Stall Speed & Bank Angle
Why Does Stall Speed Increase With Bank Angle?
When you bank while maintaining altitude, your stall speed increases. And it’s something that you need to be aware of, especially when you’re in the traffic pattern. So why does stall speed increase when you start rolling left or right?
WHAT HAPPENS WHEN YOU BANK
When you’re flying straight and level, the lift your wing produces points straight up, opposing gravity.
But when you start to bank, that lift vector starts moving too.
At this point, your lift vector is pointed to the left. And as you can see in the diagram above, you now have two components of lift: a vertical component, and a horizontal component. When you combine the two, you get a total (or resultant) lift vector.
The horizontal component of lift is what makes your airplane turn, and the vertical component is what makes your airplane maintain altitude.
MAINTAINING ALTITUDE IN A TURN
Let’s say you enter a 30 degree banked turn and you don’t change the amount of lift your wing is producing. In the banked turn, some of the lift that was keeping your plane at altitude is now working to turn your plane, and you have less vertical component to maintain altitude.
So how do you turn and maintain altitude? You need to increase the total amount of lift your wing is producing. And to do that, you need to pull back on the yoke, which increases the angle-of-attack that your wing is flying at. This part is important, because when you increase your angle-of-attack, you get closer to critical angle of attack, which is the point when your wing stalls (regardless of airspeed or attitude).
LOAD FACTOR IN TURNS
Another thing that happens in a constant altitude, coordinated turn is load factor.
Load factor is measured in Gs. So if your load factor in a turn is 2 Gs, you feel twice as heavy as you really are (and your arms want to flop down to your seat). The same goes for your airplane – it ‘feels’ twice as heavy.
But what does load factor have to do with stall speed? Stall speed increases in proportion to the square root of load factor.
You can see from the diagram above that as load factor increases, stall speed increases at an exponential rate.
If your eyes started crossing at the mention of ‘square roots’, don’t worry. Here’s a pretty simple example: if your normal stall speed is 40 knots, and you put a load factor of 4 Gs on your airplane, your plane will stall at 80 knots. Here’s the math on that: the square root of 4 is 2. And 2 X 40 knots = 80 knots.
Now 4 Gs is quite a bit, and it’s beyond the limit load factor for a normal category airplane like a Cessna 172 or a Cirrus SR-22, which is 3.8 Gs. But here’s a real world example that you could experience on your next flight: a 60 degree banked turn produces 2 Gs of load factor. And since the square root of 2 is 1.41, that means that your stall speed will be 41% faster in a 60 degree, constant altitude coordinated turn than it would be in straight and level flight.
So if the stall speed (Vs – clean config) in your Cessna 172 is 48 knots, then your stall speed at 60 degrees of bank is 48 knots X 1.41, which equals just over 67 knots.
PUTTING IT ALL TOGETHER
When you turn, you need to increase your total lift to maintain altitude. You increase your total lift by increasing your angle of attack, which means you’re closer to stall than you were in wings-level flight. And, your stall speed increases in proportion to the square root of your load factor.
So the more you bank, at altitude or in the traffic pattern, the more you need to be aware of an accelerated stall. As long as you understand and have a healthy respect for the relationship between bank angle and stall speed, you’ll keep yourself safe and stall-free.