Vertical Take â€œ off and Landing
Twenty years after ever jet flight took place, vertical takeoff and landing (VTOL) emerged as a concept, which was soon to be successfully implemented. Dr A.A Griffith, in 1941 who was then the chief engineer of Rolls Royce ltd. put forward this idea in his paper to the aeronautical research council. He proposed the idea of using small lift engines for lift only and not for normal flight. He claimed that there was great advantage in using a wing configuration and propulsion system without reference to take off and landing. This was vertical take off and landing not for its own sake but as a means to an end being a more efficient aircraft.
The 60â„¢s and 70â„¢s saw the most exciting phase of development in VTOL where in the five important criteria of high thrust\weight ratio , low efflux velocity , low fuel consumption and low machinery noise were successfully dealt with.
Rolls Royce has been working on VTOL power plants for more than 35 years. A large fund of practical knowledge on the installation and operation of a variety of VTOL power plants has been accumulated in the form of experimental aircrafts and test rigs.
TYPES OF VTOL
The various possibilities of facilitating VTOL using jet lift are as follows.
1. WHOLE AIRCRAFT IS TILTED: also referred to as vertical attitude take off and landing, the aircraft is made to takeoff in a vertical position. After gaining sufficient altitude the aircraft flattens out to a normal horizontal flight. This seemed to be the most natural idea and it has its share of advantages and disadvantages. The advantage lays in the fact that no movable wings or propellers are needed. On the other hand complicated devices for launching were needed. Great thrust too was required at no forward speed. Thus if engine failure occurred at an initial stage there was no chance of making a safe landing.
2. ENGINES OR PROPELLERS ALONE ARE TILTED: This design did away with the earlier design complexities but it too had to content with various challenges. The location of the engine posed a great difficulty. The condition was to locate the engines in such away that the propellers slipstream or jet efflux would clear the aircraft for all positions of the engines
Propellers Alone are Tilted
3. SLIPSTREAM OR JET EFFLUX IS DEFLECTED: The propellers produce thrust by forcing the air backwards and the stream of air which flows over the fuselage, tail unit and other parts of the aircraft. In case of jet powered aircrafts instead of a slipstream a jet efflux is present. Thus in these cases the slipstream or jet efflux which are normally in the horizontal direction are deflected in the downward direction to obtain vertical lift. The HARRIER aircraft is a classic example of an aircraft, which employs swiveling nozzles to deflect the issuing jet efflux.
4. TWO COMPLETELY DIFFERENT SETS OF ENGINES: As this implies two different sets of engines are employed. Besides the engines\engines for normal flight several engines solely for lift are used. The BALZAC is an aircraft, which employs this system. The main disadvantage is the weight penalty involved. During vertical flight the horizontal engines are useless dead weight and during horizontal flight the other way round.
MERITS OF VTOL:
Jet lift refers to the lifting system employed in all aircrafts capable of VTOL. The term jet is taken to include not only pure jet engines but also bypass and turbo fan engines. The merit of a lifting system can be mathematically given as
Thrust/ (Engine weight + Fuel weight)
For propulsion engines of long-range aircrafts fuel weight is eight times the engine weight. Since the emphasis is on reducing fuel consumption for the same effectiveness there is an increase in engine weight. For lift engines engine weight is three or four times the fuel weight. Since the emphasis here is on lightweight engines an increase in fuel consumption is acceptable.
The other ideas of VTOL include the use of rockets, which on account of high fuel conception was rejected, remotely driven fans, retractable helicopter rotors which remains an engineerâ„¢s ultimate dream.
The history of flight by jet lift goes back to July 53, when Rolls Royce Ëœflying bed steadâ„¢ first became air born. Two engines where placed back to back with the pilot seated above them. Air bled from the engine compressor was fed to four downward pointing nozzles which, where used to control in pitch roll and yaw. Mean while, in France SNEMA developed the ATAR VOLANT. It had an advanced system of auto stabilization and a very neat form of pneumatic deflection of the jet efflux to give control above the horizontal axis.
This led to the first VTOL aircraft the COLEOPTRE, which made its maiden flight in, may 59 and was unfortunately lost in the same year.
The tail-sitting configuration was also used in ËœRyan X 13â„¢. X 13 was a single engine delta aircraft, which was the first aircraft to make a successful transition from jet born hovering to wing born flight and back to hovering and landing.
