by on March 15, 2012 Leave a Comment

How Vertical Takeoff and Landing works in an Aircraft

VERTICAL-TAKEOFF-AND-LANDING AIRCRAFT (VTOL)
Aircraft capable of vertical takeoff and landing (usually abbreviated as VTOL) have important military and civil potentialities and are under active development by aircraft firms in a number of countries. One general advantage claimed for these aircraft is their greater safety in that the hazards associated with conventional takeoff and landing are eliminated. A further advantage is that they do not require airfields with long and expensive runways, so that despite the higher direct operating costs of VTOL aircraft; an overall saving is effected in comparison with conventional aircraft.

Of course, the helicopter has essentially solved the problem of vertical takeoff and landing, but its applications are limited by its relatively low speed. In recent years a large number of new VTOL aircraft types have emerged. Vertical takeoff without the use of large rotors became practicable with the advent of the gas-turbine engine because it could generate much more thrust for a given weight than the piston engine. Many new problems have had to be solved: e.g., in connection with stability and control during hovering and the transition from vertical to forward flight, and vice-verse, since conventional control surfaces are ineffective at low forward speeds.

Depending on the position of the aircraft during the takeoff, a distinction is made between “tail sitters” and “flat risers.” In the first-mentioned category the whole aircraft rests with its tail on the ground and its nose pointing vertically upwards. After takeoff, it is gradually brought into the normal flying position by operation of the controls. The “flat riser” takes off in the normal position, i.e., with the fuselage parallel to the ground. In this last-mentioned category of VTOL aircraft, the propulsion engines may be swiveled from the vertical position for takeoff and landing to horizontal for forward propulsion. With turbojet propulsion, the propulsion engines can be used for takeoff and landing by suitably directing the jets downwards. In addition to the propeller VTOL aircraft and the turbojet VTOL aircraft, a third type is based on the ducted fan, this being a propeller or fan within a duct or shroud, which in some types can be tilted in the same manner as the propeller engine. It is a combination of a ducted fan and a jet engine. Each of the two wings of the aircraft may be provided with such a fan, “buried” in the thickness of the wing. The jet engine provides the propulsion in the normal way when the aircraft is in forward flight. For takeoff and landing, the jet exhaust is deflected to drive the fan, which thus develops a powerful vertical thrust.

The present trend of development is toward the direct utilization of the thrust developed by turbojet. In a case where separate lift engines are provided in addition to the propulsion engine there is of course the problem of extra weight due to having two sets of engines, only one of which is in use at any particular time. In this respect the arrangement where only one set of engines is provided, which can be swiveled from vertical to horizontal, and vice-verse, or where the jets themselves can be deflected to produce a thrust in the desired direction is advantageous. This is especially true in high-speed fighter aircraft, whose engines produce a large thrust which can be utilized for vertical takeoff. On the other hand, separate lift engines or a combination of swiveling jet engines and a set of auxiliary lift engines may be more advantageous for other types of aircraft, such as civil aircraft, with lower cruising speeds.

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by on March 14, 2012 2 Comments

How Ramjet Engine Works in an Aircraft?

RAMJET PROPULSION
The ramjet engine began to compete with the turbojet when aircraft speeds increased beyond mach 2, thus entering a range where the compression produced by the airspeed becomes sufficient to perform the function of the compressor of the turbojet. For this reason the development of the ramjet also known as the “athodyd,” a contraction of “aerothermodynamic duct “has come into prominence in recent years. The air rushes into the inlet at supersonic speed and enters the combustion chamber, where it is heated by the combustion of fuel injected into the chamber. The heated air and the gases of combustion are discharged from a nozzle, thus producing the thrust. The main technical problem presented by the ramjet is to ensure steady combustion. For this it is generally necessary to have airflow speeds of less than about 100 m/sec (330 ft/sec) in the combustion chamber. This is a requirement difficult to fulfill at high air-speeds. For increasingly high speeds the ramjet evolves into something more resembling a rocket-propulsion Unit. In such engines the pressure developed in the combustion chamber is of the order of 100 atm. (about 1500 lb/in2), and the nozzle from which the jet emerges has to be made larger and larger. For very high speeds, in excess of mach 6, the engine evolves into the kind of system, where the inlet cone has become a specially shaped central body surrounded by an annular combustion chamber. As a result of allowing the gases of combustion to expand around the circumference of the conically tapering “tail” of the central body, a saving in overall construction weight of the engine is affected. From this example it is apparent how future high-speed ramjet engines are likely to become increasingly incorporated into the structure of the aircraft and thus become an integral feature thereof. The logical further development of the athodyd would consist in external combustion of fuel behind a shock wave.

The shock wave is formed at the nose of the aircraft and is associated with an abrupt increase in pressure. It could therefore serve theoretically as the “front wall” of a combustion chamber, fuel being injected into the air behind the shock wave. The fuel would ignite spontaneously in consequence of the high temperature that always develops behind the shock wave. External expansion of the gases of combustion at the rear part of the aircraft provides the propelling thrust. The appropriately shaped surfaces may be conceived as part of the aircraft’s fuselage or combination of fuselage and wing. This form of propulsion is in turn a transition to the athodyd with ultrasonic combustion. The main problem encountered here is that of stability of the flame. This may be achieved by enclosing it within a recirculation zone close to the surface of the aircraft. Alternatively, the propulsion system may take the form of a rocket motor which emits a stream of fuel-enriched gas into which air is injected and which is then brought to combustion. The main difference in relation to the conventional ramjet with subsonic combustion is that, instead of having to reduce the supersonic speed of the intake air to a subsonic value low enough to permit flame stability in the combustion chamber, the greater part of the kinetic energy of the intake air is now not converted into potential energy by adiabatic compression. This compression prior to combustion in the conventional ramjet reduces the efficiency of the ramjet at high mach numbers.

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