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Conventional short take-off and landing aircraft; could they be really on their way out?

Conventional Short Take-Off and Landing Aircraft

Could They Be Really on Their Way Out?

When considering the problem of short take-off and landing aircraft (called STOL for the sake of convenience), two main questions arise. In the first place what criteria apply to planes in the STOL category? In the second place, when taking a look at the list of aircraft of this type currently available on the market and comparing it with the much wider choice offered at the turn of the 1960s and 70s, one may well ask whether in the event of a conflict the armed forces of the West will really be capable of moving around their troops and equipment in sufficient numbers and with sufficient speed.

The best way to illustrate what makes an aircraft a STOL aircraft is to compare the rolling take-off runs of two planes that perform totally different tasks but that have more or less the same maximum take-off weight. Obviously one cannot take passenger capacity as a criterion when comparing the two since commercial airliners and transport planes usually have very different fuselage configurations. The ATR 72, an up-to-date aircraft jointly developed by the French company Aerospatiale and the Italian aircraft manufacturer Aeritalia, has a maximum take-off weight of about 20 tonnes. It requires about 1250 metres to lift off from the runway at this weight. The de Havilland Canada DHC-5 Buffalo, an ancient machine whose original version made its maiden flight in 1964, lifts its 22 tonnes weight with the greatest of ease after rolling a mere 700 metres. This gives one a very rough idea of what makes a STOL aircraft different from a normal aircraft. Of course this is not all. If one requires a STOL aircraft at all, generally speaking it is because one wants to be able to operate in difficult conditions where landing strips are scarce or totally non-existent. Added to which a short takeoff distance usually means that the airfield is surrounded by obstacles. More often than not these are trees, which is why STOL aircraft designers always lay down a second take-off distance, according to the so-called "15 metres" rule of thumb. In the case of the dear old Buffalo this is 838 metres, meaning that 838 metres after releasing the brakes the aircraft has already attained a height of 15 metres above ground level (given an identical maximum take-off weight). Dedicated short take-off and landing aircraft in the 15 to 20-tonne category are usually tasked for the transport of troops and equipment in special deployment missions and are very seldom (only on rare occasions) used as airliners. The reason is quite simple: if there is a call for a regular service for 40-50 passengers a proper runway and passenger-handling organisation will be required, and the particular features that distinguish the STOL aircraft (and which incidentally increase its running costs) will no longer be necessary. STOL aircraft are thus meant more for the "utility" aspect of air transport. This partly explains why they have more or less the same lines - a squat figure with a turned-up tail for ease of loading via a rear ramp and a pot belly to permit easy access to the floor of the aircraft.

Another feature that STOL aircraft have in common is that they are high wing planes. For an aircraft to be able to take off in a shorter distance than another of the same weight one needs to resort to some sort of artifice. In the first place the aircraft will need more powerful engines (although not necessarily, as will be seen later, to increase the rolling speed) and, more important still, very special wings. It would be a truism to say that in order to lift off a greater mass it would surely be enough to increase the lifting surface. The trouble in aerodynamics is that any increase in the lifting surface is penalised by increased drag, which is quite unacceptable at cruising speed. In order to get round this problem, the length of the chord of the wing (the distance between the leading and trailing edges) and the camber are artificially increased by means of retractable flaps. These devices, which are present on the majority of modern aircraft but are relatively larger on the STOL type, are called high-lift flaps or devices. Once the aircraft has lifted off, thanks to the increased lift they give it and due to the gathering airspeed, they are retracted inside the wing. These particular flaps have evolved over the years and are now called slats because they extend in such a way as to create open spaces between the wing and the actual flap (or flaps since these are extendable) which enable the overall lift to be augmented even more as it allows a proportion of the air on the forward section of the underwing surface to flow over the upper surface of the next section. Having at their disposal this sort of aerodynamic aid the experts soon realised that the airflow over the wing surface could be further increased artificially by making use of the propeller race. This gave birth to the concept of the blown wing.

This of course postulates very large diameter propellers, which means in turn mounting the wings fairly high up - yet another of the distinguishing features of nearly all short take-off aircraft. Mutatis mutandis, a short landing is naturally facilitated by the slow speed of approach permitted by the above-mentioned flaps, added to the sharp deceleration once the plane has touched down due to the fact that the propellers' pitch can be reversed. On some of the planes this deceleration is so spectacular that a number of people (including television crews) filming the landing of a Pilatus PC-6, for example, during an air show have been caught unawares by its sudden disappearance from their viewer due to their failure to pan more slowly.

