel (1,005 kg).

Engine	Max consumption
Garrett TFE 731-2-2J	771 kg/h (1700 lbs/hr)
Garrett TFE 731-5-1J	907 kg/h (2,000 lbs/hr)
Ivchenko AI-25TL	1,005 kg/h (2,215 lbs/hr)

So - after all that, I guess the only concrete information I have to present is that the Soviet-designed L-39 engine is a bit older, louder and hungrier. Who would have thought…

It’ll be more interesting once I get to actually testing performance, I promise

P.S. - on another note I’ll need to tidy up and consolidate the units I use, I know…it is interesting trying to compare a metric aircraft with an imperial one.

[PaulRix]

Nice write up so far! I’d say a candidate for a Mudspike article for sure!

Suck squeeze bang blow, with a pinch of Pure ehhhhh ‘Friendly’ Magic (PFM). That’s pretty much all you need to know about how a jet engine works.

[Bearhedge]

Just touching these trainers a bit for variety…I’d like to get some testing done in a more organised manner, but haven’t quite managed that yet.

I have, however, done a little bit of testing on an ad hoc basis…and the results are quite interesting so far.

I have been aware that I really like the L-39 module, which appears to put a bias on my experience…I think it is because I’ve flown it a lot, so everything seems a bit easier and more predictable and therefore more pleasant. What’s quite interesting is, that lead me to subconsciously think / perceive that the L-39 performs better.

I have done some combat climb tests (basically a zoom climb after a simulated strafe run) and the C-101CC cleans the L-39 in a similar combat loadout by quite a substantial margin (I need to find my testing numbers from earlier in the year, but the difference is notable). It didn’t feel like it in the cockpit! But the numbers don’t lie - the C-101 sustains a higher climb rate.

Anyway, tonight I decided to test another one of my favorite things to do in the Albatros - short landings. There used to be an MP server where you could pick up downed pilots from safe houses and score points for your team that way…obviously meant for helicopters, but the L-39 was great for the purpose. Land on the road in front of the safe house, get a pilot to hop in the instructor seat and fly back to base.

Again, I assumed that surely the L-39 would perform better in this test - dirtied up in a light fuel load, she sits at just above 80kts as though on rails and as long as you don’t drop the airspeed too much further, the nose sits relatively low so you can see over it almost until touchdown.

Although the minimum approach speed of the L-39 is lower at a tad under 80kts (absolute minimum) and the C-101 somewhere around 90-100kts (stall speed in the manual is 84kts but I found sub-90kts ‘uncomfortable’, once on the ground, the L-39 seems to take a longer to decelerate!

Both planes need to be planted firmly on all 3 wheels as soon as possible to minimise braking distance.

After a few tries, it looks as though the C-101 performs just as well or even slightly better than the L-39! I’ll see if I can find some landing distance data, but certainly from my practical testing it seems that the C-101 would perform just as well at road landings than my trusted L-39.

L-39:

C-101:

[Aginor]

Good comparison!
What I noticed most about them is the difference in low and slow flying. Both fly well, but the point at which the L-39 suddenly falls out of the sky is a bit harder to predict (especially in a turn) and the slower spool up time of the engine makes the situation a bit more dangerous than in the C-101. I did have a few very close calls in the L-39, especially when I still had some fuel and external stores with me while landing.

[BeachAV8R]

I love these kind of aircraft comparisons! Great job. I had always intended to write an L-39 vs C-101CC article…just hasn’t come to fruition yet. Love your analysis though!

Out of curiosity - which do you think is the most “combat effective” in their intended role of light attack / COIN ops?

[Fridge]

Nice job @Bearhedge. Love some to see some comparisons like this - certainly when it challenges your assumptions

[Bearhedge]

@BeachAV8R

which do you think is the most “combat effective” in their intended role of light attack / COIN ops?

I need to do a lot more testing for a properly qualified answer but to be fair I think it has become clear to me quite quickly that the C-101 is more effective.

The pilot interface for the weapons (which you did a very good review of in your article) of the C-101 is undoubtedly superior - in the L-39, unless you’ve been flying it all day every day for the last week and remember it back to front, you’ll still have to hope that you got the switches right and that the right things happen when you pull the trigger, as the ambiguous green bulbs of the lower middle console only tell you so much.

I’ve screamed down the chute into harms way many times only to find that a trigger pull didn’t result in the expected reassuring ‘clunk’ followed by a ballooning bird and I’m instead trying to climb my way out of the wasps’ nest with all the payload still attached and slowing me down. Pilot error no doubt but the clear, home appliance like selectors of the C-101 do make that situation less likely.

Better zoom climb means less time spent inside the enemy weapons’ effective envelope.

More hardpoints, and depending on how worried you are of airframe bending, you have options to overload the aircraft with a lot more serious ordnance than the L-39. You can do this to the point where the bird is unflyable…but even one pair of heavy bombs does provide the ability to effect armoured targets that can’t effectively be touched by the L-39. Need to know your iron bombing run profiles, of course - but I guess that goes without saying in this category.

