(Part 3) Longitudinal Stability Of Aircraft | Aircraft Axles Stability | Lecture 38
Summary
TLDRThis script delves into the concept of aerodynamic damping and its impact on aircraft stability, particularly during pitching maneuvers. It explains how changes in tailplane angle of attack generate damping forces and discusses the significance of the neutral point and maneuver point in assessing stability. The script further explores the relationship between stick force, load factor, and CG (Center of Gravity) position, emphasizing the importance of maintaining an optimal CG range for adequate maneuverability and control. It also touches on devices like downspring and bob weight for tailoring control forces and concludes with the criticality of longitudinal static stability for safe flight operations.
Takeaways
- 🛫 Aerodynamics play a crucial role in aircraft stability, with damping effects influencing how the tailplane and wing interact during pitching maneuvers.
- 📉 The tailplane's angle of attack changes more significantly than the wing's during pitching, which is essential for understanding static stability.
- 📚 A vector diagram is used to illustrate the relationship between the tailplane, true airspeed, and the speed of tailplane down travel during pitching.
- 🔼 Aerodynamic damping generates an upward force when the aircraft's nose is pitched up, which is vital for maneuver stability.
- ✈️ The true airspeed's effect on tail force changes with altitude, being less pronounced at higher altitudes due to reduced aerodynamic damping.
- 📍 The neutral point and maneuver point are critical concepts in aircraft stability, with the latter always being aft of the former, affecting stability and control.
- 💡 An aircraft's maneuver stability is indicated by a steady increase in stick force with an increase in load factor, which should be neither too high nor too low for optimal control.
- 📉 The stick force per g can be calculated knowing that level flight is at one g, and higher load factors correspond to specific stick force increases.
- 🔑 The position of the aircraft's center of gravity (CG) significantly affects the stick force gradient, with further aft positions reducing the required force for the same load factor.
- 🛠 Designers can adjust stick forces using devices like the downspring for stability at higher airspeeds and the bob weight to proportion stick force to load factor.
- 🚀 Adequate controllability is as important as stability, with the forward CG limit set to ensure sufficient control power during critical flight conditions like maneuvering, takeoff, and landing.
Q & A
What is aerodynamic damping and how does it affect static stability during an aircraft's pitch?
-Aerodynamic damping is the force generated by an increase in the angle of attack of the tailplane during the pitching of an aircraft. It affects static stability by creating an upward force that resists the change in pitch, with the tailplane angle of attack changing more than the wing's, ensuring stability once the pitch angle stops changing.
Why is the tailplane angle of attack change greater than the wing's during pitching?
-The tailplane angle of attack change is greater than the wing's during pitching because the tailplane moves down as the aircraft pitches, which results in a more pronounced angle of attack change compared to the wing.
What is the significance of the vector diagram in the study of maneuver stability?
-The vector diagram is significant in the study of maneuver stability as it illustrates the relationship between the true airspeed vector and the speed of tailplane down travel vector. It helps to understand the effective airflow direction and the resultant increase in angle of attack during pitching, which is crucial for analyzing aerodynamic damping.
How does the true airspeed affect the change in tail force during pitching at higher altitudes?
-At higher altitudes, the true airspeed is faster, which reduces the change in tail force for the same rate of aircraft pitch. This is because the increased speed alters the airflow dynamics and the resultant forces acting on the tailplane.
What is the neutral point in the context of aircraft stability?
-The neutral point is the position at which the center of gravity (CG) would need to be for the destabilizing wing moment and the stabilizing tailplane moment to be the same, ensuring no net moment on the aircraft.
What is the maneuver point and how is it related to the neutral point?
-The maneuver point is the CG position at which the wing moment and tailplane moment balance each other out when considering the change in tail lift due to a gust. It is always aft of the neutral point because it accounts for the additional tail lift during maneuvers.
Why is the aircraft always more maneuver stable than statically stable?
-The aircraft is always more maneuver stable than statically stable because the distance from the maneuver point to the aircraft CG is greater than the distance from the neutral point to the CG, providing a larger margin for control during maneuvers.
What is the relationship between stick force and load factor in an aircraft with positive maneuver stability?
-In an aircraft with positive maneuver stability, there should be a steady increase in stick force with an increase in load factor or 'g'. The stick force per 'g' must be positive, indicating a direct correlation between the force applied to the stick and the load factor experienced by the aircraft.
How do various CG positions affect the stick force gradient?
-The CG position affects the stick force gradient such that a CG further aft reduces the stick force required for the same load factor, while a CG further forward increases the stick force required, affecting the aircraft's controllability.
What is the effect of altitude on stick force per 'g'?
-At higher altitudes, the stick force gradient is reduced due to reduced aerodynamic damping, which in turn affects the control forces felt by the pilot during maneuvers.
What are some devices used to tailor control forces in an aircraft?
