(Part 3) Longitudinal Stability Of Aircraft | Aircraft Axles Stability | Lecture 38

Summary

This 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

00:00

🛫 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.

05:02

🔍 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.

Keywords

💡Aerodynamic Damping

Aerodynamic damping refers to the resistance encountered by an aircraft when it changes its angle of attack, particularly during maneuvers such as pitching. This phenomenon generates an additional force on the aircraft, which in this context is discussed as a stabilizing up force that occurs while the pilot is pitching the aircraft nose up. The concept is crucial in understanding how aerodynamic forces contribute to the stability and control of the aircraft during flight.

💡Static Stability

Static stability is the inherent ability of an aircraft to return to its original flight path after a disturbance. The video explains that aerodynamic damping can affect static stability, especially during pitching movements. A key point is that maneuver stability, which involves changes in angle of attack during such movements, often results in an aircraft having greater maneuver stability than static stability.

💡Tailplane

The tailplane, or horizontal stabilizer, is the small wing-like structure located at the tail of the aircraft, which helps to stabilize the aircraft in flight. The script discusses how the tailplane’s angle of attack changes during pitching maneuvers and how this change is greater than that of the wing, influencing the overall aerodynamic stability of the aircraft.

💡Neutral Point

The neutral point is the specific location along the aircraft’s longitudinal axis where the center of gravity (CG) would need to be positioned for the destabilizing wing moment and the stabilizing tailplane moment to be equal. The video discusses the neutral point in the context of aircraft stability, particularly how it relates to the maneuver point, which is always aft of the neutral point.

💡Maneuver Point

The maneuver point is the CG position where the destabilizing moments from the wing and the stabilizing moments from the tailplane balance when considering additional factors such as aerodynamic damping during maneuvers. The maneuver point is critical because it always lies aft of the neutral point, and it is associated with greater maneuver stability compared to static stability.

💡Stick Force per G

Stick force per G refers to the amount of force required on the aircraft's control stick to achieve a change in load factor, or 'G'. The video explains that a positive stick force per G is necessary for maintaining control, but it must not be too high or too low. The explanation includes how different CG positions and altitudes affect this gradient, impacting the ease or difficulty of flying the aircraft.

💡CG (Center of Gravity)

The CG, or Center of Gravity, is the point where the aircraft's mass is considered to be concentrated. The video emphasizes the importance of the CG's position, which affects both the maneuver stability and static stability of the aircraft. It discusses how the CG's position influences stick force per G, maneuverability, and the aircraft's overall stability.

💡Bob Weight

The bob weight is a device attached to the pitch control system of an aircraft, consisting of an eccentric mass that increases the stick force in proportion to the load factor during maneuvers. This system helps prevent the pilot from applying excessive G-forces, thereby preventing potential overstressing of the aircraft. The video explains the role of the bob weight in maintaining control and stability.

💡Downspring

The downspring is a pre-loaded spring in the aircraft's control system designed to increase stick force stability, especially in the airspeed range. It contributes an additional pull force independent of control deflection or airspeed, ensuring that the pilot maintains sufficient control forces to avoid unintentional aircraft maneuvers.

💡Longitudinal Static Stability

Longitudinal static stability refers to the aircraft's resistance to pitch changes from its equilibrium flight path. The video discusses this in the context of CG limits, explaining that the forward CG limit is set by controllability needs, while the aft CG limit is determined by stability requirements. The concept is crucial for understanding how an aircraft maintains its flight attitude and how CG adjustments impact overall flight safety.

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