Como poner el centro de gravedad de un avion RC. Estabilidad estatica longitudinal. Punto neutro.

Summary

This video script delves into the complexities of determining the center of gravity in model airplanes, debunking the oversimplified rule that it should be between the first quarter and third of the wing. It explains the concept of static longitudinal stability, the role of the neutral point, and how modern combat aircraft like the F-16 are intentionally designed with relaxed stability for maneuverability. The script also emphasizes the importance of balancing stability with aerodynamic efficiency and the need to consider both horizontal and vertical positioning of the center of gravity.

Takeaways

• 😀 The script discusses the common question of where to place the center of gravity in model airplanes and explains that a basic rule is to place it between the first quarter and first third of the wing.
• 🔍 It emphasizes that this basic rule is too simple and a more complete answer is needed, starting with a reminder of how the moment of force is calculated in a two-dimensional plane.
• 🛫 The center of gravity in model airplanes with batteries does not change and depends only on the distribution of the aircraft's mass, unlike commercial airplanes where it can slightly shift as fuel is consumed.
• 📍 The script defines the center of pressure on the wing and explains how it changes with the angle of attack, affecting the aircraft's stability.
• ⚖️ It explains the concept of longitudinal static stability, stating that an airplane is longitudinally stable if it naturally tends to recover its horizontal position after being inclined by the pilot.
• 🔄 The Wright brothers' first airplane was longitudinally unstable, but its slow speed and gentle response allowed the pilot to counteract any movements.
• 🚀 Modern combat aircraft like the F-16 are intentionally designed to be longitudinally unstable to enhance maneuverability, which is managed by computer-assisted flight systems.
• 📉 The script introduces the term 'relaxed stability' and explains how it provides maneuverability but requires quick pilot response, especially at high speeds.
• 📚 Longitudinal stability is determined by the position of the center of gravity relative to the neutral point, which is where the airplane neither accentuates nor counteracts changes in the angle of attack.
• 📉 The script provides a method to find the neutral point and adjust the center of gravity for optimal stability, involving trial flights and adjustments to the control surfaces.
• 🛠 It highlights that excessive longitudinal stability can lead to undesirable behavior, such as oscillation, and that the efficiency of an airplane decreases with increased longitudinal stability.
• 🔄 The script also addresses the importance of vertical center of gravity placement, noting that the thrust line's position relative to the center of gravity affects the airplane's natural inclination to pitch up or down.

Q & A

What is the basic rule for placing the center of gravity in an aeromodeling aircraft?

The basic rule for placing the center of gravity in an aeromodeling aircraft is to position it between the first quarter and the first third of the wing.

What is the concept of the moment of forces in the context of aeromodeling?

The moment of forces is calculated as the product of the force's magnitude and the perpendicular distance from a given point in the plane to the force, known as the force arm.

Why does the center of gravity not change in a battery-powered aeromodeling aircraft?

The center of gravity does not change in a battery-powered aeromodeling aircraft because it only depends on the distribution of the aircraft's masses and does not change with the consumption of energy sources like batteries.

How does the position of the center of pressure on an airplane wing change with the angle of attack?

The position of the center of pressure on an airplane wing changes with the angle of attack, affecting the lift force and the moments of forces acting on the aircraft.

What is the purpose of having the center of gravity in front of the center of pressure in an airplane?

Having the center of gravity in front of the center of pressure allows the main wing's lift force to produce a negative moment, or to the right in straight and level flight, which helps maintain stability.

What are the two methods to achieve a stabilizing moment in the tail of an airplane?

The two methods to achieve a stabilizing moment in the tail are: 1) designing the horizontal stabilizer with a certain downwash, or 2) not using any downwash and adjusting the trim tab so that in its neutral position, it provides the necessary stabilizing moment.

What is meant by longitudinal static stability in an airplane?

An airplane is longitudinally stable if, after the pilot moves the elevator to incline the airplane and then returns it to the neutral position, the natural tendency of the aircraft is to recover the horizontal position, either when inclining upwards or downwards.

Why were the Wright brothers' airplanes longitudinally unstable?

The Wright brothers' airplanes were longitudinally unstable because they were very slow and responded gently, allowing the pilot to always counteract any movement of the airplane in time.

What is the term for modern combat aircraft like the F-16 that are longitudinally unstable?

The term for modern combat aircraft like the F-16 that are longitudinally unstable is 'relaxed stability,' which provides maneuverability essential in military confrontations.

How can a pilot counteract the high maneuverability of a longitudinally unstable aircraft at high speeds?

At high speeds, the tendency of a longitudinally unstable aircraft to exaggerate the pilot's maneuvers can be so abrupt that the pilot may not have time to counteract. Modern military aircraft have computer-assisted flight systems to prevent any catastrophes due to their high maneuverability.

What is the term for the point where the airplane has no tendency to either increase or decrease the angle of attack if the center of gravity is placed there?

The term for this point is the 'neutral point,' where placing the center of gravity results in neither an increase nor a decrease in the airplane's tendency to change the angle of attack.

What is the term for the distance between the center of gravity and the neutral point?

The distance between the center of gravity and the neutral point is called the 'static margin.'

Why might an airplane with too much longitudinal stability exhibit undesirable behavior?

An airplane with too much longitudinal stability might exhibit undesirable behavior because the restoring moment can be too strong, pushing the tail beyond its equilibrium point, causing it to oscillate up and down until the airplane recovers straight and level flight.

How does the position of the center of gravity affect the aerodynamic efficiency of an airplane?

The more forward the center of gravity is with respect to the neutral point, the more the airplane will have to be inclined upwards, creating more drag and reducing aerodynamic efficiency.

What is the effect of the thrust line being above or below the center of gravity on an airplane's natural tendency to pitch?

If the thrust line is above the center of gravity, the airplane will naturally tend to pitch down, which must be counteracted with the elevator. Conversely, if the thrust line is below the center of gravity, it will naturally tend to pitch up.

Why is it important for commercial airplanes to be designed to be stable against lateral wind?

Commercial airplanes are designed to be stable against lateral wind to prevent unwanted yawing during flight. The forces exerted by a lateral wind are proportional to the profile areas of the airplane, and these areas are designed to produce a null resultant moment with respect to the center of gravity.

Why do rear-engine airplanes need the engine to be inclined downward?

Rear-engine airplanes need the engine to be inclined downward to ensure that the thrust line passes through the center of gravity, preventing the airplane from having a natural tendency to pitch up.