It was soon followed by Bell X 14, which was a conventional aircraft with a horizontal fuselage attitude and employing a jet pope deflection on its two horizontal engines. In September 1961 the hawker p1127 carried out its first transitions both with and without auto stabilization. It used a single ËœBristol-siddelyâ„¢ Pegasus engine for lift and propulsion. The hawker was the forerunner to the famous harrier. Thus light aircrafts could be made fully controllable throughout the sequence of vertical take off, transition to normal flying and back to vertical landing.
POWER PLANTS USED
LIFT JETS: These power plants are used primarily to produce vertical thrust. The advantage lays in the fact that it is extremely simple in design. Lightweight is another feature that works in its favor. The simple design is possible because the engine operated only for limited periods.
The RB 132, which was the first design prototype from Rolls Royce, developed a thrust of 6000 pounds at a thrust to weight ratio of 16:1. Lift jets are extremely versatile and are used in a variety of airframe configuration. The engine is very adaptable and can operate in a horizontal and vertical position; it can be installed to swivel through large angles. To reduce ground erosion the normal exhaust nozzle can be replaced by a multi lobbed nozzle.
LIFT TURBOFANS: Although lift jets and the vectored thrust (described later) engines were found to be practical and suitable for forming the basis operational VTOL power plant systems is the lift turbo fans which has the inherent advantage of lower efflux velocity and lower jet noise. In the adjoining figure examples of direct gas driven lift turbo fans using a basic lift jet as the gas generator is shown. The lift jet may drive either a rear fan or a front fan where a reduction gear may or may not be used.
In the figure propulsion engine is used to supply gas compressed air or shaft power to remote fans for lift. A range of lift turbo fans, which are self-contained are derived by adding extra turbine stages to drive a fan on the front of the basic jet engine. A range of engines is considered with by pass ratio ranging from 1-5. Bypass ratio is defined as the ratio of flow through the by pass duct to the flow through the main engine.
LIFT FANS Vs LIFT JETS: The pure jet engine has the advantage it responds quickly to throttle movement. The fans with its greater inertia take a longer time to speed up and further more unlikely to the jet it gets no immediate response to the change in temperature before the rotational speed has time to change.
There is however an appreciable reduction in fuel consumption as compared to lift jets. However there are serious off setting advantages such as increased installed bulk weight and cruise drag. Also the jet noise of the lift fans can be much lower than that of lift jets, but the advantage is lost when fan blade noise and other noises generated by the machinery is taken into account. The design of the fan is thus dictated by noise considerations.
In the final analysis it is found that the ultimate lift power plant is one, which will have to be, integrated with the airframe to a considerable effect to effect reduction in airframe structure and power plant bulk weight.
LIFT PROPULSION ENGINES: This dual-purpose lift and propulsion engines offers one of the neatest and simplest ways of achieving vertical take off and landing. With its swiveling nozzles it provides means of changing from jet lift to wing lift and of controlling the retardation on approach. These engines operate on the principles of vectored thrust in which the exhaust stream is deflected 90 degrees by swiveling nozzles and cascade vanes so that the thrust vector passes through the center of gravity of the aircraft. The directional change in thrust is obtained either by a deflector system consisting of four swiveling nozzles or by a switch in deflector system as shown in the figure.
The switch in deflector system consists of a heavily reinforced door, which lies flush with the jet pipe wall when the engine is operating in forward thrust. When deflected thrust is selected, the door moves to blank off the conventional propelling nozzle, thus diverting the exhaust gases so enabling the intermediate positions to be selected. Swiveling nozzle deflector system is used in by pass engines as mentioned in operational VTOL military strike aircraft. The nozzles can be rotated through 98.5 degrees.
On some engine installations engine thrust is augmented by a method known as plenum chamber burning (PCB). Plenum chamber burning is a method of boosting engine thrust by burning fuel in a fan air stream in a combustion chamber or plenum chamber immediately forward of the fan air, thus boosting the thrust for takeoff and landing, transonic acceleration and super sonic flight. The advantage of plenum chamber burning is that it makes possible the use of smaller engines and improves the matching of engine performance to aircraft high-speed requirements. This result in an appreciable saving in power plant weight and gives an increased range or pay load capacity.
