Look at early design studies of British aircraft in the late 1940s to the late ’50s and the you’ll see a lot of ‘T’ tails. But why? I asked Jim Smith to find out.
Well, it’s quite an interesting story. This will be a longer explanation than you expect, because we need to look first at some civil aircraft to understand ‘why T-tails at all?’, and why the Royal Aircraft Establishment might have been interested. Let’s start with the Comet, we’ll move on to the Caravelle, 1-11 and DC-9. After which, to the Javelin.
Comet and Caravelle
The Comet was designed with a very clean wing, and quite big flaps. Why? Because initial engine power was quite low, and there was a desire to operate the aircraft all over ‘The Empire’. Operating into shorter airfields in hot conditions, it is helpful to have an efficient wing, with effective high-lift devices (flaps and slats). All the more necessary with relatively low thrust.
This is still a design driver today. To be fuel efficient in the cruise, you need the smallest possible wing, and the smallest (well, least thirsty) engines possible. But a small wing increases approach speed and take-off and landing distances. This in turn affects payload – if your approach and take-off speeds are high, and you have to design to operate out of (say) Denver on a hot day, then your payload is going to be limited to what you can lift while complying with take-off and landing airworthiness rules at Denver, on that hot day. So efficient airliners have good high lift systems.
Why do Airbus airliners have the best wings in the world? Because they are designed by British Aerospace, who partnered with the RAE, and later DERA, in the UK National High-Lift Program, which generated hugely useful data enabling high-lift design to be better systematised and understood.
Before the well-known structural failures, there had been two Comet accidents on take-off in hot conditions (One at Rome and one at Karachi). The cause is thought to have been over-rotation on take-off, compounded by loss of control effectiveness in the wake of the wing. On the Comet, the fix for the take-off problem was to droop the leading edge slightly and add a wing fence, both of these measures delaying the stall to an incidence above that reached in the take-off rotation.
However, when Sud Aviation designed the Caravelle, two rear-mounted engines were used, again maximising the efficiency of the wing, but requiring the tailplane to be moved, to a new position half way up the fin. The Caravelle. was intended to operate into regional airports, some with minimal infrastructure and relatively short runways, and the clean, efficient wing will have helped cruise efficiency and take-off and landing performance. Other benefits of the fuselage mounted engines and raised tail were the ability to incorporate a rear airstair into the fuselage design, a quieter cabin, and less likelihood of foreign object ingestion (FOI) into the engines. Sud Aviation drew on de Havilland experience with the Comet, because they had an information sharing agreement with each other, leading to the Comet cockpit design being re-used for the Caravelle.
Current thinking for airliner configurations, is that two engines in nacelles on the wing is a better solution, because the weight of the engines offsets lift and manoeuvre loads, allowing a lighter structure – this is known as ‘inertia relief’. Airfields are no longer austere, so built-in airstairs are not needed, runways are longer, and FOI is not generally a problem. But wing-mounted engines do have some disadvantages. A taller undercarriage is needed, which adds weight, and the engines can interfere with the airflow over the wings, complicating high lift device design, and stability and control (737Max).
BAC 1-11, Trident and DC9
On to the BAC 1-11, Trident and DC-9. All the same ideas are in play, plus the use of a T-tail. The T-tail makes fin, rudder and tailplane design simpler and more efficient, partly because the tailplane is clear of the wake from the wing and fuselage in all normal flying conditions, partly because the tailplane acts like an endplate to the fin and rudder, increasing its effectiveness (or allowing a smaller, lighter design), and partly because the fin structure is simpler. The DC-9 also incorporated rear-airstairs, as did some 727s, because early aircraft were still operating into small regional airfields.
Because civil aircraft payload-range is critically dependent on the weight that can be lifted from hot and high airfields, take-off and landing performance is important. Landing performance is strongly dependent on the approach speed, which determines the kinetic energy at touch down, after which, it’s all down to lift dumpers, brakes, and so on, to determine the maximum weight for safe landing operations at the worst-case design airfield. The approach speed is normally required to be no less than 1.3 x the demonstrated minimum control speed of the aircraft in the landing configuration, and this will be determined by the wing area, landing weight and high-lift performance of the wing.
From this, one can see why a clean wing with good high-lift devices might be a good idea, and lead to a T-tail design. However, with the BAC 1-11 (and later the Javelin among others), one problem arose with a T-tail design. Determining the minimum control speed requires the aircraft to be flown as slowly as possible, without stalling, and this has to be demonstrated i.e. actually flown, not established using some aerodynamic model.
This turns out to be a highly marginal exercise. As the aircraft slows down and is held in level flight by increased power and increased incidence, there may come a point where the T-tail moves into the disturbed airflow over the wing, and if this happens the aircraft can enter an irrecoverable condition known as a ‘deep stall’. In a ‘deep stall’, the tailplane and elevator lose their effectiveness, and it may be impossible to reduce incidence to recover.