Although the concept has not been seriously pursued in series production, it is perhaps worth mentioning three innovative jet-powered STOL aircraft types. These are the quadrijet Douglas YC-15, the twin-jet Boeing YC-14 and the twin jet Antonov An-72. The Douglas first flew on 26 August 1976 and created a sensation at the 1977 Farnborough Show and the Salon du Bourget the following year. This mammoth plane, whose four JT-8D/17s expelled their gases over titanium flaps, could take off and land in a distance of 300 m. The YC-15 had been designed as a replacement for the C-135 Hercules, but in spite of its astonishing performance and even the adoption by way of an experiment of a high-bypass CFM-56 turbojet, it was not selected by the US Department of Defense. The Soviet twin jet for its part was designed to replace the AN-26. Two prototypes of the AN-72 were built and its maiden flight took place on 22 December 1977. The engines are mounted well forward of the leading edge and high up on the wing so that all the exhaust flow passes over the upper wing surface. This has necessitated the use of a titanium sheathing for the wing as well as for the high lift flaps. The latter, which posses double slats in the line of the flow and triple slats on the outer wing surface, can be deflected downwards by 60 [degrees]. Over and above this, a very large portion of the upper engine cowling acts as a thrust reverser so as to ensure efficient braking on landing, as does reversing the pitch of the propellers on turboprop-powered aircraft. With a payload of 3 500 kg (the maximum payload being 10 tonnes), the An-72 lifts off after rolling a distance of 470 metres under the 63.74 kN thrust of each of its Lotarev D-36 engines. In 1984 the Russians unveiled an improved version of the plane, the An-74, with a longer fuselage and a considerably larger wing.

Not to be outdone the Japanese have also developed a blown wing jet STOL. They have modified the airframe of the twin jet Kawasaki C-1 and mounted four jets on top of the wing.

With the exception of the JATO systems (Jet-Assisted Take-Off) - in which case the "L" of STOL is dropped - there are at present two other ways of shortening the take-off distance of STOL aircraft over and above those mentioned earlier. One is the vectored thrust jet engine, typified by the British Aerospace Harrier, of which a detailed account appears in this issue, and the other is the tilt-rotor (or tilt-prop) system. In point of fact the latter is akin to a prop-powered version of the former. Although used more often nowadays as a STOL aircraft, the Harrier more properly belongs to the VTOL category (Vertical Take-Off and Landing). However, the United States are currently endeavouring to develop a supersonic vectored thrust fighter aircraft with the object of giving it exclusively STOL performances combined with greater manoeuvrability in flight. Pratt & Whitney and General Electric are both in the running for the engine contract, the former with its F-100-PW-220, now being flight tested on a F-15, and the latter with its F-120, whose initial flight trials are due to begin in the early part of next year. In both cases, and in answer to the performance specifications, the prop flow can be deflected 20 [degrees] both upwards and downwards in the space of one second. With their nozzles pointed downwards both engines should be able to reduce the take-off distance by about 25%.

Of the two questions raised at the beginning of the article, one still remains unanswered. Indeed, one may well ask oneself why it is that today's armed forces appear to have lost interest in genuinely short take-off and landing transport aircraft. There is no denying the fact that the number of new designs available has definitely shrunk over the last few years. Even the splendid tilt-rotor V-22 Osprey very nearly fell a victim to this trend, although it now appears to have been salvaged by Congress. The answer no doubt lies at the root in the tremendous development of helicopter technology, a development which has gone on uninterruptedly since the seventies.

In the old days helicopters were horribly expensive not only to buy but to operate and maintain. Not only this, their maintenance required the setting up of special facilities. Nowadays, however, industry is in a position to offer helicopters with a decent payload that can not only offload men and equipment more rapidly than STOL aircraft (they only remain stationary on the ground for a very short time and can take off immediately) but that can deliver them to totally inaccessible places, equally inaccessible, one presumes, to the enemy. It should not be forgotten that western defence equipment is mainly designed for use in Europe. In addition, the idea is not to come too close to the enemy with slow-flying planes because of the danger posed by the latest generation of MANPADS. The Stingers in Afghanistan showed what these were capable of against Soviet aircraft.

Does this mean the end of the STOL aircraft? By no means. Not at any rate as far as the smaller ones are concerned since they are still less expensive than helicopters and can be used for a vast number of logistic tasks, i.e. in places where by definition there is already a defence structure in place. They can thus release helicopters for more delicate missions and moreover deliver the necessary ground equipment to prepare airstrips for the really heavy cargo planes.

