Aerodynamic Balance Of Aircraft | Aircraft Aerodynamic Balance | Lecture 43

Summary

This script delves into aircraft control surfaces, explaining how flap type controls generate moments by altering aerofoil camber. It contrasts these with all-moving slab tail planes and spoilers, which adjust angle of incidence and disrupt airflow, respectively. The script further discusses hinge moments, stick force, and the various types of aerodynamic balances and tabs that assist pilots in controlling large and fast aircraft, emphasizing the importance of manual checks for control locks and the trade-offs between control effectiveness and reduced hinge moments at different airspeeds.

Takeaways

• 🔧 Aircraft are equipped with flap type controls that generate the required moment about the center of gravity (CG) by altering the camber of the aerofoil.
• 🛫 There are alternative control surfaces like the all-moving slab tail plane, which is used for pitch control when an elevator is insufficient.
• ⚙️ The angle of incidence of the tail plane is changed by the pilot to generate the required pitching moment, either nose up or nose down.
• 💨 Spoiler surfaces can be utilized to assist ailerons in creating the necessary rolling moment by disrupting airflow and increasing drag.
• 💪 The aerodynamic force resisting control surface movement is known as hinge moment, which the pilot must oppose to achieve the desired control deflection, also referred to as stick force or feel.
• ✈️ Larger aircraft require larger control surfaces, which in turn increase the hinge moment, potentially making it difficult for pilots to operate without assistance.
• 🔄 Aerodynamic balance techniques like inset hinges, horn balances, and internal balances are used to reduce the hinge moment by adjusting forces around the hinge line.
• 📊 Tabs are devices that reduce hinge moment by generating forces after the hinge line; they include balance tabs, anti-balance tabs, servo tabs, and spring tabs, each with different effects on control effectiveness and hinge moment.
• 🔄 A balance tab moves opposite to the control surface, reducing hinge moment but also control effectiveness.
• 🔝 An anti-balance tab moves in the same direction as the control surface, increasing hinge moment and control effectiveness.
• 🛠️ A servo tab can be used for manual reversion in the event of hydraulic failure, reducing hinge moment but also control effectiveness unless compensated by a spring tab at low IAS.
• 🔒 Servo tabs require pilots to physically check for control locks before flight to ensure full control surface movement is possible.

Q & A

What is the purpose of flap type controls on an aircraft?

Flap type controls on an aircraft are used to generate the required moment about the center of gravity (CG) by changing the camber of the aerofoil, thereby allowing the pilot to control the aircraft's pitch.

How does an all-moving slab tailplane differ from a traditional elevator in pitch control?

An all-moving slab tailplane is used instead of a traditional elevator when greater control power is needed. It allows the pilot to change the angle of incidence of the entire tailplane to generate the required pitching moment, whereas a traditional elevator only moves a portion of the tail surface.

What role do spoilers play in controlling an aircraft's roll?

Spoilers help the ailerons generate the required rolling moment by disrupting the airflow over the wing, which decreases lift and increases drag on that side of the wing.

What is hinge moment and why is it important in aircraft control?

Hinge moment is the product of the aerodynamic force acting on a control surface and the distance from the hinge line to the center of pressure. It is important because the pilot must oppose this moment to make and hold the required control deflection, affecting the 'stick force' or 'feel' experienced by the pilot.

How does an inset hinge reduce hinge moment?

An inset hinge reduces the hinge moment by positioning the hinge line aft of the control surface's leading edge, thereby reducing the distance between the hinge line and the center of pressure, which in turn reduces the hinge moment.

What is the function of a horn balance in aircraft control surfaces?

A horn balance reduces the hinge moment by placing a small area of the control surface forward of the hinge line at the outboard end. This area generates an aerodynamic force that opposes the hinge moment, helping to reduce the pilot's required input force.

What is an internal balance and how does it function?

An internal balance reduces the hinge moment by using the pressure difference between the upper and lower surfaces of the aerofoil. A seal between the leading edge of the control surface and the trailing edge of the aerofoil helps manage these pressure differences, effectively reducing the hinge moment.

What is a balance tab and how does it affect control surface effectiveness?

A balance tab is a small aerofoil mounted on the trailing edge of the main control surface. When the pilot moves the control surface, the balance tab moves in the opposite direction, reducing the hinge moment and stick force. However, it also reduces the effectiveness of the control surface by opposing the change in camber.

