Aerodynamics & Performance
Aerodynamics is the branch of dynamics dealing with the motion of air and other gases that provide the performance aircraft need to fly.
Aerodynamics & Performance Introduction
- Aerodynamics is the branch of dynamics dealing with the motion of air and other gases, which gives us the performance we need to fly.
- Understanding basic aerodynamic concepts is essential to understanding the operation of the components and subcomponents of an aircraft.
- The principal aerodynamic concepts are that of the four forces that affect aircraft.
- It can be associated with the forces acting on an object in motion through the air or with a stationary object in a current of air.
- Several factors affect aircraft performance, including the atmosphere, aerodynamics, and aircraft icing.
- Pilots need an understanding of these factors to provide a sound basis for anticipating aircraft response to control inputs, which come in the form of performance.
Lift & Basic Aerodynamics
- Aircraft components and structure create the physical form we know as an aircraft.
- Four forces act upon an aircraft, making up the Principles of Flight.
- Understanding how these forces are created and, more importantly, impact each other allows pilots to control an aircraft in flight.
- These principle forces are thrust, drag, weight, and lift: [Figure 1]
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Thrust:
- Thrust is the forward force produced by the powerplant/propeller.
- It opposes or overcomes the force of drag.
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Drag:
- Drag is a rearward, retarding force caused by disruption of airflow by the wing, fuselage, and other protruding objects.
- Drag opposes thrust and acts rearward parallel to the relative wind.
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Weight:
- Weight is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage.
- The weight pulls the aircraft downward because of the force of gravity.
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Lift:
- Lift opposes the downward force of weight, produced by the dynamic effect of the air acting on the wing and acts perpendicular to the flight path through the wing's center of lift (CL).
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- Engineers design aircraft with different handling characteristics in mind, which determine aircraft stability.
- An aircraft moves in three dimensions and is controlled by moving it about one or more of its axes:
- The longitudinal, or roll, axis extends through the aircraft from nose to tail, with the line passing through the center of gravity.
- The lateral or pitch axis extends across the aircraft on a line through the wing tips, again passing through the center of gravity.
- The vertical, or yaw, axis passes through the aircraft vertically, intersecting the center of gravity.
- All control movements cause the aircraft to move around one or more of these axes and allow for the control of the aircraft in flight. [Figure 3]
- Aircraft performance designs are dependent upon operating within weight and balance limitations.
- NASA - Aerodynamics Index
Aircraft Performance
- The performance or operational information section of the Aircraft Flight Manual/Pilot's Operating Handbook (AFM/POH) contains the aircraft's takeoff, climb, range, endurance, descent, and landing data.
- Using this data in flying operations is mandatory for safe and efficient operation.
- Stall Performance
- Turn Performance
- Wake Turbulence
- Takeoff Performance
- Pilots evaluate climb performance to determine the appropriate parameters and procedures to safely and efficiently clear obstacles and transition to the enroute environment.
- Cruise Performance
- Descent Performance
- Landing Performance
- Cold Temperature Operations
- Warm Temperature Operations
- To precisely calculate aircraft performance, pilots utilize charts, tables, and data such as those provided in the airplane flight manual.
Aircraft Performance Charts, Tables, & Data
- Performance Calculations
- Aircraft performance is tested under specific conditions that are later compiled in performance charts
- Pilots must reference these performance charts to calculate the anticipated performance for a given operation
- Calculate crosswind component for takeoff and landing
- Calculating short-field takeoff and climb distance
- The conditions under which performance chart data is determined may be included on the chart, but generally speaking it is flown with proper pilot technique, under standard atmospheric conditions, to a level, dry, paved runway, and without winds present.
- The difference between charted data and flown data becomes the difference between expected performance and actual performance.
- Note too that to achieve different performance on landing and takeoff, the aircraft configuration is altered per the procedure (i.e., soft-field uses flaps).
- Different configurations therefore will achieve different performance or, said differently, required performance requires different configurations.
- Necessary climb performance calculations will depend on the situation, but include:
- Top-of-climb for flight planning, and;
- Climb gradient to ensure regulatory compliance, and;
- Rate-of-climb to determine expected performance;
Performance Calculations Versus Reality
- Charts contain performance data based on expected performance under a set of conditions.
- Said differently, pilot operating handbook numbers require the pilot operating handbook technique.
- The conditions present during calculation will always differ from the charts and graphs to a degree, sometimes a large degree.
- Factors to consider include mechanical, environmental, skill, geographic location or altitude, etc.
- It is important to use conservative numbers.
- Consider always rounding up, not interpolating but using the higher number, etc.
- Realize obstacles like trees may be more than 50'; therefore, charts are wholly inadequate to calculate by numbers.
- Consider adding 50-100% buffers to anything calculated and monitor performance while in flight.
- If numbers were overly conservative, adjust for subsequent flights.
Aerodynamics & Performance Conclusion
- Resources like Real Engineering - The Plane That Will Change Travel Forever describe complex aerodynamical topics clearly and concisely.
- Consider actual versus realized performance when doing any performance calculations.
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