-Devices such as the downspring and the bob weight are used to tailor control forces. The downspring increases airspeed stick for stability, while the bob weight provides an increment of stick force proportional to the load factor, helping to prevent the pilot from applying too much 'g' during maneuvers.
What are the critical longitudinal control power requirements for an aircraft during different flight conditions?
-The critical longitudinal control power requirements for an aircraft include having sufficient pitch control power to achieve maximum usable lift coefficient during maneuvers, enough elevator control power for rotation to the takeoff attitude, and adequate elevator control power to rotate the aircraft to the required nose-up attitude during landing.
Why are the forward and aft CG limits set in an aircraft?
-The forward CG limit is set by the minimum permissible controllability to ensure the aircraft can be adequately maneuvered, while the aft CG limit is set by the minimum permissible stability to prevent the aircraft from becoming unstable. These limits ensure safe and effective flight operation.
Outlines
🛫 Aerodynamics and Maneuver Stability
This paragraph delves into the concept of aerodynamic damping and its impact on aircraft stability. It explains how the tailplane angle of attack changes during pitching, creating a moment that affects the aircraft's stability. The discussion moves to maneuver stability, where the tail force changes during pitching maneuvers, generating aerodynamic damping. The text introduces the 'maneuver point' concept, which is the CG position required for balance during gusts, and explains how it relates to the 'neutral point'. The paragraph concludes with insights on stick force gradients and how they're affected by CG position and altitude, emphasizing the importance of maintaining the CG within safe limits for adequate maneuverability.
🔍 Control Forces and Aircraft Stability
The second paragraph focuses on the control forces in an aircraft and how they can be modified for better maneuverability. It discusses the use of devices like the downspring and bob weight to adjust stick forces and prevent overstressing the aircraft during maneuvers. The paragraph also covers the importance of longitudinal static stability and the critical control power requirements during different flight conditions, such as maneuvering, takeoff, and landing. It highlights how the forward CG limit is determined by the minimum permissible controllability, while the aft CG limit is set by the minimum permissible stability. The summary underscores the necessity of keeping the aircraft's CG within the prescribed limits for safe and effective flight operations.
Mindmap
Keywords
💡Aerodynamic Damping
💡Static Stability
💡Tailplane
💡Neutral Point
💡Maneuver Point
💡Stick Force per G
💡CG (Center of Gravity)
💡Bob Weight
💡Downspring
💡Longitudinal Static Stability
Highlights
Aerodynamic damping was first discussed in the context of its effect on static stability during aircraft pitching.
The tailplane angle of attack changes more than the wing's during pitching, due to its downward movement.
A vector diagram illustrates the effective airflow direction during pitching, showing the increase in angle of attack and the resultant up force.
At higher altitudes, the true airspeed is faster, reducing the change in tail force for the same pitch rate.
The neutral point is defined as the CG position where the destabilizing wing moment equals the stabilizing tailplane moment.
The maneuver point is always aft of the neutral point and is critical for understanding aircraft stability during maneuvers.
Aircraft with a CG position further aft have more maneuver stability than static stability due to the greater distance from the maneuver point.
The stick force per g must be positive, neither too high nor too low, to ensure adequate aircraft control during maneuvers.
Calculating stick force per g is essential, remembering that a 2.5 load factor maneuver is only a 1.5 g increase from level flight.
The effect of CG position on stick force gradient is significant, with an aft CG reducing the required stick force for the same load factor.
Altitude affects stick force per g, with higher altitudes reducing the stick force gradient due to decreased aerodynamic damping.
The forward CG limit could be set to prevent excessive stick force gradients, ensuring adequate aircraft maneuverability.
Designers can modify stick forces using devices like the downspring for increased stability and the bob weight for load factor proportionate resistance.
Aircraft must have adequate controllability for critical flight conditions such as maneuvering, takeoff, and landing.
Longitudinal control power is critical for achieving maximum lift coefficient during maneuvers and for takeoff and landing control.
The forward CG limit is set by minimum permissible controllability, and the aft CG limit by minimum permissible stability.
Pilots must keep the aircraft CG within the forward and aft limits at all times for safe and effective flight.
