Stalls

Question 1

Explain why an aircraft stalls?

Answer 1

As the AoA is increased and exceeds the critical AoA (independant of IAS) the laminar airflow over the upper surface of the wing become turbulent and starts to break off at an earlier stage. This disrupts the pressure pattern on the upper surface reducing the pressure differential between the upper and lower surface of the wing and therefore reducing the amount on lift produced by the aerofoil.

Question 2

What factors affect an aircraft’s stall speed and how do they affect it?

Answer 2

• Weight increases Vs
• Load factor increases Vs
• AoB increases Vs
• Ice increases Vs
• Damage to wing increases Vs
• Wing configuration (i.e. flaps, slats,…) decreases Vs when deployed
• Thrust decrease Vs
• CoG forward increases Vs

Question 3

How does the stall speed vary with weight?

Answer 3

Weight increases the stall speed of an aircraft. For a constant AoA between two aircraft a heavier aircraft will have to fly faster in order to create more lift to counter the extra weight. If both aircraft then approach the critical AoA (an aerofoil will always stall at the same AoA) the heavier will reach this at a higher speed. Vs is proportional to the square root of the aircraft’s weight.

Question 4

What wing design features can be used to prevent the breakup of airflow and cause of a stall?

Answer 4

• Wing slots: they re-energize the airflow over the wing preventing turbulent air over the rear part of the wing and maintaining the pressure pattern.
• Lower angle of incidence and a greater camber for a particular wing section.

Question 5

How does altitude affect Vs?

Answer 5

Altitude does not affect the stall speed of an aircraft when measures in IAS.
The TAS Vs on the other hand will increase with an increase in altitude as density decreases and TAS decreases. Therefore for the same AoA and IAS the TAS will be higher.
Compressibility errors at higher altitudes and speed also affect the EAS of an aircraft and its instruments.

Question 6

How does CoG change the stall speed of an aircraft?

Answer 6

The further forward the CoG the higher the stall speed of an aircraft. This is so, because a forward CoG will induce a nose down pitching moment, which in turn needs to be countered by the tailplane creating a downward force. As this adds to the effective weight of an aircraft the aircraft now needs to fly at a higher speed (for a given AoA) to counter this added effective weight.

Question 7

What is a super stall?

Answer 7

A super stall is a stall which is very difficult or impossible to recover from. This is usually created by the increased nose-up pitching tendency of swept wing (wing tips stall first), rear engine aircraft where the nose area of the fuselage also aid in the nose-up pitching moment and the inefficiency of the T-tail to recover the stall due to it protruding into the turbulent airflow of the main wing.

Question 8

What systems protect against a stall?

Answer 8

• Training
• Stall warning such as stick shakers
• Stick pusher