Longitudinal Stability and Neutral Point

AE-705 Introduction to Flight Lecture-15 Capsule-08

AE-705: Introduction to Flight

Vijay Gaikwad

VJTI, Mumbai

Longitudinal Stability

and

Neutral Point


Reaction of aircraft when disturbed

https://www.nasa.gov/centers/glenn/home/

• Returns to its initial positionStable

• Departs from equilibriumStatic

Instability

• Oscillates with larger amplitudeDynamic

Instability


STATIC STABILITY

Initial Reaction on disturbing an aircraft !


Definition: When disturbed from an equilibrium position, the aircraft tends to

return to its original position, without any action from the pilot

Static Stability

Types :

I. Positive static

II. Neutral static

III. Negative static

www.boldmethod.com

positive-static.jpg

neutral-static.jpg

negative-static.jpg


Ball remains in new position when disturbed

Neutral Static Stability

NEUTRAL


Positive Static Stability

STABLE

Ball returns to starting position when disturbed


Ball moves away from starting position when disturbed

Negative Static Stability

UNSTABLE


Flight Conditions

http://history.nasa.gov


https://youtu.be/NciePa8sY_U

Example of an unstable aircraft


DYNAMIC STABILITY

Final Outcome of disturbing an aircraft !


Dynamic Stability

Positive

Oscillations decrease in amplitude

Neutral

Oscillations constant in amplitude

Negative

Oscillations increase in amplitude

www.boldmethod.com

negative-dynamic.jpg

neutral-dynamic.jpg

positive-dynamic.jpg

neutral-dynamic.jpg

negative-dynamic.jpg


https://www.youtube.com/watch?v=YJ1Tt1zSyH4

A Quick Summary


THE 3 CENTRES OF AN AIRCRAFT

Their relative location affects Stability


https://zh.scribd.com/document/334413404/XFLR5-and-Stability-analysis-pdf

The Three Key Centers

TWO

THREEONE



CENTER OF GRAVITY

TWO

THREE



CENTER OF PRESSURE

THREECENTER OF GRAVITY



Centre of Pressure



NEUTRAL POINT

CENTER OF PRESSURE

CENTER OF GRAVITY


• position of CG where the aircraft would be Neutrally stable.

Neutral Point

The basics of R/C model Aircraft design . Author–Andy Lennon ,ISBN:0-911295-40-2. chapter -06,page 27.



All three Centers together!

CENTER OF PRESSURE

NEUTRAL POINTCENTER OF GRAVITY


THE FOURTH CENTRE !

Aerodynamic Center



Aerodynamic Centre


EFFECT OF RELATIVE LOCATION

Example of a toy plane

Source: Understanding Flight, D. F. Anderson, S.Eberhardt , 2nd ed, McGraw-Hill, Ch-4, pp. 102 ,ISBN-13: 978-0071626965


Perturbation in flight increase Angle of Attack

Lwing ↑ but Ltail ↓, hence net Nose down Moment created

Aircraft is Stable due to this restoring torque


When CG is ahead of NP


Position of AC is independent of angle of attack

Moments due to ↑ Lwing & ↑ Ltail balance

There is no restoring torque

Aircraft is neutrally stable due to this balance of moments

Source: Understanding Flight, D. F. Anderson, S.Eberhardt , 2nd ed, McGraw-Hill, Ch-4, pp. 102, ,ISBN-13: 978-0071626965

When CG is at NP


As the nose pitches up, ↑ Lwing > ↑ Ltail

The aircraft nose pitches up further

Aircraft is unstable due to unbalanced moments

When CG is behind NP


WHY ??


EFFECT OF CG POSITION

w.r.t. AC and NP


CG ahead of AC and NP

Andy Lennon, The basics of R/C model Aircraft design, Chapter-06, pp. 27, ISBN:0-911295-40-2

Aircraft is HIGHLY stable !



Aircraft is stable !

CG at AC but ahead of NP



Aircraft is LESS stable !

CG behind AC but ahead of NP



Aircraft is NEUTRALLY STABLE !

CG behind AC and at NP


In summary : Effect of CG Position



EFFECT OF CG ON FLIGHT

Effect of CG on Longitudinal Stability of plane


Longitudinal stability

Payload position CG Position

BOTTOM VIEWTOP VIEW

5

4

3

2

1

1 2 3 4 5


1

BOTTOM VIEWTOP VIEW

BACK

Payload Location CG Location

CG Position at 1

0

100

200

300

0 100 200 300 400 500 600

C:/Data/dee/official/acads/aactive_sem/AE705/lectures/Capsule 08_Stab_Control/Long_Stat_Stability/w_1.mp4


2

BOTTOM VIEWTOP VIEW

BACK


CG Position at 2

0

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300

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3

BOTTOM VIEWTOP VIEW

BACK


CG Position at 3

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4

BOTTOM VIEWTOP VIEW

BACK


CG Position at 4

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5

BOTTOM VIEWTOP VIEW

BACK


CG Position at 5

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Flights with different CG position

BACK

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CG at 1

CG at 2

CG at 3

CG at 4

CG at 5


Why ??

Lets see !


Vs α graph

𝛼

2

3

4

5

𝛼

1


Comparison of Flight Path with Graph

0

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CG at 1

CG at 2

CG at 3

CG at 4

CG at 5

Different CG positions Corresponding α positions

𝛼

1

3

4

5

2


NEUTRAL POINT

Longitudinal stability concept explanation


Longitudinal stability

Payload position CG Position

BOTTOM VIEWTOP VIEW

5

4

3

1

1 3 4 5


Comparison of flights (CG at 1)

without Vertical Gust with Vertical Gust

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C:/Data/dee/official/acads/aactive_sem/AE705/lectures/Capsule 08_Stab_Control/Long_Stat_Stability/h_1.mp4


Comparison of flights(CG at 3)


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Back




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Back

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Flights with changing AOA

Angle of attack = 4°

Angle of Attack = 8°

Angle of Attack = 12°

Angle of attack = 0°

aoa4.mp4

aoa8.mp4

aoa12.mp4

aoa0.mp4


Comparison of flights

Documents

Longitudinal Stability and Neutral Point