There are also various other ways to increase the thrust developed by the aircraft. One such way is by using thrust augmenters. The fan engine is one of the forms of thrust augmentors which by increasing the mass flow of air increase the thrust and reduces the efflux velocity. Another form which uses the principle
of air ejectors was developed in France and in USA. In the fuselage are fitted jet diverter valves, which in normal position allows straight through passage of the jet exhaust. For take off and landing the diverter passes whole of the jet gases into the return flow ducts which are mounted side by side along the fuselage center line. Branch nozzle pipes from these ducts direct the flow downwards and so entrap the secondary air through a large opening in the top of the fuselage and out through a similar opening at the bottom. The fuselage center is therefore in the form of a mixing chamber.
The claims made for thrust augmentors are that it saves fuel, generates less noise and causes less ground erosion as compared to the pure jet lift. However it is not as easy as one might thing to augment the thrust without a proportionate increase in installed weight. A large mass means a large frontal area of the lifting system and so by increasing the drag may result in greater fuel conception and so it is a disadvantage.
A turbofan engine is the most modern variation of the basic gas turbine engine. As with other gas turbines, there is a core engine, whose parts and operation are discussed on a separate page. In the turbofan engine, the core engine is surrounded by a fan in the front and an additional turbine at the rear. The fan and fan turbine are composed of many blades, like the core compressor and core turbine, and are connected to an additional shaft. All of this additional turbo machinery is colored green on the schematic. As with the core compressor and turbine, some of the fan blades turn with the shaft and some blades remain stationary. The fan shaft passes through the core shaft for mechanical reasons. This type of arrangement is called a two-spool engine (one "spool" for the fan, one "spool" for the core.) Some advanced engines have additional spools for even higher efficiency.
How does a turbofan engine work? The incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the core compressor and then the burner, where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle, as in a basic turbojet. The rest of the incoming air passes through the fan and bypasses, or goes around the engine, just like the air through a propeller. The air that goes through the fan has a velocity that is slightly increased from free stream. So a turbofan gets some of its thrust from the core and some of its thrust from the fan. The ratio of the air that goes around the engine to the air that goes through the core is called the bypass ratio.
Because the fuel flow rate for the core is changed only a small amount by the addition of the fan, a turbofan generates more thrust for nearly the same amount of fuel used by the core. This means that a turbofan is very fuel-efficient. In fact, high bypass ratio turbofans are nearly as fuel efficient as turboprops. Because the fan is enclosed by the inlet and is composed of many blades, it can operate efficiently at higher speeds than a simple propeller. That is why turbofans are found on high-speed transports and propellers are used on low speed transports. Low bypass ratio turbofans are still more fuel-efficient than basic turbojets. Many modern fighter planes actually use low bypass ratio turbofans equipped with afterburners. They can then cruise efficiently but still have high thrust when dog fighting. Even though the fighter plane can fly much faster than the speed of sound, the air going into the engine must travel less than the speed of sound for high efficiency. Therefore, the airplane inlet slows the air down from supersonic speeds.
The problems of operating the jet lift aircraft have received much attention. There are questions of how best to use jet lift aircraft, logistic and training problem. For operation of VTOL aircraft from fixed basis some design characteristics have to be incorporated. The surface below should be a grillage, which allows the exhaust gases to pass through and then spread truly in all directions except up wind where they are prevented by shutters at the edge of platform. This equipment practically eliminates re-circulation of hot gases into the engine intakes, prevents ground erosion and reduces ground suction. It also protects the tires from the heat or the jet exhaust and so allows the engine to be run for exhausted period when they are being set up or tested. It is therefore well adapted to use at a fixed operating or maintenance base for operation is forward areas the ideal would be to have no ground equipments at all.
Vertical takes off by jet lift have been made from ordinary uncured concrete with no protection. The damage to the concrete was slight and indicated the procedure was acceptable but could not be repeated too often from the same place. Very simple protection in the form of simple metal plates stacked in the grounds immediately below the jets in all that is necessary to prevent erosion and with this protection it is possible to operate from otherwise unprepared sites, although there may be some loss of thrust due to circulation of exhaust gases. Landing presents no problem on concrete or turf but some protection will be needed on softer ground.
