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.

Outlines

00:00

🛫 Aerodynamics of Flight Controls

This paragraph discusses the aerodynamic balance of flight controls, explaining how aerodynamic forces act on control surfaces and create hinge moments. It details how the size of the control surface and the speed of the aircraft affect these forces. The paragraph also introduces the concept of reversible controls and the need for mechanical or hydraulic assistance in larger aircraft. Various methods to achieve aerodynamic balance, such as inset or setback hinges, horn balance, and internal balance, are explored. These methods aim to reduce the hinge moment and assist pilots in operating manual flight controls with a reasonable amount of feel.

05:00

🔍 Control Surface Balancing Techniques

Paragraph 2 delves into different types of trailing edge tabs used to balance control surfaces: balance tabs, anti-balance tabs, spring tabs, and servo tabs. It explains how these tabs function to either reduce or increase stick force, depending on their design. The balance tab is highlighted for reducing control forces, while the anti-balance tab is used to increase them, which is beneficial for preventing over-control especially in high-speed aircraft. The servo tab assists the pilot by moving the main flying control surface, and the spring tab adjusts assistance based on the aerodynamic load. The paragraph emphasizes how these systems help manage control forces and maintain control effectiveness.

10:07

🔧 Advanced Control Surface Balancing and Flutter Prevention

The final paragraph focuses on advanced balancing techniques, particularly mass balancing, which is crucial for preventing control surface flutter at high speeds. Flutter is an oscillation that can lead to structural failure if not managed. This section describes how mass balancing, achieved by adjusting the center of gravity of the control surface, can mitigate flutter. It also mentions that this method is applicable to various control surfaces like elevators, ailerons, and rudders. The paragraph concludes with a summary of the lesson, emphasizing the importance of aerodynamic balance in flight controls and providing a list of balanced systems discussed.

Keywords

💡Aerodynamic balance

Aerodynamic balance refers to the equilibrium of forces acting on an aircraft's control surfaces, ensuring stable and predictable flight behavior. In the context of the video, achieving aerodynamic balance is crucial for the pilot to effectively control the aircraft without excessive forces on the cockpit controls. The video discusses various methods to achieve this balance, such as using inset hinges, horn balance, and internal balance, which are all designed to reduce the hinge moment and make the controls more manageable for the pilot.

💡Control surface

A control surface is a movable part of an aircraft's wing or tail that is used to control the aircraft's flight path. Examples include ailerons, elevators, and rudders. The video script explains how aerodynamic forces acting on these surfaces are managed to achieve the desired balance and control. The size and angle of deflection of control surfaces directly influence the aerodynamic forces and moments, which in turn affect the aircraft's maneuverability.

💡Hinge moment

The hinge moment is the turning force produced by the aerodynamic force acting on a control surface around its hinge. It is calculated as the force multiplied by the distance from the hinge to the center of pressure. In the video, reducing the hinge moment is a key aspect of achieving aerodynamic balance, as it affects the stick force required by the pilot to manipulate the control surfaces. Methods like inset or setback hinges are used to reduce this moment, making the controls feel lighter and more responsive.

💡Center of pressure

The center of pressure is the point on a control surface where the net aerodynamic force is considered to act. It is a critical concept in the video as it influences the hinge moment and the effectiveness of the control surface. The video explains that the position of the center of pressure relative to the hinge line is crucial for maintaining control feel and preventing control reversal, which could lead to loss of control.

💡Reversible controls

Reversible controls are manual flight control systems that allow some movement of the control surfaces in response to aerodynamic forces, even when the pilot is not actively controlling them. This is in contrast to irreversible systems, which only move in response to pilot input. The video mentions that manually operated flight controls are reversible, meaning they can provide feedback to the pilot about the aerodynamic forces acting on the aircraft, enhancing the pilot's feel for the aircraft's state.

💡Inset or setback hinge

An inset or setback hinge is a design feature where the hinge of a control surface is positioned rearward, closer to the center of pressure. This arrangement helps reduce the hinge moment, as explained in the video, by positioning the center of pressure closer to the hinge line. This design improves the control's feel for the pilot and reduces the force needed to move the control surface, contributing to aerodynamic balance.

💡Horn balance

Horn balance is a method of achieving aerodynamic balance by incorporating a 'horn' or extension on the control surface forward of the hinge line. When the control surface is deflected, aerodynamic forces act on the horn to produce a moment that opposes the hinge moment, as described in the video. This reduces the overall hinge moment and the stick force required by the pilot, while maintaining control effectiveness.

💡Internal balance

Internal balance is a technique where a balancing area is enclosed within the rear of the main aerofoil section of a control surface. The video explains that this design uses a pressure differential within a divided chamber to create a moment that opposes the hinge moment. This method is advantageous as it reduces hinge moments without increasing drag, unlike external balancing methods like horn balance.

💡Trailing edge tabs

Trailing edge tabs are small, hinged surfaces at the rear of a control surface used to adjust the aerodynamic forces and moments acting on the control surface. The video discusses four types of tabs: balance tabs, anti-balance tabs, spring tabs, and servo tabs. Each serves a different purpose, such as reducing or increasing stick force, or assisting the pilot in moving the control surfaces. These tabs are crucial for fine-tuning the control forces and enhancing the pilot's ability to manage the aircraft.

