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POF > Longitudinal Stability > Flashcards

Longitudinal Stability Flashcards

1
Q

Forward CG On stick force and elevator authority

A

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

Less elevator authority
More stick force

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2
Q

Aft CG on stick force and elevator authority

A

Decreased longitudinal stability
Stick force per g decreases

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3
Q

Restoring Moment depends on

A

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

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4
Q

Longitudinal Stability acts around

A

Longitudinal axis

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5
Q

Pitching Moment coefficient

A

Moment = IAS2 x S x Cm

M = moment about the CG

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6
Q

Altitude on longitudinal stability

A

Reduces with altitude

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7
Q

CM/CL Grpah

A

Absolute angle of attack plotted with pitching moment (CM)

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8
Q

Static Margin

A

Aft CG limit to the AC neutral point

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9
Q

What is needed for longitudinal stability (CG position)

A

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

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10
Q

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

A

Represents positive static longitudinal stability

Steeper the slope greater the size of the resorting moment

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11
Q

CM/CL graph - Negative stability

A

Positive gradient

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

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12
Q

Longitudinal Dihedral - Factor for longitudinal stability

A

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

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13
Q

Longitudinal stability factors

A

Neutral point - CG must be in front

Tailplane elevators - most important factor - forward cg less effective

Nacelle position - destabilising effect of thrust line

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14
Q

Elevator effectiveness - longitudinal stability

A

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

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

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15
Q

Effects of flaps on longitudinal stability

A

Reduce effectiveness of tailplane
Destabilising - reduced longitudinal stability

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16
Q

Stick Position Stability

A

Controls must move in same direction to trim

17
Q

Calculating stick force g gradient

A

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

18
Q

Stick for per g meaning

A

More stability means more stick force to destabilise needed

19
Q

Short Period Oscillations

A

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

20
Q

Pilot Induced Oscillation

A

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

21
Q

Phugoid oscillations (long)

A

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

22
Q

The pitching moment is much greater at higher IAS because

A

Dynamic pressure is proportional to V2

23
Q

Factors that increase pitching moment coefficient

A

Dynamic pressure
Wing area
MAC increase

Think increasing forces that pitch the aircraft up

24
Q

Fuselage gives what pitching moment

A

Unstable pitching moment as cp is ahead of CG

25
Q

Stick Force Stability

A

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

26
Q

CG movement on stick force stability

A

Moving CG forward will increase stability and reduce manoeuvrability and increase stick force stability

Far forward CG - higher stick force per g

27
Q

Manoeuvring and stability

A

More stable due to the increase in force

More stick for per g needed

28
Q

High Altitude on stick force

A

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

29
Q

Engine nacelles on stability

A

Mounted aft of ac above CG - Nose down pitch - positive stability

Mounted below the CG - Nose up pitch - negative stability

30
Q

Increase airspeed requires a what force

A

Increased airspeed means a push force is required on the controls

31
Q

What force is required for a decrease is speed

A

A pull force required to decrease airspeed

32
Q

How is stick force per g provided to the pilot

A

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

POF (25 decks)

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