Whenever lift is increased, for example in a turn, the load factor increases and this causes the stall speed to increase – but why is this?
The lift is split into two components – one vertical and one horizontal. In a turn, the total lift is increased until the vertical component of lift is equal to weight so that the aircraft maintains altitude.
The horizontal component of lift is what makes the aircraft turn, but this also creates a force that’s equal and opposite to the horizontal component – known as the centrifugal force. The centrifugal force may be easier to understand if you think about driving around a corner a bit too fast in your car – you can feel a force pushing your body in the opposite direction to the turn. This is the same centrifugal force.
If we combine the centrifugal force and the weight, we get one force that is equal and opposite to lift – called the apparent weight. The apparent weight effectively makes the aircraft heavier, and as we know an increase in weight leads to an increase in stall speed.
So whenever load factor is increased, the apparent weight of the aircraft is higher and stall speed increases.
Put another way – any time you can feel g-force in the cockpit, stall speed is increased.
The same effect can be seen when pulling out of a dive or in gusts and turbulence.
You will remember from earlier lessons that:
The stall speed always increases by the square root of the load factor – shown on this graph.
In a 60° bank turn the load factor is 2. √2 = 1.41
So in a 60° bank turn, the stall speed increases by 41%.
If the stall speed were 70 knots in straight and level flight, in a 60° bank turn the stall speed would be:
70 x 1.41 = 99 knots
Let’s try another example:
Lift has increased by 1.41 (41%), and weight has remained the same. Therefore, the load factor is 1.41.
√1.41 = 1.19
Straight and level stall speed is 47 knots. So the stall speed in a 45° bank turn is:
47 x 1.19 = 56 knots
Imagine an aircraft that is in a dive when the pilot abruptly pulls back on the control column and raises the nose to try to recover into a climb. The nose rises but because of the aircraft’s inertia, it initially continues along the downwards flight path.
With the nose raised and the aircraft descending, the angle of attack has rapidly increased. The critical angle of attack may be exceeded and the aircraft will stall – despite it still being at high speed!
This is another example of a stall at increased airspeed due to load factor. The rapidly increasing angle of attack will initially generate more lift, increasing load factor. However, as soon as the critical angle is exceeded the aircraft will stall – no matter what the IAS is!