💡Mass balancing

Mass balancing is a method used to prevent control surface flutter, which is an oscillation that can lead to structural failure if not controlled. The video explains that by adding weights to the control surface to move its center of gravity onto the hinge line, the inertia moments about the hinge are reduced. This technique is particularly important at higher speeds where flutter is more likely to occur, and it is applied to various control surfaces like elevators, ailerons, and rudders.

Highlights

The aerodynamic force on a control surface creates a hinge moment that pilots must balance.

Aerodynamic force varies with control surface angle, size, and speed squared.

Pilots need to overcome hinge moments to move and hold control surfaces in position.

For large and fast aircraft, hinge moments can be too high, requiring assistance for control.

Mechanical advantage, power operation, or moment reduction systems can assist with control.

Modern large aircraft use hydraulic power for flight control systems.

Smaller aircraft rely on mechanical systems where pilots provide the motive force.

Manually operated flight controls are called reversible controls due to aerodynamic forces.

Aerodynamic balance helps reduce hinge moments, improving control manageability.

Inset or setback hinge placement reduces hinge moments by positioning near the center of pressure.

Horn balance uses a forward part of the control surface to reduce hinge moments.

Internal balance employs a chamber inside the wing to counteract hinge moments.

Trailing edge tabs are small sections that can adjust control surface forces and moments.

Balance tabs reduce stick force but can slightly decrease control effectiveness.

Anti-balance tabs increase stick force to prevent over-controlling on large surfaces.

Servo tabs assist pilots by moving the main flying control through tab adjustments.

Spring tabs provide variable assistance based on stick force and aerodynamic loads.

Mass balancing reduces control surface flutter by moving the center of gravity onto the hinge line.

Flutter can lead to structural failure if not controlled by mass balancing or other means.

