The Tejas LCA: improving performance with the current F-404 engine

(Concluding part of a two-part series on the Tejas LCA's engine)

by Ajai Shukla

The selection of the Tejas LCA’s new engine in October --- the choice (as the previous post deals with) is between the Eurojet EJ200 and the GE F-414 --- will provide an extra 10 KiloNewtons of thrust to the Tejas. The new engines, however, will start being fitted onto the third Tejas squadron; the first two squadrons, comprising 40 aircraft, would already be in service with the GE F-404 IN-20 engines.

And so the Indian Air Force (IAF) has asked the Aeronautical Development Agency (ADA) and Hindustan Aeronautics Limited (HAL) to urgently improve the performance of Tejas LCAs fitted with the GE F-404 IN-20 engines. These will power the first two LCA squadrons consisting of 40 fighters.

I visited HAL’s Bangalore Complex to see how that is being done. HAL has adopted a three-fold strategy:

1.  Improving the air intake. 

Top HAL decision-makers pooh-pooh the IAF’s contention that the LCA’s air intakes are incorrectly designed, resulting in oxygen starvation and incomplete burning and, therefore, sub-optimal engine power from the F-404s. At the same time, however, steps are being taken to improve air intake, without getting into major redesign that could set back the programme by years. Instead, auxiliary air intakes are being provided on the sides of the Tejas engine housing --- similar to those on the Jaguar (see photos).

These auxiliary air intakes comprise of spring-loaded panels that open when engine suction is very high and provide an additional route for airflow into the engine intakes. As you can see in the photos, the spring-loaded panels can be pushed in by manual pressure.

At critical stages in the flight envelope, such as during take-off, rapid climb, sustained turn… and in any case, when afterburners are on… the heavy suction from the engines would open the auxiliary air intakes. When the demand for air goes down, such as in level flight, the auxiliary air intakes would close.

HAL designers aver that this would improve the engine performance only in some portions of the flight envelope. They say that during the most critical moments --- which are during sustained turns, in aerial combat --- the auxiliary air intakes would provide only marginally improved performance, if any at all.

A top HAL designer told me, “There is some merit in [the IAF’s idea]… the designers are considering it. There has been a debate for quite some time… will it really improve to that extent. Where it really matters it may not give added thrust.. in other places it will give.”

Nevertheless, the fitment of auxiliary air intakes is going ahead, partly because this does not require major re-engineering, nor will it delay the Tejas induction in any way. According to HAL, this will take six months to engineer; later LSPs will incorporate the auxiliary air intakes.

2.  Reduction of Tejas' weight. 

The LCA’s designers say that the removal of telemetry instrumentation, which is essential during flight testing, will bring the Tejas’ weight down by as much as 300-400 kilos. Re-engineering some of the displays and sub-systems within the cockpit will lop off another 300 kilos; the weight reduction of 600-700 kilos is expected to allow the carriage of more weapons.

There is a lack of understanding about what the Tejas’ weight is, since all kinds of figures are bandied about. Let me clarify: The 10.5 tons that I wrote about in my last post is the total weight of the Tejas, with full fuel on board; all 7 pylons fitted but not carrying weapons; and two outboard missiles being carried. The maximum payload of the Tejas is 3.5 tons… carried on its pylons. This could be armament or external fuel tanks; if external fuel tanks are fitted, the weight of fuel will correspondingly bring down the weapons load carried.

But there’s a catch! The maximum take-off weight of the Tejas is 13 tons. So if you load the maximum payload of 3.5 tons onto the 10.5 ton fighter, your weight of 14 tons is beyond the maximum take-off weight. So you’ll have to shed one ton… or either internal fuel or external fuel/armaments. That’s what happens when a fighter’s weight goes beyond what was originally planned.

So the reduction of 600-700 kilos may not actually go into making the Tejas more manoeuvrable. This shaved off weight may be made up by allowing the Tejas to carry (close to) its full capacity of external fuel-cum-armament.

3.  Increasing control surfaces. 

The designers say they are considering adding an auxiliary wing (similar to the Eurofighter) to the front portion of the fuselage to increase the control surfaces, and therefore manoeuvrability. This involves major re-engineering, which cannot be done for the first two squadrons. However, it will be grouped along with the re-design that will be necessary for fitting in the new engine for Tejas No 41 onwards.

The Tejas designers are not unanimous about the utility of an auxiliary wing. Some are of the opinion that the added power that will come from the new engine might make the additional control surfaces superfluous. But the option remains on the table.