Transcripts
aerodynamic damping was first discussed
when you were studying drag we will now
see how it can affect
static stability
while the pilot is pitching the aircraft
the change in tailplane angle of attack
is greater than the change in angle of
attack of the wing
because the tail plane is moving down
as soon as the pitch angle stops
changing the tail plane and the wing
have effectively the same angle of
attack
for the study of maneuver stability we
need to consider the period during the
change in angle of attack
study the vector diagram it shows the
tail plane
with the true air speed vector and the
speed of tail plane down travel vector
the effective airflow direction is the
resultant of the aircraft through
airspeed
and the speed of tailplane downtravel
while pitching
you can now see the increase in angle of
attack while pitching
which generates an up force that we know
is aerodynamic damping
aerodynamic damping gives a change in
tail force
in this case upwards while the pilot is
pitching the aircraft nose up
at higher altitude at the same indicated
airspeed
the true airspeed is faster
which reduces the change in tail force
for the same rate of aircraft pitch
change
let's consider the diagram we used
earlier to define the neutral point
the neutral point is the position at
which the cg would need to be
for the destabilizing wing moment and
the stabilizing tail plane moment
to be the same if we now add the change
in tail lift
to the change in tail plane lift due to
a gust
the cg would have to be further aft for
the wing moment in the tail plane moment
to balance this cg position
is called the maneuver point
the maneuver point will always be aft of
the neutral point
if we now look again at the aft cg limit
and a sample aircraft cg position
you can see that the aircraft will
always have more maneuver stability
than static stability
this is because the distance from the
maneuver point to the aircraft cg
is greater than the distance from the
neutral point to the cg
we can now take what we have learned
about maneuver stability
and apply it to a study of stick force
versus load factor
an aircraft with positive maneuver
stability should give a steady increase
in stick force
with increase in load factor or g
the stick force per g must be positive
but neither too high
nor too low if the stick force per g is
too high
the aircraft will be difficult and
tiring to fly
if the stick force per g is too low the
aircraft may be accidentally
overstressed
because of the light control forces
you may be asked to calculate stick
force per g
so you need to remember that in level
flight the aircraft is at one g
and a maneuver that gives a load factor
of 2.5
is only a one and a half g increase
the graph shows a typical stick force
gradient
a load factor of 1.6 g
requires a 10 pound pull force
and a load factor of 2g requires a stick
force of 18 pounds
these stick force values don't represent
any particular aeroplane
and are just sample values
let's now consider the effect of various
cg positions on the stick force gradient
starting with the cg at 30 percent mac
with the cg further aft at 40 percent
mac
the stick force gradient is reduced in
that
for the same load factor the stick force
is reduced
with the cg forward at 10 mac
the stick force gradient is increased
for the same load factor the stick force
is higher
we can now consider the effect of
altitude on stick force per g
at higher altitude the stick force
gradient is reduced
this is due to reduced aerodynamic
damping
in conclusion the possibility exists
that the forward cg limit could be set
to prevent an excessive stick force
gradient
and thereby an inability for the pilot
to maneuver the aircraft adequately
as the cg moves aft the stig force
gradient decreases
and the lower limit of stick force
gradient may be reached
and the pilot could be in danger of too
readily being able to
accidentally overstress the aircraft
designers can modify the stick forces if
required
to give adequate maneuverability
throughout the entire flight envelope
the first device that can be used to
tailor the control forces
is the downspring the downspring
is used to increase the airspeed stick
for stability
the downspring consists of a pre-loaded
spring attached to the control system
that tends to rotate the elevator down
the effect is to contribute an increment
of pull force
independent of control deflection or
airspeed
the second device used to tailor the
control forces
is the bob weight the bob weight
consists of an eccentric mass attached
to the pitch control system
the bob weight mass is subjected to the
same change in load factor as the
aeroplane
thus the bob weight will provide an
increment of stick force
in direct proportion to the load factor
applied
this will prevent the pilot applying too
much g during maneuvers
the more the pilot pulls back the more
resistance the bob weight adds to the
control system
an aeroplane must have adequate
controllability
as well as adequate stability
as previously mentioned an airplane with
high longitudinal static stability
will have great resistance to
displacement from equilibrium
there are three conditions of flight
that provide the critical requirements
of longitudinal control power
maneuvering takeoff and landing
any one or a combination of these
conditions can determine the position of
the forward cg limit
we will look first at the maneuvering
control requirements
the aeroplane should have sufficient
pitch control power
to achieve the maximum usable lift
coefficient during maneuvers
the illustration shows that with the cg
forward
at 10 mac the maximum elevator
deflection
is not capable of trimming the airplane
at cl max
but with the cg slightly further aft at
18
mac maximum elevated deflection
is now capable of trimming the airplane
at cl max
now for a look at the takeoff control
requirements
at takeoff the airplane must have enough
elevator control power
to be rotated to the takeoff attitude
anything that increases the nose down
pitching moment
will increase the amount of elevator
deflection required
for the same pitch attitude for example
if the cg is further forward
or the rolling resistance is increased
by for instance
runway contamination
now we can look at landing control
requirements
when flaring to land enough elevator
control power must be available
to rotate the aircraft to the required
nose-up attitude
anything that increases the nose-down
pitching moment
will increase the amount of elevator
deflection required
for the same pitch attitude such as
idle thrust full
flap forward cg
and the aircraft in ground effect
each of the longitudinal control power
requirements are critical
for high longitudinal static stability
if the forward cg limit were
accidentally exceeded
the aeroplane may encounter insufficient
controllability
in any of these conditions
to conclude our study of longitudinal
static stability
it must be remembered that the forward
cg limit
is set by the minimum permissible
controllability
and the aft cg limit is set by the
minimum permissible stability
you must keep your aircraft cg within
the forward and half limits
at all times during the flight
you
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