Difference Between Subsonic and Supersonic

Subsonic vs Supersonic

The velocity of the air flow plays a critical role in determining the characteristic of air flow. Air flowing at low speeds can be considered as a viscous fluid with incompressible properties, like water. When the velocity of the air flow increases, the properties related to the compressibility change significantly resulting in a change, in the aerodynamic forces around a body within the flow.

In the context of relative motion, the aircraft can be considered as a body, which is stationary in a flow of air, for the analytical purposes. The speed of the aircraft becomes the relative velocity of the air flow, commonly used as airspeed. An aircraft designed to fly below the speed of sound is known as subsonic aircraft, while aircraft designed to fly faster than the speed of sound is known as supersonic aircraft. This speed is usually expressed by the Mach number (M) which is the ratio between the air speed and the speed of sound. If an aircraft is subsonic its maximum speed is less than 1 (M <1) and, for supersonic aircrafts, it is greater than 1 (M >1).

More about Subsonic Aircraft

Most of the aircrafts produced are subsonic aircrafts, which are designed to fly below Mach 0.8. Small lightweight airplanes have lower Mach numbers, which is around Mach 0.2. Business jets and commercial airliners can fly at maximum speeds up to Mach 0.85.

Light weight subsonic aircrafts use piston engines as the power plant, while business jets and commercial airliners use turboprop or high bypass turbofan engines. Structurally the loading on the airframe varies from airplane to airplane. The wings are usually straight or with low sweep angle. Airplane skin is built with aluminium and airframe may consist of aluminium and steel. With the advancement of composite material technology, fibre reinforced composite materials with high strength and low weight are introduced.

More about Supersonic Aircraft

Supersonic regime is also divided into supersonic (1<M<3), high supersonic (3<M<5) and hypersonic (M>5) classes.

Supersonic aircrafts are mostly military aircrafts, designed for combat (ex. F-15E, Su 27, Dassault Rafale, Eurofighter Typhoon). They use low bypass turbofan engines as power plant, and the structure is designed to withstand vibrations and loads occurring at the supersonic speeds.

The airframe is mostly made of high grade titanium and aluminium to withstand the high loading in maneuvers and damage during combat. The airframe has been aerodynamically optimized to minimize compressibility effects and drag effects. The local air density varies due to shock waves, expansion, and flow choking resulting in a drastic change from subsonic flight conditions.

Supersonic Transport (SST) is a realized, but an expensive, aviation challenge. Only two types of supersonic transport have ever been built, and both exceeded the average cost of a flight. Those are Concorde and Tu-144, designed as passenger aircrafts, but operation abandoned due to high expenses.

High supersonic aircrafts are mostly recon airplanes, and hypersonic aircrafts are highly experimental aircrafts (with the exception of the space shuttle).

What is the difference between Subsonic and Supersonic?

• Subsonic aircrafts fly below the speed of sound while supersonic aircrafts fly faster than the speed of sound.

• Supersonic aircrafts use low bypass turbofan engines as the propulsion system, while subsonic aircrafts use propellers driven piston engines, turboprop engines, or high bypass turbofan engines.

• Supersonic aircrafts use swept wings or delta wings, while subsonic aircrafts use straight wings or wings with a smaller sweep angle.

• Supersonic aircrafts are mainly built of titanium, while subsonic aircrafts are built with titanium, aluminium, and carbon fibre reinforced polymers or other composite materials.

• Usually supersonic aircrafts are military aircrafts used for combat or reconnaissance operations, while subsonic aircraft are used for transport and travel.