Mercedes Mullins
The fuel economy of aircraft is influenced by several factors, including the type of engine, airspeed, and drag. While airspeed alone does not determine fuel consumption, it can impact other variables that influence fuel efficiency. At slower urban speeds, friction and rolling resistance are the primary factors contributing to fuel consumption. However, as speed increases, aerodynamic drag becomes more significant, affecting fuel economy. Aircraft engines, such as shaft engines and jet engines, have different efficiencies based on their designs and optimal speed ranges. Additionally, new technologies and design improvements, such as those aimed at reducing parasitic drag and improving aerodynamics, can enhance fuel efficiency.
| Characteristics | Values |
|---|---|
| Aerodynamic drag | Increases in proportion to the square of the speed |
| Air density | Cannot be controlled |
| Airspeed | Not a primary factor in fuel flow |
| Air distance | Positively associated with fuel consumption |
| Fuel efficiency | Improved by maximising lift-to-drag ratio |
| Fuel economy | Improved by new technology, aerodynamics, and operational efficiency |
| Jet aircraft | Twice the fuel efficiency of the earliest jet airliners |
| Parasitic drag | Grows with the square of the speed |
| Piston airliners | More energy-intensive than jet airliners |
| Propeller planes | More efficient than jets |
| Speed | Requires twice the energy to travel at 60mph compared to 30mph |
| Urban speeds | Friction and rolling resistance burn most fuel |
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What You'll Learn
Aerodynamic drag increases with speed
Aerodynamic drag is the force of the air acting to slow down a body moving through it. The faster an object moves, the more air it has to push out of the way, and the more the air pushes back. This is why drag increases with speed.
In fluid dynamics, drag is a force acting opposite to the direction of motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers, two solid surfaces, or between a fluid and a solid surface. Drag forces tend to decrease fluid velocity relative to the solid object in the fluid's path. Unlike other resistive forces, drag force depends on velocity. Drag force is proportional to the relative velocity for low-speed flow and is proportional to the velocity squared for high-speed flow. This distinction between low and high-speed flow is measured by the Reynolds number.
Parasitic drag, which is constituted by form drag and skin-friction drag, increases because the fluid is flowing more quickly around protruding objects, increasing friction or drag. At even higher speeds, wave drag enters the picture. Each of these forms of drag changes in proportion to the others based on speed. The combined overall drag curve, therefore, shows a minimum at some airspeed. An aircraft flying at this speed will be at or close to its optimal efficiency. Pilots will use this speed to maximize endurance (minimum fuel consumption).
Aerodynamic drag is a significant factor in the fuel economy of aircraft and road vehicles. In the case of aircraft, the maximum range is determined by the level of efficiency with which thrust can be applied to overcome aerodynamic drag. Similarly, at urban speeds, friction and rolling resistance are responsible for most of the fuel burned in road vehicles, but at freeway speeds, aerodynamic drag will account for more than half of the fuel used.
To reduce aerodynamic drag, aircraft and road vehicles can be designed with streamlined shapes, rounded edges, and optimized grill openings to reduce turbulence. In the case of aircraft, aerodynamic wheel shapes and "active" aerodynamic controls that operate as the speed increases, including lowering air dams, pop-up rear spoilers, and lowered ride height, can also be used.
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Jet engine efficiency
Jet engines have changed very little in the past 60 years. They pull air in, squeeze it, heat it, and then exhaust it. This process is known as the engine's core or powerhouse. While these engines are finely tuned, there is still room for improvement in terms of efficiency.
The efficiency of jet engines is calculated by dividing the aircraft's airspeed by its thrust-specific fuel consumption and the specific energy of the fuel. Jet engines are more efficient at high speeds, with an exhaust gas velocity of around 1100 kph, making them suitable for subsonic flight. However, at lower speeds, propeller engines are more efficient.
To improve jet engine efficiency, NASA is working on reducing the size of the engine's core. By shrinking the core, the bypass ratio of the engine increases, which means the fuel burn rate remains relatively unchanged despite the addition of a larger inlet fan. As a result, the engine produces more thrust while burning roughly the same amount of fuel, making it more efficient.
Additionally, new technologies can further enhance jet engine efficiency. These include higher pressure and bypass ratios, geared turbofans, open rotors, and hybrid or fully electric propulsion. Improving the aerodynamics of the aircraft can also contribute to efficiency gains. This can be achieved through retrofits, better materials and systems, and advanced aerodynamics, such as minimizing parasitic drag and induced drag.
Overall, increasing jet engine efficiency not only reduces fuel consumption and operational costs but also helps lower emissions, contributing to a more sustainable aviation industry.
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Airframe efficiency
An aircraft's wings are engineered to create lift by generating a pressure differential between the upper and lower surfaces. This lift enables the plane to overcome gravity and ascend. The wing design is a key factor in airframe efficiency, as it determines the aircraft's lift-to-drag ratio. A higher lift-to-drag ratio means the aircraft can generate more lift with less drag, resulting in improved fuel efficiency.
