SUBSONIC AIRCRAFT
A subsonic aircraft is an aircraft whose highest speed is less than the speed of sound.
Mach Number: It is very important to define this quantity. In fluid dynamics, the Mach number is basically a dimensionless quantity. It depicts the ratio of flow velocity past a boundary to the local speed of sound.
M = u / c
where:
M is the Mach number,
u is the flow velocity with respect to the boundaries and
c is the sound’s speed in the medium.
This ratio determines the magnitude of many of the effects of compressibility. Because of the importance of this ratio, scientists have named it the Mach number to honor the gas dynamics physicist Ernst Mach.
Subsonic aircrafts have a Mach number typically less than 1. Their speed is less than 250 metres per hour. The aircrafts that come under this category are most of the commercial/personal aircrafts that are currently used to transport passengers and cargo. The speed is just below the speed of sound. Engines these days, are lighter and more powerful and can travel rapidly with bulky loads of people or goods.
Usually, high speeds are most desirable in an aircraft. Hence supersonic flights should have been more desirable. But it is not so. Supersonic flight needs a much bigger engine. It also consumes a lot of fuel and requires much more advanced materials than a subsonic flight. Thus, a subsonic aircraft is far more cost-efficient than the equivalent supersonic design. It also has a greater range and causes very less damage to nature.
The less fierce subsonic atmosphere also permits a much broader range of aircraft types, such as balloons, airships, and rotorcraft, allowing them to fill a much broader range of roles. At lower subsonic speeds, compressibility factor can usually be neglected.
Propulsion of subsonic flights:
The propeller is one of the most effective sources of thrust available. It is very common on subsonic planes and airships. At times, it is bounded in the form of a ducted fan. At higher subsonic speeds and at high altitudes, which is acquired by most of the airliners, the high-bypass turbofan becomes very essential. Pure jets such as the turbojet and ramjet are unproductive at subsonic speeds and are mostly not often used.
Wing design
For the characteristics of lift, both, the span and area of a wing, are very essential. They are linked by the aspect ratio, which is the ratio of the span, measured from tip to tip, to the average chord, which is measured from leading edge to trailing edge.
The drag of a wing consists of two components:
- The induced drag : It is linked to the production of lift.
- Profile drag : Largely due to skin friction which is contributed to by the entire wing area.
It is hence desirable for a wing to have the minimum area compatible with the wanted lift characteristics. This is best accomplished with a high aspect ratio. Also, most of the high-performance aircrafts mostly have this type of wing.
But other considerations like lightweight, structural stiffness, maneuverability, ground handling and so on, most often are advantaged by a smaller span and, accordingly a less efficient wing. Small, low-altitude general aviation planes characteristically have aspect ratios of 6 or 7; airliners, 12 or more; and high-performance sailplanes, 30 or more.
In theory, induced drag is at its least value when the span-wise distribution of lift is elliptical in shape. But in practice, not all wings are elliptical in shape. The wings of jet airliners, which are highly enhanced for efficiency, are far from elliptical in shape.
