Longitudinal Stability

Question 1

Forward CG On stick force and elevator authority

Answer 1

Increased positive longitudinal static stability
Decreased manners ability and control response - increase moment arm

Less elevator authority
More stick force

Question 2

Aft CG on stick force and elevator authority

Answer 2

Decreased longitudinal stability
Stick force per g decreases

Question 3

Restoring Moment depends on

Answer 3

Tailplane area
Distance between the CG and the tailplane (moment arm)

Question 4

Longitudinal Stability acts around

Answer 4

Longitudinal axis

Question 5

Pitching Moment coefficient

Answer 5

Moment = IAS2 x S x Cm

M = moment about the CG

Question 6

Altitude on longitudinal stability

Answer 6

Reduces with altitude

Question 7

CM/CL Grpah

Answer 7

Absolute angle of attack plotted with pitching moment (CM)

Question 8

Static Margin

Answer 8

Aft CG limit to the AC neutral point

Question 9

What is needed for longitudinal stability (CG position)

Answer 9

CG has to be in front of the neutral point by at least the static margin

Question 10

CM/a Curve - Negative slope
(What stability is this?)

Answer 10

Represents positive static longitudinal stability

Steeper the slope greater the size of the resorting moment

Question 11

CM/CL graph - Negative stability

Answer 11

Positive gradient

Increase in AOA results in a positive pitching moment (divergent from equilibrium)

Question 12

Longitudinal Dihedral - Factor for longitudinal stability

Answer 12

Angle of incidence of wing and stabiliser differ - vary the trim point

Question 13

Longitudinal stability factors

Answer 13

Neutral point - CG must be in front

Tailplane elevators - most important factor - forward cg less effective

Nacelle position - destabilising effect of thrust line

Question 14

Elevator effectiveness - longitudinal stability

Answer 14

Forward CG greatest arm to tailplane AC - large moment therefore greatest longitudinal stability

However large control input are then required (pitch control reduced)

Question 15

Effects of flaps on longitudinal stability

Answer 15

Reduce effectiveness of tailplane
Destabilising - reduced longitudinal stability

Question 16

Stick Position Stability

Answer 16

Controls must move in same direction to trim

Question 17

Calculating stick force g gradient

Answer 17

Stick force / LF - 1

Gs x lb/g

Example utility ac has 160lb/g what is the stick force per g

4.4 - 1 = 3.4gs

3.4 x 160 = 544lbs

Question 18

Stick for per g meaning

Answer 18

More stability means more stick force to destabilise needed

Question 19

Short Period Oscillations

Answer 19

Large rates of change of pitch attitude and AOA - 1 sec to a few seconds

No change in airspeed or altitude

Can provide excessive load factors and damage ac

Question 20

Pilot Induced Oscillation

Answer 20

Short period oscillation
Let go of controls or hold rigid until oscillations cease

Question 21

Phugoid oscillations (long)

Answer 21

Oscillations lasting 1-2 minutes
Attitude/height and airspeed change

Little change in aoa and load factor
Only weakly damped due to less structural risk

Question 22

The pitching moment is much greater at higher IAS because

Answer 22

Dynamic pressure is proportional to V2

Question 23

Factors that increase pitching moment coefficient

Answer 23

Dynamic pressure
Wing area
MAC increase

Think increasing forces that pitch the aircraft up

Question 24

Fuselage gives what pitching moment

Answer 24

Unstable pitching moment as cp is ahead of CG

Question 25

Stick Force Stability

Answer 25

The force on the stick required to change the aircraft’s load factor by 1G

Question 26

CG movement on stick force stability

Answer 26

Moving CG forward will increase stability and reduce manoeuvrability and increase stick force stability Far forward CG - higher stick force per g

Question 27

Manoeuvring and stability

Answer 27

More stable due to the increase in force More stick for per g needed

Question 28

High Altitude on stick force

Answer 28

Decrease in stick force as less density at altitude Pitch damp in reduces at altitude

Question 29

Engine nacelles on stability

Answer 29

Mounted aft of ac above CG - Nose down pitch - positive stability Mounted below the CG - Nose up pitch - negative stability

Question 30

Increase airspeed requires a what force

Answer 30

Increased airspeed means a push force is required on the controls

Question 31

What force is required for a decrease is speed

Answer 31

A pull force required to decrease airspeed

Question 32

How is stick force per g provided to the pilot

Answer 32

Mass attached to the control surface or control column (bob weight) as g increases so too does the weight force produced