This happened in the flight testing of the BAC 1-11, leading to the loss of G-ASHG in October 1963; this also occurred with Trident G-ARPY in June 1966, and had previously occurred with Javelin WD808 back in June 1953. Consequently, T-tail configured aircraft are often fitted with a stick-pusher, triggered if the aircraft is at too high an incidence to the airflow. Normal air traffic operations should not take aircraft anywhere near this condition, but the systems are there as a protection, just in case.
A Hawker-Siddeley test engineer I worked with in the 5m wind tunnel had flown as an observer on some of the Trident minimum control speed tests. He very much admired the flying skills of the Company test pilot John ‘Cats-Eyes’ Cunningham, but hated being on board when he was flying, because he could, and did, fly the aircraft 5knots slower than anyone else. The whole experience was heightened by being able to observed the stalled airflow over the wings, as they were covered with wool tufts and filmed as part of the test process.
As a quick aside, take-off performance is also important for civil aircraft design. The accelerate-stop rejected take-off case will be important in lift dumper, brakes, thrust reverser and undercarriage design. From the aerodynamic perspective, the required climb gradient after the loss of one engine is generally more important, as it is likely to determine propulsion system requirements. The requirement to achieve a certain climb gradient (generally 3%) with one engine failed is the main reason why twin-engine aircraft are seen to climb much more steeply than four-engine aircraft after take-off. They have to demonstrate the climb gradient requirement on 50% power, not the 75% available to four-engine aircraft, and consequently can achieve a much greater climb gradient when all engines are operating.
The Royal Aircraft Establishment and the Lightning
At the time of the BAC 1-11, Trident and Javelin deep stall accidents, and before that the Comet take-off accidents and structural failures, the RAE was deeply involved in assisting British Industry to understand all aspects of advanced aircraft design, and assisting with resolving issues, whether they be related to handling qualities, performance shortfalls or accidents. The RAE’s main base and low-speed aerodynamic research facilities were at Farnborough in Hampshire, and this location was also shared with the RAF Institute of Aviation Medicine, and the Air Accident Investigation Branch. The RAE was directly involved with resolving the problems of the Comet, and would also have assisted with the investigation of the BAC 1-11, Trident, and, possibly, Javelin accidents. They also, as indicated earlier, were directly involved with the understanding of high-lift system design.
In the design of the Lightning, they were concerned to ensure stability and control of this ‘notched delta’ design would meet requirements, and the Shorts SB5 experimental aircraft was designed to investigate low-speed handling. The SB5 could be configured with three different wing-sweep angles, and could have either a T-tail or low mounted tailplane. Testing showed that the low-mounted tailplane recommended by English Electric for the Lightning was preferable. I have not seen anything definitive on why the T-tail was not, but would assume that interaction with the wing leading edge vortices and/or the wing /fuselage flows more generally, would have driven undesirable handling characteristics.
The Javelin – why a T-tail. Well, like almost all weird design outcomes, the answer lies in the specified requirements. The requirements for the Javelin, in addition to the main requirements for a two-seat all weather fighter, required that the aircraft take-off from runways no more than 4500 ft in length. To quote Wikipedia:
“The specification called for a two-seat night fighter, that would intercept enemy aircraft at heights of up to at least 40,000 feet. It would also have to reach a maximum speed of 525 kn at this height, be able to perform rapid ascents and attain an altitude of 45,000 feet within ten minutes of engine ignition.”
Additional criteria given in the requirement included a minimum flight endurance of two hours, a takeoff distance of 1,500 yards, structural strength to support up to 4g manoeuvres at high speed and for the aircraft to incorporate airborne interception radar, multi-channel VHF radio and various navigational aids.”
Given the payload and endurance requirements, the Javelin was always going to be a large aircraft. I assume Gloster selected a delta wing to allow large wing area (to meet altitude performance requirements), and to provide significant internal volume (to meet endurance requirements). Operation off a short runway (4500 ft is just over half the normal NATO runway of 7000 ft – and many are longer than that) would require some form of high lift system, something that is not normally possible on a pure delta, because of the difficulty of trimming the aircraft once flaps are deployed. The relatively thick wing section was probably also selected to allow a slower approach speed for landing.
Without today’s computer technology, an unstable canard solution would not have been available, and Tejas-like LEVCONs had not been invented. So, a tailplane and elevators were used, freeing up the wing trailing edge to allow large flaps and airbrakes to meet the take-off and landing requirement, as well as ailerons for roll control. In the end, that ‘take-off distance of 1500 yards’, or more importantly, the implicit landing on to the same runway, drove the whole design. Another good example of the importance of getting the requirements right in the first place.
Who knows, without that requirement, and with a thin wing, the UK might have had a Mirage-like world-beater in the mid-fifties. Realistically, though, that would not have happened, because the big radar, two seats, the high endurance requirement, and lack of knowledge of area rule, and of supersonic intake design, would all have worked against such a concept.
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