How Center of Gravity Affects Flight | Tail Down Force | Aircraft Stability

Summary

This script delves into the balance of forces on an aircraft during straight and level flight, focusing on the equilibrium between lift and weight. It explains the concept of the center of lift and center of gravity (CG), and how their positions relative to each other affect the aircraft's stability and stall characteristics. The video discusses how a forward CG enhances stability but increases drag, while a rearward CG improves efficiency and reduces stall speed, albeit at the cost of stability. The script also touches on the practical implications of CG placement on takeoff and the importance of adjusting elevator input accordingly.

Takeaways

• ✈️ In straight and level flight, the lift force equals the weight force, and these forces are in equilibrium.
• 🔄 The center of lift (center of pressure) is roughly at the midpoint between the leading and trailing edges of the wing.
• 🎢 The center of gravity is typically in front of the center of pressure, enhancing the aircraft's stability.
• 📉 If the center of pressure is behind the center of gravity, the aircraft will have a tendency to pitch down.
• 🛫 The horizontal stabilizer creates a tail down force to counteract the pitch down tendency.
• 📈 Increasing the angle of attack can increase lift, which is necessary if the center of gravity moves forward.
• 📉 A forward center of gravity increases stability but can also increase stall speed and reduce efficiency.
• 🚀 A more rearward center of gravity can improve aerodynamic efficiency and potentially allow for faster speeds.
• 📉 The placement of the center of gravity affects the stall speed, with a forward CG leading to higher stall speeds.
• 📏 The center of gravity is defined relative to a reference point, often near the engine firewall, and can be adjusted by moving weight.
• 🛣️ On the ground, the aircraft's nose heaviness due to a forward center of gravity requires additional back elevator input during takeoff.

Q & A

What are the four forces acting on an aircraft in straight and level unaccelerated flight?

The four forces acting on an aircraft in straight and level unaccelerated flight are lift, weight, thrust, and drag.

Where is the center of lift, or center of pressure, located on an aircraft?

The center of lift, or center of pressure (CP), is roughly at the midpoint between the leading and trailing edges of the wing and can move around in flight.

What is the significance of the center of gravity (CG) in an aircraft?

The center of gravity (CG) is the pivot point of the aircraft, similar to balancing a model airplane on a finger. It's typically located in front of the center of pressure for stability.

Why is it important for the CG to be in front of the CP in a well-loaded aircraft?

Having the CG in front of the CP ensures stability by creating a pitch-down tendency when the CP is behind the CG, which is counteracted by the tail down force (TDF).

How does the horizontal stabilizer help maintain level pitch attitude in an aircraft?

The horizontal stabilizer maintains level pitch attitude by exerting a downward force on the tail, known as tail down force (TDF), which resists the pitch-down movement.

What happens to the required lift force when the CG is moved forward in an aircraft?

When the CG is moved forward, the pitch-down tendency increases, requiring greater tail down force and, consequently, a higher angle of attack and more lift to maintain equilibrium.

How does the position of the CG affect the stall characteristics of an aircraft?

A forward CG increases stability and protects the aircraft in a stall by causing the aircraft to pitch down more rapidly when the lift decreases, aiding in stall recovery.

What is the trade-off of having a more rearward CG in terms of aerodynamic efficiency and stall speed?

A more rearward CG can increase aerodynamic efficiency and allow for faster flight speeds by reducing the required tail down force and angle of attack, but it can also make the aircraft less stable and more difficult to recover from a stall.

How does the placement of the CG affect the stall speed of an aircraft?

A forward CG increases the stall speed because it requires a larger tail down force and a higher angle of attack, while a rearward CG decreases the stall speed by reducing the angle of attack.

What are 'arms' in the context of aircraft loading and how are they measured?

Arms are positions on the long axis of an aircraft, measured as distances in inches from a predetermined reference point, often near the engine firewall, with positions in front of this point having negative measurements.

Why is the CG's position relative to the main wheels important during takeoff in a tricycle gear aircraft?

A well-loaded aircraft typically has its CG in front of the center of pressure, which also places it in front of the two main wheels in a tricycle gear. This makes the aircraft nose-heavy on the ground, requiring extra back elevator input to pitch up during takeoff.