How does an anti-balance tab differ from a balance tab?

An anti-balance tab differs from a balance tab in that it moves in the same direction as the control surface. This increases the hinge moment and stick force, and also enhances the control surface's effectiveness by increasing the change in camber.

What are the advantages and disadvantages of using a servo tab?

The advantage of a servo tab is that it reduces the hinge moment, stick force, and control feel by moving in the opposite direction to the control surface. However, at lower airspeeds (IAS), its effectiveness is reduced, and it requires physical checks to ensure control locks are removed before flight. It can also be used for manual reversion in case of hydraulic failure.

Outlines

00:00

🛩️ Aircraft Control Surfaces and Hinge Moment

This paragraph discusses various types of aircraft control surfaces, such as flap type controls, all-moving slab tail planes, and spoilers, which are used for pitch and roll control. It explains how these surfaces generate the required moment about the center of gravity (CG) and the concept of hinge moment, which is the aerodynamic force that pilots must oppose to make and hold control deflections. The paragraph also touches on the challenges of increased hinge moment in larger and faster aircraft, and the solutions like inset hinges, horn balances, and internal balances to reduce this moment and assist pilots in control.

05:02

🔄 Types of Tabs for Control Surfaces

This section delves into the different types of tabs used to adjust the hinge moment of aircraft control surfaces. It describes the balance tab, which reduces the hinge moment but also decreases control effectiveness; the anti-balance tab, which increases the hinge moment and control effectiveness, suitable for aircraft with low stick forces; and the servo tab, which reduces hinge moment and stick force but also control effectiveness, and is used for manual reversion in case of hydraulic failure. The disadvantages of servo tabs, such as reduced effectiveness at low indicated airspeed (IAS) and the necessity to check for control locks, are also highlighted.

10:04

⚙️ Spring Tabs and Manual Reversion

The final paragraph introduces the spring tab, which, like the servo tab, is moved by the pilot's input but also incorporates a spring mechanism. At low IAS, the spring tab moves with the control surface for maximum effectiveness. As IAS increases, the spring tab moves opposite to the control surface, reducing the hinge moment and stick force but also control effectiveness. The paragraph emphasizes the importance of checking for control locks before flight when using spring tabs and their utility in manual reversion scenarios in the event of powered flying control hydraulic failure.

Keywords

💡Flap Type Controls

Flap type controls are aeronautical devices used on aircraft to generate the required moment about the center of gravity (CG) by changing the camber of the aerofoil. They are integral to the aircraft's maneuverability, allowing for adjustments in lift and drag during flight. In the script, flap type controls are contrasted with other control surfaces that do not change the camber of the aerofoil, highlighting their unique function in aircraft control systems.

💡All Moving Slab Tail Plane

The all moving slab tail plane is an alternative to the traditional elevator used for pitch control in aircraft. It is employed when an elevator would not provide sufficient power. By changing the angle of incidence of the entire tail plane, pilots can generate the necessary pitching moment. The script explains how this mechanism works in generating nose-up or nose-down pitching moments, emphasizing its role in aircraft stability and control.

💡Spoiler Surfaces

Spoiler surfaces are used to disrupt the airflow over a wing, thereby decreasing lift and increasing drag. They assist ailerons in generating the required rolling moment. The script describes how spoilers 'spoil' the airflow, which is crucial for understanding their function in aiding the aircraft's roll and contributing to the overall control dynamics.

💡Hinge Moment

The hinge moment is the product of the aerodynamic force and the distance from the hinge line to the center of pressure of the control surface. It is the force that the pilot must oppose to make and hold the required control deflection. The script discusses how hinge moments affect the stick force or 'feel' that pilots experience, and how various control mechanisms are designed to manage these moments for effective flight control.

💡Aerodynamic Balance

Aerodynamic balance refers to the design features that reduce the hinge moment in aircraft control surfaces. The script outlines different types of aerodynamic balance, such as inset hinges, horn balances, and internal balances, explaining how each contributes to reducing the force required from the pilot to manipulate the control surfaces, thereby enhancing control efficiency.

💡Inset Hinge

An inset hinge is a type of aerodynamic balance where the control surface hinge is positioned behind the leading edge, reducing the distance that the aerodynamic force must act upon, thus reducing the hinge moment. The script describes inset hinges as a common feature on ailerons, elevators, and rudders, emphasizing their importance in manual control systems.

