31 Limitations

Question 1

What is the Limit Load

Answer 1

Maximum load expected during normal service

Question 2

What is the Ultimate Load?

Answer 2

The failing load of the structure

Question 3

What is the Factor of Safety and what is it for commercial AC?

Answer 3

• The ratio between the ultimate load and the limit load

- For commercial AC this is 1.5

Question 4

What is the maximum and minimum Load Factor for a normal AC?

Answer 4

2.5 g and -1 g

Question 5

What is the maximum and Load Factor for a Utility AC?

Answer 5

4.4 g

Question 6

What is the maximum and minimum Load Factor for an aerobatic AC?

Answer 6

6.0 g and -3.0 g

Question 7

What is the definition of Va?

Answer 7

‘The maximum speed at which a sudden, full-up elevator pitch can be made without exceeding the Limit Load Factor’

I.e. The max speed which can be flown where with increasing Load Factor the the AC will stall before it reaches the Limit Load Factor

Question 8

Why is Va slower than the intersection of CLmax and Limit Load Factor?

Answer 8

This is due to when the AC pitches up rapidly, the tail plane has a downward velocity. RAF now comes from below creating a large upward load and load factor on the horizontal stabiliser.

To allow for this Va is slightly slower then the intersection of the stall speed and the limit load factor

Question 9

How does Va vary with weight?

Answer 9

As an AC becomes heavier, Va increases

Question 10

What is the effect of altitude on Va?

Answer 10

• As altitude increases, Va increases

- This is due to compressibility effects increasing the stall speed at altitude

Question 11

What happens to the max load factor and the manoeuvre envelope with flaps extended?

Answer 11

• Flaps are only designed to work up to s certain speed
• The max load factor reduces to 2.0 g
• The manoeuvre envelope reduces and is limited by Vfe

Question 12

Define Vb

Answer 12

• Design speed for the maximum gust intensity
• Speed at which the AC can withstand the greatest expected vertical gusts
• • or - 66 fps at 20,000ft

Question 13

Define Vc/Mc

Answer 13

• Design speed for maximum cruising speed
• Used to determine the structural strength of the AC
• If a shallow dive was caused from Vc/Mc then the AC would not exceed Vd

Question 14

Define Vd

Answer 14

• Design dive speed

- Selected such that if an upset caused the AC to enter a shallow dive from Vc/Mc then Vd would not be exceeded

Question 15

What is the definition of Operating Speeds?

Answer 15

The maximum speeds that an AC can intentionally be flown under CS25 and CS23

Question 16

Define Vmo

Answer 16

• Maximum operating speed that must not be exceeded at any phase of the flight
• EAS which must be less than Vc
• At high altitude Vmo is replaced by Mmo which becomes the limiting factor

Question 17

Define Mmo

Answer 17

• Maximum operating Mach Number

- At high altitudes this replaces Vmo as the limiting factor

Question 18

Define Vne

Answer 18

• Velocity never exceed
• Applicable to smaller AC
• Indicated by the red radial line at the fast end of the yellow sector on the ASI
• 0.9 x Vd

Question 19

Define Vno

Answer 19

• Maximum structural cruise speed
• Applicable to smaller AC
• Must not be greater than the lesser of Vc and 0.89 x Vne
• At the slow end of the yellow section and the fast end of the green section on the ASI

Question 20

Define Vle

Answer 20

• Maximum landing gear extended speed

- Maximum speed the AC can be flow with the landing gear down and locked

Question 21

Define Vlo

Answer 21

• Maximum landing gear operating speed
• Maximum speed the AC can be flow with the landing gear deploying
• Sometimes slower than Vle as the doors are not as strong when not locked

Question 22

Define Vfe

Answer 22

• Maximum flap extension speed

- Maximum speed an AC can be flown with the flaps extended in prescribed position

Question 23

What happens to the AOA on an aerofoil when it is hit with a sudden:

• Up Gust
• Down Gust

Answer 23

• Up gusts cause the AOA to increase

- Down gusts cause the AOA to decrease

Question 24

What is the effect of altitude and the density ratio on the Gust Load Factor?

Answer 24

• Gust load factor decreases with increasing altitude
• Climbing at constant IAS, TAS increases with altitude
• The same size vertical gust will therefore increase the AOA by a greater amount at a lower altitude
• Therefore at a lower altitude, gust load factor is greater

Question 25

What is the effect of Aspect Ratio and Sweep back on Gust Load Factor?

Answer 25

- High aspect ratio and straight wings have a much steeper CL to AOA curve compared to low aspect and swept wings - Therefore the change in AOA caused by a vertical gust is much more significant on straight, high aspect ratio wings - The gust load factor decreases with: - Increasing sweep back - Decreasing aspect ratio

Question 26

What is Wing Loading?

Answer 26

Wing Loading is the weight per unit area of wing

Question 27

What is the effect of Wing Loading on Gust Load Factor?

Answer 27

- Increasing Wing Loading, decreases Gust Load Factor | - I.e. A light AC with large wing has a higher Gust Load Factor

Question 28

What is the effect of Airspeed on Gust Load Factor?

Answer 28

- An increase in airspeed, increases the Gust Load Factor - An AC flying at a higher IAS has a lower initial CL value - A vertical gust which increases CL by a specific amount, will have a greater percentage increase in CL for the high IAS AC - E.g. A high IAS AC has an initial CL of 0.4 experiences a gust increasing CL by 0.2. This is a greater percentage increase than a low IAS AC with an initial CL of 0.9.

Question 29

AC with a higher Gust Load Factor are more or less susceptible to turbulence?

Answer 29

More susceptible

Question 30

The factors which tend to increase Gust Load Factor are:

Answer 30

- Low altitude - High speed - High aspect ratio - Straight wings - Low wing loading (i.e. Lighter with bigger wings)

Question 31

What speed should be flown during turbulence?

Answer 31

Vra / Mra

Question 32

What causes Flutter and how can it be delayed?

Answer 32

- Flutter occurs when the frequency of the aerodynamic-elastic couple matches the natural frequency of the structure Delayed by: - increasing stiffness (weight penalty) - moving the wing mass (and therefore the flexural axis) forward

Question 33

What is Control Surface Flutter?

Answer 33

Result of the inertia of the control surface interacting with the torsional twisting of its associated main surface

Question 34

How can Control Flutter be prevented?

Answer 34

- Mass Balancing | - Moving the CG of the control surface onto or forward of the hing line

Question 35

What should be done if control flutter is experienced?

Answer 35

Reduce speed immediately