In Africa and South America the situation is slightly different. In these regions the helicopter is much affected by the heat, the altitude and of course the lack of cash. Here the STOL aircraft can profit from its flying qualities and relatively low cost to overcome these disadvantages.

It would be difficult to list all the present-day aircraft that fall in the STOL category or that lay claim to belonging to it. We shall therefore confine ourselves to singling out, somewhat arbitrarily it is true, a few types that belong to very different categories. Some of these are no longer produced but are, and will for a long time remain, essential for the transport of men and materials. Others are mainly used as civilian commuter aircraft but would be bound to be roped in for military service in case of need. The planes are listed by manufacturer in alphabetical order.

* Aeritalia.

The Italian firm of Aeritalia resulted from the merger in 1972 between Fiat's aeronautical division (whose origins date back to the beginning of the century) and IRI-Finmeccanica. Its latest developments are the G-91 fighter and the G222. * G222. This STOL-type aircraft designed by Fiat made its maiden flight in 1970. The Italian air force took delivery of the first of a batch of 44 planes in 1978, but in fact the first series-produced G222 was delivered to Dubai (UAE) in 1976. More than 70 of these G222s of 14.6 tonnes unladen weight and fitted with a pressurized cabin were ordered by various countries, although it would seem that Italy is the only western nation to possess any. Libya, which ordered a score, had the privilege of receiving a special version: as the standard 2 530 kW General Electric T64-GE-4PD engine driving a three-blade reversible propeller came under the US embargo against the Ghadaffi regime, Aeritalia got round the problem by adopting the 3 620 k W Rolls-Royce Tyne. The 222T is immediately recognisable by its four-blade (and also reversible pitch) propellers. The G222, which can take off and land from unprepared airstrips, lifts its 28-tonne payload in a distance of only 660 metres (1 000 metres to a height of 15 metres). The "T" model lifts one more tonne over a rolling distance of 630 metres. Its size enables it to carry 53 fully equipped troops or 42 para-troops. A rear ramp allows the loading of vehicles. An APU feeds the airconditioning system and powers the loading equipment while the aircraft is on the ground.

* Bromon Aircraft

Totally unknown a few years ago as an aircraft manufacturer, Bromon Aircraft of Las Vegas has embarked on the ambitious project of developing a new STOL aircraft from scratch. The manufacturers are aiming at the most undeveloped areas of the world where maintenance facilities are practically non-existent. Bromon also says that its BR-2000 almost perfectly matches a future US Air Force requirement. * The BR-2000 has been deliberately designed as a totally unsophisticated aircraft, at least as far as materials and systems are concerned. The prime aim of the manufacturers being to guarantee the best possible mix of excellent performance and ease of maintenance, hydraulic systems, for example, are kept down to a bare minimum and the unpressurised aircraft is of conventional all-metal construction. In its civil 46-passenger guise, the BR-2000 can be converted in less than one hour into a cargo transport capable of carrying three "A"-size containers. The military version, thanks to its rear loading ramp, can accommodate two HMMWV vehicles or 36 combat troops, or two spare engines for an F-15. Maximum take-off weight is 14 300 kg with a maximum payload of 5 765 kg -- a weight it is designed to lift after a 580-metre run under the 1 750 horse-power of its two General Electric CT7-9B turboprop engines. Compared with earlier plans, the BR-2000 programme has slipped by more than a year, but the manufacturer now schedules to get the first of five planned prototypes airborne towards the beginning of next year and to obtain FAA certification by early 1991.

* CASA.

Construcciones Aeronauticas SA first started to build airplanes under licence in 1923, among which Junkers, Buckers and Heinkels, and more recently BO 105 helicopters. The Spanish firm's first practical "home-made" development was the 212 Aviocar STOL aircraft, which was followed by the C-101. * CASA 212. The very least one can say is that the Aviocar has been an unqualified success story. The 120th unit rolled off the assembly line six years after its maiden flight in 1971, and Indonesia started up its own production line. Today, counting all its versions, sales have reached the total of over 400 units. The light, twin-engine plane has a rear loading ramp. The Series 200 version (from which were derived ASW versions) is powered by two four-blade reversible pitch propeller Garrett TPE331-10R-511 engines developing each 670 kW. It can carry 23 fully equipped paratroops. Its unladen weight is 3 780 kg: fully laden it lifts 7 450 kg over a distance of 440 metres (650 metres to a height of 15 metres). It can land within an astonishing 200 metres.

