India has a new light fighter, the indigenous Tejas (which translates as ‘radiance’). Other than its small size, there are two more unusual things about Tejas: large control surfaces where the front of the wing meets the aircraft’s body and a naval variant with a pure delta wing.
“As a supplement to my recent piece on the Tejas programme, Hush-Kit have asked me to contribute an item about leading-edge vortex controllers (LEVCON) on the Tejas, seeking to explain what these are, how they work, and why they might be used on Tejas. As explained in my previous article, this piece is from an outsider’s perspective, but in this case, from the background of an aerodynamicist experienced in combat aircraft performance and configuration design.
I’m going to write about LEVCONs in the context of the Navy variant of Tejas, and seek to explain why they are being used on that aircraft. The picture below shows the Navy version of Tejas in its recent land-based arrested landing trials, and the second picture provides a clearview of the LEVCON behind some of the trials team. Pictures are from the Indian Navy via swarajyamag.com.
To understand LEVCON, and their application to Tejas, it is first necessary to discuss the advantages and disadvantages of delta wing planforms for combat aircraft.
In the context of supersonic aircraft, the delta wing is an attractive design option. The key source of drag in supersonic flight is wave drag. Wave drag arises as a consequence of the pressure distribution on wing and body surfaces in supersonic flows. Wave drag is extremely sensitive to the thickness to chord ratio of the wing, and also to leading edge radius. In aeronautics, the thickness-to-chord ratio, compares the maximum vertical thickness of a wing to its chord (the distance from the front to the back of a wing). A thick wing, with a rounded leading edge, will have a very high wave drag if it experiences transonic or supersonic flows. In addition, wing sweep reduces the local mach number and delays the occurrence of wave drag in transonic flight. This is why fast aircraft tend to have highly swept wings.
From this brief discussion, some of the advantages of a delta wing planform for a supersonic fighter aircraft become apparent. The delta planform offers the possibility of using a low thickness to chord ratio wing, which simultaneously provides good wing area, a highly swept leading edge, and low transonic and supersonic wave drag, while still providing reasonable internal volume for structure and fuel.
The validity of this approach is illustrated by the large number of fighter aircraft using delta, or near delta planforms, some as pure delta wings without additional stabilising or control surfaces, others with either tailplanes or canards. Examples of pure deltas include the Dassault Mirage series and the Convair F-102 and F-106. Examples of canard deltas include Typhoon and Rafale, while the Lockheed Martin F-22 and even the McDonnell F-15 may be considered as tailed near-deltas.
What then might be the downside of using a delta planform? There are two potential issues with the use of a delta wing planform, both arising out of its inherently low aspect ratio. Aspect ratio is defined as ‘span-squared divided by wing area’, and it provides an indication of the efficiency of the wing – its ability to deliver lift with minimum drag. High aspect ratio wings are seen on sailplanes, and the best of these deliver lift to drag ratios of more than 60 to 1. A 60-degree sweep pure delta wing has an aspect ratio of ~2.3, compared to a sailplane which might have an aspect ration of 30, or a typical airliner, which might have an aspect ratio of 9.5.
Low aspect ratio increases lift-dependent drag of the aircraft, and this in turn reduces sustained turn rate performance. The maximum sustained turn rate for a high-performance combat aircraft will generally be limited by structural design limits, except at high altitude or at supersonic speeds, where aerodynamic limitations are more likely. This is because, for much of the flight envelope, a high-performance aircraft might be physically capable of turning at more than 9g, but pilots cannot sustain this loading for sustained periods. Consequently, to save weight, the structure would generally be designed to a 9g limit. Outside the structurally limited region, higher lift-dependent drag will result in the aircraft maximum thrust being reached at a lower sustained turn rate than for a higher aspect ratio configuration.
The other impact of having a low aspect ratio highly swept wing is that the lift curve slope of the aircraft is low. What does this mean? Well, the lift curve slope is the amount of lift generated by the aircraft for a given angle to the airflow. A low lift curve slope means that the aircraft has to fly at a higher angle to produce a given amount of lift than a configuration with a higher lift curve slope. Alternatively, the aircraft might have to fly at a higher approach speed to generate the same lift.
