C-101 and L-39: Trainer Comparison


#1

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. :slight_smile:


#2

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.


#3

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


#4

Nice read so far! :slight_smile:


#5

Looking forward to your next observations.

Wheels


#6

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 :smiley:

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.


#7

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. :wink: