Ch 6 - The Effect Of Mass An Its Distribution

Question 1

If an aircraft is too heavy:

Answer 1

V speeds will be impacted

Rate of climb decreases

Gradient of climb decreases

Lower cruise ceiling therefore decreased efficiency

Increased Fuel Burn

Decreased Range

Question 2

Too Heavy effects 2:

Answer 2

Landing roll will be longer

Lower approach speed, margins above the threshold will be lower

Increased breaks and tire wear

Increased landing speed

Question 3

CG Too Far Aft effects

Answer 3

Pitching up moment which could be a problem for TO - Tailstrike

Decreased longitudinal Static Stability

Decreased cruise fuel burn due to there being less trim drag

Stall speed will reduce

Poor landing characteristics

Range and endurance increased

Insufficient nose down authority

Question 4

Forces Experienced by an airframe

Answer 4

Twisting (Torsion)

Tension (stretching)

Compression

Shear

An AC will be exposed to multiple stress inputs at the same time

If there is a large enough stress - the airframe could fail (break/snap)

Question 5

Ultimate Load Limit

Answer 5

Is based on the load factor

The airframe will start to fail at this point

Design load limit has a 50% safety factors over and above the ultimate load limit

Question 6

Fatigue

Answer 6

Metal fatigue is caused by multiple applications of stress below the ultimate load limit

Can cause the structure to eventually fail well below the ultimate limit

Cycles of compression/tensions produce fatigue rapidly when the stresses are larger

A med-long term source of damage

Question 7

Fatigue Life

Answer 7

Determined by the manufacturer based on the number of hours flown or number of fatigue cycles (TO and landings)

Question 8

How do we protect against failure

Answer 8

Observe mass limits

AC have multiple load bearing paths so if something breaks somewhere, the structure isn’t totally failed

Question 9

Overloading on performance

Answer 9

The basic stall speed will increase along with all other v speeds apart from V1

Reduced acceleration and deceleration

Reduces the rate and angle of climb

Question 10

Overloading on TO

Answer 10

Reluctant to rotate and lift off

Takes longer to accelerate and rotate

Fail to become airborne

Question 11

Overloading on Climb

Answer 11

Unable to achieve the required obstacle clearance

Lift off point closer to the obstacles

Climb gradient becomes shallower

Question 12

Overloading on Cruise

Answer 12

Decreased Max speed

Decreased max ceiling

Increased Drag

Increased Fuel consumption

Decreased range and endurance

Decreased manoeuvrability

Question 13

Overloading on Landing

Answer 13

Faster approach speed

Increased landing speed

Longer landing run

Brake fade caused by heating of the breaks

Question 14

Overloading on Landing Gear

Answer 14

Can cause SEV tyre wear

Tyres may fail

Severe break wear

Wing spar failure (in the most extreme cases)

Question 15

CG and CG position

Answer 15

Where the mass is considered to be concentrated

Weight acts through CG

Pitches/rolls/yaws around CG (in flight)

CG changes with mass distribution (fuel consumption)

Acceptable range of CG is determined by manufacturer (%MAC)

Question 16

CG In flight (fwd)

Answer 16

Too far froward;
Too Stable
Controls heavy and unresponsive
Cristal stage of flight where the most effect is felt

Cruise:
 Increased fuel burn (elevator drag/trim drag)
Decreased range and endurance
Decreased max speed 
Increased stalling speed

Question 17

CG in flight (aft)

Answer 17

Decreased stall speed
Decreased longitudinal stability 
Decreased control authority (effective elevator)
Could cause uncontrollable pitch up
Insufficient nose down authority 

Controls light and overly reponsive
Insufficient pitch down authority
Spin recover may be more difficult or impossible

Question 18

Desirable CG Position

Answer 18

Operator and the pilot will opt for a slightly aft CG position (within safe limits) to reduce the drag and fuel used

This does however open the AC up to tail strikes in which case they opt for a slightly fwd cg

Question 19

CG on the ground

Answer 19

CG aft of the main gear which can lead to nose up

Should be kept forward of the main gear at all times - care has to be taken when loading to unloading to ensure that this is the case

Tail support struts are sometimes needed

Aft CG - light nose wheel steering

Fwd CG - nose wheel collapse

Question 20

Conclusion

Answer 20

CG effects both performance and handling in flight and on the ground

CG has to remain within limits