Aerodynamic Balance Of Aircraft | Aircraft Aerodynamic Balance | Lecture 43
Summary
TLDRThis 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
🛩️ 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.
🔄 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.
⚙️ 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.
Mindmap
Keywords
💡Flap Type Controls
💡All Moving Slab Tail Plane
💡Spoiler Surfaces
💡Hinge Moment
💡Aerodynamic Balance
💡Inset Hinge
💡Horn Balance
💡Internal Balance
💡Tabs
💡Balance Tab
💡Anti-Balance Tab
💡Servo Tab
💡Spring Tab
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
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