3.3 Aircraft Performance Parameters and Calculations

Aircraft performance is all about efficiency and capability. Key parameters like lift-to-drag ratio and thrust determine how well a plane flies. These factors influence range and endurance, which are calculated using Breguet's equations.

Various factors affect aircraft performance. Altitude, weight, and environmental conditions impact how a plane operates. Specific fuel consumption measures engine efficiency, with lower values indicating better performance and improved range and endurance.

Aircraft Performance Parameters

Key performance parameters

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• Lift-to-drag ratio ($L/D$) measures aerodynamic efficiency with higher values indicating better performance influenced by aircraft design, wing shape (swept or delta), and airspeed
• Thrust generated by the propulsion system (jet engines or propellers) overcomes drag to maintain or change aircraft velocity measured in units of force (Newtons or pounds)
• Power required represents energy per unit time needed to maintain flight calculated as the product of thrust and velocity ($P = T \times V$) measured in units of power (Watts or horsepower)

Range and endurance calculations

• Breguet's range equation $Range = \frac{V}{c} \times \frac{L}{D} \times ln(\frac{W_1}{W_2})$ calculates maximum distance an aircraft can fly with a given amount of fuel considering aircraft velocity ($V$), specific fuel consumption ($c$), lift-to-drag ratio ($L/D$), initial weight with fuel ($W_1$), and final weight without fuel ($W_2$)
• Breguet's endurance equation $Endurance = \frac{1}{c} \times \frac{L}{D} \times ln(\frac{W_1}{W_2})$ calculates maximum time an aircraft can stay airborne with a given amount of fuel using the same variables as the range equation

Factors Affecting Aircraft Performance

Factors affecting aircraft performance

• Altitude decreases air density reducing lift and engine performance (turbojet or turboprop) requiring higher true airspeed to maintain the same indicated airspeed and affecting maximum range and endurance
• Weight increases required lift and thrust reducing climb performance and maneuverability (rate of turn) and affecting maximum range and endurance
• Environmental conditions impact performance:
  1. High temperatures decrease air density reducing lift and engine performance
  2. High humidity decreases air density reducing lift and engine performance
  3. Headwinds reduce ground speed and increase flight time while tailwinds increase ground speed and decrease flight time

Specific fuel consumption concept

• Specific fuel consumption ($c$) measures engine efficiency defined as the fuel flow rate per unit of thrust ($c = \frac{\dot{m}_f}{T}$) with lower values indicating better engine efficiency
• Impact on aircraft efficiency:
  • Lower specific fuel consumption leads to improved range (nautical miles) and endurance (hours)
  • More efficient engines (high-bypass turbofans) reduce fuel consumption and operating costs
  • Advancements in engine technology (geared turbofans) aim to minimize specific fuel consumption