💡Horn Balance

A horn balance is a small area positioned forward of the hinge line on a control surface. It generates an aerodynamic force that opposes and decreases the hinge moment. The script explains that the horn balance is carefully calculated to prevent overbalance and is used specifically on the rudder and elevator, contributing to the aircraft's pitch control.

💡Internal Balance

Internal balance utilizes the pressure difference between the upper and lower surfaces of an aerofoil to reduce the hinge moment. It involves a seal that connects the leading edge of the control surface to the trailing edge of the aerofoil. The script notes that internal balance is very effective and is still used in modern aircraft, including those with powered flying controls, to relieve structural stress.

💡Tabs

Tabs are devices that reduce the hinge moment by generating forces after the hinge line. The script introduces different types of tabs, such as balance tabs, anti-balance tabs, servo tabs, and spring tabs, each serving a specific purpose in adjusting the control feel, hinge moment, and effectiveness of the control surfaces.

💡Balance Tab

A balance tab is a small aerofoil mounted on the trailing edge of the main control surface that moves in the opposite direction to the control surface. The script explains that the aerodynamic force on the balance tab reduces the hinge moment, thereby reducing the stick force and control feel, but at the cost of some control effectiveness.

💡Anti-Balance Tab

An anti-balance tab moves in the same direction as the control surface, increasing the hinge moment, stick force, and control feel. The script describes how the anti-balance tab enhances control effectiveness by increasing the change in camber commanded by the pilot, but it is used cautiously due to its potential to increase control forces required from the pilot.

💡Servo Tab

A servo tab is a control surface that is free to rotate and is moved by the pilot's input, which in turn moves the main control surface. The script details how the servo tab reduces the hinge moment, stick force, and control feel, but also reduces control effectiveness due to its movement in the opposite direction to the control surface. It also discusses the importance of checking control locks when using servo tabs to prevent flight control issues.

💡Spring Tab

A spring tab operates similarly to a servo tab but includes a spring mechanism that provides additional force at low indicated airspeed (IAS), ensuring maximum control effectiveness. The script explains how the spring tab adapts its behavior based on IAS, providing reduced hinge moment at high speeds while maintaining control effectiveness at low speeds, and its utility in manual reversion scenarios.

Highlights

Flap-type controls generate the required moment about the center of gravity by changing the camber of the aerofoil.

The all-moving slab tail plane is used instead of an elevator for pitch control when more power is needed.

Backward movement of the control column decreases the incidence of the tail plane, generating a nose-up pitching moment.

Spoilers help ailerons generate the required rolling moment by disrupting airflow over the wing.

The hinge moment is the product of aerodynamic force and the distance from the hinge line to the center of pressure.

Inset hinges reduce hinge moments by shortening the distance between the hinge line and the center of pressure.

Horn balances on control surfaces reduce hinge moments by generating opposing aerodynamic forces.

Internal balance uses pressure differences between upper and lower aerofoil surfaces to reduce hinge moments.

Balance tabs reduce hinge moments but also slightly decrease control effectiveness.

Anti-balance tabs increase hinge moments and control effectiveness by moving in the same direction as the control surface.

Servo tabs reduce hinge moments and stick force by moving in the opposite direction to the control surface.

Servo tabs allow manual reversion in case of hydraulic failure, ensuring control at high speeds.

Spring tabs provide maximum control effectiveness at low IAS and act like servo tabs at higher IAS.

The design of control tabs must account for both high and low IAS conditions to ensure consistent performance.

Control locks must be checked and removed before flight, especially on aircraft with servo tabs.

