How does an airplane fly? How does it gain "lift"? What's the mechanism?
ChatGPT:
The ability of an airplane to fly is based on the fundamental principles of aerodynamics. Here's a detailed explanation of how an airplane gains lift and achieves flight:
Four Forces of Flight
- Lift: The upward force that counteracts the airplane's weight.
- Weight (Gravity): The downward force due to the airplane's mass and Earth's gravity.
- Thrust: The forward force produced by engines or propellers.
- Drag: The air resistance opposing the airplane's forward motion.
For an airplane to fly, lift must overcome weight, and thrust must overcome drag.
How Lift Is Generated
Lift is primarily generated by the wings of the airplane. The key concepts are:
1. Shape of the Wing (Airfoil Design)
- Airfoil: The cross-sectional shape of a wing.
- The top of the wing is typically curved, while the bottom is flatter. This design creates a difference in airflow speed above and below the wing.
2. Bernoulli's Principle
- Bernoulli's Principle states that as the speed of a fluid (air, in this case) increases, its pressure decreases.
- As air flows over the wing:
- The air on top of the wing travels faster due to the longer curved path, resulting in lower pressure.
- The air below the wing moves slower, resulting in higher pressure.
- This pressure difference generates an upward force—lift.
3. Newton’s Third Law of Motion
- As the wing pushes air downward (action), the air exerts an equal and opposite upward force on the wing (reaction), contributing to lift.
Role of Angle of Attack
- The angle of attack is the angle between the wing's chord line (a straight line from the leading to the trailing edge) and the oncoming airflow.
- Increasing the angle of attack increases the lift—up to a certain point. Beyond this, airflow separation occurs, leading to stall (a sudden loss of lift).
Mechanics of Flight
- Engines produce thrust, propelling the airplane forward.
- Wings interact with the oncoming air, and lift is generated due to the airfoil shape and proper angle of attack.
- Once lift exceeds weight, the airplane takes off and stays airborne as long as the forces remain balanced for sustained flight.
Other Contributing Factors
- Flaps and Slats: Extendable parts of the wing that increase surface area or change the wing's shape to enhance lift during takeoff and landing.
- Speed: Faster airflow increases the lift. Airplanes need to reach sufficient speed on the runway before takeoff to generate the necessary lift.
By balancing these principles, an airplane achieves flight and maintains stability in the air.
あなた:
What's the proportion of the lift created by airfoil to that by the angle of attack?
ChatGPT:
The proportion of lift generated by the airfoil shape (aerofoil) versus the angle of attack is not a fixed value, as it depends on several factors, including the design of the wing, the flight conditions, and the speed of the aircraft. However, we can break down the relationship qualitatively:
Airfoil Shape Contribution
- The shape of the airfoil primarily contributes to lift at lower angles of attack and during steady, level flight.
- A well-designed airfoil takes advantage of Bernoulli's Principle by creating a natural pressure difference between the upper and lower surfaces. This "baseline" lift exists even when the angle of attack is zero or very small.
Example:
- On commercial airplanes with carefully engineered airfoils, the shape alone can generate a significant portion (50% or more) of the total lift at cruise conditions.
Angle of Attack Contribution
- As the angle of attack increases, the wing produces additional lift due to Newton’s Third Law of Motion. The wing deflects more air downward, creating a stronger upward reaction force.
- At higher angles of attack, the contribution of lift from this deflection becomes dominant, especially during takeoff, climbing, and low-speed flight, when the wing relies on the angle of attack to generate sufficient lift.
Example:
- At higher angles of attack (but below the stall angle, typically around 10–15° for many aircraft), up to 70–80% of the lift can come from the angle of attack rather than the airfoil shape.
Overall Trends
-
Low Angle of Attack:
- Airfoil shape dominates (e.g., during cruising flight).
- Contribution: Airfoil ≈ 70–90%, Angle of Attack ≈ 10–30%.
-
High Angle of Attack:
- Angle of attack dominates (e.g., during takeoff, landing, or slow-speed maneuvers).
- Contribution: Airfoil ≈ 20–50%, Angle of Attack ≈ 50–80%.