As an alternative to metal piece experiments have been made in the USA with the use of plastics to consolidate the ground and this also shows considerable promise. Where a concrete road or even an area of good turf is available the aircraft can be allowed to roll forward at a speed of 15 KMS in which the case tests show that it will cause no recirculation of hot gases.
The Most Successful VTOL aircraft - the HARRIER
The Harrier which was conceived in the late 60 after a few modification over the gases in perhaps the most remarkable aircraft ever built which can surpass any other aircraft in terms of maneuverability.
Need for a VTOL combat aircraft
Most modern jet propelled aircraft have to be a forward speed of at least 195 kilometers/hour before they generate enough lift. This requires long runways. During war times the first intention would be to destroy the runways. Thus the need for an aircraft which did not depend on runways for operating became imperative.
Various designs were put forth starting from the 50's. In the late 50's Hawker Aircrafts Ltd. was looking for an aircraft to succeed the 'Hunter'. All earlier proposals had separate engines for lift and protection. Once lift was achieved, the lift engines were useless dead weight especially in the case of a combat aircraft. In 1957 Bristol siddly engine Corporation offered a developed version of the Bristol Orpheus engine, which was equipped with rotating nozzles. In the control aspect the auto stabilizers needed to be replaced by a manual control system controlled by the pilot. The auto stabilizers needed to be replaced by a manual control system controlled by the pilot. In 1966 after various experimental prototypes the 'Harrier' was induced into the Royal Air force.
Heart of the Harrier:
The heart of the Harrier is the engine and system nozzle which enables vertical take off to be achieved. The Harrier is powered by a Rolls Royce 'Pegasus' engine. Over the years the engine has been continually developed and the latest versions output a thrust of 9525 kg.
Â¢ Wingspan: 9.25 meters - 30 feet, 4 inches
Â¢ Length: 14.12 meters - 46 feet, 4 inches
Â¢ Height 3.55 meters - 11 feet, 7.75 inches
Â¢ Max weight: 13,494 kilograms - 29,750 pounds
Â¢ One 9,979 KN (22,000 lb st) Rolls-Royce Pegasus 11-21 (F402-RR-406) turbofan
Until air starts to flow over the control surfaces they cannot produce any loads to alter the attitude of the aircraft. In the Harrier air can be tapped off from the high-pressure compressor of the engine and connected to nozzles, which are vertical, present at the front and rear of the Fuselage and at the wing tips. The nozzles in the fuselage control the aircraft in pitch and at the wing tips at roll.
Specialty of the nozzles used:
The four nozzles through which the air issuing from the engine is directed are geared together so that they all point in the same direction at the same time. The pilot has a control in the cockpit through which the four nozzles and the mechanism for rotating them divert the thrust in the required direction (thrust vectoring).
Recirculation of Hot gases:
One of the most difficult problems, which had to be solved, was the re-circulation of hot gases issuing from the engines back to intakes. A semicircular design was found to be optimal design, which also takes into account for ground erosion as well.
VTOL when it emerged as a concept way back in 1953 was something much ahead of its times. Although military application seem to be more imperative, much of the future lies in exploring the prospects of using VTOL for Civil application. In this respect we have to look far into the future. It is no longer sufficient that the aircraft should perform well; it must also be economical and really safe. Civil applications as has always been the case follow after some years of development and refined in military services.
A glimpse into what the future holds as far as military aircraft are concerned in indeed very exiting. Advanced supersonic CTOL aircraft will bring the benefits of VTOL to a wider Varity of missions. By the year 2000 they could be replacing types like the F-16 and Harrier-II for offensive support, moving quickly to suppress threatening artillery or missile system engaging enemy support aircraft and hitting armor with multiple warhead ammunition. They will operate with relative immunity from counter air operations.
Advanced electronic system, with their capacity for self-diagnosis and their ability to function after partial failures, will make the VTOL tighter reliable and rugged for off base operation. This sort of technology in search of ultimate performance against high values targets will be adapted to give it quick reaction and high reliability. It will be a tough, independent, 'kick the starter and jump abroad fighting machine.
1. Aerodynamics for Engineering Students â€œ Hought and Brock
2. Fundamentals of Aerodynamics â€œ Anderson
3. Mechanics of Flight â€œ A.C. Kermode
4. http://www.vtol.org.lockheed propulsion system.htm
5. SEMINAR TOPIC FROM :: http://www.edufive.com/seminarstopics.html