Fineness ratio
Encyclopedia
Fineness ratio is a term used in naval architecture
and aerospace engineering
to describe the overall shape of a streamlined body. Specifically, it is the ratio of the length of a body to its maximum width; shapes that are "short and fat" have a low fineness ratio, those that are "long and skinny" have high fineness ratios. Aircraft that spend time at supersonic
speeds generally have high fineness ratios, a canonical example being Concorde
.
At speeds below critical mach, one of the primary forms of drag is skin friction. As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface", which generally implies the fuselage should be somewhat "egg shaped", with a fineness ratio about 4.5. A good example of such a design is the Questair Venture
.
Most aircraft have fineness ratios significantly greater than this, however. This is often due to the competing need to place the tail control surfaces at the end of a longer moment arm to increase their effectiveness. Reducing the length of the fuselage would require larger controls, which would offset the drag savings from using the ideal fineness ratio. An example of a high-performance design with a "non-perfect" fineness ratio is the Lancair
. In other cases the designer is forced to use a non-ideal design due to outside factors such as seating arrangements or cargo pallet sizes. Modern airliner
s often have fineness ratios much higher than ideal, a side effect of their cylindrical cross-section which is selected for strength, as well as providing a single width to simplify seating layout.
As an aircraft approaches the speed of sound
, shock wave
s form on areas of greater curvature. These shock waves radiate away energy that the engines must supply, energy that does not go into making the aircraft go faster. This appears to be a new form of drag —referred to as wave drag
— which peaks at about three times the drag at speeds even slightly below the critical mach. In order to minimize the wave drag, the curvature of the aircraft should be kept to a minimum, which implies much higher fineness ratios. This is why high-speed aircraft have long pointed noses and tails, and cockpit canopies that are flush to the fuselage line.
More technically, the best possible performance for a supersonic design is typified by two "perfect shapes", the Sears-Haack body
which is pointed at both ends, or the von Kármán ogive, which has a blunt tail. Well known examples include the Concorde
, F-104 Starfighter
and XB-70 Valkyrie
, although to some degree practically every post-WWII interceptor aircraft
featured such a design. Missile designers are even less interested in low-speed performance, and missiles generally have higher fineness ratios than most aircraft.
The introduction of aircraft with higher fineness ratios also introduced a new form of instability, inertial coupling. As the engines and cockpit moved away from the center of the aircraft to locations ever further from the center of mass
, the roll inertia of these masses grew to be able to overwhelm the power of the aerodynamic surfaces. A variety of methods are used to combat this effect, including oversized controls and stability augmentation systems.
Naval architecture
Naval architecture is an engineering discipline dealing with the design, construction, maintenance and operation of marine vessels and structures. Naval architecture involves basic and applied research, design, development, design evaluation and calculations during all stages of the life of a...
and aerospace engineering
Aerospace engineering
Aerospace engineering is the primary branch of engineering concerned with the design, construction and science of aircraft and spacecraft. It is divided into two major and overlapping branches: aeronautical engineering and astronautical engineering...
to describe the overall shape of a streamlined body. Specifically, it is the ratio of the length of a body to its maximum width; shapes that are "short and fat" have a low fineness ratio, those that are "long and skinny" have high fineness ratios. Aircraft that spend time at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds generally have high fineness ratios, a canonical example being Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
.
At speeds below critical mach, one of the primary forms of drag is skin friction. As the name implies, this is drag caused by the interaction of the airflow with the aircraft's skin. To minimize this drag, the aircraft should be designed to minimize the exposed skin area, or "wetted surface", which generally implies the fuselage should be somewhat "egg shaped", with a fineness ratio about 4.5. A good example of such a design is the Questair Venture
Questair Venture
The Questair Venture is an American-built light sporting aircraft of the 1980s.The Questair Venture is an American-built light sporting aircraft of the 1980s.The Questair Venture is an American-built light sporting aircraft of the 1980s....
.
Most aircraft have fineness ratios significantly greater than this, however. This is often due to the competing need to place the tail control surfaces at the end of a longer moment arm to increase their effectiveness. Reducing the length of the fuselage would require larger controls, which would offset the drag savings from using the ideal fineness ratio. An example of a high-performance design with a "non-perfect" fineness ratio is the Lancair
Lancair
Lancair International, Inc. is a U.S. manufacturer of general aviation aircraft kits. They are well known for their series of high-performance single-engine aircraft that offer cruise speeds that surpass many twin-engine turboprop designs...
. In other cases the designer is forced to use a non-ideal design due to outside factors such as seating arrangements or cargo pallet sizes. Modern airliner
Airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...
s often have fineness ratios much higher than ideal, a side effect of their cylindrical cross-section which is selected for strength, as well as providing a single width to simplify seating layout.
As an aircraft approaches the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
, shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
s form on areas of greater curvature. These shock waves radiate away energy that the engines must supply, energy that does not go into making the aircraft go faster. This appears to be a new form of drag —referred to as wave drag
Wave drag
In aeronautics, wave drag is a component of the drag on aircraft, blade tips and projectiles moving at transonic and supersonic speeds, due to the presence of shock waves. Wave drag is independent of viscous effects.- Overview :...
— which peaks at about three times the drag at speeds even slightly below the critical mach. In order to minimize the wave drag, the curvature of the aircraft should be kept to a minimum, which implies much higher fineness ratios. This is why high-speed aircraft have long pointed noses and tails, and cockpit canopies that are flush to the fuselage line.
More technically, the best possible performance for a supersonic design is typified by two "perfect shapes", the Sears-Haack body
Sears-Haack body
The Sears–Haack body is the aerodynamic body shape with the lowest theoretical wave drag. Aircraft designed to operate at high subsonic or supersonic speeds have their cross-sectional areas designed to match as closely as possible the proportions of Sears-Haack body.By Whitcomb's area rule, the...
which is pointed at both ends, or the von Kármán ogive, which has a blunt tail. Well known examples include the Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...
, F-104 Starfighter
F-104 Starfighter
The Lockheed F-104 Starfighter is a single-engine, high-performance, supersonic interceptor aircraft originally developed for the United States Air Force by Lockheed. One of the Century Series of aircraft, it served with the USAF from 1958 until 1969, and continued with Air National Guard units...
and XB-70 Valkyrie
XB-70 Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the proposed B-70 nuclear-armed deep-penetration strategic bomber for the United States Air Force's Strategic Air Command...
, although to some degree practically every post-WWII interceptor aircraft
Interceptor aircraft
An interceptor aircraft is a type of fighter aircraft designed specifically to prevent missions of enemy aircraft, particularly bombers and reconnaissance aircraft. Interceptors generally rely on high speed and powerful armament in order to complete their mission as quickly as possible and set up...
featured such a design. Missile designers are even less interested in low-speed performance, and missiles generally have higher fineness ratios than most aircraft.
The introduction of aircraft with higher fineness ratios also introduced a new form of instability, inertial coupling. As the engines and cockpit moved away from the center of the aircraft to locations ever further from the center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
, the roll inertia of these masses grew to be able to overwhelm the power of the aerodynamic surfaces. A variety of methods are used to combat this effect, including oversized controls and stability augmentation systems.