Airframe design also plays a crucial role in minimizing parasitic drag. Parasitic drag is the resistance created by the aircraft's body and surfaces moving through the air. Efficient airframe design aims to streamline the aircraft's exterior to reduce parasitic drag, which in turn lowers the amount of fuel required to overcome this resistance.
Additionally, the shape and configuration of the aircraft's fuselage, tail, and control surfaces all contribute to airframe efficiency. These components are designed to minimize drag while providing stability and control during flight. Advanced materials and manufacturing techniques can also be employed to reduce weight, further enhancing airframe efficiency by decreasing the overall weight that needs to be lifted.
Overall, the efficiency of an aircraft's airframe is a key determinant of its fuel economy. Through careful design, engineering, and the application of aerodynamic principles, aircraft manufacturers strive to create airframes that optimize lift, minimize drag, and enhance fuel efficiency. This not only reduces operating costs but also contributes to more environmentally sustainable aviation practices.
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Fuel injection methods
The term "fuel injection" is vague and covers various distinct systems with fundamentally different functional principles. The only thing all fuel injection systems have in common is the absence of carburetion.
Internal and External Mixture Formation Systems
There are two main functional principles of mixture formation systems for internal combustion engines: internal and external. A fuel injection system that uses external mixture formation is called a manifold injection system, which can be multi-point (or port) and single-point (or throttle body) injection. Internal mixture formation systems can be separated into several different varieties of direct and indirect injection, the most common being the common-rail injection, a variety of direct injection.
Direct Injection
Direct injection means that the fuel is injected into the main combustion chamber of each cylinder. As air and fuel are mixed only inside the combustion chamber, air alone is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual). Fuel is injected directly into the combustion chamber either with a blast of air or hydraulically. Typically, hydraulic direct injection systems spray fuel into the air inside the cylinder or combustion chamber.
Indirect Injection
Petrol-engined cars use indirect fuel injection. A fuel pump sends the petrol to the engine bay, and it is then injected into the inlet manifold by an injector. There is either a separate injector for each cylinder or one or two injectors into the inlet manifold. The fuel/air mixture then enters the combustion chamber.
Continuous Injection
Continuous injection involves the fuel being squirted into the inlet port all the time the engine is running. The injector acts as a spray nozzle to break up the fuel into a fine spray. The amount of fuel sprayed is increased or decreased by a mechanical or electrical control unit.
Timed Injection
Also known as pulsed injection, timed injection involves the fuel being delivered in bursts to coincide with the induction stroke of the cylinder. As with continuous injection, timed injection can also be controlled either mechanically or electronically.
Common-Rail Injection
In a common-rail system, fuel from the fuel tank is supplied to a common header (called the accumulator) and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator has a high-pressure relief valve to maintain pressure and return the excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve.
M-System
The M-System, used in some diesel engines from the 1960s to the 1980s, sprayed the fuel onto the walls of the combustion chamber, as opposed to most other direct-injection systems that spray fuel into the middle of the chamber.
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Airspeed and thrust
For jet engines, the efficiency is determined by dividing airspeed by the thrust-specific fuel consumption and the specific energy of the fuel. Higher airspeeds generally require greater thrust, which increases fuel consumption. However, the relationship is not linear, and other factors, such as altitude and air density, also come into play.
Turboprop engines, which combine a shaft engine with a propeller, have an optimal speed below 460 miles per hour (740 km/h). While propeller planes are generally more fuel-efficient than jet aircraft, they have lower maximum speeds. The efficiency of turboprop engines is inversely proportional to their brake-specific fuel consumption.
The maximum range of an aircraft is determined by how efficiently thrust is applied to overcome aerodynamic drag. Aerodynamic drag is influenced by two main types of drag: parasitic drag and induced drag. Parasitic drag includes form drag and skin-friction drag, which increases with the square of speed. Induced drag is influenced by factors such as airframe size, fuel and payload weight, and wing design.
By reducing drag and improving aerodynamics, aircraft can achieve better fuel economy. This can be achieved through design changes such as rounded edges, optimised grill openings, and aerodynamic wheel shapes. Additionally, maintaining lower cruise speeds can augment range and reduce fuel consumption.
In summary, airspeed and thrust are intricately linked and play a significant role in aircraft fuel economy. While increasing airspeed generally requires more thrust and fuel, improvements in aerodynamics, engine technology, and design can help mitigate fuel consumption.
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Frequently asked questions
Airspeed does not directly impact fuel flow, but it can influence other variables that may have an impact. As airspeed increases, net thrust also increases, which requires more fuel.
Aircraft efficiency is improved by maximising the lift-to-drag ratio. As lift and drag are functions of airspeed, their relationship is a major factor in determining an aircraft's design efficiency.
At low speeds, friction and rolling resistance are responsible for most of the fuel burned. However, at higher speeds, aerodynamic drag becomes more significant, accounting for more than half of the fuel used.
Modern jet aircraft have twice the fuel efficiency of the earliest jet airliners. For example, jet airliners from the 1990s are 40-80% faster and 1-28% more fuel-efficient than those from the 1950s.
Wind speed can impact the fuel economy of aircraft. Pilots can take advantage of existing winds to achieve more economical efficiency by setting different flight levels and speeds.