Factors Influencing the Proportions
-
Wing Design:
- High-lift wings (e.g., gliders): Airfoil contributes more to lift.
- Flat or symmetric wings (e.g., aerobatic planes): Angle of attack has a larger role.
-
Speed:
- At high speeds, airflow over the airfoil plays a greater role, and the lift generated by shape increases.
- At low speeds, the angle of attack becomes more critical to generate lift.
-
Flaps and Slats:
- Deploying flaps increases the effective curvature of the airfoil, boosting the contribution from airfoil shape.
Summary
- There is no universal ratio, but a general estimate suggests:
- Airfoil contributes more lift at low angles of attack and high speeds.
- Angle of attack contributes more lift at high angles of attack and low speeds.
- Engineers and aerodynamicists use precise calculations, including computational fluid dynamics (CFD) and wind tunnel testing, to determine these contributions for specific wing designs and operating conditions.
あなた:
Some people seem to be saying something that contradicts the explanation of flying based on airfoil and Bernoulli's Principle. What are they saying?
ChatGPT:
The disagreement you're referring to likely stems from a debate in the aviation community about the role of Bernoulli's Principle versus Newton's Third Law in explaining lift. Here's a breakdown of the controversy and what those who challenge the Bernoulli-based explanation might argue:
The Conventional Explanation (Bernoulli and Airfoil Shape)
- Bernoulli's Principle: Faster airflow over the curved top surface of the wing leads to lower pressure above, while slower airflow below results in higher pressure, creating lift.
- Airfoil Design: The curved upper surface of the wing naturally causes air to flow faster, which plays a key role in lift generation.
This explanation emphasizes the pressure differential and often portrays it as the primary reason airplanes fly.
The Challenge to the Conventional Explanation
Critics argue that the conventional explanation is incomplete or misleading for several reasons:
1. Misinterpretation of Airflow Speeds
- Equal Transit Time Fallacy: Many simplified explanations suggest that air traveling over the top of the wing must "meet up" with air traveling under the wing at the trailing edge. This implies that air on top must travel faster because the path is longer.
- Reality: Air on top does move faster, but not because it "meets up" with the air below. Experiments show that air on the top actually arrives at the trailing edge before the air below.
- Critics' Point: The pressure difference exists, but it cannot be solely attributed to this "equal transit time" idea.
2. Neglect of Newton's Third Law
- Critics emphasize that lift can be explained through Newton's Third Law of Motion: the wing deflects air downward (action), and the air pushes back upward on the wing (reaction). This explanation is observable and does not rely on the abstract pressure differential argument.
3. Flat Surfaces Can Fly
- Wings with flat or symmetric shapes (like those on stunt planes or kites) can also generate lift if angled correctly.
- This shows that lift does not always require a specific airfoil shape, challenging the notion that Bernoulli's Principle is the primary mechanism.
- Instead, angle of attack and the deflection of air downward (Newton's laws) become critical.
4. Simplifications Ignore Complex Flow Patterns
- Real-world airflow involves vortices, turbulence, and circulation that cannot be fully explained by Bernoulli's Principle alone.
- Computational fluid dynamics (CFD) and wind tunnel studies show that lift is a result of both pressure differentials and airflow deflection, making the process more complex than a simple Bernoulli-based explanation.
So What Are They Really Saying?
Critics are arguing that:
- Bernoulli's Principle is real, but it is not the sole or even the dominant explanation for lift in all situations.
- Newton's Third Law and airflow deflection are just as important, if not more so, particularly in situations involving high angles of attack or unconventional wing shapes.
- Simplified explanations (like equal transit time) are misleading and fail to capture the full complexity of how lift works.
Revised Understanding of Lift
Most modern explanations now combine Bernoulli's Principle and Newton's Laws:
- Lift is created by a combination of:
- Pressure differences (Bernoulli-based explanation).
- Air deflection and momentum change (Newton-based explanation).
- Vortex and circulation effects, which play a significant role in generating and sustaining lift.
This integrated view avoids oversimplifying the phenomenon and acknowledges that different aspects dominate under different conditions.