Transcripts

this lesson covers the reasons for

aerodynamic balance of flight controls

and the various methods that are

employed to achieve this balance

the aerodynamic force acting on a

control surface through its center of

pressure will tend to rotate the control

around its hinge in the direction of the

force the moment produced will be the

force F multiplied by the distance D

from the hinge to the center of pressure

this is known as the hinge moment

the aerodynamic force will vary with the

angle of deflection of the control

surface

the size of the control surface and the

speed squared the size of the control

surface is usually fixed however as

speed increases the aerodynamic force

generated greatly increases

to move the flying control surface to

the required angular displacement and

maintain it in that position the pilot

has to overcome and then balance the

hinge moment by applying a force to the

cockpit control the cockpit control load

or stick force will therefore depend on

the size of the hinge moment up to a

point this is a good thing as it

produces feel in the controls

however for large and fast aircraft the

resulting force could give Hinch moments

and related stick forces which will be

too high for easy operation of the

controls the pilot will require

assistance to move the controls in these

conditions and this can be done by using

mechanical advantage or by power

operation both of which will be

discussed in another lesson or by using

a system to reduce or oppose the moment

modern large aircraft have flight

control systems operated by hydraulic

power

however smaller aircraft will have a

mechanical system where the pilot

provides the motive force having moved

the control surface he will have to

maintain a force on the control to

oppose the aerodynamic force which will

be attempting to reverse the operation

and return the surface to the neutral

position for this reason manually

operated flight controls are known as

reversible controls

aerodynamic balance is a common method

of helping the pilot of an aircraft with

manual controls to move the controls

while still leaving him with a

reasonable amount to feel aerodynamic

balance involves using the aerodynamic

forces on the control surface to reduce

the hinge moment it may be achieved in

several ways and we will now have a look

at some of the methods used

the first method we should look at is

called the inset or setback hinge

setting the hinge back in the control

surface puts it near at the center of

pressure causing the hinge moment to be

reduced furthermore the airflow strikes

the surface forward of the hinge

exerting a force which opposes the hinge

moment and helps the pilot to move the

control

setting the hinge back does not reduce

the effectiveness of the control only

the hinge moment of the force is reduced

not the force itself

it is important that the center of

pressure is not moved too close to the

hinge line as when a control is operated

its center of pressure moves forward if

this was to take the center of pressure

forward beyond the hinge line an all

feel would be removed or even reversed

the principle of horn balance is similar

to that of the set back hinge the horn

is part of the control surface and is

forward of the hinge line

in flight when the control surface is

displaced as shown in the diagram

aerodynamic forces will be generated

both fore and aft of the hinge line this

produces turning moments about the hinge

reducing the overall hinge moment once

again control effectiveness is

unaffected

design of the horn balance will mean

that during operation the horn extends

above or below the surface of the main

aerofoil section this will cause an

increase in drag

horn balance is commonly found on

rudders and elevators

internal balance works on the same

principle as the setback hinge but the

balancing area is enclosed inside the

rear of the main aerofoil section this

forms a chamber split into two halves by

a flexible diaphragm the areas of which

will feel the same changes in pressure

as a produced above and below the

control surface the pressure

differential inside the chamber will

produce a moment in opposition to the

hinge moment

in our example the control surface is

moved down pressure above the aerofoil

is reduced and pressure below it is

increased the reduced pressure is felt

on the upper surface of the balance and

the increased pressure on the lower

surface

on the balance therefore gives a hinge

moment which is in opposition to the

hinge moment on the main control surface

and the overall hinge moment is reduced

the advantage of this system over the

previous two is that because the

balancing area is inside the wing there

is no increase in drag

all of the previous solutions provide

balance by causing some of the pressures

on the control surface to act forward of

the hinge line stick force can also be

changed by small aerofoil tabs

positioned at the rear of the control

surface but these do alter the

effectiveness of the flying control

there are four main types of trailing

edge tab device they are

the balance tab

the anti balanced tab

the spring tab

and the servo tab

these tabs are small error for sections

hinged at the trailing edge at the

flying control surface their actual size

will vary from aircraft aircraft we will

now take a look at each of these tabs in

greater detail

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we will first consider the balance tab

the pilot has no direct control over tap

movement the pilots inputs to the flying

control are transmitted by linkage to

move the balance tab in the opposite

direction to the flying control surface

the pilot moves the control surface the

control surface moves the tab

the balance tab generates an aerodynamic

force in the opposite direction to the

flying control surface this reduces the

hinge moment and stick force which will

also give some reduction in control

effectiveness

aircraft fitted with large elevators or

stable aters a capable of generating

very powerful aerodynamic forces due to

their large surface area for relatively

small control deflections especially at

high speeds the forces produced could

cause the pilot to over control

if this were the case an increase in

stick force would reduce the possibility

of over controlling this can be achieved

by the use of an anti balance tab

you can see from the graphic the

similarity between a balance and an anti

balance tab the difference is that the

arrangement of the control linkage

causes the anti balance tab to move in

the same direction as the control

surface it can be seen from the graphic

that both the tab and the control

surface force act in the same direction

anteye balance tabs are commonly used on

all flying tail planes otherwise known

as stable ators remember the balance tab

reduces stick force while the anti

balance tab increases it

the servo tab is used to assist the

pilot in moving the control surface it

differs from the previous two tab

controls in that the pilot actually

controls the tab and not the control

surface movement of the tabs generates

an aerodynamic force which moves the

main flying control

view the sequence of events in the

graphic which depicts an elevator with a

servo tap attached as the pilot selects

the nose up pitch note that depicts the

tab moves down causing the elevator to

move up this does not affect the way the

pilot operates his cockpit controls

you can use your mouse to step through

the operation of the servo tab each

click in the box will move it forward

one step

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if the aircraft is stationary on the

ground movement of the cockpit control

will give no movement of the control

surface only of the tab the control

becomes effective as speed increases

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it should also be noted that if external

control locks are fitted to the control

surface the copic control will still be

free to move as the tab is not normally

locked

servo tabs tend to be used on larger

aircraft where the weight due to the

physical size of the flight controls let

alone their aerodynamic forces will

produce very large stick forces the

Boeing 737 for instance normally uses

hydraulic power for its flight controls

but has servo tabs as a backup in the

event of hydraulic system failure

the final tab we are going to look at is

the spring tab it is a modification of

the balanced tab

such that the tap movement is

proportional to the applied stick force

maximum assistance is therefore obtained

when the stick forces are greatest this

is achieved by putting a spring in the

linkage to the tab

the spring tap is used mainly to reduce

control loads at high air speeds

the pilots control movement is

transmitted to a lever pivoted on the

primary control surface but not directly

operating it operation of the control

surface is via Springs

with low aerodynamic loads the springs

are strong enough not to be compressed

so the control surface moves with

control input and there is no change in

the position of the trim tab relative to

the control surface the full load is

felt by the pilot

as the aerodynamic loads increase the

spring force will be overcome causing

the balance tab to move in the opposite

direction to the primary control surface

thus reducing the stick forces

there is one more form of balancing that

is applicable to flying control surfaces

but this time it has nothing to do with

assisting the pilot to operate the

controls or to reduce stick force it is

known as mass balancing and its purpose

is to reduce control surface flutter

is a phenomenon associated with the

higher speeds of aircraft it is an

oscillation of the control surface

coupled with an oscillation and bending

or twisting of the wing fin or tail

plane it can occur on aircraft with

manual or powered flight controls

flutter is likely to occur if the center

of gravity of the control surface is

behind the hinge line uncontrolled

flutter can cause structural failure

flutter can be prevented by moving the

center of gravity of the control surface

onto its hinge line by adding weights to

the control this reduces the inertia

moments about the hinge line

this is known as mass balancing and it

can be applied to elevators ailerons and

the rudder common methods of mass

balancing are shown in the picture

this picture shows the mass balancing of

the aircraft's ailerons

that is the end of the lesson on

aerodynamic balance remember that

manually operated flight controls are

known as reversible controls shown on

the screen is a list of all the balanced

systems discussed in this lesson you can

click on a system to see a short summary

of its purpose and operation

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