* de Havilland Canada.

A daughter company of de Havilland's in the UK, this Canadian firm was founded in 1928 in Ontario. In 1986 de Havilland Canada as it was called became a division of Boeing of Canada. Even so, for as long as it perpetuated the memory of its founder George de Havilland, the firm will remain inextricably associated with one of the most illustrious families of famous aircraft such as the Tiger Moth, Mosquito, Vampire and Comet. * DHC-5. A whole issue of Armada International would not suffice to encompass a detailed description of all the many versions of the remarkable Buffalo. The DHC-5A Buffalo was originally built for the US Army and Canadian armed forces, but it was also ordered by the Brazilian and Peruvian air forces. Its most recent version is the DHC-5D, which is powered by two 2 336 kW General Electric CT64 turboprops and has been sold to a very large number of countries in Africa, the Arabian Gulf and Latin America. Initial deliveries started in 1976 but, in contrast to the previous versions, the 5D found no takers among the armed forces of the West, thereby signalling the first sign of the latter's growing lack of interest in this type of aircraft. Even so, in those countries in which it operates, it offers a virtually irreplaceable solution to the problem of transporting men and equipment in difficult, hot and/or high altitude regions. The Egyptian DHC-5Ds are equipped with a clever system, LAPES, which enables them to drop loads of up to 2 250 kg strapped to sleds without landing and with the utmost precision. It works in the following way. The rear loading ramp is opened and a small parachute of about 4.5 metres in diameter attached to the load, which is still lashed down to the aircraft, is deployed, slowing down the plane. The pilot then comes down to 1.5 or 2 metres above ground level, at which moment the parachute extracts a second and even larger parachute that quite literally plucks the load out of the aircraft. The load then slides to a halt on the ground on its skids.

Unladen the DHC-5D weighs about 11 1/2 tonnes. In offensive action, taking off from an unprepared strip and with a take-off weight of 17 tonnes, the Buffalo lifts off in 290 metres (346 metres to a height of 15 metres). In a transport operation and using a prepared strip, the maximum take-off can be augmented to nearly 20 tonnes, and the take-off distances are 700 and 840 metres respectively. Landing distances with the different payloads are respectively 170 metres and 260 metres.

The Buffalo, of which some 120 units were sold, is no longer being built. * DHC-6. Called the Twin Otter, this aircraft has a maximum take-off weight of 5.67 tonnes and what makes it famous is its enormous success. The DHC-6 STOL aircraft has been sold to the tune of 825 units all over the world, including the People's Republic of China. Granted, this figure includes a large number of civilian and governmental users (for aerial mapping, surveillance of forests and lakes, etc.), but it was adopted in 1976 by the the US Army's Alaska Army National Guard for supply missions. The latest version of the DHC-6, called Twin Otter Series 300, is powered by 462 kW Pratt & Whitney Canada PT6As. Depending on its weight it takes off in a distance of 210 or 260 metres. It carries 20 passengers but its small dimensions do not warrant a rear ramp: however, a wide twin-door loading bay permits the loading of bulky equipment. * DHC-7. More commonly known as the Dash Seven and in the main used as a commercial liner, this four-engined aircraft never fails to surprise the spectators at air shows and is worth a mention because of its STOL characteristics and its great success. Ten years after its maiden flight in 1975, and when the world economy was not in the best of shapes, de Havilland Canada delivered its hundredth Dash Seven. Wardair Canada operates it in the very harshest climates of the Far North. Rocky Mountains lands it on airstrips at 3 000 metres altitude and its serviceability rate is 99%. With a maximum take-off weight of 18.6 tonnes the 50-passenger aircraft lifts off (at sea level) in 690 metres. A wide side door can be fitted for awkward loads.

* Israel Aircraft Industries.

