Aircraft Performance Assessment - Essay Example

Overall engine efficiency is cycle efficiency x propulsion efficiency (Hunecke, 2010). Propulsion efficiency (np) = 2/(1+c/v) where c is exhaust speed and v is aircraft speed. Thus, maximum efficiency is achieved in a balanced equation. Looking at the equation, at takeoff, c exceeds v and thus np is low and as V increases np will increase, however, to extend endurance, c is reduced (power reduction), once again affecting efficiency. In considering overall efficiency, propulsion efficiency must also be considered in relation to the aircraft itself. One such method is the Oswald efficiency factor (FAA, 2001) whereby:

CD = CD0 + (CL)2/ e AR where:

CD is the overall drag coefficient,

CD0 is the zero-lift drag coefficient,  

CL is the aircraft lift coefficient,

 is the aircraft circumference-to-diameter ratio

e is the Oswald efficiency number, and

AR is the aspect ratio. (FAA, 2001). 

From this equation in conjunction with the propulsion equation can be seen the multiplicity of factors that affect efficiency.

The dilemma in performance versus efficiency arises from both design and operating environment. For example, a fighter jet may have low efficiency (a high SFC) however, the aircraft performance is high. Designers would attempt to maximize payload, time on station, maneuverability, and speed to intercept (FAA, 2001). With respect to the operating environment, aircraft efficiency is increased at higher altitudes as less fuel is used (drag is decreased) however, as air density decreases with altitude, thrust performance is decreased as incident airflow is less (Dole and Lewis, 2000). Management of the latter is demonstrated by operating techniques such as stepped climb that provide the optimal range (performance) for SFC (efficiency).

An increase in performance does not necessarily equate to an increasing inefficiency. An inverse relationship normally exists. There are considerable trade-offs between the two and designers aim to achieve a balance relative to the aircraft's intended role.

Engine Designs          

Engine designs have increased the efficiency of engines through design modifications including combustion chambers, compressors, turbines, and air intakes. Examples of different engine designs and advances include:

• Development of turbofan engines to increase efficiency at higher speeds.
• Development of turbojets over earlier piston engines to increase efficiency.
• Axial flow instead of centrifugal compressors for smaller diameter engine housings to improve performance.
• Use of reverse thrust technology to decrease stopping distances.
• Use of after-burners to increase acceleration performance.
• Use of carbon-fiber turbine blades to decrease heat energy loss and weight thereby improving performance and efficiency
• Advances in fuel nozzle designs such as de Laval nozzle to increase efficiency.
• Use of multi-stage turbines with each stage operating at its optimal efficiency.