Aerodynamic Balance - Flight Controls - Airframes & Aircraft Systems #29
Summary
TLDRThis 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
🛫 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.
🔍 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.
🔧 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.
Mindmap
Keywords
💡Aerodynamic balance
💡Control surface
💡Hinge moment
💡Center of pressure
💡Reversible controls
💡Inset or setback hinge
💡Horn balance
💡Internal balance
💡Trailing edge tabs
💡Mass balancing
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
you
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
you
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
you
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
you
you
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