Founded in 1952, IAI has now grown into a multi-division enterprise employing more than 20 000 people. IAI is renowned as the constructor of the Kfir fighter aircraft, the Westwind and the Arava STOL. The cancellation of the Lavi project came as a stunning blow, but the Israeli firm remains a major subcontractor in a number of civilian and military programmes. * Arava. A small dimension STOL aircraft (about four tonnes unladen weight), the Arava distinguishes itself from its contemporaries by a squat and somewhat peculiar body due to its twin-boom configuration. This design allows one to open the whole of the rear cone of the fuselage and to make full use of the interior section, the empennage being mounted on the tips of the booms. The Arava 102 carries 16 paratroopers in addition to two cabin crew. In the case of the extended 202 version (90 cm longer) these figures are 20 and 3 respectively. As with other aircraft of this type, its favourite customers are the African and Latin American countries. Curiously enough, these countries ordered it before the Israeli forces did (apart from three planes leased in 1973 by the IAF): it was only in 1984 that they put it into service, a good dozen years after its maiden flight. Up to now IAI has produced about one hundred units. With its maximum take-off weight of 6.8 tonnes the 201 lifts off after rolling 298 metres and is 15 metres above ground after 463 metres. With the same weight it lands in 250 metres.

* Pilatus.

Founded in 1939 the Swiss Pilatus Flugzeugwerke came to notice in the STOL field shortly after the war by developing the SB-2 Pelican. Fitted with flaps that considerably increase the camber of the wings, it can take off in 110 metres. However, it is the PC-6 Porter, followed by the PC-6A Turbo Porter, that have made Pilatus famous. * PC-6A Turbo Porter. For any who have seen it performing during demonstration flights (when aircraft carry a minimum payload), the Turbo Porter must surely typify what one expects from a STOL aircraft: indeed, it appears to be catapulted upwards by an invisible spring. With a take-off weight of 2 200 kg, the Swiss plane, weighing only 1 200 kg empty, takes off in a distance of only 110 metres. Noted for such exploits as being able to land on glaciers or snow-covered slopes when fitted with skis, the PC-6A (passenger capacity: 7 persons) quickly won worldwide renown. It has even been license-produced by Fairchild for the US Army under the designation UV-20A Chiricahua.

* Short Brothers

Short Brothers is another one of those firms whose lifespan coincides with that of aviation. The Irish company, recently taken over by Bombardier of Canada, is now making its mark as a specialist in the production of missiles and civil and military short take-off and landing transport planes. * Shorts SC.7 Skyvan 3M. This aircraft has a very characteristic silhouette owing to the square cross-section of its fuselage which permits the fullest possible use of the available stowage space in the cabin (and which has earned it the nickname of flying breadbox). The Skyvan, which first flew in 1970, has a rear door which can be opened in flight for the dropping of palletized stores. Some of the aircraft have in addition been equipped with the Rolamat roller conveyor system for shifting stores around. The twin turboprop (two 533 kW Garrett TPE 331-2-201A) lifts its payload of 6 125 kg after a run of 240 metres and comes to a halt on landing in 210 metres. Its favourite customers are all in Africa, Latin America and South-East Asia. * Shorts 330 UTT. A military version of the 330 short-haul transport aircraft, itself derived from the Skyvan, the UTT (Utility Tactical Transport) made its appearance in 1982. It takes off over a distance of 415 metres.

PHOTO : The G222 is offered in a number special duty versions like the fire-fighting G222AMA

PHOTO : (above), the electronic warfare G222VS, and the navaid calibration G222RM.

PHOTO : The American jet-powered STOL project was dropped in the late 1970s, but the technology

PHOTO : developed by McDonnell Douglas for the YC-15 will be used in the C-17.

PHOTO : The Bromon BR-2000 is a newcomer in the STOL transport clan. Its key features are

PHOTO : ruggedness and ease of maintenance.

PHOTO : CASA's first in-house development, the C-212 Aviocar paved the Spanish manufacturer's way

PHOTO : for a more ambitious project - the CN-235 developed with Nurtanio.

PHOTO : Son of the de Havilland Caribou, the DH-5 Buffalo enjoys an equally remarkable career as a

PHOTO : workhorse around the world.

PHOTO : The early 1970s IAI Arava has sold quite successfully in Latin America, but joined the

PHOTO : Israeli Air Force as late as 1984.

PHOTO : The list of duties performed by the Pilatus PC-6 Turbo-Porter is beyond belief. This

PHOTO : demonstrator is equipped with a weather radar and long-range underwing fuel tanks.

PHOTO : Production of the Shorts Skyvan (above) has now ceased, but has given way to that of the

PHOTO : larger Shorts 330, 330-UTT utility transport and US Air Force C-23A Sherpa.

PHOTO : To Bell and Boeing (and to many others), the V-22 Osprey is the way to go now when large

PHOTO : playloads have to transported rapidly to the most remote or inaccessible areas.
COPYRIGHT 1989 Armada International
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Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Biass, Eric H.
Publication:Armada International
Date:Oct 1, 1989
Words:4391
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