Transcripts

it has been stated that aircraft are

fitted with

flap type controls and that flap type

controls

generate the required moment about the

cg by changing the camber of the

aerofoil

there are two other types of control

surface that do not

change the camber of the aerofoil the

all moving

slab tail plane is sometimes used

instead of an elevator for pitch control

when an elevator would not be powerful

enough

the pilot changes the angle of incidence

of the whole tail plane to generate the

required pitching moment

the tail plane pivots around a hinge

when the pilot moves the control column

for and aft backward movement of the

control column decreases the incidence

of the tail plane to generate an

aircraft nose up pitching moment

forward movement of the control column

increases the incidence of the tail

plane to generate an

aircraft nose down pitching moment

spoiler surfaces can be used to help the

ailerons

generate the required rolling moment

spoilers literally spoil or disrupt the

airflow over the wing

which decreases lift and increases drag

spoiler surfaces

will be explained in detail later

the force the pilot must apply to move

the control surfaces into the airflow or

hold the required angle of deflection

must not be too high

when the pilot moves a control surface

into the airflow the airflow resists

an aerodynamic force will try to rotate

the surface around the hinge in the

direction

in which the force is acting the product

of the aerodynamic force and the

distance from the hinge line to the

center of pressure of the control

is called the hinge moment it is the

hinge moment the pilot must oppose

to make and hold the required control

deflection

the pilot refers to the hinge moment as

stick force

or feel if the control deflection is

increased

so is the hinge moment

as the ias is increased the hinge moment

is increased for the same control

deflection

larger aircraft require control surfaces

of larger area

and this will also increase the hinge

moment

so the greater the displacement ias or

surface area

the greater the hinge moment for large

and fast aircraft therefore

the hinge moment could become so high

that the pilot would have

great difficulty in making and holding

the required control input

therefore the pilot may require

assistance to move the controls

this is done either by using some form

of aerodynamic balance or power flying

controls

the first type of aerodynamic balance we

will consider is the inset hinge

we have seen that the hinge moment is a

product of the aerodynamic force

f and the distance of the centre of

pressure from the hinge line

d during manufacture

the control surface hinge is positioned

after the control surface leading edge

this reduces distance d and thus reduces

the hinge moment

most aircraft with manual controls have

inset hinges on the ailerons

elevator and rudder

the designer must make sure that the

control surface center of pressure can

never move forward of the hinge line

this would cause the control surface to

self select or increase in deflection

this is known as over balance and can

never

be allowed to happen

a typical inset hinge looks like this

the area of the control surface forward

of the hinge line

is shielded to prevent it protruding

into the airflow

the second type of aerodynamic balance

we will consider is the horn balance

a small area of the control surface is

positioned forward of the hinge line

at the outboard end the horn generates a

small aerodynamic force that opposes and

therefore decreases

the hinge moment the area of the horn

balance

is carefully calculated so that over

balance is prevented

the horn balance is used on the rudder

and elevator but not the ailerons

the third type of aerodynamic balance we

will consider

is the internal balance which uses the

pressure difference between the upper

and lower surfaces of the aerofoil

to reduce the hinge moment

the internal balance consists of a seal

between the leading edge of the control

surface and the trailing edge of the

aerofoil

the seal is in the form of metal panels

with appropriate hinges to allow

movement

the internal balance is very effective

and is still used on some modern jet

transport aircraft and business jets

even those with powered flying controls

as they relieve the stresses on the

structure where the hydraulic actuators

are attached

the preceding three types of aerodynamic

balance reduce the hinge moment by

adjusting the aerodynamic force

or the arm forward of the hinge line

we will now consider devices that reduce

the hinge moment by generating forces

after the hinge line

these devices are collectively known as

tabs

a small aerofoil mounted on the trailing

edge of the main control surface

the following illustrations are

schematic they just show the operating

principle of the device in reality the

mechanisms will be inside the structure

the first tab we will consider is the

balance tab

the pilot input is connected to the

control surface

so that when the pilot moves the cockpit

control he or she

is directly moving the control surface

there is a link between the fixed

structure and the balance tab

such that when the pilot moves the

control surface the balance tab moves in

the

opposite direction

please note it is movement of the

control surface

that moves the balance tab therefore if

for some reason

the main control surface can't be moved

the balance tab

can't move it is the aerodynamic force

on the balance tab

acting in the opposite direction to the

one on the control surface

which reduces the hinge moment

because the balance tab moves in the

opposite direction to the control

surface

the effectiveness of the control is

reduced

this is because the opposite motion of

the balance tab reduces the change in

camber commanded by the pilot

in practice the designer will compensate

for this

to summarize the balance tab reduces the

hinge moment

stick force and feel the pilot moves the

control surface

the control surface moves the balance

