Part 2: Forces & Moment, Read Airfoil Plots | How To Choose Airfoil | Airplane Design

Summary

This video explains how airfoils generate lift, drag, and pitching moments, and how engineers use non-dimensional aerodynamic coefficients (CL, CD, CM) to compare and choose profiles. It introduces the aerodynamic center—near 25% of the chord—where pitching moment stays constant with angle of attack, simplifying analysis. The transcript covers wind-tunnel measurement of coefficients, reading CL vs. AOA curves to find CL_max and stall angle, and using the drag polar (CD vs. CL) to identify minimum-drag operating points for cruise. Practical selection advice and the importance of pitching moment, stall speed, and wing incidence are highlighted, with more detail promised in later parts.

Takeaways

• 😀 Lift is the force that keeps an aircraft in the air, acting perpendicular to the free stream flow, not the airfoil cord line.
• 😀 Drag is the resistance force experienced by the airfoil, acting parallel to the flow and defined relative to the flow, not the cord line.
• 😀 The zero lift pitching moment is the turning force that affects aircraft stability, even when no lift is produced.
• 😀 The aerodynamic center is the point where the pitching moment remains constant regardless of the angle of attack.
• 😀 The center of pressure changes with the angle of attack, while the aerodynamic center remains fixed, simplifying aerodynamic analysis.
• 😀 Aerodynamic coefficients (CL, CD, CM) are non-dimensional numbers used to model the behavior of an airfoil, independent of physical dimensions.
• 😀 The lift coefficient (CL) is crucial for understanding how an airfoil performs under different conditions like shape, angle of attack, and airspeed.
• 😀 The drag coefficient (CD) is used to calculate drag and is related to the airfoil's resistance to the airflow.
• 😀 The pitching moment coefficient (CM) is key to understanding the moment that causes an airfoil's nose to pitch up or down, with negative values indicating nose-down moments.
• 😀 The drag polar, a plot of CD vs. CL, is one of the most useful tools for analyzing airfoil performance, helping to determine minimum drag and the best airfoil for specific flight conditions.

Q & A

What is the primary force generated by an airfoil, and how does it relate to the aircraft's flight?

The primary force generated by an airfoil is lift, which acts perpendicular to the free stream flow. Lift is the force that counteracts gravity, keeping the aircraft in the air. It is crucial for maintaining altitude during flight.

What is drag, and how is it different from lift?

Drag is the resistance force experienced by the airfoil as it moves through the air. Unlike lift, which is perpendicular to the flow, drag acts parallel to the airflow, opposing the aircraft's motion.

What is the 'zero lift pitching moment' and why is it important in aircraft design?

The zero lift pitching moment is the torque or turning force on the airfoil when no lift is being produced, such as at a certain angle of attack. It is important in aircraft design because it affects the trim drag and structural loads on the wings, influencing the aircraft's stability.

What is the aerodynamic center, and why is it a key point in analyzing airfoil behavior?

The aerodynamic center is a point on the airfoil's chord line where the pitching moment remains constant regardless of changes in the angle of attack. It is crucial for simplifying the analysis of aerodynamic forces, as it allows for a consistent reference point when studying lift and moment distribution.

How do changes in angle of attack affect the airfoil's pressure distribution?

When the angle of attack changes, the pressure distribution over the airfoil's surfaces changes as well. This alters the lift force and the location of the center of pressure, which can complicate the analysis. The aerodynamic center helps mitigate this issue by providing a fixed point for the analysis.

What are aerodynamic coefficients, and why are they useful in airfoil selection?

Aerodynamic coefficients are non-dimensional numbers used to describe an airfoil's behavior in terms of its shape, angle of attack, and flow conditions. These coefficients, such as CL (lift), CD (drag), and CM (moment), allow for easier comparison of different airfoils regardless of their physical size or properties.

How does the lift equation help in calculating the lift force of an airfoil?

The lift equation calculates the lift force by multiplying the lift coefficient (CL), airspeed, air density, and the wing area. The lift coefficient itself is a model that incorporates various factors like airfoil shape and angle of attack to determine the lift produced under specific conditions.

What is the drag polar, and why is it important in analyzing airfoil performance?

The drag polar is a plot that shows the relationship between the drag coefficient (CD) and the lift coefficient (CL). It is an essential tool for assessing the performance of an airfoil, as it helps identify the conditions under which the airfoil operates at minimum drag, which is important for fuel efficiency and overall aircraft performance.

What does the CL vs. AOA plot tell us about the airfoil's performance?

The CL vs. AOA (Angle of Attack) plot shows how the lift coefficient varies with the angle of attack. It helps identify the maximum lift coefficient (CL max), which corresponds to the stall angle of attack. This plot is crucial for determining the airfoil's stall characteristics and for selecting the appropriate incidence angle for the aircraft's wings.

How does the lift coefficient (CL) affect the stall speed of an aircraft?

The higher the CL max of an airfoil, the lower its stall speed. A lower stall speed is desirable for safe takeoff and landing, as it allows the aircraft to operate efficiently at lower speeds.