It is easy to see that this latter aspect of a delta wing could pose some issues for a naval fighter. Landing on to a carrier is a high-intensity operation, and even today remains one which is largely dependent on the visual picture presented to the pilot as he approaches the ship; that picture being enhanced by shipboard landing systems and Deck Landing Officer guidance. A high angle of attack for landing raises the nose of the aircraft, making this critical picture more difficult to see. The alternative of landing at a higher speed may simply not be possible as the energy to be absorbed by the arrester system of the aircraft and ship will increase with landing-speed squared, imposing higher loads on the aircraft structure and undercarriage.
High lift devices are systems which increase the lift available to an aircraft, generally in the landing configuration, but occasionally also used to improve sustained turn rate. Higher lift available to the aircraft reduces the approach speed and angle to the airflow of the aircraft. In the context of highly-swept naval aircraft designs, extreme examples may be seen in the variable incidence system used for the F-8 Crusader, and, in a sense, the variable sweep solution for the F-14 Tomcat, transforming a low aspect ratio highly swept wing into a conventional moderate aspect ratio, low sweep configuration.
Many approaches to providing higher lift in the landing configuration have been used. The most traditional approach, seen on almost all commercial aircraft is the use of slats and flaps which extend from the trailing and leading edges of the wing to both increase wing area, and wing camber (the curvature of the lifting surfaces) to increase lift. In some naval aircraft, most notably the Buccaneer, high pressure air from the engine is blown over flap surfaces to further increase lift.
One aspect of these devices is that in addition to producing more lift, they also change the centre of lift on the wing, generally resulting in strong nose-down forces that need to be balanced through a tailplane or canard deflection to provide the required opposing force to balance, or trim, the aircraft. These flap-and-slat high-lift devices are used on aircraft with delta, or near-delta, planforms, examples being the A-4 Skyhawk or F-4 Phantom, both of which use a tailplane to balance the aircraft. Similarly, Rafale and Typhoon use canard control surfaces to trim the aircraft on approach.
Another approach to increase lift is to use a leading-edge strake. This is a sharp-edged, very highly-swept extension to the aircraft leading edge at the fuselage side, introduced to great effect on the F-16 and F-18. This has the effect, at high incidence, of generating a powerful vortex over the inboard portion of the wing, and can increase both instantaneous and sustained turn rate substantially. In the landing configuration, Concorde, which had a slender delta planform, exploited vortex lift generated over its blended wing design, to reduce approach speeds, although the relatively high incidence required led to the need to also droop the nose of the aircraft, to provide visibility on landing.
The leading-edge strake has been widely adopted, with several combat aircraft being fitted with LERX, or leading-edge root extensions, as a simple way of modifying the aircraft to improve instantaneous or sustained turn rate, or both. Examples may be seen on many aircraft, but the Harrier LERX modification is a good example of a modification to an existing design.
The LEVCON uses the idea of a passive strake to generate a powerful vortex over the wing, but does so in an active sense. In other words, it is a device for modifying the vortex flow over the wing which may be used to increase lift, to control the aircraft, or both.
The basic principles are described in US Patent US5094411A:
“The use of vortex flaps as a leading edge device for reducing the lift-dependent drag of highly-swept, thin wing aircraft that are prone to leading edge flow separation and vortex formation, has been extended and adapted for aircraft control, particularly at high angles of attack where conventional trailing edge surfaces lose effectiveness. Down-deflected vortex flaps capture the vortex suction on their upper surfaces to generate an aerodynamic thrust force component that results in drag reduction. Conversely, up-deflection of flaps magnifies the vortex to thereby increase wing lift accompanied by a drag force on the flaps. The present invention combines the advantageous features of up and down deflected vortex flaps to induce thrust and drag forces in order to generate directional control moments. Similarly, the differential operation of the flaps creates unequal lift increments on the wing panels to generate lateral moments. The segmented, differentially actuated flaps of the present invention thereby improve the ability and agility of high-swept thin wing aircraft during manoeuvring at high angles of attack.”