tab

the balance tab moves in the opposite

direction to the control surface

giving a reduction in control

effectiveness

the second type of tab we will consider

is the anti-balance tab

the pilot input is connected to the

control surface

so that when the pilot moves the cockpit

control he or she

is directly moving the control surface

there is a link between the fixed

structure and the anti-balance tab

but this time when the pilot moves the

control surface the anti-balance tab

moves in the same direction

please note the anti-balance tab is the

only tab that moves in the same

direction as the control surface

it is the aerodynamic force on the

anti-balance tab

acting in the same direction as the one

on the control surface

that increases the hinge moment stick

force and feel

because the anti-balance tab moves in

the same direction to the control

surface

the effectiveness of the control is

increased

this is because the motion of the tab

now increases the change in camber

commanded by the pilot

the anti-balance tab is fitted to

aircraft that otherwise would suffer

from either a lack of control

effectiveness

or stick forces that are too low

to summarize the anti-balance tab

increases the hinge moment

stick force and feel the pilot

still moves the control surface directly

and it is the control surface

that once again moves the anti-balance

tab

the anti-balance tab moves in the same

direction as the control surface

giving an increase in control

effectiveness

the third type of tab we will consider

is the servo tab

the servo tab is very different from the

preceding two tabs

the horn is merely pivoted on the

control surface

and is free to rotate

the pilot input directly moves the horn

but this time the link is connected from

the horn to the servo tab

thus when the pilot makes a control

input it is the servo tab that is being

moved

without any mechanical input to the

control surface

it is only when the aircraft has

sufficient ias

the aerodynamic force generated by the

servo tab

can move the control surface

the aerodynamic force on the servo tab

moves the control surface in the

opposite direction

because the pilot is only moving the

servo tap into the airflow

the hinge moment stick force and feel

are reduced

if the aircraft has powered flying

controls and a hydraulic failure occurs

a servo tab can be used for what is

called manual

reversion that is the pilot will still

be able to move the servo tab

and hence the control surface even at

high ias

and maintain control of the airplane

because the servo tab moves in the

opposite direction to the control

surface

the effectiveness of the control is

reduced

the servo tab has other disadvantages

the first disadvantage is that when

flying at lower ias

the smaller aerodynamic force from the

servo tab

will give reduced control surface

movement when in fact the pilot requires

an

increased control surface movement

in practice the designer will compensate

for this

the second and more significant

disadvantage of the servo tab

concerns the fitment of control locks

if your aircraft is fitted with servo

tabs you must

physically check that the control locks

have been removed

before flight because the pilot only

moves the servo tab

the full and free control movement check

in the cockpit

will not alert you to the fact that the

control locks are still engaged

and you will not be able to control the

aircraft

the control locks of some aircraft can

be engaged and disengaged from the

cockpit

and when engaged will prevent access to

a vital item

such as engine start in order to

minimize accidentally trying to take off

with the control locks engaged

to summarize the pilot can move the

servo tab

only the servo tab moves in the opposite

direction to the control surface

and therefore reduces control

effectiveness

the servo tab reduces the hinge moment

the stick force and control feel

the servo tab has reduced effectiveness

at low ias

and you must physically check that

control locks have been removed before

flight

because the pilot is only moving the

servo tab and not the control surface

the servo tab can be used for manual

reversion in the event of hydraulic

failure to powered flying controls

this will be discussed in detail in the

next segment

to overcome the reduced effectiveness of

the servo tab at low ias

a spring tab can be fitted the fourth

type of tab

the horn is merely pivoted on the

control surface and is free to rotate

the pilot input

directly moves the horn

the link is connected from the horn to

the spring tab

when the pilot makes a control input the

spring tab is being moved directly by

the pilot

without any direct input to the control

surface

so far exactly the same as the servo tab

but now you can see there is a spring

fitted between the horn and the control

surface

so that when the pilot makes an input at

low ias

the control surface and the spring tab

move together

giving maximum effectiveness

progressively as the ias increases there

is a greater aerodynamic

force on the control surface that

resists the attempt of the pilot

to deflect it into the airflow causing

the spring to be compressed

compression of the spring makes the

spring tab move in the opposite

direction to the control surface

so the hinge moment stick force and feel

is reduced

the higher the ias the greater the

spring tab deflection

and the more assistance the spring tap

provides

to summarise at low ias

the spring tab doesn't move relative to

the control surface

the pilot moves the control surface and

the spring tab together

for maximum effectiveness at higher ias

the spring tab

acts as a servo tab by moving in the

opposite direction to the control

surface

and therefore reducing control

effectiveness

at high ias the servo tab reduces the

hinge moment

the stick force and control feel

the spring tab can also be used for

manual reversion in the event of

hydraulic failure to powered flying

controls

this will be discussed in detail in the

next lesson