Aerodynamic Balance - Flight Controls - Airframes & Aircraft Systems #29

Summary

This lesson delves into the aerodynamic balance of flight controls, crucial for aircraft stability and maneuverability. It explains how aerodynamic forces impact control surfaces and the hinge moments they create. The discussion covers various methods to achieve balance, such as inset hinges, horn balances, internal balances, and different types of tabs like balance, anti-balance, servo, and spring tabs. These mechanisms assist pilots in managing control forces and maintaining aircraft stability, especially at high speeds. The lesson also touches on mass balancing to prevent control surface flutter, ensuring flight safety.

Takeaways

• 🔧 The aerodynamic balance of flight controls is crucial for maintaining stability and control during flight.
• ⚖️ Hinge moment, which is the force multiplied by the distance from the hinge to the center of pressure, affects the control surface's rotation.
• 📏 The size of the control surface and the speed of the aircraft are key factors in determining the aerodynamic force generated.
• ✈️ Pilots need to overcome hinge moments by applying force to cockpit controls, which can be challenging in large and fast aircraft.
• 💡 Mechanical advantage, power operation, and moment reduction systems can assist pilots in operating controls in large aircraft.
• 🛠️ Aerodynamic balance techniques like inset or setback hinge, horn balance, and internal balance help reduce hinge moments and improve control feel.
• 🔄 The center of pressure's position relative to the hinge line is critical; it should not move beyond the hinge line to avoid loss of feel or reversed feel.
• 📊 Trailing edge tabs such as balance tabs, anti-balance tabs, spring tabs, and servo tabs are used to adjust control forces and effectiveness.
• 🛂 Balance tabs reduce stick force, anti-balance tabs increase it, and servo tabs assist in moving the control surfaces, while spring tabs adjust assistance based on stick force.
• 🏋️‍♂️ Mass balancing is used to prevent control surface flutter, a high-speed oscillation that can lead to structural failure, by adjusting the center of gravity.

Q & A

What is the hinge moment in the context of flight controls?

The hinge moment is the moment produced when the aerodynamic force acting on a control surface through its center of pressure tends to rotate the control around its hinge in the direction of the force. It is calculated as the force F multiplied by the distance D from the hinge to the center of pressure.

How does the aerodynamic force on a control surface vary with different factors?

The aerodynamic force on a control surface varies with the angle of deflection of the control surface, the size of the control surface, and the speed squared.

Why is it necessary to balance the hinge moment in flight controls?

Balancing the hinge moment is necessary to reduce the force required by the pilot to move and maintain the control surface in a specific position, ensuring easier operation and providing a sense of 'feel' in the controls.

What is an inset or setback hinge and how does it help in reducing hinge moment?

An inset or setback hinge is a method where the hinge is set back in the control surface, positioning it near the center of pressure. This reduces the hinge moment because the airflow strikes the surface forward of the hinge, exerting a force that opposes the hinge moment.

How does horn balance reduce the hinge moment, and where is it commonly found?

Horn balance works by generating aerodynamic forces both fore and aft of the hinge line, which produce turning moments that reduce the overall hinge moment. It is commonly found on rudders and elevators.

What is internal balance and how does it differ from setback hinge and horn balance?

Internal balance involves a balancing area enclosed inside the rear of the main aerofoil section, which forms a chamber split by a flexible diaphragm. The pressure differential inside the chamber produces a moment in opposition to the hinge moment. Unlike setback hinge and horn balance, it does not increase drag because the balancing area is inside the wing.

What are the four main types of trailing edge tab devices mentioned in the script?

The four main types of trailing edge tab devices are the balance tab, the anti-balance tab, the spring tab, and the servo tab.

How does a balance tab assist in reducing control stick forces?

A balance tab moves in the opposite direction to the flying control surface when the pilot moves the control. This generates an aerodynamic force that opposes the hinge moment, thereby reducing the stick force required by the pilot.

What is the purpose of an anti-balance tab, and how does it differ from a balance tab?

An anti-balance tab moves in the same direction as the control surface. It is used to increase the stick force, which can help prevent over-controlling, especially in aircraft with large control surfaces capable of generating powerful aerodynamic forces.

How does a servo tab assist the pilot in controlling the aircraft?

A servo tab is controlled directly by the pilot, and its movement generates an aerodynamic force that moves the main flying control surface. This assists the pilot, especially in larger aircraft where the control forces can be significant.

What is the purpose of mass balancing in flight control surfaces?

Mass balancing is used to reduce control surface flutter, which is an oscillation that can lead to structural failure. It involves moving the center of gravity of the control surface onto its hinge line by adding weights, thereby reducing inertia moments.