Translated, this shows the concept of deflecting leading edge surfaces down to reduce drag and, for example, improve Sustained Turn Rate, or to deflect them up to generate a stronger vortex and additional lift. An advantage of the latter approach is that upward deflected LEVCON can force the formation of a leading-edge vortex at lower incidences, where a fixed strake would not generate significant lift. The patent also describes the possible use of such devices to provide yaw and roll control.
The pictures shown earlier reveal that in making its recent arrested landing, Tejas was using an upward deflected LEVCON. This innovation makes a lot of sense for Tejas, because, as a pure delta with no balancing tail or canard surface, a conventional slat-and-flap high lift system cannot be used.
Moreover, the large upward deflection of the LEVCON will force the development of the leading-edge vortices, and the associated increase in lift, to occur at low incidence, allowing the view over the nose of the aircraft to be maintained for the approach to landing. Because the additional lift is developed over the whole length of the wing, it is likely that the pitching moment generated is less than would have been seen with a conventional system, and, on the approach, might even require a small droop of the trailing edge surfaces, which would also increase lift.
The only photographs I have seen of Tejas with LEVCON deployed are for the naval variant. The ski jump trials were conducted with the LEVCON more or less in line with the wing, increasing lift slightly, but with little effect on drag, whereas the recent arrested landing with upward deployed LEVCON would have generated significant lift and drag.
Subsequent development of the aircraft may see LEVCON integrated into the control system to improve manoeuvre capability for both variants, but whether this will be implemented remains to be seen.
Tejas is indeed an interesting little aircraft, and, in my view, is the first carrier aircraft to use a pure delta planform. I recognise that some might disagree with this, pointing to the Douglas F4D Skyray as having this distinction. In the Skyray, however, the landing approach speed problem was resolved using slats on the outboard wing, and large triangular trimming tail surfaces, forming the junction between the wing trailing edge and the fuselage, to cope with the slat-induced pitching moment. The Skyray was aerodynamically, in effect, a tailed-delta rather than pure delta.
As indicated in my earlier article there are plenty of developments to watch in this program, some of which may be observed via the official website.
Why the Viggen-like wing?
The wing of Tejas is reminiscent of of the Swedish Viggen, a tactical fighter designed in the 1960s. Both wings have a inner section that sweeps back at a more shallow angle than the outer section – why has Tejas opted for this ‘Viggen-style’ wing?
This is what DelhiDefenceReview has to say about the Tejas planform , written in the context of an article about the Medium Weight Fighter (MWF):
“As mentioned earlier, MWF retains the main wing from MK1 with minor modifications. It has the same iconic double delta wing featuring lower sweep angle for the inboard section. In a pure delta wing, the LE vortex, which constitutes a large portion of the total lift, starts forming right from the apex, the point where the wing LE attaches with the fuselage. The lower sweep on the inboard section results in the wing LE vortex forming slightly downstream of the apex. This pushes the CoL slightly aft-ward and helps bring down the static instability to a manageable range. This wing configuration also allows the designers to have a significantly larger wing area for the same LE sweep angle, length of fuselage and static instability margin. Figure 14 shows the blue outline of a pure delta wing which would need to have its apex downstream to maintain the same level of instability. In addition, the leading edge portion of the inboard section is lifted up a bit to provide the required clearance between the air intakes and the lower surface of the wing.”
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Clearly this sort of thinking may have influenced the wing design for Viggen, but one must also bear in mind that Viggen was not designed as an unstable platform. Short take-off and landing (STOL) requirements were, I believe, a strong driver for Viggen, and the canard foreplane is important in this regard. The trailing edge flap on the Viggen provides a nose-up pitch control on rotation for STO which adds to aircraft lift. The alternative of using elevons only at the rear of the delta wing would reduce lift, resulting in a longer take-off run.
The article on Tejas suggests that the planform essentially allows a larger wing area for a given level of stability – again a useful property given STOL requirements for Viggen. However, one must also consider the canard, and particularly how its trailing vortices interact with the flow over the wing. This is likely to be a more complex aerodynamic situation than for Tejas.
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Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes from ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon. He was also Britain’s technical liaison to the British Embassy in Washington, covering several projects including the Advanced Tactical Fighter contest. His latest book is available here.