The available weapons selection is perhaps a bit unfair as the C-101 devs approach appears to be a bit more liberal to what you’d actually be allowed to hang from the wings…but if we assume that if it can be loaded, it is there to be used, the C-101 has more effective options. Some of the L-39 weaponry is fun in principle but not very effective in practice (such as the machine gun pods) and it suffers from having smaller rockets than the C-101.

The illumination bombs that L-39 has do enable limited night ops, which as you noted, is hard with illumination rockets. I’ve tried those in the Huey but at jet speeds determining the distance and angle in the dark would be some next level stuff.

The C-101’s more organised cockpit with big friendly gauges for readability helps in maintaining awareness of speed and altitude while mostly paying attention to the outside. I’m so used to the L-39 that it’s not a big issue for me but I have to acknowledge that the C-101CC cockpit is miles better.

Gun rounds counter is pretty handy too.

I guess all in all on the numbers the C-101 will outperform the L-39 in just about every category, just because it’s a more modern aircraft. Energy, payload, SA, range.

The funny thing is, I still feel more effective in the L-39, despite all that! It’s just such a great bird to get amongst with despite it’s shortcomings…but, I’m starting to think that’s just a function of experience and that I just need more time in the C-101 to get the most out of it.

[Bearhedge]

@Aginor

L-39 suddenly falls out of the sky is a bit harder to predict (especially in a turn) and the slower spool up time of the engine

That’s true. Slow flight with the airbrake out and a bit more throttle helps with the spool time (from memory that’s also the standard final procedure from the POH) but it’s not a perfect fix for the slow spool up.

Agree also that the stall speed - bank angle couple is a killer if you’re not careful. The Albatross generally stalls one wing first and remains controllable but that’s a treacherous place, as levelling the wings back without pushing the nose well down will have you enter this wallowing stall state and the spool up is too slow for a low altitude recovery with power alone…so if you go too deep into that and don’t have the altitude to trade in, it’s not a great story.

There is a state where she goes ‘mushy’ before dipping a wing - pulling in the airbrake and adding power while still in the ‘mushy’ stage works…but just having an extra 10kts is safer and simpler, of course.

I haven’t done much slow flight with the C-101 yet but the stall horn is nice and the spool up quicker, I think.

[BeachAV8R]

@Bearhedge

I’ve screamed down the chute into harms way many times only to find that a trigger pull didn’t result in the expected reassuring ‘clunk’…

Good to hear that I’m not the only one that has had that happen…look over the shoulder and wait for the beautiful explosions…but…uh…nothing came off the racks…

[RedBravo65]

Absolutely beautiful write-up! Bravo.

[wheelsup_cavu]

@BeachAV8R

but…uh…nothing came off the racks…

Sam City (Flight of the Intruder)

Flight of the Intruder Sam City

Wheels

[komemiute]

*Cue ‘Downtown’ song"

[Bearhedge]

Guns, guns, guns

Let’s be honest, if you come home from a COIN mission with rounds left in the gun, you have to ask yourself - did I try hard enough?

Let’s have a look at the fuselage-mounted gun options. The L-39 comes with a twin-barrelled 23 mm Gryazev-Shipunov GS-23L autocannon. The C-101 has the options of a single 30 mm DEFA 553 autocannon or two AN-M3 machine guns. Some general statistics of the guns are shown below:

	Caliber (mm)	Rate of fire	Muzzle velocity	Cartridge	Rounds carried	Rounds per sec	Total firing time
GS-23L	23	3400 (max)	715 m/s (2350 ft/s)	23x115 mm AM-23	150	56.67	2.65
DEFA 553	30	1300	815 m/s (2670 ft/s)	30x113 mm NATO	130	21.67	6
AN-M3	12.7	1200	890m/s (2910 ft/s)	12.7x99mm NATO	440	40	11

Due to the DCS damage model, I’m going to focus on the autocannons - if I get around to it, I might play with the machine guns but I don’t think the combat effectiveness of the MGs is likely to be on par, so we’ll compare (French) apples with (Soviet) apples first.

The GS-23L and the DEFA 553 are quite different:

• The GS-23L is a ‘Gast gun’, i.e. a twin-barreled weapon in which the firing action of one barrel operates the mechanism of the other. It is powered by the recoil of the floating barrels, hence it needs no external power source to operate.

• The DEFA cannon is a gas-operated five-chamber revolver cannon using pyrotechnic cocking and electrical ignition.

GS-23L at the top, DEFA at the bottom

So - the designers have approached the same problem with quite different solutions. Does it matter in the cockpit?

Looking at the numbers, the biggest difference on paper is the rate of fire. The twin-barrel Gast gun mechanism gives the GS a substantially higher rate of fire, 3400 vs 1300 rounds per minute. You can look at this from quite different perspectives - on one hand, you get more passes with the DEFA as your total trigger-down time is a lot longer, on the other, in theory with the GS a shorter burst will do and you’ll get more rounds on target in a shorter space of time.

This will require some field testing. Stay tuned.

[Chuck_Owl]

Pretty interesting series of posts. Keep it up!

[Wes]

We need some accuracy data if that’s available!
Can’t wait for the test results, the idea of adding a trainer to my roster has been on my mind and this thread will likely set my choice on the matter.